DESC:F O R M 2
THE PATENTS ACT 1970
(Act 39 of 70)

COMPLETE SPECIFICATION
Section 10

POWER GENERATION AUGMENTATION IN HIGH PRESSURE COGENERATION UNIT

Reotech Pulp & Paper Projects Pvt. Ltd, an Indian Company, 28-30, Pantheon Plaza, Pantheon Road, Egmore, Chennai 600 008, Tamil Nadu, India

The following specification describes the invention:
FIELD OF INVENTION:

The present invention seeks to enhance power generation in a steam turbo generator of a High pressure Cogeneration unit, specifically in Pulp and Paper mills, as well as other industries.

PRIOR ART

Cogeneration as the name implies is defined as generation of steam and power from a single source. Chemical recovery boilers along with Coal fired boilers are being used in pulp and paper mills to generate high pressure [HP] steam. Black liquor [with around 70 to 75% concentration solids] is the fuel in Chemical recovery boiler to generate steam, which in turn is led to a steam turbo-generator[STG] for producing power, and for producing medium and low pressure steam for process use-after catering to steam and power for the cogeneration unit. The steam from the boiler outlet fed into the steam turbine is at a high pressure and temperature. In the steam turbo-generators, power is generated because of expansion of steam from higher pressure and temperature to lower steam pressure and temperature (with attendant enthalpy drop) and conversion of thermal energy to mechanical (rotation) and then finally to electrical energy, leading to the generation of electrical power, as well as medium pressure [MP] and low pressure [LP] steam flows. The pressure of the MP and LP steam leaving the respective nozzles of the steam turbine is then further reduced to the desired steam pressure and cooled to the desired steam temperature [above saturation state] by small pressure reducing de-superheating [PRDS] units using boiler feed water as spray - the process of which is known as de-superheating, and is then supplied for processes throughout the plant -apart from meeting auxiliary steam requirement of the HP boiler.

It is also relevant to note that in many cogeneration units, multiple steam turbo generators [STG] are operated such that each turbine produces a specified quantity of lower pressure steam and power, and one or more extraction nozzles are normally present in each steam turbine arranged in battery or otherwise , which extract steam at medium and low pressures.

However, there are certain challenges that are faced by the current process. Standard industry practice is that the boiler and the steam turbo-generator are situated away from each other, and the steam so generated in the boiler is transported to STG through insulated steam pipelines. While steam is being transported, the temperature difference between the steam carrying insulated pipe and the surrounding ambient temperature causes a drop in steam temperature, due to radiation and convective heat losses. This drop in steam temperature causes the steam in the steam turbine inlet to be at a lower temperature, than what is leaving the boiler main steam outlet, thus reducing the power generation capability due to steam being at a lower temperature at inlet to steam turbine. Prior art references relate to the thermal insulation of steam pipelines, wherein pipeline insulation is designed based on the design rated full steam load, so as to restrict this drop in steam temperature to within 10oC. Needless to say, any further reduction in steam temperature drop between the boiler outlet and steam turbine inlet would be beneficial in increasing the power generation by the STG.

Further, splitting of extraction MP steam flows between 2 turbines connected in battery leads to increased extraction steam pressures and temperatures at the turbine nozzles with lowered individual extraction flows from each steam turbine, resulting in marginal increase in steam enthalpies at nozzle outlet. Steam pressures and temperatures are lowered through external de-superheating to suit process and boiler use; Because of the above splitting of steam flows, significant thermal energy is not available within the steam turbines, resulting in reduced power generation.

Due to throttling of LP steam pressure to pre-set values at the nozzle exhaust of each STG, power generation is being further reduced.

Although high efficiency STGs are available in the market, they still require increased steaming conditions i.e., in terms of higher steam pressure and temperature to be fed into the STG inlet, in order to achieve increased power generation from STG.
It is therefore evident that the key to maximising power generation by the STG in a cogeneration unit without increasing the intake of fuel into the boilers (and their operating conditions) connected to the STG, is to maximise the steam thermal energy bound within the steam turbine, by ensuring that the steam entering the steam turbine is at the highest optimal pressure, temperature and load combined, and that the extraction/exhaust steam flow is at the lowest possible temperature at turbine nozzle exit thus maximising the thermal energy bound [Thermal energy of steam at turbine inlet– Thermal energy of steam at turbine extract and exhaust nozzles] within the steam turbine to generate more power on a continuous basis.
In light of the foregoing, it is an object of this invention to maximize /optimize power generation by the STG, without disturbing the fuel intake into the boiler, and maximising thermal energy bound within the steam turbine [turbine inlet to nozzle exit] i.e., maximizing the product of corresponding steam flow and the enthalpy difference between turbine inlet to nozzle exit.
It is another object of the invention to minimize high pressure [HP] steam temperature drop from the boiler main steam outlet to the steam turbine inlet to less than 5oC-at normal and full load operation of the boiler- so as to achieve marginal increase in power generation, by re-insulation with increased thickness (and higher density) of advanced insulation mattress on the steam pipeline, based on the innovative concept of energy insulation.
It is another object of the invention, not to split the extraction steam flows between the two steam turbines in battery, but to avoid/minimise MP extraction steam flow in either of the steam turbine, and to extract the above MP steam from the other steam turbine, thus lowering the LP steam extraction temperature and pressure [lowered steam enthalpy at nozzle exit] of such other steam turbine, and to maximise the enthalpy difference from turbine inlet to LP steam nozzle exhaust to generate more electrical power.
It is yet another object of the invention to enhance high pressure steam generation in the boiler without fuel intake, with lowered boiler steam outlet temperature through increased steam attemperation in the superheater section of the boiler within a narrow band of boiler main steam pressure, and setting the turbine LP steam exhaust pressure lower than that designed by the STG manufacturer, thereby enhancing high pressure steam generation from HP Boiler which is led to the steam turbine, resulting in increased power generation in the STG, and consequently reducing de-superheating of the extraction/exhaust steam after leaving the steam turbine.
The present invention thus seeks to address the aforesaid objectives by enhancing power generation in a three-part manner, the first part being within the Cogeneration units as taught in this invention; the second part being the minimization of steam temperature drop connecting the High pressure boiler and STG by the use of advanced insulation concept of steam pipelines; and the third part increased steam attemperation within the boiler resulting in increased HP steam flow to the steam turbine and by lowering the STG LP exhaust nozzle pressure, all of which together resulting in increased thermal energy bound within the turbine, resulting in significant increase in power generation by the STG and increased overall cycle efficiency.

SUMMARY OF THE INVENTION:
The present invention thus relates to enhancing power generation in a High Pressure Cogeneration unit encompassing an extraction Steam Turbo-generator with HP boiler by maximizing the steam enthalpy cum energy [steam flow] from turbine inlet to the LP exhaust nozzle and also avoiding/minimizing MP steam extraction in the steam turbine. The present invention discloses a 3-part process to achieve enhanced power generation, wherein (i) steam at higher temperature is being made available to the steam turbine without increased consumption of fuel and operating conditions of the boiler; by minimising the steam temperature drop between the boiler and the steam turbine; (ii) steam enthalpy differential between the turbine inlet and the STG nozzle is maximised due to lowered nozzle exhaust steam pressure/temperature resulting from increased extraction steam load; and (iii) additional high pressure steam is generated within the Boiler, through increased attemperation in superheater zone of the boiler leading to more steam availability for power generation, and the pressure of the LP exhaust nozzle of the STG is lowered, all of which combined result in the increased generation of power by making available to the STG additional steam flow at a higher optimal temperature and pressure without any additional fuel consumption. This results in the continuous generation of additional power from the STG, at the cost of lowered water spray for de-superheating after extraction of steam flow from the STG due to reduction in steam extraction temperatures.

A method and process for increasing power generation in two or more high pressure cogeneration units (7,8), is disclosed, by way of a three step method:
increasing HP steam generation within the boiler (25) without consuming additional input fuel; without changing the fuel characteristics or without changing the operating boiler conditions;
Lowering the drop in steam temperature between the main steam outlet (32) of the boiler (25) and the inlet (2) of the turbine (33);
Increasing the energy bound within the turbine (33) by manoeuvring the steam extraction flows in nozzles (34/35/17/16) of the turbines (33/15) of one or more cogeneration units; resulting in steam with lower pressure and temperature being extracted from the turbine (15,33);
all of the above resulting in enhanced power generation by 7to 8% on a continuous basis in each cogeneration unit (8).
The process for increase in power generation using the aforesaid three-step method comprises:
increasing the quantity of high pressure steam produced by the boiler (25) without consuming additional input fuel, changing fuel characteristics or boiler operating conditions, by increasing boiler feed water spray attemperation of steam through an inter-stage attemperator (30) located between the secondary superheater (29) and the tertiary superheater (31), in the superheater section in the boiler (25); such increased spray attemperation resulting in generating additional high pressure steam of 1 to 1.5% within the Boiler (25) and being fed to the steam turbine [33] at optimal high steam temperature;
increasing the thickness of insulating mattress over the steam pipeline, being of higher resistivity [due to increased density of the mattress]; calculating the said insulation thickness based on normal continuous boiler operating conditions instead of design steam flow rating of the boiler (25); resulting in minimising the drop in steam temperature by approximately 40% between the boiler steam outlet (32) and the inlet (2) to the Steam turbine (33);
Maximising the thermal energy bound within the steam turbine (33) by maximising exhaust steam flow in one nozzle (35) of the turbine (33) and lowering nozzle exhaust steam pressure; resulting in exhaust steam with lowered temperature being extracted from the steam turbine (33);
Such lowered steam temperature extracted from the turbine (33) requiring lowered de-superheating using boiler feed water spray;
all of the above stated, resulting in additional power generation by 7 to 8% by the steam turbine (33) on a continuous basis.
The exhaust steam flow from one of the nozzles (16,17, 34, 35) of one turbine of one of the cogeneration unit (7/8) is maximized by:
Avoiding/Minimising the steam extraction flow in the MP extraction nozzle (16/ 34) of one STG (15/ 33) of one Cogeneration Unit (7/ 8), preferably maximizing the MP extraction flow in the steam turbine with higher inlet steam enthalpy [pressure and temperature] conditions;
maximising the steam extraction flow in the MP extraction nozzle (16/ 34) of the turbine (15/33) of the other Cogeneration unit (7/8); altering the steam load, temperature and pressure of such STG (15/33); extracting the deficit steam from nozzle (16/34) of the STG (15/33) in the other Cogen unit (7/8);
Maximising LP steam exhaust (48/ 47) flow in a Cogeneration Unit (7/8) and reducing corresponding LP steam exhaust (48/ 47) flow in the other Unit (7/8);
the total MP steam extraction (44) and total LP steam exhaust (45) hence remaining unchanged excepting marginal reduction due to lowered turbine exhaust/ extraction steam temperatures.
Resulting in increased steam flow with reduced temperatures and reduced pressure, being extracted from the turbines (15,33); maximising steam thermal energy differential between the inlet (2) and the nozzle(16,17,34,35) and enabling the turbine (15/33) to generate more power.
Where a single cogeneration unit (8) operates in isolation, power generation can be increased by (i) increasing generation of HP steam within the boiler (25) without consuming additional input fuel or without changing the operating boiler conditions or without changing the fuel characteristics; (ii) lowering the drop in steam temperature between the main steam outlet (32) of the boiler (25) and the inlet (2) of the turbine (33), all of the above resulting in enhanced power generation by 4% on a continuous basis in such unit (8).
The process and method for reducing the drop in steam temperature from the boiler main steam outlet (32) to the inlet (2) of steam turbine (15/33), is by increasing thickness and density of the advanced Insulation mattress over the main steam pipeline and securing the same with metal cladding, the increased insulation thickness being calculated based on normal steam flow rate as the base value and not based on design /maximum steam load; effecting reduction in radiation and convection losses at all loads of operation and preserving the heat in HP steam to the steam turbine; capable of being retrofitted while the boiler (9/25) is in operation without shutting the HP boiler (9,25) down; manoeuvring the boiler main steam outlet (14/32) temperature such that the difference in steam temperature between the boiler main steam outlet (14/32) and the inlet (2) to the turbine (15/33) is brought down to under 5°C, resulting in maintaining high pressure steam at higher temperature entering the turbine (15/33). This enables the steam turbine (15/33) to generate additional power of 2 to 3%.
The increase in insulation mattress thickness is computed at normal steam load by:

Insulation thickness at Normal steam load = [Design Steam flow /Normal steam flow]*Insulation thickness computed for Design steam load; all other conditions remaining unaltered

The additional power [P] generated by the steam turbine (33) as a function of lowering the drop in steam temperature (DTs) from the boiler main steam outlet (32) to the inlet of the steam turbine(33) and HP steam (Win) in connecting steam carrying pipe-line being computed by:
P [MW] = Win [TPH] * DTs [°C ] * 0.65 * 1.1

The method and process for maximising/increasing energy bound within the steam turbine (15/33) of two or more cogeneration units (7,8) by maximising thermal energy differential between the inlet (2) to the steam turbine (15/33) and the extraction (16/34) and exhaust (17/35) nozzles, comprises
Avoiding/minimising the steam extraction flow in the MP extraction nozzle (16/ 34) of one STG (15/ 33) of one Cogen Unit (7/ 8);
maximising the steam extraction flow in the MP extraction nozzle (16/ 34) of the turbine (3) of the other Cogen unit (7/8); altering the steam load, temperature and pressure of such STG (15/33); extracting the deficit steam from the MP nozzle (16/34) of the STG (15/33) in the other Cogen unit (7/8);
Maximising the LP steam exhaust (48/ 47) in a Cogeneration Unit (7/8) and reducing the LP steam exhaust (48/ 47) in the other Unit (7/8);
the total MP steam extraction (49) and total LP steam exhaust (50) remaining unchanged excepting marginal reduction due to lowered turbine exhaust/extraction steam temperatures; and
Resulting in increased LP steam flow with reduced temperatures and reduced re-set pressure, being extracted from the turbines (15,33); maximising steam enthalpy differential between the inlet (2) and the turbine exhaust nozzle(17/35), and enabling the steam turbine (15/33) to generate additional power of 2 to 3%.
The maximisation/increase of extraction steam flow and its corresponding effect on generation of power is computed by
Mass Balance –
Win[TPH]= Wext + Wexh [at Turbine Nozzle exit] [TPH]
Wherein
Win = Steam flow entering Steam turbine;
Wext = Steam flow at steam turbine extraction nozzle
Wexh = Steam flow at steam turbine exhaust nozzle,

Power Balance:
. PT [MW] = [Pext [MW]+ Pexh[MW]]* AF
Wherein
# Pext = K1 * Wext [TPH]
# Pexh = K2* Wexh [TPH]
where
K1= 0.06[Lower extraction steam load] to 0.075[Highest extraction steam load]
K2 = 0.122 [Lower exhaust steam load] to 0.129 [Highest exhaust steam load]
AF : Ageing factor [deterioration] of Steam turbine, wherein AF = 0.999* Number of years in operation since complete overhauling of turbine internals].

The method and process to increase the quantity of HP steam produced by a boiler (9/25) without consuming additional input fuel, comprises increasing attemperation of steam in the inter-stage attemperator (29) in the superheater section (31) of the boiler (9/25) resulting in increased HP steam being generated by the Boiler (9/25) and being made available to the steam turbine (15/33) to generate additional power of 1 to 2 % , and requiring lesser water spray for de-superheating, thereby generating reduced amount of wetness in steam for use in process.
It is relevant to note that the process and method as disclosed in the present invention can be implemented for increased generation of power in captive power plants as well as in cogeneration plants.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 represents the working of a regular Back pressure extraction STG, comprising.
Main HP Steam pipe from the boiler
Steam turbine inlet
Steam turbine
MP steam extraction nozzle
LP Steam exhaust nozzle
Generator

Figure 2 represents a HP Cogeneration unit integrated with another Cogeneration unit with Steam attemperation as taught in the present invention comprising,
High Pressure Cogeneration Unit 1
High Pressure Cogeneration Unit 2
Unit 1 HP Boiler
Unit 1Steam Drum
Unit 1 Primary Superheater
Unit 1 Primary Superheater Attemperator
Unit 1 Secondary Superheater
Unit 1 Main Steam Outlet
Unit 1 Steam turbine
Unit 1 MP Steam turbine extraction nozzle
Unit 1 LP Steam turbine exhaust nozzle
Unit1 Pressure reducing De-Superheater of MP steam
extraction
Unit 1 Pressure reducing De-Superheater of LP steam
exhaust
Unit 1 Generator
Unit 1 Turbine exhaust steam to condenser
Unit 1 Water Cooled Condenser integrated to Steam turbine
Unit 1 Cooling Tower
Unit 1 STG Condenser to Cooling tower cooling water flow
circuit
Unit 2 HP Boiler
Unit 2 Steam Drum
Unit 2 Primary Superheater
Unit 2 Primary Superheater Attemperator
Unit 2 Secondary Superheater
Unit 2 Secondary Superheater Attemperator;
Unit 2 Tertiary Superheater;
Unit 2 Main Steam Outlet;
Unit 2 Steam Turbine
Unit 2 MP steam extraction nozzle
Unit 2 LP Steam exhaust nozzle
Unit 2 Pressure reducing De-Superheater of MP steam
extraction;
Unit 2 Pressure reducing De-Superheater of LP steam
exhaust
Unit 2 Generator
MP Steam to process and boiler
LP Steam to process and boiler
Unit 1 STG Condenser related Condensate Pump

Figure 3 represents a graphic comparison of turbine exhaust nozzle steam temperatures at various steam loads of STG (33) in Unit 2 (8).
Unit 2 LP steam flow from turbine nozzle [TPH]
Unit 2 LP steam temperature at turbine nozzle [°C ]

Figure 4 represents the levelized power generation drop in STG (33) in Unit 2 (8) corresponding to various exhaust steam loads.
Unit 2 Levelized power reduction due to increased exhaust
steam temperature with lowering of exhaust steam flow[Mkcal/h]

Figure 5 represents the schematic principle underlying the maximisation of steam enthalpy differential between the turbine inlet (2) and the extraction/exhaust nozzle (16,17,34,35), from MP steam shift from STG (33) in Unit 2 (8) to STG (15) in Unit 1 (7), wherein:
Unit 2 MP steam extraction flow [TPH]
Unit 1 MP steam extraction flow [TPH]
Unit 2 LP steam exhaust flow [TPH]
Unit 1 LP steam exhaust flow [TPH]
Total MP steam extraction [TPH]
Total LP steam exhaust [TPH]

Figure 6 represents the process flow chart to check the steam temperature differential between the boiler outlet (32) and steam turbine inlet (2) across the main steam pipeline wherein
Unit 2 Boiler Main HP steam outlet temperature [°C]
Unit 2 HP steam temperature at Turbine inlet [°C]
HP Steam Temperature difference from Boiler outlet to steam
turbine inlet in Unit 2 [°C]

Figure 7 represents the advanced insulation concept related to energy conservation as contemplated in the present invention, comprising,
HP Steam flow in the main steam pipeline connecting Recovery Boiler and 16 MW Steam turbine of Unit 2 [TPH]
HP steam temperature drop between boiler outlet and steam turbine inlet of Unit 2 [ °C].

DETAILED DESCRIPTION OF THE DRAWINGS:

Figure 1 describes the regular working of a Back Pressure STG wherein high pressure steam generated in the boiler is fed into the steam turbine (3) at the turbine inlet (2) through a connecting inlet pipe (1). As that steam flows past the turbine’s (3) rotating blades, the steam expands and the potential energy of the steam is thus turned into kinetic energy in the rotating turbine’s blades, which in turn generates power output (6). Upon expansion, the steam starts cooling, thus resulting in steam with lowered pressure and temperature. This steam flow is then extracted from the MP Steam Extraction nozzle (4) and the LP Steam exhaust nozzle (5) as per pre-set values.

Figure 2 represents Cogeneration unit 1 (7) connected in battery with a second cogeneration unit (8) with steam attemperators as taught in the present invention, in order to extract MP steam and extract LP steam for use in process. High pressure Cogeneration Units 1 and 2 (7, 8) are units that are in battery, and that independently perform the cogeneration process. In terms of Cogeneration Unit 1 (7), boiler feed water at saturation temperature stored in the steam drum (10) is superheated in the boiler (9) using a primary superheater (11) and the secondary superheater (13). The steam outlet temperature is controlled using an inter-stage attemperator (20) which sprays a precise amount of feed water into the superheated steam. The superheated HP steam so generated, is transported from the boiler (9) at the main steam outlet (14), and is fed into the steam turbine (15) at the turbine inlet (2) through an inter-connecting insulated pipe (1). This steam flow, with reduced temperature and pressure upon expansion, is then extracted from the MP Steam Extraction nozzle (16) and the LP Steam Extraction nozzle (17) as per pre-set values, and sent to the respective de-super heaters (18;19) for further temperature/pressure reduction before such MP/LP steam is sent for use in process (21). Remaining portion of the steam is allowed to expand further to vacuum in the turbine (15) and is finally led to a water-cooled Condenser (22). Steam at vacuum is then condensed using cooling water in closed circulation with the cooling tower. Steam condensate is returned to the boiler (9) as feed water using a condensate pump (41), and warm water from the condenser (22) is sent to the cooling tower (23) for cooling. The cooled water from cooling tower then returns to the condenser (22) and the cycle continues. Electrical Power is produced in the Generator (20) connected to the steam turbine (15).

In terms of Cogeneration Unit 2 (8), the process in the HP boiler (25) has been set as taught in the present invention, such that the boiler feed water (at saturation) stored in the steam drum (26) is superheated at three levels, using a primary superheater (27), secondary superheater (29) and a tertiary superheater (31). Superheated steam temperature is controlled at a pre-set value with the help of inter-stage steam attemperation. This aspect is carried out at 2 levels viz., Superheated steam temperature is reduced at two levels using a primary attemperator (28) and a secondary attemperator (30). From the tertiary superheater (31), the high pressure steam so generated is then sent to the Back pressure extraction steam turbine (33). The turbine (33) is provided with an MP steam extraction nozzle (34) as well as a LP steam exhaust nozzle (35), Pressures and temperatures of Extraction/Exhaust steam are lowered using pressure reducing de-super heaters (36/37) before they are sent for use in the process and for boiler auxiliaries (39/40). Electrical Power is produced in the generator (38) unit connected to the Steam turbine (33).

In terms of Figure 3, Extraction steam temperatures from the turbine (33) of Unit (2) at various steam loads can be seen, where the X-axis represents Exhaust steam load (42), and the Y-axis represents exhaust steam temperature at turbine nozzle (43). The graph clearly shows that the lower the extraction load, the extraction nozzle steam temperature is higher; and that with increased exhaust steam flow, steam temperature at nozzle drops.

Figure 4 represents the impact of increase in exhaust LP steam flow in the turbine (33) in Unit 2 (8), on power generation enhancement, resulting from lowered exhaust steam temperature at turbine nozzle (35), wherein the X-axis represents turbine exhaust steam flow rate(42), and the Y-axis represents corresponding increase in power generation with load (44).

In terms of Figure 5, the principle of additional power generation resulting from lowered exhaust steam flow based on increased differential steam enthalpy is represented such that:
the steam temperature and pressure leaving the turbine extraction nozzle (34, 35) gets altered so as to minimise /avoid the Unit 2 MP extraction steam (45) in Cogeneration unit 2 (8), and maximise Unit 1 MP steam extraction (46) in Cogeneration unit 1 (7). The deficit steam is compensated for by extracting an equivalent amount of steam from the steam turbine (15) in cogeneration unit 1 (7), i.e. the total MP steam extraction (49) from the MP extraction nozzle (16) of the steam turbine (15) in Unit 1(7) is the sum of Unit 2 MP extraction (45) and Unit 1 MP extraction (46), the quantity of total MP steam extraction thus remaining unchanged; and
The Unit 2 LP steam exhaust (47) in Cogen Unit 2 (8) is maximized and the Unit 1 LP steam exhaust (48) in Unit 1 (7) is accordingly reduced to that extent, i.e. the total LP steam extraction (50) from LP exhaust nozzles (35,17) of the STGs (33,15) in Unit 1 (7) and Unit 2 (8) is the sum of Unit 1 LP exhaust (48) and Unit 2 LP exhaust (47), the quantity of total LP steam exhaust thus remaining unchanged.

Figure 6 represents the process to check the temperature differential between boiler main steam outlet (51) and the steam turbine inlet (52) across the main steam pipeline wherein the preferred range of HP steam temperature differential (53) is 4- 5oC. If the temperature differential is not within such range, the steam flow rate should be checked, and if not in order, the health of the insulation of pipeline needs to be checked from time to time and if called for, corresponding changes are to be made to the insulation of the main steam line.

Figure 7 relates to the advanced insulation approach to be designed for normal operating main HP steam flow [90-110 TPH] (54) in connecting steam pipe and not for the conventional age old method of designing insulation thickness for design steam flow rating [140 TPH] for achieving lowered steam temperature drop between the two sections (55).

DETAILED DESCRIPTION OF THE INVENTION:

The present invention is described in detail hereinbelow with reference to the preferred process for enhancing power generation in a High Pressure [HP] Cogeneration Boiler integrated to an extraction Back pressure Steam Turbo-generator by maximizing the thermal energy bound within the steam turbine, i.e. difference between thermal energy at the turbine inlet and the thermal energy at MP and LP turbine nozzle exit), and by minimizing /avoiding MP steam extraction in at least one Back Pressure Steam turbo-generator. A key aspect of the invention is that it achieves enhancement in power generation, not by increasing input fuel, but by maneuvering the existing process to generate more power with the same Cogen unit and operating conditions without disturbing input fuel [rate and characteristics]. This is accomplished using a three part process explained hereinbelow using Figure 2.

In terms of Figure 2, two High pressure cogeneration units (7, 8) are described, comprising two HP boilers (9, 25) integrated with their respective steam turbines (15,33) generating power, such that the steam so generated in each boiler (9,25) is fed to the respective steam turbines (15,33). It is to be noted that Figure 2 represents an embodiment of the invention, based on the cogeneration unit where the invention was tested, and that its principle and working extends to all other kinds of cogeneration units as well, whether they contain one or more STGs, and whether such multiple STGs are connected in battery or not.

A key aspect, forming the first part of the three-part process of this invention is minimising the temperature drop of HP steam from the HP boiler (25) to the Steam turbine (2) at normal operational and design rated steam loads. As mentioned hereinabove, drop in steam temperature from the boiler (25) to the steam turbine (33) causes the steam input to steam turbine inlet (2) to be of a lower temperature than what is sent through the boiler outlet (32), thus reducing the power generation capability due to lesser expansion of steam within steam turbine (33). The present invention seeks to address this challenge by the use of advanced thermal insulation concept, i.e. redesigning the insulation of the steam pipe by increasing thickness of the Insulation mattress (as also the density of mattress packing) keeping normal steam flow with lower temperature drop as the base values for calculation, and not designing the thickness of the insulation mattress based on maximum steam load, which is the general principle for insulation design in prior art references. This novel design thus effects significant reduction in radiation and convection losses at all loads of operation; thereby preserving the heat in HP steam, which otherwise would have been wasted to the atmosphere through radiation and convection losses. For higher temperatures, increased density of the insulation (effecting lowered thermal conductivity) mattress aids in combating radiation heat. In order to ensure that the mattress is secured for a long time stretch, a new cladding method had been successfully employed. It has been demonstrated that such insulation can be retrofitted even when the boiler (25) is in operation; this being implemented without shutting down the operation of the boiler- which is the first of its kind in this regard. Such insulation also helps in manoeuvring the boiler main steam outlet (32) temperature. In a preferred embodiment of the invention, the difference in steam temperature is brought down to under 5°C as opposed to >10°C, which is the average steam temperature drop observed earlier, from the boiler to steam turbine.
This reduction in steam temperature drop causes HP steam entering the turbine (33) to be at a higher temperature, thereby leading to enhanced power generation. The present invention has further sought to automate control of the parameters of the transmission of steam from the boiler outlet (32) to the steam inlet (2) of the turbine (33). The Algorithm for additional power [P] generated in the STG, related to reduction in difference in steam temperature from the boiler (25) to the steam turbine (denoted as DTs below;33) and HP steam [denoted as Win] in the connecting steam carrying pipe-line is computed by:
P [MW] = Win [TPH] * DTs [°C ] * 0.65* 1.1

Referring to Figure 6, it is possible that at lower steam loads of operation, the set temperature drop limit might be exceeded. In all these cases, it is to be ensured that there is no HP steam being led from the HP Boiler (25) through the main pressure reducing de-superheater unit bypassing the STG (33) in operation. The de-superheater (36,37) shall be functional only to allow the LP/MP steam leaving the nozzles of the steam turbine (33) being sent for process use-instead of venting the steam to atmosphere. Therefore, such advanced insulation of the main connecting steam pipe-line keeps steam pressure and temperature drop limited to a small band width, resulting in increased additional Power generation from the STG (33), along with marginally reduced LP steam generation with lowered de-super heating before being sent for use in process (38).

Once the HP steam enters the steam turbine (33), it expands due to high temperature and pressure, which causes the turbine (33) blades to rotate, thereby generating electric power. The steam turbine (33) is provided with an MP steam extraction nozzle (34) as well as a LP steam exhaust nozzle (35), wherefrom the extraction steam and exhaust steam are respectively removed, and then temperature and pressure are lowered using de-super heaters (36,37) before it is sent for use in process (39,40).

Another key aspect of this invention which forms the second part of the inventive process disclosed herein below resulting in enhanced power generation is the alteration of steam temperature and pressure leaving the STG extraction nozzles (16,17, 34, 35) so as to avoid/minimise the steam extraction flow in the MP extraction nozzle (16/34) of the STG (15/33) of one Cogeneration Unit (7/8), and to maximise the steam extraction flow in the MP extraction nozzle (16/ 34) of the turbine (3) of the other Cogeneration unit (7/8). The deficit steam, i.e. the steam not extracted from the nozzle (16/34) of one STG (15/ 33) of a Cogeneration unit (7/8) is compensated for by extracting an equivalent amount of steam from the nozzle (16/34) of the STG (15/33) in the other Cogeneration unit (7/8) by altering the steam load, temperature and pressure of such STG (15/33). Similarly, the LP steam exhaust (47/48) in a Cogeneration Unit (7/8) is maximized and the LP steam exhaust (47/48) in the other Unit (7/8) is accordingly reduced to that extent.

To explain the process by way of an example, it is assumed that the turbines (15,33) each draw 100 TPH of HP steam, such that the MP steam extraction (40, 41) through the MP nozzles (16,34) will be 20 TPH in each turbine (15,33), and the LP steam exhaust (42,43) through the LP nozzles (17,35) will be 80 TPH in each turbine (15,33). In terms of the present invention,
Unit 2 MP extraction (45) will be reduced to 0;
Unit 1 MP extraction (46) will increase by an amount equivalent to the deficit steam not extracted from Unit 2 MP nozzle (34), i.e. 20 TPH. Therefore, the Unit 1 MP extraction (46) will be 40 TPH;
Unit 2 LP exhaust (47) will be 100 TPH (since there is no MP extraction from the turbine (33) in Unit 2 (8); and
Unit 1 LP exhaust (48) will stand reduced by an amount equivalent to the additional steam extracted from the MP nozzle (16) of the turbine (15) of Unit 1 (7), i.e. its exhaust will be 60 TPH.
Thus, the total MP steam extraction (49) in both turbines (15,33) will be 40 TPH, i.e. the same as before, and the total LP exhaust (50) will be 160 TPH, which is the same total quantity as before (less a marginal reduction due to lowered exhaust steam temperature). However, using the present invention, Unit 1 MP extraction (46) is maximised in Cogen Unit 1 (7), Unit 2 MP extraction (45) is reduced to 0, and Unit 2 LP exhaust (47) is maximised, and Unit 1 LP exhaust (48) is accordingly reduced.

Figure 3 which studies the relationship between the relationship between extraction steam flow and the exhaust steam temperature reveals that reduced turbine extraction/exhaust steam flow results in increased extraction/exhaust steam temperature. This reduces power generation capacity as steam is extracted from the turbine (15/33) at a higher temperature when the extraction flow is lesser. The present invention therefore teaches the minimizing of exhaust steam temperature, i.e. enthalpy to the extent practicable, by maximising the extraction/exhaust flow from one MP nozzle (16/34, 17/35) of each STG (15/33) as shown hereinabove. This is further confirmed by Figure 3, which shows that at higher extraction flows, subject to the maximum extraction of the STG (15/33), the exhaust steam temperature is lesser, leading to steam enthalpy resulting in increased power generation (8) within the STG (15/33).

Generally, temperature of steam extracted from the nozzles (16, 17) of steam turbine (15) is much higher than what is required for the process(20). Extraction steam is therefore passed through a pressure reducing de-superheater (18,19), to reduce the temperature and pressure to the desired level. This would be the case for Cogeneration Unit 2 (8) as well. In terms of the present invention, by maximising thermal energy within the turbine (33) and steam enthalpy differential from the turbine inlet (2) to extraction nozzle (34, 35) of the turbine (3) of Cogeneration Unit 2 (8), the extraction/exhaust steam is at a lower temperature, thereby leading to reduced use of the de-superheater.
The third part of the inventive process in the present invention is contemplates partially shifting the de- superheating principle after e STG (33), and instead applying it within the Boiler (25), i.e. increased attemperation of steam in interstage attemperator (30) of superheater section(31) of the Boiler (25) . The principle underlying this shift is that the cogeneration unit (8) should be designed such that maximum quantity of steam at the optimum temperature and pressure is generated in the boiler (25) and fed into the STG (33) in order to maximise power generation, and that any steam generated after the STG (33) is power lost, as it cannot be utilised to generate any power. By adopting the aforesaid shift, the boiler (25) is able to generate additional steam without consuming any additional fuel.
Through increased steam attemperation in superheater section of the boiler (25), steam flow entering turbine inlet (2) increases, and this combined with the aforesaid processes results in lowered steam temperature at steam turbine exhaust nozzle (34, 35) [Figure 5 and Figure 6]. In addition to the foregoing, lowering the pressure of the LP exhaust nozzle (35) than that designed by the STG manufacturer further reduces de-superheating of the extraction/exhaust steam after leaving the steam turbine (3).
This additional high pressure steam from the Boiler (25) when fed into the steam turbine (33), in addition to minimal steam temperature drop owing to enhanced insulation, coupled with maximised steam enthaply differential between the turbine inlet (2) and the extraction /exhaust nozzle (34,35), shall result in an increased quantity of steam at a higher pressure and temperature being utilised in the STG (33), thereby resulting in enhanced power generation. The three-pronged approach as taught in this invention when applied results in increased power generation (8) by the STG (33) without increasing the input fuel into the boiler (25). It is relevant to note that in terms of Figure 2, Cogeneration Unit 2 (8) has been set in terms of the inventive process taught herein. However, it is to be noted that it is not necessary that only 1 unit be so modified, and that it is possible to modify the process in any or all Cogeneration Units in the manner as disclosed herein, so as to enhance power generation.

PREFERRED EMBODIMENT OF THE INVENTION:

The working of the present invention has been validated in a Chemical Recovery Boiler integrated with a 16 MW Back pressure Steam Turbo generator (shown as Cogeneration Unit 2 in Figure 2), in M/s Seshasayee Paper and Boards Limited, Erode unit. The three-pronged approach of the present invention has been applied to the design of one of the 2 Cogeneration units -Unit 2 (8), and results of the working of the plant and the power so generated have been obtained. In order to better explain the present invention, the working of the invention as demonstrated in the plant is explained hereinbelow. It is to be noted that the below mentioned parameters merely constitute a preferred embodiment of this invention, and various modifications and changes may be made without departing from the invention, by those skilled in the art, in order to suit different cogeneration units across different industries.

The Steam turbine is as shown in Figure 2, wherein two cogeneration units are present, with Cogen unit 1(7) containing a 21 MW STG and Unit 2 (8) containing a 16 MW STG, Cogeneration Unit 2 (8) being the unit where the modifications have been made in terms of this Invention. As regards Unit 1 (7), the steam generated in the boiler (9) is transported from the boiler (9) at the main steam outlet (14), and is fed into the turbine (15) for cogeneration. Upon power generation, steam flow with reduced temperature and pressure is then extracted from the MP Steam Extraction nozzle (16) and the LP Steam Extraction nozzle (17) as per pre-set values, and sent to the respective de-super heaters (18, 19) for cooling before they are sent for use in process (20). The original design varies extraction and exhaust steam flows from the turbine (15) with fixed steam pressure and temperature. The design is such that the extraction steam pressure at the MP steam extraction nozzle (16) is designed for 12 ksc at the highest design extraction steam load. Exhaust steam flows shall keep on varying. The nozzle steam pressure is set at 6.1 ksca design. At lower extraction steam loads, the steam pressure as well as the nozzle steam temperature at the turbine nozzles (16,17) would proportionately be higher. The exhaust steam temperature would be well over saturation temperature, thus leading to increased steam for de-superheating in order to lower steam temperature just above saturation condition. Essentially, the high-pressure steam is not fully utilised to maximise power generation.

In terms of the STG (33) integrated with the HP Boiler (25) in Cogeneration Unit 2 (8), the existing design steaming conditions are as follows:


A. Chemical Recovery HP Boiler -EAPL

Parameter Value Units
HP Steam Generation 60 to 140 TPH
Steam outlet Pressure 65 Kscg
Steam outlet Temperature 465±5 °C
Feed Water Temperature 135 °C
Fuel Black liquor [ Biomass]

B. Steam Turbo-Generator [STG]-BHEL
Parameter Value Units
Design Steam inlet Flow 130 TPH
Steam inlet Pressure 63 Kscg
Steam Inlet Temperature 455 °C
MP Extraction Flow [max] 40 TPH
MP steam extraction pressure 12-20 Kscg
MP steam extraction temperature 283- 320 °C
LP steam exhaust flow [max] 130 TPH
LP steam exhaust Pressure 5.1 Kscg
LP Steam Exhaust Temperature 183-207 °C
Rated Power Generation @ 0.80 Power Factor & 50 Hz ( Frequency) 16 MW

The present invention sought to modify the original design by applying its three-pronged approach to enhance power generation in Unit 2 (8), i.e. (i) minimising steam temperature drop between the boiler (25) and the STG (33); (ii) maximising steam enthalpy differential in the STG (33) by increasing extraction flows at the STG nozzle (34, 35); and (iii) partially shifting the de- superheating principle after the STG (33), and instead applying it within the Boiler (25), i.e. increased attemperation of steam in inter-stage attemperator (30) located between the secondary superheater (29) and the tertiary superheater (31) of the superheater section of the Boiler (25). The details of each step are explained herein below:
MINIMISE- Steam temperature drop
The present invention minimises steam temperature drop between the boiler main steam outlet (32) and the STG inlet (2) by the use of advanced thermal insulation reducing radiation and convection losses to a minimum, thereby preserving the heat which otherwise would have been wasted to the atmosphere through radiation and convection. Such insulation can be retrofitted even when the boiler (25) is in operation; thus being implemented without shutting down the operation of the plant.
Originally, in terms of lower steam load operation (80 TPH), the steam temperature drop from the Main steam outlet (32) to the STG inlet (2) was around 15oC. Through strengthening of the thermal Insulation of the Steam pipe-line, this steam temperature differential has been reduced to around 5°C at economic continuous rating, i.e. at 100 TPH steam flow of operation. This is achieved by application of increased re-insulation to the main HP steam pipeline, being a re-insulation mattress density of 144 kg/m3 coupled with enhanced thickness of 210 mm for the superior grade insulation mattress, i.e.45% increase in insulation effectiveness.

The insulation parameters taught by the present invention are significant because they are a clear departure from standard insulation practices which prescribe 100 kg/m3 mattress density and 144 mm thickness, and recommend insulation based on the design principle of arriving at insulation thickness for design maximum steam flow (i.e. at 140 TPH) in the carrying main steam pipeline.

The following table serves as best illustration for the present case study.

Impact of HP steam flow variation in pipeline on steam temperature drop
Main Steam flow Steam temperature drop from Boiler outlet to Steam turbine inlet
TPH °C
140 3 4 5 6 7 8
130 3.2 4.3 5.4 6.5 7.5 8.6
120 3.5 4.7 5.8 7 8.2 9.3
110 3.8 5.1 6.4 7.6 8.9 10.1
100 4.2 5.6 7 8.4 9.8 11.2
90 4.7 6.2 7.8 9.3 10.9 12.4
80 5.2 7 8.8 10.5 12.2 14

The increase in insulation mattress thickness is related to Normal HP steam flow in the steam pipeline is given by the formula as under:

Insulation thickness at Normal steam load = [Design Steam flow /Normal steam flow]*Insulation thickness computed for Design steam load; all other conditions remaining unaltered

In the present invention, increased insulation thickness and mattress density of superior grade in terms of effectiveness, had been applied, over which metal cladding is installed to secure the insulation and protect from rain, etc. All of the above has been carried out at an elevated level, while the boiler (25) was in operation. Further, the thermal insulation of HP steam carrying pipe at the STG (33) end was strengthened.

It is seen that the use of increased and advanced insulation as set out herein above, lowers the steam temperature drop to 5oC at operating steam load of 112 TPH.
Operational performance data is set out hereinbelow.
Parameter Units Recovery Boiler Main steam outlet 16 MW STG inlet Drop
Steam flow rate TPH 112 112 -
Steam Pressure Kscg 63.5 63 0.5
Steam Temperature °C 456 451 5
The increase in power generation by lowering steam temperature drop between the boiler (25) and the STG (33) as per the invention is captured in the table above.
The Algorithm for Additional Power generation [P] related to reduction in differential steam temperature[DTs] from the boiler (25) to the steam turbine (33) for specific steam load[Win] is computed by:
P [MW] = Win [TPH] * DTs [°C] * 0.65 *1.1

Specific heat of superheated steam = 0.65 kcal/kg °C
This results in additional Power generation of 0.325 MW for 112 TPH HP steam generation, at 63 kscg pressure and 451oC temperature.
Given that radiation and convection heat losses vary only with change in temperatures of ambient and insulated surface of pipe carrying HP steam, and remain constant for steam flow variation in the main steam pipe-line, the steam temperature drop is expected to further reduce to ~4°C with rated design steam flow of 130 TPH.
The present invention has further sought to automate control of the parameters of the transmission of steam from the boiler main steam outlet (32) to the steam inlet (2) of the turbine (33), such that a very small increase in steam temperature differential would be related to lowered steam loading/flow and vice-versa; and (i) in the event of increased steam temperature difference without corresponding steam flow change, insulation weakness/shortfall can be identified and suitably remedied.
In this manner, the steam temperature difference shall be reduced/minimal at all times of operation, and power generation enhancement to the tune of 0.3 to 0.35 MW [~3%] is achieved on a continuous basis.

SHIFT- Maximising steam enthalpy differential between Steam Turbine inlet and turbine exhaust/extraction nozzles
The second prong of this invention is the maximisation of steam enthalpy difference between steam turbine inlet (2) and turbine extraction and exhaust nozzles (34, 35) resulting in steam at higher temperature and pressure being utilised at inlet to STG (33) and exiting the turbine nozzles at lower temperature and pressure, to generate additional power. This is achieved by the alteration of steam flow, steam temperature and pressure leaving the STG extraction nozzles (34, 35) so as to minimise /avoid the steam extraction flow in the MP extraction nozzle (34) of the STG (33) of Cogeneration Unit 2 (8), and to maximise the steam extraction flow in the MP extraction nozzle (16) of the turbine (15) of Cogen unit 1 (7); and to maximise the LP steam nozzle (35) of the turbine (33) of Cogen Unit 2 (8). The deficit steam, i.e. the steam not extracted from the MP nozzle (34) of STG (33) of Cogen unit 2 (8) is compensated for by extracting an equivalent amount of MP steam from the extraction nozzle (16) of the Steam turbine (15) in Cogeneration Unit 1 (7) by altering the steam load, temperature and pressure of such steam turbine (15).

In terms of validation of the aforesaid concept in Cogeneration Unit 2 (8), the alteration of extraction/exhaust steam has been done such that more steam is extracted from the STG of Unit1. As mentioned earlier, in the demonstration (Validation) testing plant, Cogeneration Unit 1 (7) has an STG of 21 MW capacity related to Double extraction Condensing, whereas Cogeneration Unit 2 (8) has a 16 MW STG related to Back pressure extraction. Hence, when testing the aforesaid principle, extraction flow of steam was maximised such that:
the steam temperature and pressure leaving the STG extraction nozzle (34/ 35) was altered so as to minimise /avoid MP steam extraction flow in the extraction nozzle (34) of the STG (33) of Cogeneration unit 2 (8; since it uses black liquor, a Renewable Biomass, as fuel in Boiler), and maximise MP steam extraction flow in the extraction nozzle (16) of the STG (15) of Cogeneration Unit 1 (7). Deficit steam, i.e. MP steam not extracted from the nozzle (34) of the STG (33) of Cogeneration Unit 2 (8), is compensated for by extracting an equivalent amount of MP steam from the nozzle(16) of the steam turbine (15) in cogeneration unit 1 (7) by altering the steam load, temperature and pressure of such steam turbine (15);
Both STGs (15, 33) are connected as a battery in operation. Thus, by preferably avoiding MP steam extraction in the STG of Cogeneration Unit 2 (8), and extracting an amount of steam equivalent to the deficit steam, from the MP steam extraction nozzle (16) of the steam turbine (15) of Unit 1 (7), the extraction flow from the nozzle is maximised- instead of splitting the medium pressure steam flows between the 2 Steam turbines. The crux of maximising power generation (by 3 to 4 %) is to have all MP steam extracted from either one of the two Steam turbines (15/33) in operation, and the other STG unit would have zero/minimal MP steam extraction.
The LP steam exhaust flow from the LP nozzle (35) of Cogeneration Unit 2 (8) is maximized and the LP steam exhaust flow from the LP nozzle (17) of Cogeneration Unit 1 (7) is accordingly reduced to such extent. The High pressure and temperature steaming conditions are adjusted to suit such varying steaming flows. The exhaust steam pressure at nozzle exit is set lower, at 5.7 ksca [i.e. 0.4 ksc lower than design rating of 6.1ksca] This increased exhaust steam flow and lowered turbine nozzle set pressure results in lowered exhaust steam temperature and enthalpy at the STG exhaust nozzle (16, 17, 34, 35), thereby resulting in increased energy bound within the turbine (15,33) and reduced De-superheating steam after the turbine nozzle (16, 17, 34, 35); and
The sliding algorithm for computation of individual Power as also total Power generation (8) by the STG (15, 33) is developed such that exhaust steam loads can be maximised at all times for enhanced power generation.
The algorithm for calculating power Generation through the STG (15, 33) is as follows:
Mass Balance –
Win[TPH]= Wext + Wexh [at Turbine Nozzle exit] [TPH]
Wherein
Win = Steam flow entering Steam turbine;
Wext = Steam flow at steam turbine extraction nozzle
Wexh = Steam flow at steam turbine exhaust nozzle,

Power Balance:
PT [MW] = [Pext [MW]+ Pexh[MW]] * AF
wherein
# Pext = K1 * Wext [TPH]
# Pexh = K2* Wexh [TPH]
K1 = 0.06 [Lower ext. steam load] to 0.075 [Highest ext, steam load]

K2 = 0.122[Lower exh. steam load]to 0.129[Highest Exhaust steam load]

AF : Ageing factor related to efficiency reduction due to deterioration of steam turbine internals over the years , being of the order of 0.1 % per year of operation, i.e.
AF = 0.999 * N ; where N : Number of years of STG in operation.

Steam temperature and corresponding steam enthalpy differential with saturation in exhaust nozzle (35) of the STG (33) in Cogeneration Unit 2, as designed for various turbine exhaust loads is set out below.

Levelized Power Reduction with lowering of exhaust LP steam load
Steam Turbine Inlet Pressure : 64 ksca ;
PN : 6.1 ksca ; Tsat : 159°C ; hi : 658.45 kcal/kg.
E2- MPS
TN DT [TN-Tsat] hN Dh [hN-hN130] Levelized Power reduction
TPH °C °C Kcal/kg Kcal/kg
Mkcal/h
48
207 48 684.2 13.0 0.62
62
200 41 680.5 9.3 0.58
72
194 35 677.4 6.2 0.45
82
190 31 675.3 4.1 0.34
105
183.3 24 671.7 0.5 0.05
130
182.5 23 671.2 Basis Basis
Set out hereinbelow is the impact on power generation upon varying the LP steam exhaust flow in the STG (33) of Unit 2 (8) based on the original design parameters.

E2 E2 Nozzle Pressure E2 Nozzle Temp hE2
hi—hE2
Power Conversion factor Unit Power reduction
TPH Ksca °C Kcal/kg Kcal/kg - %
48 6.1 207 684 105.5 1.195 11
62 6.1 200 680.5 109.2 1.236 8
72 6.1 194 677.5 112.3 1.271 5
82 6.1 190 675 114.4 1.296 3
105 6.1 183.5 671.7 118.0 1,335 0.5
130 6.1 182.5 671.2 118.3 1.34 Base

Note: The actual operational figures are one of higher steam temperatures at steam turbine exhaust at nozzle end and before PRDS. Accordingly, the Power generation figures vary.
The validation of the increase in power generation from the 16 MW Back pressure steam turbine (33) corresponding to maximised steam enthalpy differential between the boiler main steam outlet (32) and the turbine inlet (2) is shown below, the testing conditions being (i) Zero Extraction from the MP nozzle (34) in 16 MW STG in Unit 2 (8); (ii) HP Steam Inlet Pressure: 62 to 63 kscg ; and (iii) Steam Inlet Temperature : 450 °C
Parameter Units A B C D
HP Inlet steam flow TPH 104 103 102 111
%Rated Steam flow to STG % 80% 79% 78% 85%
MPS Extraction flow TPH 0 0 0 0
LPS Exhaust Flow TPH 104 103 107 111
LP Steam Exhaust Pressure Kscg 4.7 4.7 4.7 4.7
LP Steam Exhaust Temperature °C 189 189 189 188
Power Generation MW 12.5 12.3 12.9 13.3
Specific Steam Consumption [SSC] MW/TPH 0.12 0.11 0.121 0.12
SSC corrected* for inlet steam temperature MW/TPH 0.123 0.114 0.124 0.124
SSC Corrected* for lowered HP steam temperature [DT] of ~4 to 5°C [455 -450/451°C ] as compared to Design rating at Steam Turbine inlet is :
=SSC+ DT* Specific heat * 1.1 /HP Steam flow rate =SSC+ [4 to 5°C] * 0.65 * 1.1/[104 to 111TPH] =SSC+ 0.003 to 0.0035 MW/tph=0.124 MW/TPH
Thus with shifting MP steam extraction totally to Cogeneration Unit 1 (7) and maximizing the LP exhaust steam load in the STG (33) in Cogeneration Unit 2 (8), power enhancement realization is over 0.5 to 0.6 MW [4-5%] for approximately 20 TPH MP steam shift to LP steam exhaust in STG (33) of Unit 2 (8) and corresponding changes in steam split in Steam turbine (15) of Unit 1 (7).
It should be noted that validation of the present invention in 16 MW STG (33) has been carried out well after a decade of commissioning of the STG [year :2008] and continued operation of the unit (8) with annual ageing degradation factor, probable harmonic losses and generator efficiency to be reckoned with for performance analysis.
Set out hereinbelow is an extract from Steam Tables of steam enthalpies [h] at various steam pressures and temperatures related to both the Cogeneration units [Unit 1 and Unit 2].

STEAM TABLES [ Extract]- Steam Enthalpy [kcal/kg]
LP Steam
P[kscg] 3.0 3.5 4.0 4.5 5.0 5.5
P[ksca] 4.0 4,5 5.0 5.5 6.0 6.5
Tsat°C 143 147 151 155 158 161
hsat[kcal/kg] 654 655 656 657 658 659
144°C 654 - - - - -
150°C 658 657 - - - -
160°C 663 662 661 660 659
170°C 668 668 667 666 665 664
180°C 673 673 672 671 670 669

MP Steam
P[kscg] 10.0 11 12 13 14 15 16 17 18
P[ksca] 11 12 13 14 15 16 17 18 19
Tsat°C 183 187 191 194 197 200 203 205 208
hsat[kcal/kg] 664 665 666 666 667 667 668 668 669
190°C 668 667 - - - - - - -
220°C 686 684 683 682 681 680 679 678 677
250°C 702 701 700 699 698 697 696 695 694
280°C 717 716 716 715 714 712 713 712 711
310°C 732 732 731 730 730 729 728 727 726

HP Steam -Unit2
P[kscg] 60 61 62 63 64 65
P[ksca] 61 62 63 64 65 66
440°C 782 782 782 781 781 780
445°C 785 785 784 784 784 783
450°C 788 788 787 787 787 786
455°C 791 791 790 790 790 789
460°C 794 794 793 793 793 792
465°C 797 797 797 796 796 795

HP Steam -Unit 1
P[kscg] 98 100 102 104 106
P[ksca] 99 101 103 105 107
490°C 800 800 799 798 797
500°C 806 806 805 805 804
510°C 812 812 811 811 810

SWITCH- Increased attemperation within the HP Boiler- Unit 2
The third prong of the invention contemplates partially shifting the de- superheating load after the steam turbine (33), and instead applying it within the Recovery Boiler (25), i.e. increased attemperation of steam in the inter-stage attemperator[30] of the superheater section of the Boiler (25) superheater (31). The principle underlying this shift is that the cogeneration unit (8) should be designed such that maximum quantity of steam at the optimum temperature and pressure is generated in the boiler (25) and fed into the STG (33) in order to maximise power generation, and that any steam generated after the STG (33) is power lost, as it cannot be utilised to generate any power. By adopting the aforesaid shift, the boiler (25) is able to generate additional steam without consuming any additional fuel.
The validation of this aspect of the invention has been shown in the 16 MW steam turbine (33) in Unit 2(8). The STG (33) is designed to handle HP steam of 63 kscg pressure and temperature of 455 °C and the Recovery Boiler (25) at 65 kscg and 465°C. The Recovery Boiler (25) main steam temperature had been operated at sometimes even around 10 to 15 °C lower i.e., 450 °C also to safeguard boiler superheater not being exposed to higher steam temperatures for ensuring continued availability of the boiler (25).
In terms of the present invention, since Chemical Recovery boiler-Unit 2 is operated at lower boiler main steam outlet temperatures, increased attemperation within the boiler (25) is possible through second inter-stage steam attemperator (30) located between the secondary superheater (29) and the tertiary superheater (31). This led to increased HP steam generation within the Boiler (25) superheater section. Given that such increased thermal energy is fed to the steam turbine (33), this led to increase in Power generation, without increasing input fuel consumption. Thus, the effect is that desuperheating of steam after the turbine (33) exit is lowered, and instead, the steam generated within the boiler (25) is increased, without increasing the consumption of the fuel, thereby generating more power at the cost of marginal reduction in water spray for final desuperheating, and no additional fuel consumption.
The additional steam quantity realised using this approach, and the increased HP steam available to the STG is around 0.6 TPH as shown herein below:
Additional HP Steam generation with lowered Main steam temperature setting
Parameter Units Value Computation
HP Steam Flow at-Boiler outlet TPH 112
Steam temp. inlet to Steam turbine °C 451
Design steam temp. at Steam turbine inlet °C 456
Increase in Superheater inter-stage Attemperation TPH 0.57 =112 * [456-451) * 0.65/ [787-140]

When the boiler (25) is set at 455 °C at the main steam outlet (32), the increase in HP steam generation through Superheater inter-stage steam attemperation would be doubled at ~ 1.1 TPH. In addition, LP steam turbine exhaust nozzle is reset at lower pressure of 4.7 kscg [as against design pressure of 5.1 kscg] resulting in lowered exhaust steam temperature and consequent enthalpy reduction to a marginal extent. The additional HP steam generation increasing thermal energy within the turbine (33) due to the above, results in enhanced power generation in the STG (33) to the tune of 0.15 MW [1-1.5%]. This aspect, combined with the aforesaid step of maximising steam enthalpy differential between the STG inlet (2) and extraction nozzles (34, 35) further ensures that the extraction steam is at a lower temperature, thereby resulting in reduced water spraying for de-superheating. It is preferable to minimize desuperheating of steam to the maximum extent possible reducing possibility of wet steam for process use.
Thus, when all three aspects are combined, total thermal energy to the
steam turbine is maximised and the enthalpy differential from the turbine inlet to the LP/MP nozzle exit is maximised. Thermal energy is strengthened at the turbine inlet with increased steam temperature / additional steam flow; followed by maximizing thermal energy from turbine inlet to nozzle exit. (16, 17, 34, 35).

Through control automation system available in DCS, the following inference algorithm [AI] is advocated for enhanced productivity on a sustained basis:
To Adjust the steam pressure leaving the Boiler between 61 and 65 kscg to suit an increase or decrease in turbine exhaust steam flow, and
To Vary the steam temperature [through final inter-stage steam attemperation in superheater section] leaving the boiler main steam outlet and entering the turbine to achieve lower turbine exhaust steam temperature to the extent practicable, at the lower re-set turbine exhaust pressure.

Therefore, the process contemplated in this invention, seeks to maximise power generation in a high pressure cogeneration unit using the aforesaid three-pronged approach, wherein (i) steam at optimally higher temperature is made available to the STG (33) without increased consumption of fuel, by minimising the steam temperature drop between the boiler (25) and the STG (33); (ii) steam enthalpy differential between the turbine inlet (2) and the STG nozzle (16,17,34,35) is maximised due to lowered nozzle exhaust steam temperature and lowered exhaust nozzle set pressure resulting from increased extraction steam load achieved by minimizing/avoiding MP steam in one turbine maximising the MP steam flow extraction in one turbine (33) instead of splitting it between turbines (15,33) [ Fig.5 and Fig.6]; and (iii) additional high pressure steam is generated within the Boiler (25) through increased attemperation in superheater zone of the boiler (25) leading to more HP steam availability for power generation, all of which combined result in the increased generation of power by making available to the STG increased steam load at a higher optimal temperature and pressure without any additional fuel consumption. This results in the continuous generation of additional power, while reducing the water to be sprayed for de-superheating after extraction of steam flow from the STG (15, 33) due to steam extraction temperatures being reduced. The increase in power generation using the aforesaid three-pronged approach upon validation showed additional Power Generation of over 0.8 to 0.9 MW, i.e. an annual Increase in power generation by approximately 6 to 7 Million Units/annum. Thus, the enhancement in power generation using the present invention is estimated at around 7 to 8 % on a continuous basis, and the emission reduction accrued is to the tune of ~ 6000 tCO2e/annum.
Extending the aforesaid principle, it is possible to generate additional power through further lowering of exhaust steam temperature by setting the Steam turbine exhaust nozzle pressure to a value closest to that required for use in process (39,40; say 3.5 ksca +1 ksc & -0 ksc], thus eliminating the need for PRDS after turbine nozzles and requiring only a small desuperheating unit for lowering steam temperature. This is further supported by the fact that with attendant increase in turbine exhaust vacuum in STG of Unit 1, resulting in lowered exhaust steam enthalpy as taught in the present invention, increased power generation had been realized.
The present invention disclosed herein can be replicated in plants in any industry, having HP Cogeneration units connected in battery as well as in stand-alone Cogeneration units as also Captive Power plants. Where more than one HP Cogeneration unit is in use, the present invention can be applied in one or both of such Cogeneration units, so as to increase power generation, subject to the capacity of such Cogeneration units. The present invention can also be extended to cogeneration and condensing units with turbines for power generation. The present invention can also be applied to any and all the boilers integrated to steam turbo-generators for enhanced power generation.

The described process and arrangement has been advanced merely by way of explanation and many modifications may be made thereto wither to without departing from the spirit and scope of the invention which includes every novel feature and combination of novel features herein disclosed. While a preferred embodiment of the invention has been shown and described hereinabove, various modifications and changes may be made by those skilled in the art without departing from the true spirit and scope of the invention.

,CLAIMS:WE CLAIM
1. A method and process for increasing power generation in two or more high pressure cogeneration unit (7,8), comprising a three step method:
(i) increasing HP steam generation within the boiler (25) without consuming additional input fuel; without changing the fuel characteristics or without changing the operating boiler conditions;
(ii) Lowering the drop in steam temperature between the main steam outlet (32) of the boiler (25) and the inlet (2) of the steam turbine (33);
(iii) Increasing the energy bound within the turbine (33) by manoeuvring the steam extraction flows in nozzles (34/35/17/16) of the turbines (33/15) of one or more cogeneration units; resulting in steam with lower pressure and temperature being extracted from the turbine (15,33);
(iv) all of the above resulting in enhanced power generation by 7-8% on a continuous basis in each cogeneration unit (8).

2. A method and process for maximising/increasing power generation in two or more high pressure cogeneration units (7,8), comprising:
(i) increasing the quantity of high pressure steam produced by the boiler (25) without consuming additional input fuel, changing fuel characteristics or boiler operating conditions, by increasing attemperation of steam through an inter-stage attemperator (30) located between the secondary superheater (29) and the tertiary superheater (31)in the boiler (25); such increased spray attemperation resulting in generating additional high pressure steam of 1-1.5% within the Boiler (25) and being fed to the steam turbine [33] at optimal high steam temperature;
(ii) increasing the thickness of insulating mattress over the steam pipeline, being of higher resistivity [due to increased density of the mattress]; calculating the said insulation thickness based on normal continuous boiler operating conditions instead of design steam flow rating of the boiler (25); resulting in minimising the drop in steam temperature by approximately 40% between the boiler steam outlet (32) and the inlet (2) to the Steam turbine (33);
(iii) Maximising the thermal energy bound within the steam turbine (33) by maximising exhaust steam flow in one nozzle (35) of the turbine (33) and lowering nozzle exhaust steam pressure; resulting in exhaust steam with lowered temperature being extracted from the steam turbine (33);
(iv) Such lowered steam temperature extracted from the turbine (33) requiring lowered de-superheating using boiler feed water spray;
(v) all of the above stated , resulting in additional power generation by 7 to 8% by the steam turbine (33) on a continuous basis.

3. The said process and method of maximising the thermal energy bound with the steam turbine (33) as claimed in Claim 2 being implemented in the Cogeneration units (7,8) comprising maximising the exhaust steam flow from one of the nozzles (16,17, 34, 35) of one turbine (15/33) of one of the cogeneration unit (7/8) by:
(i) Avoiding/Minimising the steam extraction flow in the MP extraction nozzle (16/ 34) of one STG (15/ 33) of one Cogeneration Unit (7/ 8), preferably maximizing the MP extraction flow in the steam turbine with higher inlet steam enthalpy [pressure and temperature] conditions;
(ii) maximising the steam extraction flow in the MP extraction nozzle (16/ 34) of the turbine (15/33) of the other Cogeneration unit (7/8); altering the steam load, temperature and pressure of such STG (15/33); extracting the deficit steam from nozzle (16/34) of the STG (15/33) in the other Cogen unit (7/8);
(iii) Maximising LP steam exhaust (48/ 47) flow in a Cogeneration Unit (7/8) and reducing corresponding LP steam exhaust (48/ 47) flow in the other Unit (7/8);
(iv) the total MP steam extraction (39) and total LP steam exhaust (40) hence remaining unchanged excepting marginal reduction due to lowered exhaust steam temperature;
(v) Resulting in increased steam flow with reduced temperatures and reduced pressure, being extracted from the turbines (15,33); maximising steam thermal energy differential between the inlet (2) and the nozzle(16,17,34,35) and enabling the turbine (15/33) to generate more power.

4. A process and method for reducing the drop in steam temperature from the boiler main steam outlet (32) to the inlet (2) of steam turbine (15/33), by increasing thickness and density of the advanced Insulation mattress over the main steam pipeline and securing the same with metal cladding, the increased insulation thickness being calculated based on normal steam flow rate as the base value and not based on design /maximum steam load; effecting reduction in radiation and convection losses at all loads of operation and preserving the heat in HP steam to the steam turbine; capable of being retrofitted while the boiler (9/25) is in operation without shutting the HP boiler (9,25) down; manoeuvring the boiler main steam outlet (14/32) temperature such that the difference in steam temperature between the boiler main steam outlet (14/32) and the inlet (2) to the turbine (15/33) is brought down to under 5°C, resulting in maintaining high pressure steam at higher temperature entering the turbine (15/33), and enabling the steam turbine (15/33) to generate additional power of 2 to 3%.

5. Steam temperature drop from the boiler main steam outlet (14/32) to the inlet (2) of the turbine (33) as claimed in Claims 1,2 and 4, being <50C.

6. The increase in insulation mattress thickness as claimed in Claims 1,2 and 4 being computed at normal steam load by:

Insulation thickness at Normal steam load = [Design Steam flow /Normal steam flow]*Insulation thickness computed for Design steam load; all other conditions remaining unaltered

7. The additional power [P] generated by the steam turbine (33) as a function of lowering the drop in steam temperature (DTs) from the boiler main steam outlet (32) to the inlet of the steam turbine(33) and HP steam (Win) in the connecting steam carrying pipe-line as claimed in Claims 1,2 and 4 being computed by:
P [MW] = Win [TPH] * DTs [°C ] * 0.65 * 1.1

8. A method and process for maximising/increasing energy bound within the steam turbine (15/33) of two or more cogeneration units (7,8) by maximising thermal energy differential between the inlet (2) to the steam turbine (15/33) and the extraction (16/34) and exhaust (17/35) nozzles, comprising
(i) Avoiding/minimising the steam extraction flow in the MP extraction nozzle (16/ 34) of one STG (15/ 33) of one Cogen Unit (7/ 8);
(ii) maximising the steam extraction flow in the MP extraction nozzle (16/ 34) of the turbine (3) of the other Cogen unit (7/8); altering the steam load, temperature and pressure of such STG (15/33); extracting the deficit steam from the MP nozzle (16/34) of the STG (15/33) in the other Cogen unit (7/8);
(iii) Maximising the LP steam exhaust (48/ 47) in a Cogeneration Unit (7/8) and reducing the LP steam exhaust (48/ 47) in the other Unit (7/8);
(iv) the total MP steam extraction (39) and total LP steam exhaust (40) remaining practically unchanged but for marginal reduction due to lowered exhaust steam temperature;
(v) Resulting in increased LP steam flow with reduced temperatures and reduced re-set pressure, being extracted from the turbines (15,33); maximising steam enthalpy differential between the inlet (2) and the turbine exhaust nozzle(17/35), and enabling the steam turbine (15/33) to generate additional power of 2 to 3%.

9. The maximisation/increase of extraction steam flow and its corresponding effect on generation of power as claimed in Claims 1,2, 3 and 8 computed by

Mass Balance –
• Win[TPH]= Wext + Wexh [at Turbine Nozzle exit] [TPH]
Wherein
Win = Steam flow entering Steam turbine;
Wext = Steam flow at steam turbine extraction nozzle
Wexh = Steam flow at steam turbine exhaust nozzle,

Power Balance:
. PT [MW] = [Pext [MW]+ Pexh[MW]]* AF
Wherein
# Pext = K1 * Wext [TPH]
# Pexh = K2* Wexh [TPH]
where
K1= 0.06[Lower extraction steam load] to 0.075[Highest extraction steam load]
K2 = 0.122 [Lower exhaust steam load] to 0.129 [Highest exhaust steam load]
AF : Ageing factor [deterioration] of Steam turbine, wherein AF = 0.999* Number of years in operation since complete overhauling of turbine internals].

10. A method and process to increase the quantity of HP steam produced by a boiler (9/25) without consuming additional input fuel, comprising increasing attemperation of steam in the inter-stage attemperator (29) in the superheater section (31) of the boiler (9/25) resulting in increased HP steam being generated by the Boiler (9/25) and being made available to the steam turbine (15/33) to generate additional power of 1 to 2 %, and requiring lesser water spray for de-superheating, thereby generating reduced amount of wetness in steam for use in process.

11. The method and process as claimed in Claim 1, comprising a single cogeneration unit (8) in isolation, by
(i) increasing generation of HP steam within the boiler (25) without consuming additional input fuel or without changing the operating boiler conditions or without changing the fuel characteristics;
(ii) lowering the drop in steam temperature between the main steam outlet (32) of the boiler (25) and the inlet (2) of the turbine (33),
(iii) all of the above resulting in enhanced power generation by 4% on a continuous basis in such unit (8).

12. The process and method as claimed in any of the preceding claims being implemented for increased generation of power in captive power plants.

Dated this the 17th day of September, 2020.

Sumitha Vibhu
Patent Agent
IN/PA 383