PROCESS AND APPARATUS FOR MANUFACTURE OF PORTLAND
CEMENT
FIELD OF INVENTION
[001]. The present invention relates broadly to a process and apparatus for
manufacture of Portland cement or lime which is adapted to facilitate carbon
capture.
BACKGROUND
[002]. Portland cement production is an industry producing 3,400 million tonnes of
cement powder per annum, and is the second largest source of man-made CO2
emissions, with approximately 0.8 tonne of CO2 produced per tonne of cement. Of this,
about 60% derives from the CO2 emitted from processing limestone CaC0 to lime CaO
in the production of cement c!inker in a process known as calcination, and 40% arises
from the burning of fossil fuels to produce the cement. The reduction of CO2 emissions
to reduce global warming is required, and the Portland Cement industry is under
pressure to reduce its CO2 emissions.
[003]. Incremental improvements to the efficiency of the production process are one
approach, but substantial reductions are not possible by incremental improvements. The
other approaches that have been developed include the application of post-combustion
capture in which the C0 2 from the exhaust gas stream, which contains the CO2 from
both carbonate calcination and fuel combustion. Established processes, such as amine
stripping, are too capital expensive, and the recent focus has been on using lime, CaO,
as a high temperature CO2 sorbent, in a process called Calcium Looping. This process
is at the pilot stage of demonstration. It has the advantage that the spent CaO sorbent is
consumed in the Portland cement. This process has the disadvantage that the capture
process is carried out at about ambient pressure and the size and cost of the plant would
be very large, approaching that of the Portland cement plant itself. A significant concern
is the penalty arising from the consumption of additional energy for the Calcium Looping
Process. This is a cost, and adds to the scale of the plant.
[004]. Another approach is Oxy-fuel combustion in which pure oxygen is used for
combustion instead of air, in which case the exhaust gas is CO2 and steam, which allows
the CO2 to be captured by condensing the steam. The cost of a cryogenic separation
plant is a very large cost, and the Portland cement plant has to be significantly
redesigned to account for the very different flue gas flows through the kilns and another
processes.
[005]. Portland cement plants typically use coal and waste materials as a fuel, rather
than natural gas, so that approaches based on pre-combustion capture to produce a
hydrogen gas stream from natural gas are generally not applicable.
[006]. Lime production is similar to that of Portland cement, except that a higher
quality of limestone is used, and sand and clay is excluded to produce a lime product
[007], There is a need for a process that can significantly reduce the C0 2 emissions
from a Portland cement or lime process without the requirement for large additional
processing plants described above. In all the CO2 reduction schemes considered above,
the CO2 must be compressed for sequestration.
[008]. Portland cement production now uses the "dry process" in which lime and sand
particles fuse in the rotary kiln, compared with the "wet process" previously used in
which the limestone, sand and other additives are pressed into a pellet. The dry process
has a lower energy demand than the wet process. This invention is directed to the dry
process.
[009]. In the dry process, limestone is received as rocks, which are crushed and
ground to a particle size of less than 100 microns, and uniformly mixed with sand that
has also been ground to less than 100 microns. Other ground materials, such as clay
and iron oxide may be added for a particular cement formulation. Generally, the different
particles streams are mixed in a hopper designed for efficient mixing to give a
homogenous mixture. The dry cement process relies on an efficient mixing to promote
fusion and reaction in a rotary kiln.
[0010]. In the conventional dry process, the mixed powder is pre-heated by the flue gas
exhaust from kiln using a pre-heater cyclone stack, which is a bank of cyclones in series.
In each stage, the colder particles are heated by mixing with the hotter flue gas steams,
and the equilibrated gas and particles are separated in a cyclone. This process is
repeated many times in a sequence in which the temperature of the particles are raised
and that of the gas is reduced. A modern plant may have up to six of these stages to
achieve high heat recuperation efficiency, and thereby lower the energy demand. This
staged approach of mixing and demixing approximates a counter fiow heat exchanger in
which the temperature of the solids is raised and that of the gas is lowered. As the
temperature rises during these pre-heating stages, the calcination reaction of limestone
will proceed to a extent that the C0 2 partial pressure is not higher than the equilibrium
pressure of the calcination reaction. Up to about 30% reaction may be achieved in the
pre-heater cyclone stack.
[0011]. The accumulated pressure drops in each cyclone stage is high as the particles
are accelerated in each stage. These pressure drops accumulate and present a
significant energy penalty for operating the blowers for forcing the flue gas into the Preheater
cyclone stack and for drawing out the flue gas.
[0012]. In the conventional approach, the powder from the pre-heater cyclone stack is
injected into a flash calciner where they are mixed with the hot flue gas steam from the
clinker kiln described and coal. The hot flue gas stream has excess air which combusts
with the coal to drive the calcination reaction towards completion such that 95%
calcination is achieved and the exhaust temperature is about 900 C. The exhaust gas
temperature is held below that at which the sand will begin to vitrify and calcium silicates
begin to form. The solids are once again separated from the flue gas stream, adding to
the pressure drop penalty. The pre-heated homogenously mixed lime and sand powders
are ready for processing in the rotary kiln.
[0013]. It will be appreciated that the conventional approach uses powder-gas mixing
for each of the stages in the pre-heater stack and in the flash calciner. This gives very
efficient heat transfer, but has the undesirable attributes of requiring many stages of
powder-gas mixing and separation to achieve an over-all system thermal efficiency.
[0014]. A disadvantage of the powder-gas mixing is that the exhaust flue gas may have
large amounts of cement dust that needs to be separated, and re-injected into the
process in order to meet emissions standards. The cost of the filter units is scaled to the
gas flow, and the wear of the filter units is associated with the entrained powder. These
are disadvantages of the conventional process.
[00 5] . The production of lime is generally carried out in kilns, which are not amenable
to C0 2 capture described in the present disclosure. However, ground limestone, or lime
kiln dust, is calcined in flash calciners similar to that described above for Portland
cement. In that case, the pre-heater stack and the flash calciner are augmented by a
cyclone cooler which is used to pre-heat the air for combustion. It will be understood by
a person skilled in the art that the benefits described in this invention in detail for
Portland cement are also applicable with respect to lime production with C0 2 capture.
[0016]. In the case of Portland cement, the pre-heated calcined hot particles are
injected into the clinker kiln, which is a rotating kiln fired by a counter-current of flue gas
produced from the combustion of coal to a temperature of about 50 with pre-heated
air. At these temperatures, the sand fuses with the lime and the particles begin to
agglomerate into granules in much the same way as in silica glass manufacturing. In the
granules, the reactions proceed to form the calcium silicates that define the composition
of Portland cement and the granules sinter. The fusion, reaction and sintering lead to an
exhaust stream of calcium silicates in the form of clinker granules of about 10-30 mm in
diameter. The clinker granules are cooled by a forced air pre-heater, and then ground to
form cement powder. The heated air is used in the combustion process described
above. The amount of pre-heated air is sufficient to completely combust the fuel in the
rotary kiln and in the flash calciner. This is a large volume of gas that flows counter to
the input particles and the growing granules, and the propensity of the lighter particles
and granules to be entrained in the gas stream requires a careful design of the rotary
kiln.
[0017]. The rotary kiln flue gas also contains volatile impurities, and an advantage of
the mixing in the pre-heater cyclone stack is that these impurities, principally sulphur
oxides, react with the raw feed and are oxidised to gypsum, and sequestered in the
cement.
[0018]. The flue gas stream exhausting from the pre-heater cyclone stack is the result
of the first and second combustion processes, and contains the carbon dioxide (C0 2)
from the calcination process. This gas stream has a propensity to comprise a significant
amount of carbon monoxide generated in the combustion of the fuels in the presence of
the CO2. Carbon monoxide is toxic, and its emissions are regulated. The energy
efficiency of the Portland cement process is reduced by the excess air that has to be
injected into the combustion process and heated by it. The presence of the CO from
limestone calcination is a disadvantage of the process.
SUMMARY OF THE INVENTION
[001 9J. The invention aims to provide improvements to processes and apparatus for
Portland cement manufacture which may overcome some or all of the above-described
deficiencies of the conventional process, including without limitation one or more of:
a . facilitating carbon dioxide capture;
b. limiting the pressure drops, which allows use of a lower forced air pressure,
with a reduction of power consumption;
c . allowing the input air volume flow to be optimised to achieve efficient
processing in the kiln, rather than being constrained to ensure that the flue
gas has sufficient excess oxygen to drive the combustion of fuel in the flash
calciner;
d. minimising the volumetric flow of flue gas with entrained particles, which
allows for a reduction in the filter size and cost for flue gas emissions;
e . providing additional pre-heated air to the combustion gas stream, which
allows for the reduction of carbon monoxide emissions.
[0020]. A first aspect of the present invention may include: A process for producing
Portland cement clinker from at least crushed limestone and crushed sand including
the steps of: Mixing the limestone, and the sand to form a mixed powder; calcining
the mixed powder in a calciner reactor, wherein the calciner reactor is adapted to
apply indirect heat generated from the combustion of a first fuel input to produce the
mixed powder, and wherein the calciner reactor pre-heats the mixed power in a first
segment, and reacts the pre-heated powder in a second segment to generate a first
gas stream of carbon dioxide from the calcination of limestone and a separate
second gas stream from the combustion of the first fuel input; Introducing the
calcined mixed powder into a kiln using direct heating to produce Portland cement
clinker, where the kiln is fueled by the combustion of a second fuel input mixed with
air that is pre-heated by hot Portland cement clinker exiting the kiln.
[0021]. Preferably, first gas stream may be cooled and compressed, and stored.
The process may also include a further step in which the first fuel input is a gas
mixed with air, wherein the air has been pre-heated by heat exchange with the
cooling the first gas stream. More preferably, this heat exchange occurs within the
solids pre-heater of the calciner reactor, such that the input mixed power and air are
pre-heated by the first and second hot gas streams, without mixing of the solids with
these gas streams.
[0022]. Preferably, the first gas stream includes a slip stream of the exhaust gas
stream from the kiln, which has sufficient excess air to provide complete combustion
of that fuel. Preferably, the sand may include various setting additives including
setting catalysts and retardants.
[0023]. A second aspect of the present invention discloses a means of production of
Portland cement in which the C0 2 from the carbonate calcination is available for capture
as pure gas stream by a change in the process flow of a conventional Portland cement
plant. This invention does not capture the CO2 from the fuel combustion, and is
therefore limited to reduction of emissions by 60%. This is a very significant reduction.
The invention may operate with any carbon capture processes for the flue gas stream. If
the fuel used was a biofuel waste, then zero-emissions may be achieved.
[0024]. A third aspect of the present invention may include a process for producing
lime from limestone powder, including the steps of. Calcining the limestone powder
in a calciner reactor, wherein the calciner reactor is adapted to apply indirect heat
generated from the combustion of a fuel input to produce a preheated mixed
powder, and wherein the calciner reactor generates a first gas stream of carbon
dioxide from the calcination of limestone and a separate second gas stream from the
combustion of the first fuel input; Cooling the calcined powder in a heat exchanger,
such that the preheated air is used in the combustion.
[0025]. Preferably, the powder is preheated prior to calcining the powder.
Preferably, the first gas stream is cooled and compressed, and stored. More
preferably, the first fuel input is a gas mixed with air, wherein the air has been pre
heated by heat exchange with the cooling the first gas stream.
[0026]. A fourth aspect of the present invention may include a means of production of
Portland cement in which the amount of flue gas that has entrained cement particles is
significantly reduced from the conventional process in which the flue gas from
combustion of the first and the second combustors are mixed with the input powder
stream. The first combustor generally consumes at least around 60% of the fuel, and
the second combustor consumes at least around 30% of the fuel. In the present
disclosure, powder is mixed only with the second flue gas stream, and this cement
powder is separated using conventional cyclones such that the size of the flue gas filter
unit is significantly decreased.
[0027]. A fifth aspect of the present invention may include a means of production of
Portland cement in which the production of carbon monoxide in the flue gas is
significantly reduced from the conventional process. In the present invention, the hot
carbon dioxide is replaced in the gas stream by pre-heated air such that the first
combustion process to completion such that carbon monoxide production is greatly
reduced.
[0028]. A sixth aspect of the invention discloses the use of a slip stream of the mixed
power that is injected into the flue gas from the kiln. This powder stream is pre-heated
by the hot flue gas to a temperature in which volatile compounds, principally oxides of
sulphur, react with the power to form non-volatile compounds, such as calcium sulphate,
which is sequestered into the cement when the powder steam is calcined and mixed with
the primary calcined powder steam, and processed to cement in the kiln.
[0029]. In preferred embodiments the raw feed crushing and grinding, and the clinker
grinding processes may be essentially unchanged from those already known. The rotary
kiln and the clinker cooler, may be substantially the same those already known in the art.
The processes of powder mixing, pre-heating and flash calcination are changed from
direct heating to indirect heating to enable carbon capture.
[0030]. Other aspects of this present invention may further disclose a modification of
the pre-heater cyclone stack and the flash calciner that uses indirect heating in which the
flue gases and the process streams are purposefully not mixed. In that context, it is
noted that the C0 from calcination is mixed with flue gas from the coal combustion and
the input gas stream from the clinker kiln, and that the use of an indirect system of
calcination and heat transfer will not mix the C0 2 from calcination and the flue gases.
[0031]. In one form, the invention disclosure provides a method and means of
manufacture of Portland cement in which the carbon dioxide from the calcination of
carbonates, principally limestone, is produced as a separate stream of carbon
dioxide, which may be compressed of liquefied for sequestration or other uses, with
the primary intent that it is not emitted.
[0032]. The disclosure provides a process in which the ground carbonate minerals,
principally limestone, are pre-heated and calcined in an indirect heated counter-flow
flash calciner reactor such that the output of the reactor is a gas stream of
essentially pure C0 2 and a solids stream of hot lime. The heat for the reaction is
provided by heat transfer across the walls of this reactor by the combustion of fuel
and pre-heated air in a separate chamber.
[0033]. Other aspects of the disclosure relate to the processes of pre-heating the
premixed sand and other additives by the flue gas stream, preferably by indirect
heating in a solids-gas heat exchanger, and the homogenous mixing of this stream
with the hot lime for injection into the rotary kiln. The subsequent processes of
fusion and sintering in the rotary kiln to produce clinker, cooling the clinker by an air
pre-heater and grinding the cooled clinker may be substantially the same as the
conventional process for production of Portland cement.
[0034]. The pre-heating of the air for the combustor of the indirectly heated is
preferably achieved by cooling of the C0 2 gas stream, with any additional preheating
coming from the ftue gas of this calciner. This means that the air demand
required for the rotary kiln is not dictated by the need to have excess air in the flue
gas exhaust for this combustion .
[0035] . In another form , the limestone, sand and other additives are mixed before
pre-heating and calcination in the indirect heated counter-flow reactor. This form
entails a larger reactor because it must pre-heat the sand and other additives. This
is not a preferred embodiment because of the potential for the sand and lime to
commence their reaction in this reactor, which mig ht lead to a build-up of a glassy
insulating layer of material on the walls of the reactor.
[0036]. The cooling of the C0 2 gas stream is preferably achieved by pre-heating of
an air stream for the combustor of the indirectly heated calciner. This air stream
augments the pre-heated air from the clinker cooler such that excess oxygen is
increased and the combustion produces minimal carbon monoxide. The
replacement of the hot C0 2 gas stream by a pre-heated air gas steam means that
the heat losses from the plant are minimised and the thermal efficiency of the
process is similar to that of the conventional process.
[0037] . Pre-heating of the solids using indirect heating from the flue gas is such that
the pressure drop of the flue gas is determined by typical gas friction effects, and is
greatly reduced compared to that of the conventional process because these gas
streams do not experience the gas pressure drops from accelerating the powders in
multiple cyclones. This reduces the power required to draw the flue gases through
the plant, for both first and second combustion processes.
[0038] . The separation of the rotary kiln flue gas from the calciner flue gas is such
that the wear on the calciner and the powder pre-heater refractories from entrained
cement particles is significantly reduced or eliminated. The powders introduced into
the pre-heater and calciner flow down the tubes at low velocities, typically around
less than 5 meters per second , such that there is very little wear on the reactor steel.
[0039]. In the production of lime from limestone, the feed is a substantially pure
limestone powder. The benefits of the invention described above for Portland
cement apply to lime, except that the limestone feed is generally of a quality that
sequestration of volatiles is not required , and would degrade the product quality.
For most limestone applications, a higher degree of calcination is required, and for
such applications a small reactor, such as a fluidised bed , would be used to reduce
the calcination from around 93-96% achieved in this invention, to 99% or higher.
The amount of CO2 lost in this process is very small, so that conventional fluidised
bed reactors may be used. The lime product may be cooled in conventional heat
exchangers to pre-heat the air for combustion.
[0040]. It would be recognized by a person skilled in the art that there are a number
of alternative process for recuperation of heat from hot flue gas and solids product
steams that may be applied to optimise the performance of the indirect counter-flow
calcination processes described herein, without departing from the production of a
separate, relatively pure CO2 exhaust stream from the flash calciner as described
herein, by contrast to the conventional flash calciner which mixes the powder, the
fuel, and in the case of Portland cement, the rotary kiln flue gas, in a single reactor
such that the C0 2 from the carbonate calcination is mixed with the flue gas
components principally nitrogen, steam and excess oxygen.
[0041]. Further forms of the invention will be apparent from the description and
drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042]. Embodiments of the invention will be better understood and readily
apparent to one of ordinary skill in the art from the following written description, by
way of example only, and in conjunction with the drawings, in which:
[0043]. Figure 1 shows a schematic drawing of a process for production of Portland
cement clinker and a relatively pure C0 stream as per a first preferred embodiment;
[0044]. Figure 2 shows a cross sectional schematic view of an example flash
calciner reactor suitable for use with the first preferred embodiment of the present
invention;
[0045]. Figure 3 shows a schematic drawing of a process for the production of lime
and a relatively pure C02 stream;
[0046]. Figure 4 shows a schematic drawing of a process for production of lime
from limestone;
[0047], Figure 5 shows a cross section schematic view of an example of a flash
calciner reactor.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0048]. The invention required to manufacture of Portland cement can be described by
consideration of the process flow of Figure 1.
[0049]. In this embodiment, the raw limestone rock 101 is crushed and ground in a
crushing and grinding plant 102 to a powder 103 with a particle size less than 100
microns. This piant 102 may be the same as used in a conventional Portland cement
process.
[0050]. The limestone powder stream 103 is processed in a counter-flow indirect preheater
and flash calciner 4 to produce a hot lime stream 105 and a separate hot C0 2
stream 106. This hot CO2 stream is cooled in a gas-gas heat exchanger 107 to produce
a cool COigas stream 108, which can be further cooled and compressed, and liquefied
if required. To reduce the carbon footprint from Portland cement production, this gas
stream 108 is not emitted.
[0051], The air input 109 for cooling the CO2 is pressurized by a fan 110 and the air
stream 111 is heated in the gas-gas heat exchanger to provide a pre-heated air stream
112 that is combusted with fuel 113 in the external combustor and heat transfer plant
1 4 to provide the energy for the pre-heating and calcining reactor 104. The pre-heated
flue gas stream 115 has significant thermal energy.
[0052]. A suitable pre-heater and calciner reactor is of the type described by Sceats, for
example in Published PCT Patent Application No. WO201 2/1 45802 {incorporated
herein by reference) that uses steam to entrain the carbonate particles into the reactor.
A schematic illustration of an example flash calciner reactor is illustrated in Figure 2
hereof.
[0053]. In one embodiment, the amount of steam injection is minimised to ensure that
the energy demand from generation and pre-heating steam is minimised, so that the
powder initially moves down the reactor initially dominated by gravitational flow, with the
gas-particle coupling becoming increasingly important as the C0 2 is evolved.
[0054]. The use of a low steam content, for example less than 5% steam to limestone
mass ratio requires an increased length of the calciner, because of the reduced steam
catalysis, and increases the requirement of the exhaust temperature of the calcined lime,
nameiy greater than about 900°C, so that the partial pressure of CO2, approximately at
ambient pressure, is less than the equilibrium C0 2 pressure. This requires a high wall
temperature of the reactor, and this specification can be met, for example, preferably by
high nickel-chromium steels, or high radiance refractory materials near the exhaust of
the reactor at the base. The limestone powder is injected at low temperature, near
ambient in this embodiment, and the upper part of the reactor is used to pre-heat the
solids. In this region, the walls can be constructed of stainless steels, and the design
may be more typical of solids-gas heat exchangers.
[0055]. It will be appreciated by a person skilled in the art that the mechanism of heat
transfer from the reactor walls to the powder will vary down the reactor, from conduction,
to convection and to radiative heat transfer and a number of baffle designs inside the
reactor can be used to maximise the heat transfer to minimise the reactor length. For
example intermediate hoppers and rotary valves can be used to hold up the solids, and
mixers can be used to increase the turbulence and to break up the gravitational
acceleration. The heated combustion gas from the external combustor and heat transfer
unit is injected into the pre-heater calciner to provide the required temperature profile
along the calciner walls, and provide the necessary heat up the reactor. The preferable
temperature profile is one in which the temperature is maximised at the exhaust of the
reactor. The counter-flow of the heating gas is such that the exhaust flue gas steam 115
is at a low temperature as possible by virtue of the heat transfer to the powder through
the reactor. The reactor may be comprised of a number of segments to enable the
transition between heat transfer regimes, and may comprise a number of downer and
riser segments.
[0056]. The sand, and other additives, 16 are mixed and ground in a crushing and
grinding plant 17 to a sand powder 118, also with a particle size also less than 100
microns. This plant may be the same as used for a conventional Portland cement
process.
[0057]. In contrast to the conventional process, in this embodiment, the powder
streams 03 and 1 8 are not homogenized at ambient conditions. Instead, the two
streams are separately processed and pre-heated, and homogenized at high
temperature. It will be further understood that the preferred approach to this
embodiment is to separate the limestone and sand streams to reduce the heating load
on the pre-heating calciner 104, to reflect the fact that indirect heating carries an
efficiency penalty. The primary benefit of indirect heating is that the pure C0 2 stream is
extracted as the hot gas stream 106, for heat recovery and sequestration by any suitable
process, to reduce the CO2 emissions.
[0058]. The sand powder 8 is pre-heated in the solids gas heat exchanger
comprising the solids pre-heating unit 1 9 to produce the pre-heated sand stream 120.
The gas side of the heat exchanger provides heat from the hot flue gas 115 from the
pre-heater calciner 104 and the hot flue gas 121 from the rotary kiln 130, which is
substantially scrubbed of the hot fines 123 in the cyclone 124 to give a hot flue gas 125
for injection into the heat exchanger unit 6 , along with the hot flue gas stream 5.
This heat exchanger design may take account of the higher temperature of the hot flue
gas stream 125 compared to the hot flue gas stream 115, for example by using less hot
flue gas 115 in a pre-heating section of the heat exchanger unit 126. The temperature of
the flue gas steam 127 after heat recuperation should be as low as possible to reflect the
energy efficiency of the Portland cement production process described herein. This gas
stream generally contains C0 from the combustion of the fuel inputs to the plant.
[0059]. The pre-heated lime powder 105, the pre-heated sand powder 20 and the hot
fines 123 are homogenously mixed in the powder mixer 28 to give a hot powder stream
129. In the rotary kiln 130, the lime and sand particles fuse, react and sinter to produce
cement clinker granules 133. Compared to the conventional dry process for production
of Portland cement, the pre-heated lime powder 105 produced from the pre-heating
calciner 04 has a larger surface area because the monotonically increasing
temperature profile of the particle flows through the reactor minimises sintering. These
particles have higher reactivity, compared to that produced by a conventional flash
calciner, and the solids-solids reaction in the calciner will occur more readily.
[0060], The kiln is fired by fuel 131 combustion, with a hot air stream 32. The hot flue
gas at about 1450°C causes the lime, sand and additives to fuse, react and sinter to
produce the calcium silicates of the Portland cement. The fusion grows the materials
into granules, and this granular stream is ejected from the rotary kiln 130 as the hot
cement clinker granules 133. The hot clinker granules 133 are cooled in the clinker
cooler 134 to give the clinker stream 136 in the clinker cooler 134, which pre-heats the
forced air stream 135 to the heated stream 132. The cooled clinker 136 is ground to
cement powder 140 in the grinder plant 139, as per the conventional process. The air
fan 137 pressurises the input air stream 138.
[0061]. In a further advantage of the described example embodiment, it will be
understood by a person skilled in the art that the pressure drop of gas from the input air
32 to the flue exhaust 127 is less than that required for the conventional process in
which the air has to drive against the solids streams for up to about six stages of solidsgas
mixing, and cyclone separation.
[0062]. In an unillustrated embodiment, the air stream 112 may contain a slip stream of
the flue gas 121.
[0063]. In another unillustrated embodiment, the limestone powder 103 and sand
powder streams 118 may be homogenously mixed before being pre-heated by the kiln
flue gas 121 and injected into the pre-heater and calciner 104.
[0064]. Referring now to Figure 3, of the present disclosure, the crusher and grinder
plant 300 receives a mixed powder 201 of raw meal with particles less than about 100
microns, from a feed 200 of limestone, sand and clay. The moisture in the raw meal
powder 201 is controlled using waste heat from the plant. In one embodiment, the
moisture is removed using the residual heat from the flue gas from the scrubber plant
306.
[0065]. The calciner tower comprises a pre-heater segment 302, a calciner segment
303, a gas-solids segment 304, a calciner combustor 305 and a scrubber plant 306. In
the calciner tower, the limestone in the raw meal 201 is first pre-heated and then
processed to lime, with the liberation and separation of C02and any excess moisture
and hydrated water from all the compounds in the raw meal 201 being released as
steam. The process output streams of the calciner tower is hot calcined powder 205 and
a cooled C0 2and steam stream 212. The segments 302, 303, 304 of the calciner tower
described herein are a unitary structure and the description of separate segments is for
clarity purposes.
[0066]. Detailed descriptions of the calciner segment 303 and the pre-heater segment
302 are shown in Figure 3 . The scrubber 306 is used to extract at least some of the
volatile compounds from the flue gas 241 from rotary kiln plant 308, and to cooi flue gas
241 so that it can be vented to the atmosphere or captured. The rotary kiln 308 uses a
slip steam of raw meal 202, which is heated and the hot solids is mixed with the primary
pre-heated meal 203 for calcination. The scrubbed flue gas 242 is used to remove
moisture in the Crusher and Grinder plant 300.
[0067]. The C0 2 processing plant 307 takes the cooled C0 2and steam stream 212 and
processes it to extract the water and compress the CO2 2 3 for sequestration. The plant
comprises coolers and compressors, with dewatering, to give a compressed CO2 steam
at about 136 bar.
[0068]. The kiln plant comprises a rotary kiln 308 that produces clinker 206, a clinker
cooler 309 and a kiln combustor 3 1. In this plant the calcined powder 205 is
transformed to bails of clinker 206 of about 30mm size. A cooled clinker stream 207 is
crushed to produce cement powder 208 in the cement crusher plant 310. The process
streams will be considered in detail below.
[0069]. The crusher grinder plant 300, kiln plant 308 and cement crusher plant 3 0 are
standard plant equipment associated with the production of Portland cement. Thus the
crusher and grinder plant 300, the kiln plant 308 and the cement crusher plant 310 are
unchanged by the invention described herein.
[0070]. The description presented below describes the processing in a single reactor
tube as shown in Figure 2 and Figure 5 . A large cement plant would commonly employ
the use of said single reactor for the production of Portland cement. In the present
embodiment, the reactor tube can process 240 tonnes per day of raw meal, therefore a
cement plant operating at maximum efficiency producing about 5,000 tonnes of cement
per day would have a reactor plant comprising a minimum of 32 reactor tubes in across
four modules, each comprising eight tubes per module.
[0071 ] . The detailed process steps shown in Figure 3 are now described. A raw meai
200 comprising a mixture of limestone, sand and c!ay are fed into crusher grinder piant
400 ground and crushed to the raw meal powder 201 , being a mixed powder 201.
[0072]. The mixed powder 201 is processed in the pre-heater segment 302 and the
secondary output stream of raw meal 202 is processed in the scrubber plant 306. The
pre-heating segment 302 of the plant pre-heats the raw meal powder 201. The preheater
302 serves a number of functions; to pre-heat the raw meal 202 to a pre-heated
meal stream 204, and to pre-heat an air stream 235 to a pre-heated air stream 236 that
is later used in the calciner combustor. The energy for pre-heating these streams comes
for the heating gas exhaust 245 and the hot C0 2 stream 2 11 from the calciner 303 are
directed through the pre-heater segment 302, to produce a cooled CO stream 2 2 and
a cooled flue gas stream 246. The flue gas stream 246 does not require a baghouse to
remove dust from the raw meal 201 because the calciner described below does not mix
the heating gas streams with the raw meal steams. The amount of air injected into the
pre-heater is controlled to deliver cooled flue gas and C0 2 at the lowest possible
temperature so as to maximise the energy efficiency of the process. The optimum
temperature of the heated meal 203 can be in the range of 650°C to 850°C, but is most
preferably about 730°C, which is below the onset of the calcination reaction. During pre
heating, residual moisture in the raw meal forms superheated primary heated meal
steam 203, such that the stream 203 is an entrained solids-steam mixture.
[0073]. The design of the pre-heater is preferably a tube-in-plate cross-fiow design.
Whilst in the calciner, the orientation is such that the solids meal stream flows vertically
down under gravity and the hot C0 2 stream 2 11 flows vertically upwards. These two
streams flow in multiple, separate pipes of typically with a 10cm to 20cm diameter, such
that the build-up of powder in the tubes is minimised. It will be appreciated that other
diameters or cross sections, such as oval or substantially rounded cross sections, may
be used to reduce build-up of powder in the tubes. At the entrance of the CO2 stream
2 11, there may be in-line micro-cyclones that filter out dust in that stream, and return any
solids to the calciner 103. This ensures that the CO2 exhaust stream 2 contains
minimal dust fines. The cooled air stream 235 and the heating gas exhaust stream 245
flow in the pre-heater through a cross flow in ducts between pairs of plates through
which the pipes described above penetrate. The ducts of these streams alternate, such
that the system can employ heating and cooling ducts. The use of tube in plate cross
flow commonly employs the use of two streams to transfer heat, however in the present
embodiment there are four streams employed to transfer heat. One of the streams is a
flow of powders entrained in steam, and the heat transfer to this stream is the notably
slower than the heat transfer to the other streams.
[0074]. Notwithstanding the heat transfer and the complexity of multiple flows, the preheater
302 has hot inputs at a lower end, and cold inputs at an upper end, such that the
overall heat transfer is that of a counter-flow system. A counter-flow system such as this
can increase the thermal efficiency of the piant. It would be appreciated by a person
skilled in the art, that the tube lengths, diameters, spacing, duct heights and widths can
be chosen to give the desired heat transfers for the mass flows specified, and are not
limited to the present embodiment. There are established correlations for heat transfer
rates between each of the gas and solids flows to the pre-heater. The metal tube and
plate thicknesses are selected to give the required structural strength for the
temperatures and temperature gradients established. In another embodiment, two preheater
segments can be used to separate the two processes. The pre-heater is
encased in refractory to minimise the loss of heat through the walls.
[0075], The calciner segment is where the innovation of this invention substantially lies.
The calciner is an indirectly heated counter-ffow reactor. A suitable pre-heater and
calciner reactor for the present invention is disclosed above. It is the indirect heating
that separates the CO2 gas generated from the calcination reaction of limestone to lime
from the heating gas that provides the energy for the calcination reaction.
[0076]. Referring now to Figure 5 , the calciner reactor 303, 403 comprises an inner
tube 501 which ducts the hot CO2 gas from the gas-solids separator adjacent to cyclone
507 at the base, through the calciner to the array of solids heating tubes 503 that form
the pre-heater. The tubes have an entrance shape and alignment that creates a vortex
flow of the CO2 gas in the calciner between the outer wall of the inner tube 501 and the
inner wall of the outer tube 502, such that powder entrained in the CO stream is ejected
onto said walls, and the solids flow is directed down the wails into the cyclone separator
507. The powders are introduced into an annulus bounded by the inner tube 501 and an
outer tube 502. The outer tube 502 can be fabricated from, for example, a metal, metal
alloy or a ceramic, or a combination of thereof.
[0077]. The powder stream comprises steam to form an entrained flow entering the
calciner reactor 303, 403 in the solid heating tubes 503 from the pre-heater 302, 402.
The shape of the pipes at the entrance to the reactor 303, 403 are shaped and arranged
to impart a helical motion to the stream in the reactor 303, 403. This helical motion is
further enhanced by helical generators 504, in the form of a deflector 504, at the base of
this annulus for the calcined stream. The calcination reaction takes place in this
annulus, with the heat being supplied from the outer wall of tube 501 and the inner wall
of tube 502. The heating gas 509 flows in through at feast one annulus formed between
a refractory 505 and the outer wall of the outer tube 502 of the reactor. The heating gas
is introduced into the calciner from the combustor through a plurality of heat injector
ports 506 arrayed from the base with preferably two injector ports 506 at each stage
offset by 80°C. The injector ports 506 are configured such that heat is applied to
approximately 30% to 50% of the tube from the lower portion of the reactor 500. This
allows the heat to drive the calcination of limestone and raise the temperature up to
between 800°C to 1000°C, but preferably in the range of 900-920°C at the exhaust
port(s) 509. The exhaust from the calciner segment comprises calcined meal and C0 2
and steam at the preferred temperature. The CO2 produced in the reactor annulus is
used to drive the helical flow in the exhaust port(s) 509. The heating gas is directed to
apply a substantially uniform heat to outer tube 502 to avoid hot spots.
[0078]. It would be appreciated by a person skilled in the art that the outer tube 502 is
under considerable thermal stress. As such, the outer tube 502 requires a high thermal
capacity and can be fabricated from, for example, a nickel-chromium alloy, or high
radiance refractory material, or any other material suitable for high temperature
environments near the exhaust port(s) 509 of the reactor 500. In some embodiments
the outer tube 502 has at least one of the following properties; a high corrosion
resistance, low thermal expansion, expansion resistance or any other desirable property
for a highly volatile environment. Heating of the reactants occurs from one or both of the
radiative and/or corrective heat flows. The gas powder separator is a cyclone system
507 in which the calcined meal is separated by centrifugal and gravitational forces
against the wall of the cyc!one 507, such that the gas forms a counter-flow vortex that
rises upwards into the inner tube 501. In the present embodiment, the gas flowing up
through the inner tube is C0 2 stream 508 and flowing into CO2 cooling tubes 5 0. The
calcined powder 51 gathers at the base of the cyclone separator 507 and is ejected by
a rotary valve or screw. In at least one embodiment the heating gas 509 can be
recycled by the process to improve efficiency.
[0079]. Returning to Figure 3 , the powder streams injected into the calciner are the pre
heated streams 203 and 209. Unlike conventional plants, the steams 203, 209 can be
processed in separate calciner reactor tubes. The temperature of these streams can be
around 800°C or less, or more particularly 730°C or less, such that primary heated meal
stream 203 contains steam from excess moisture. The limestone in the powder is
calcined to lime 204 and carbon dioxide 2 in the reactor and the exhaust temperature
in the stream 204 is in the range of about 900-930°C. The heating gas 244 from the
combustor is in the range of about 1500-1 700°C and is distributed along the reactor such
that the wall temperature distribution (on outer tube wall 502) is within the operating
range of metals and/or ceramics when under load from the absorption of heat for the
reaction. The exhaust of the heating gas 245 is preferably in the range of about 950°C
to1000°C.
[0080]. Referring to Figure 3, the hot C0 2 stream 210 from the gas particle separator
304 enters the calciner reactor 303 at the base and rises through the reactor 303 in the
central tube described above. The C0 2 stream 210 loses a portion of heat to at least
one of the solids streams 203, 209 and exhausts at the upper portion of the reactor C02
gas stream 2 11 into the array of tubes for cooling in the pre-heater segment 102. This
provides a process in which there is no mixing of the calciner combustion CO2 gas
stream 210 with the heating gas stream 245, so that there is no need for carbon capture
processes. The calcined powder and gas stream 204 is separated into the CO2 gas
stream 210 and the hot calcined powder is exhausted into the rotary kiln 308 from the
gas particle separator 304 as stream 205.
[0081]. The rotary kiln plant 308 process is similar to that used in conventional cement
plants. The calcined powder is injected into the rotary kiln 308 where it is heated to
between 1300 to 1600°C, generally the powder is heated to approximately 1450°C. The
vitrification of the silica is such that aggregation of particles takes place and the balls of
materia! are formed and agglomerate as the reaction proceeds. The energy for the
heating to form the clinker 306 is derived from the kiln combustor 3 that produces a
heating gas 240. The clinker reactions are exothermic, and additional energy is only
required to raise the temperature to approximately 1450°C. The exhaust gas 241 from
the rotary kiln 308 is approximately 1000°C. The clinker 206 is exhausted from the
rotary kiln 308 at approximately 1400°C, and is cooled in the clinker cooler 309 by air
stream 230 to give a cooled clinker stream 207 which is ground to cement powder 208 in
the cement grinder plant. Alternatively, the cooled clinker 207 can be stored before
being ground to cement powder 208.
[0082], The processes previously described can be used to increase power efficiencies.
There are many ways to arrange the process flows to deliver such efficiency, and the
one described below is a preferred embodiment. The air stream 230 is used to cool the
clinker 306, and the heated air is split into a secondary air stream 231 which is used,
with the primary air stream 233 to burn the fuel 222 in the kiln combustor 3 1. The
primary air stream 233 is a cold air stream that conveys the fuel 222, usually a solid, into
the combustor 3 11. For a low emissions cement plant, this fuel 222 is largely a biomass
or waste such that carbon emissions are minimised or eliminated from this part of the
process. The tertiary air stream 232 is used in the calciner combustor 305, 405. It is
mixed with the pre-heated air 236 from the pre-heater and is combusted in the caiciner
combustor 305, 405, along with a primary air stream 234, with the fuel 220 to produce
the heating gas stream 244. This recuperation cycle is almost identical to that of the
conventional plant. In at least one embodiment 60% to 70% of the fuel is combusted in
the calciner combustor 305, 405, and the remaining fuel is combusted in the kiln
combustor 3 11. A substantial difference from the conventional process is that the air
stream 236 has replaced the C0 2 in the combustion gas exhaust, so that the kiln
combustor 3 1 may run with additional excess air to reduce the carbon monoxide
emissions with a negligible impact on the plant efficiency.
[0083], The hot gas stream 241 from the rotary kiln 308 exhaust can contain a large
volume of volatile impurities, which can be reactive and condense on surfaces and
create equipment blockages. In this embodiment, the hot gas stream is treated by the
slip stream of solids 202 in a suspension cyclone scrubber 306. The mass flow of 241
and 202 are such that the exhaust temperature of the mixture from the scrubber 306 is
about 500°C. At this temperature, a number of the impurities in the gas react with the
meal to form solid compounds. These become sequestered in the heated meal 209,
T U2014/001054
19
which is injected into the calciner reactor 303, 403, The scrubbed flue gas 242 is routed
into the crusher and grinder plant 300 and is used to remove moisture from the raw
meal, particularly in the grinders. The crusher grinder plant 300 can also filter the flue
gas 242 prior to release into the atmosphere as filtered stream 243. The filtered stream
243 is safe to release into the atmosphere as the majority of the dust fines from the
cement have been removed prior to release.
[0084]. In a further embodiment, it will be understood by a person skilled in the art that
the pressure drop of gas from the input air 1 2 to the flue exhaust 120 is less than that
required for the conventional process in which the air has to drive against the solids
streams for up to about six stages of solids-gas mixing, and cyclone separation. This
means that the power consumption of the fans used for injecting the air (not shown),
and/or pulling the flue has through the plant are significantly reduced.
[0085]. Figure 4 shows an example embodiment of the process flow for lime
production. For convenience, the same numbering of the processes used for the
Portland cement embodiment are used. In this case, the raw meal 200 which is pure
limestone and the cooled product 208 is lime. The rotary kiln and clinker grinder plants
are replaced by a solids cooler 408. In the present embodiment the scrubber has been
removed due to the high purity of the limestone used in the process and negligible
volatile mixtures. Carbon is captured in the process similar to that described for Figure 3
in which the cooled CO2 stream 4 2 is fed into the CO2 processing plant 407. The
processing plant 407 separates and compresses the C0 2 3 from the water 214. The
crusher grinder plant 400 can also filter the flue gas 242 prior to release into the
atmosphere as filtered stream 243. The filtered stream 243 is safe to release into the
atmosphere as the majority of the dust fines from the cement have been removed prior
to release.
[0086]. In the case of lime 205, the lime 205 is cooled by air 230 in the solids cooler
408 to give a preheated air stream 232 for the calciner combustor 405. The solids
cooler 408 can be adapted to use the cooling tube 5 0 and plate cross-flow system as
described previously for the pre-heater 402. Therefore, hot lime can be fed into an array
of pipes, and a gas, such as air, can be fed into the lower portion of the cooler where it
rises through a cross-flow of an array of ducts such that air is heated in each horizontal
duct, and is then directed to the next higher duct and so on. This is an efficient counterflow
system that can deliver a cool powder and a heated air stream. The use of indirect
heating reduces the expose of the lime to the air, and the product can be bagged in
suitable containers for storage.
[0087]. In this specification, the word "comprising" is to be understood in its "open"
sense, that is, in the sense of "including", and thus not limited to its "closed" sense,
that is the sense of "consisting only of. A corresponding meaning is to be attributed
to the corresponding words "comprise", "comprised" and "comprises" where they
appear.
[0088]. While particular embodiments of this invention have been described, it will
be evident to those skilled in the art that the present invention may be embodied in
other specific forms without departing from the essential characteristics thereof. The
present embodiments and examples are therefore to be considered in all respects
as illustrative and not restrictive, the scope of the invention being indicated by the
appended claims rather than the foregoing description, and all changes which come
within the meaning and range of equivalency of the claims are therefore intended to
be embraced therein. It will further be understood that any reference herein to
known prior art does not, unless the contrary indication appears, constitute an
admission that such prior art is commonly known by those skilled in the art to which
the invention relates.

CLAIMS
1. A process for producing Portland cement clinker from at least crushed limestone and
crushed sand and clay including the steps of:
a. Mixing the limestone, and the sand and clay to form a mixed powder;
b. Calcining the mixed powder in a calciner reactor, wherein the calciner
reactor is adapted to apply indirect heat generated from the combustion of
a first fuel input to produce a pre-heated mixed powder, and wherein the
calciner reactor generates a first gas stream of carbon dioxide from the
calcination of limestone, a separate second gas stream from the
combustion of the first fuel input and a steam stream from the mixed
powder;
c. Introducing the mixed powder into a kiln using direct heating to produce
Portland cement clinker, where the kiln is fueled by the combustion of a
second fuel input mixed with air that is pre-heated by hot Portland cement
clinker exiting the kiln.
2. The process of claim 1, wherein the mixed powder is pre-heated prior to calcining
the mixed powder.
3. The process of claim 1 or claim 2 in which the first gas stream is cooled and
compressed, and stored.
4. The process of any one of preceding claims in which the first fuel input is a gas
mixed with air, wherein the air has been pre-heated by heat exchange with the
cooling the first gas stream.
5. The process of any one of preceding claims, wherein the first gas stream includes a
slip stream of the exhaust gas stream from the kiln, which has sufficient excess air to
provide complete combustion of that fuel.
6. The process of any one of the preceding claims, wherein the definition of sand
includes sand and setting additives.