DESC:FIELD OF THE INVENTION
[0001] The present invention relates to production of fuel gas from waste materials. Specifically, the invention relates to a process and design for producing a fuel gas from cellulosic waste materials with significant reduction in tar formation.

BACKGROUND OF THE INVENTION
[0002] Industrially, quality of the produced syngas plays a major role. The quality of the syngas is strongly dependent on the feedstock material, gasifying agent, feedstock dimensions, temperature and pressure inside the reactor, and design of the reactor. For example, Gasification by pure oxygen offers advantages such as similar or competitive capital cost with increased combustible components (carbon monoxide (CO) 20-32%, hydrogen (H2) 20-30% and carbon dioxide (CO2) 25-40%, CH4 5-10%, tar content 1-20%) and high heat content (10–12 MJ/Nm3) when compared with air-based gasification.
[0003] Gasification of waste materials crop residues for the production of syngas is both competitive and environmentally benign and adds economic value to the waste materials. The produced syngas offers a broad range of application from clean fuel synthesis to power generation. Recently, there has been an increase in the demand for syngas, especially in petroleum refineries. Methanol is the second largest consumer of synthesis gas and has shown remarkable growth as part of the methyl ethers used as octane enhancers in automotive fuels. The Fischer–Tropsch (FT) synthesis is the third largest consumer of syngas, mostly for transportation fuels and as a growing feedstock source for the manufacture of chemicals, including polymers. The hydroformylation of olefins (Oxo reaction), a completely chemical use of syngas, is the fourth largest use of carbon monoxide and hydrogen mixtures. In recent years, syngas (from agricultural residue via O2 gasification) is getting great attention as the precursor to synthesize bio-di-methyl-ether (bio-DME) and other chemicals having high economic and market potential.
[0004] Currently, waste materials such as lignocellulosic materials are not utilized completely for the production of high value products such as hydrogen, methanol, ammonia, methyl esters, FT fuels, ethanol, DME for fuel use etc., as high tar content of the gas from lignocellulosic feedstocks is a major hindrance to use of this feedstock. Generation of high value products from other waste materials such as municipal waste, animal manure, plastic waste materials etc., also need to be carried out.
[0005] Gasification of waste materials such as lignocellulosic and plastic materials is a thermochemical process, where the feedstock is heated to high temperatures, producing gases which can undergo chemical reactions to form syngas (combustible mixture of CO & H2). The heating is performed in the presence of a gasifying media such as air, oxygen (O2), steam (H2O) or carbon dioxide (CO2), inside a reactor called as gasifier. The gasification occurs in several steps involving heating and drying, pyrolysis, gas–solid reactions, and gas–phase reactions. During heating and drying, all feed moisture evaporates before the particle temperature increases to gasification temperatures. Pyrolysis occurs once the thermal front penetrates the particle, resulting in the release of volatile gases. In the pyrolysis step, about 70-80 % of the weight of the material is vaporized leaving behind char.
[0006] Tar consists of heavy and extremely viscous hydrocarbon compounds. After the pyrolysis step, the gases react with the particle surface, which is currently primarily char, in a series of gas–solid endothermic and exothermic reactions that increase the yield of light gases. Primarily, char reacts with oxygen, steam and carbon dioxide producing carbon monoxide, hydrogen and carbon dioxide. Finally, released gases continue to react in the gas–phase until they reach equilibrium conditions. The overall reaction in an air or oxygen in a steam gasifier can be represented by following equation, which involves multiple reactions and pathways.
CHxOy (biomass waste material) + O2 + H2O (steam) = Tar + CH4 + CO + CO2 + H2 + H2O + C (char) (1)
[0007] Products of char, oxygen reaction are carbon monoxide and carbon dioxide. The proportion of CO and CO2 formed depends on the temperature of char. At low temperature product is mostly carbon dioxide and at temperatures above 1000 C, product is mostly carbon monoxide.
C + O2 ? a CO + (1-a) CO2 ?HR = a (-110.5) + (1-a) (-393.5) kJ/mole (2)
C + H2O ? CO + H2 ?HR = 131.3 kJ/mole (3)
C + CO2 ? 2 CO ?HR = 172.5 kJ/mole (4)
In many gasifier arrangements, reaction 2 provides the heat required by reactions 3 and 4. However, such arrangement always produces gas with high tar and methane content.
[0008] Federal Emergency Management Agency (FEMA) has developed a gasifier design by modifying a traditional design. National Renewable Energy Laboratory (NREL) in the US conducted an exhaustive study of the design validating the different aspects of the details of this modified design. Several trials of the oxygen blown downflow gasifier of FEMA design validated by NREL have confirmed the ease of operation of this gasifier. A sketch of the FEMA gasifier is shown in FIG. 1. As confirmed by the NREL studies, the gasifier capacity is controlled by the throat area for the air blown unit.
[0009] Operating trials of the FEMA gasifier showed peak temperature in the furnace zone. If this temperature increased beyond 700º C, clinker formation starts. As this temperature exceeds 800º C, clinkering becomes a major challenge. At a peak temperature of 650º C-750º C in this zone, the gasifier operation is smooth. Although the gasifier is simple to operate and easy to start, it produces very large quantity of tar, making the use of gas challenging for many applications.

SUMMARY OF THE DISCLOSURE
[0010] This summary is provided to introduce a selection of concepts in a simple manner that are further described in the detailed description of the disclosure. This summary is not intended to identify key or essential inventive concepts of the subject matter nor is it intended to determine the scope of the disclosure.
[0011] In some aspects, a modified gasification process and a gasification system that is configured to provide high gas temperature for pyrolysis and gasification are disclosed. The process and equipment are modified to provide high heat-adding secondary/tertiary oxygen/air at the entry to the jacket that normally supplies heat to the reactions.
[0012] In one aspect, a gasification system for waste material gasification is disclosed. The gasification system includes a buffer and drying zone at the entry of the waste material feed, a pyrolysis start zone downstream of the buffer and drying zone, a pyrolysis completion zone downstream of the pyrolysis start zone, a gasification zone downstream of the pyrolysis completion zone, and a furnace zone downstream of the gasification zone. The system also has a jacket surrounding the pyrolysis completion zone and the gasification zone. The gasification system includes a primary oxidizer port configured to supply a primary oxidizer to the gasification zone for the combustion of the waste material feed and produce a product gas. The furnace zone is configured to move the product gas from the gasification zone to the jacket. The gasification system also includes a secondary oxidizer port. The secondary oxidizer port is configured to supply a secondary oxidizer to the product gas at an entrance to the jacket from the furnace zone to increase temperature of the product gas.
[0013] In another aspect, a process for waste material gasification using a gasification system is disclosed. The process for the waste material gasification includes supplying a primary oxidizer to a gasification zone of the gasification system during gasification for combusting a waste material feed and producing a product gas, supplying a secondary oxidizer to the product gas escaping from the gasification zone of the gasification system through a furnace zone to a jacket, for raising the temperature of the product gas for making the product gas suitable for steam reforming of tar and hydrocarbons.
[0014] Further advantages and other details of the present subject matter will be apparent from a reading of the following description and a review of the associated drawings. It is to be understood that the following description is explanatory only and is not restrictive of the present disclosure.

BRIEF DESCRIPTION OF THE FIGURES
[0015] To further clarify the advantages and features of the disclosure, a more particular description of the disclosure will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the disclosure and are therefore not to be considered limiting of its scope. The disclosure will be described and explained with additional specificity and detail with the accompanying drawings in which:
[0016] FIG. 1 illustrates a schematic process used in a FEMA plant of a prior art;
[0017] FIG. 2A shows arrangement of a low tar gasifier having a secondary oxidizer port, in accordance with an embodiment of the present invention;
[0018] FIG. 2B shows arrangement of a low tar gasifier having a secondary oxidizer port and a tertiary oxidizer port, in accordance with an embodiment of the present invention;
[0019] FIG. 3 illustrates a graph showing an expected relationship of the temperature of a solid biomass feed moving down the gasification system and the product gas in the jacket with the depth of the gasification system in a gasification system shown in FIG. 1; and
[0020] FIG. 4 illustrates an expected relationship of the temperature of the product gas under the influence of secondary oxidizer and under the influence of secondary and tertiary oxidizers, in the jacket in a gasification system shown in FIGs. 2A and 2B, in accordance with an embodiment of the present invention.
[0021] It may be noted that to the extent possible like reference numerals have been used to represent like elements in the drawings. Further, those of ordinary skilled in the art will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily drawn to scale. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help to improve understanding of aspects of the disclosure. Furthermore, the one or more elements may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skilled in the art having the benefits of the description herein.

DETAILED DESCRIPTION OF THE DISCLOSURE
[0022] For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the disclosure as illustrated therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates.
[0023] It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the disclosure and are not intended to be restrictive thereof. Throughout the patent specification, a convention employed is that in the appended drawings, like numerals denote like components.
[0024] Reference throughout this specification to “an embodiment”, “another embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of the phrase “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
[0025] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub-systems.
[0026] One or more of the embodiments of the present disclosure provide a modified design of a downflow gasifier to reduce or eliminate the quantity of tar it produces. Specifically, the modified design of the downflow gasifier disclosed herein reduces or eliminates highly aromatic and high molecular weight tar.
[0027] FIG. 1 illustrates a prior art gasifier design developed by Federal Emergency Management Agency (FEMA). In the FEMA design, the solid feed enters the gasifier at top, keeps dropping down and accumulates as ash at the bottom. Ash is occasionally removed from the gasifier. Air/oxygen flows down the gasifier converting lignocellulosic material to gas. After exiting the throat, the gas flow turns upward along the jacket and exits the gasifier about 2/3 of the way up. In the jacketed portion, the hot exiting gas heats up the downflowing solid feed, thereby drying and pyrolyzing it. The resultant char is gasified by the incoming oxygen as well as steam and CO2.
[0028] Steam is the highest weight portion of the pyrolysis products. Cellulosic waste and plastic waste release a major weight in the form of gaseous pyrolysis products. Cellulosic waste, each 100 kg of dry weight feed generates 25 kg char, 40 kg water vapor, 10 kg tar and 25 kg pyrolysis gas. The pyrolysis gas contains methane and other light hydrocarbons as well as CO and Hydrogen. In case of waste plastic, quite often the plastic is contaminated with other food/other waste materials, however, the plastic component of the waste releases as much as 95% of weight as gaseous product. Details of the products obtained from cellulosic waste and their yield using design of FEMA is provided in Table 1.

Table 1. Pyrolysis Reaction Yield
Sr. No. Component and Surrogate Weight % Yield Moles of surrogate/kg lignocellulosic solid
1 Char, Carbon 25 ---
2 H2O 40 22.22
3 Tar, Phenol (C6H5OH) 10 1.0638
4 Pyrolysis Gas 25 --
5 CH4 4.0 2.5 (2.24 to 2.725)
6 CO 19.77 7.062 (6.61-7.58)
7 H2 0.09 0.45 (0 - 0.966)
8 CO2 1.135 0.258 (0- 0.48)

[0029] FIG. 1. illustrates different functional zones created by the gasifier arrangement. The top portion contains unreacted feed. As the feed flows down, it is heated by the hot gas in the jacket. The hot solid in the jacketed portion also heats the solid above as heat rises upwards. The net result is that the solid is essentially dry before it enters the jacketed portion.
[0030] In the jacketed portion, the feed temperature keeps increasing as the feed flows down due to the existence of hotter solid below and the hot gas in the jacket. The feed gets pyrolyzed in the pyrolysis zone and char gets formed. The pyrolysis gases including tar and steam flow down into gasification zone. Below the pyrolysis zone, before entering the gasification zone, the char steam and char CO2 reactions can take place, if the suitable conditions are provided. However, in this portion, there is no oxygen available to provide heat of combustion. The reactions 3 and 4 need heat. The available heat at this portion in the FEMA designed gasification system depends on the heat transfer from below and from the jacket. The heat available from these sources are not sufficient for significant reactions to happen. Hence this zone is termed as char buffer zone in the FEMA design. Significant heat addition starts from the oxygen introduction point in the FEMA design.
[0031] Zone from oxygen inlet to throat is the main gasification zone, as all three char reactions take place at this zone, slowly raising the temperature. At the throat, the gas separates from the solid, gas flows up while solid continues to drop down. This is the hot ash and furnace zone. Highest temperature is achieved here. Gas phase reaction can take place in this furnace zone. Water gas shift would be expected to occur here as the temperature is high and significant quantity of steam and carbon monoxide is present.
[0032] In the FEMA design, peak temperature occurs in the furnace zone. At a peak temperature of 650º C - 750º C in this zone, the gasifier operation is smooth. Although the gasifier is simple to operate and easy to start, it produces very large quantity of tar. The gas leaving the solid and entering the jacket of FEMA gasifier contains oxidation products of char and pyrolysis products including tar and methane. A temperature of 700º C-750º C is too low for desirable reactions to take place to reduce tar formation, and hence they do not occur in FEMA gasifier. If the temperature at the furnace zone is increased, clinker formation starts. The clinkering becomes a major challenge as this temperature exceeds 800ºC, making the use of gas challenging for many applications. Therefore, a design of the gasification system that does not raise the temperature at the furnace zone yet providing sufficient heat for the gasification completion so as to reduce the tar formation is proposed in the current disclosure.
[0033] The desirable reactions that need to take place in a gasification system for reducing tar formation are as given below:
CnHmO + (n-1) H2O = n CO + (n-1+m/2) H2¬ (5)
CH4 + H2O = CO + 3 H2 (6)
CH4 + 2 O2 = CO2 + 2 H2O (7)
CnHmO + (n-1/2+m/2) O2 = n CO2 + m/2 H2O (8)
[0034] Reaction (5) is for reforming tar, reaction (6) is for reforming of methane, reaction (7) is for burning of methane, and reaction (8) is for burning of tar. Reactions (5) and (6) are highly desirable reactions as they remove undesirable tar, and further improve the syngas quality by increasing the hydrogen content. However, the temperature required for these reactions is higher than the gas temperature entering the jacket in a FEMA gasifier.
[0035] If all four reactions (5) to (8) takes place, no tar will be left in the product. Partial reactions of (5) to (8) also will lower the tar formation, especially that of heavy tar components. By adding secondary oxygen/air at the entry to the jacket, the temperature of reactions (7) and (8) may be increased. This is accomplished by implementing modified designs for the gasification system as shown in FIG. 2A and FIG. 2B.
[0036] FIG. 2A denotes a downward flow gasification system 100. The gasification system 100 includes a hopper 110 for feeding a waste material feed. The waste material that may be used in the gasification system 100 for gasification is any waste material including biomass. Biomass may include agro waste, forest waste, livestock manure, or other such predominantly cellulosic waste materials. Municipal solid waste may also be used as the waste material feed to the system 100. The waste material having predominantly cellulosic material can also include other waste materials that incinerate at temperatures less than about 1000ºC. A waste material may be considered as “predominantly cellulosic material” if the cellulosic material constitutes at least 60 wt.% of the waste material. The gasification system 100 also shows feasibility to use plastic waste as feedstock, if mixed with cellulosic waste in suitable proportion such as, less than 40 wt.%.
[0037] The hopper may have a convenient design for easy feeding and optimized rate of feeding. The gasification system 100 has a feed buffer and drying zone 120 immediately downward to the hopper, where the incoming feed gets dried by the heat flowing upwards from the contents further below. The dried feed enters a pyrolysis start zone 130 and consequently a pyrolysis completion zone 140 of the gasification system 100. The feed gets pyrolyzed at the pyrolysis start zone 130 and pyrolysis completion zone 140, forming char of the waste material feed introduce to the gasification system 100. A gasification zone 150 is provided below the pyrolysis completion zone 140. A furnace and ash collection zone 160 is situated below the gasification zone 150.
[0038] The hopper 110, feed buffer and drying zone 120, pyrolysis start zone 130, pyrolysis completion zone 140, and the gasification zone 150 are formed inside an outer shell 170 of the gasification system 100. In some embodiments, the shell 170 is formed using a stainless-steel material. In some embodiments, the shell 170 has one or more linings 172 on the outer surface at the bottom part, covering a portion of the gasification zone 150. The lining 172 aids in protecting wall of the gasification zone 150 from the high temperature of the exiting product gas. In some embodiments, the one or more lining 172 is formed using a fire cement material. In some embodiments, the lining 172 extends from the bottom of the shell 170 to about half the depth of the gasification zone 160.
[0039] The shell 170 has a jacket 180 covering lower portions of the shell 170 and extending further below than the shell 170 in the gasification system 100. The jacket 180 functions as an outer cover to the shell 170 at the pyrolysis completion zone 140 and the gasification zone 150. The furnace zone 160 includes a furnace and is located beyond the shell 170 in the bottom of the gasification system 100 and is essentially covered in the sides and bottom by the jacket 180. In some embodiments, the jacket 180 extends to a height span in a range from 65% to 85% of the total depth of the gasification system. In these embodiments, the top part of the jacket 180 is at a depth in a range from 15% to 35% of the total depth of the gasification system 100 from the top the gasification system 100. In other words, the jacket 180 extends from the bottom of the gasification system 100 and may extend to anywhere between 65% to 85% height of the gasification system 100, when measured from the bottom of the gasification system 100.
[0040] In some embodiments, the primary oxidizer port 182 is located at a distance (height) in a range from 70% to 85% of the total depth of the gasification system 100 from the top of the gasification system 100. In some embodiments, there are a plurality of primary oxidizer ports 182 deployed in the gasification system 100, and all the of primary oxidizer ports 182 are in a height range from 70% to 85% of the total depth of the gasification system 100 from the top of the gasification system 100. In some embodiments, the primary oxidizer ports 182 are deployed at various points surrounding the gasification zone 150, and all the primary oxidizers are in a same depth in the gasification system 100. In other embodiments, the primary oxidizers 182 are deployed at various points surrounding the gasification zone 150, and at least one of the primary oxidizer port 182 is deployed at a different height than at least one another primary oxidizer port 182, and both the oxidizer ports 182 are deployed surrounding the gasification zone 150 and are located in a range from 70% to 85% of the total depth of the gasification system 100 from the top.
[0041] The jacket 180 and the shell 170 have one or more primary oxidizer port 182 that is configured to supply a primary oxidizer to the gasification zone 150. The primary oxidizer is the oxidizer supplied for the gasification of the waste material feed in the form of pyrolyzed char that travels down from the pyrolysis completion zone 140 to the gasification zone 150. The gasification of the char in the gasification zone 150 by combining with the primary oxidizer is aided by the heat produced by the furnace in the furnace zone 160. In the gasification zone 150, combustion of the waste material feed produces a product gas. The furnace zone 160 is configured to move the product gas from the gasification zone 150 to the jacket 180. In the jacket 180, the product gas moves to the sides of the jacket surrounding the shell and exits through a product gas port 184 in the jacket 180, located near to the top of the jacket 180 in the gasification system 100. The temperature of the furnace may be regulated to provide required heat to the gasification zone 150. In some embodiments, the temperature of the furnace in the furnace zone 160 is regulated to be in a range from 650ºC to 750ºC. Ash formed by the gasification at the gasification zone 150 falls downwards and collected at the ash collection door 190 of the jacket 180 at the furnace zone 160.
[0042] The gasification system 100 of the embodiments of this disclosure further includes a secondary oxidizer port 186 in the jacket 180. The secondary oxidizer port 186 is located outside of the gasification zone 150 and configured to supply secondary oxidizer to the jacket, outside of the shell 170. In the embodiments, wherein the lining 172 is present, the secondary oxidizer port 186 is designed to supply the oxidizer to the space in the jacket surrounding the lining 170.
[0043] The location of the product gas port 184 is designed such that the product gas loses some part of its heat to the gasification zone 150 before escaping from the jacket 180 of the gasification system 100. Thus, the outgoing product gas supplies heat to the gasification zone 150 for the gasification reaction, in addition to the heat supplied from the furnace zone 160. In some embodiments, the product gas port 184 is located in the jacket 180 near the top of the jacket 180 at a distance in a range from 20% to 40% of the total depth of the gasification system 100 from the top.
[0044] The secondary oxidizer port 186 is designed to supply secondary oxidizer to the outgoing product gas. In some embodiments, the secondary oxidizer port 186 is located in the jacket 180 at the exit from the gasification zone 150, at a distance of nearly 75% of the total depth of the gasification system 100 from the top. In other words, the secondary oxidizer port 186 is located at the bottom of the gasification zone 150, supplying secondary oxidizer to the product gas in the jacket 180.
[0045] A process for waste material gasification using the gasification system 100 includes gasification of the waste material feed in the presence of a primary oxidizer to produce a product gas and supplying secondary oxidizer gas to the product gas on its way to escaping from the gasification zone 150 and furnace zone 160 to jacket zone 180 of the gasification system 100. The process specifically includes supplying the primary oxidizer to a gasification zone 150 of the gasification system 100 during gasification for combusting the waste material feed introduced through the hopper 110 of the gasification system 100. The waste material feed gets dried in the feed buffer and drying zone 120, starts pyrolyzing in the pyrolysis start zone 120, completes pyrolysis and gets charred in the pyrolysis completion zone 130, before entering the gasification zone 150 in the gasification system 100. In the gasification zone, the charred products receive heat from the furnace zone and primary oxidizer through the primary oxidizer port 182 and undergoes gasification reactions producing the product gas.
[0046] The product gas escapes through the furnace zone 160 to the jacket 180. On its way to the product gas port 184 in the jacket 180, the product gas interacts with the secondary oxidizer supplied through the secondary oxidizer port 186. The secondary oxidizer is supplied to the product gas to further complete the reactions of the product gas and increase the temperature of the product gas. The product gas with increased temperature, when moving from the bottom of the gasification system 100 through the space surrounding the shell 170 and inside the jacket 180 to the product gas port 184 outlet, heats the shell 170 and thereby supplies heat to the gasification zone 150 and to the pyrolysis completion zone 140 from the surroundings. This additional het supply from the surroundings to the gasification zone 150 and to the pyrolysis completion zone 140 aids increasing the heat energy available for the reactions in these zones than that is provided from the furnace in the furnace zone 160.
[0047] In some embodiments, the gasification system 100 further includes a tertiary oxidizer port 188 as illustrated in FIG. 2B. The tertiary oxidizer port 188 in the jacket is configured to supply a tertiary oxidizer to the product gas to further increase the temperature of the product gas. The tertiary oxidizer port 188 is located above the secondary oxidizer port 186 and below the product gas port 184 in the jacket 180 of the gasification system 100. There is a designed vertical gap in between the secondary oxidizer port 186 and the tertiary oxidizer port 188. In some embodiments, the tertiary oxidizer port 188 is located in the jacket 180 at a distance in a range from 50% to 70% of the total depth of the gasification system 100 from the top. The tertiary oxidizer port 188 is also located outside of the gasification zone 150 and configured to supply tertiary oxidizer to the jacket, outside of the shell 170. In the embodiments, wherein the lining 172 is present, the tertiary oxidizer port 188 is designed to supply the oxidizers to the space in the jacket surrounding the lining 170. The tertiary oxidizer port 188 is configured to further supply a tertiary oxidizer to the outgoing product gas, which has already reacted with the secondary oxidizer gas.
[0048] The primary, secondary, and tertiary oxidizers include oxygen. In some embodiments, oxygen is used as the oxidizer, in some other embodiments, air is used as the oxidizer. In some embodiments, air enriched with oxygen may also be used as the oxidizer. In some embodiments, the gasification system 100 includes a plurality of primary oxidizer ports, a plurality of secondary oxidizer ports, a plurality of tertiary oxidizer ports, or any combinations thereof. The plurality of oxidizers may be arranged at various points in the perimeter of the gasification system 100. The altitude of the one or more primary oxidizer ports, secondary oxidizer ports, and tertiary oxidizer ports are designed for optimal performance of the gasification system 100.
[0049] On its way to the product gas port 184 in the jacket 180, the product gas initially interacts with the secondary oxidizer supplied through the secondary oxidizer port 186, and later with the tertiary oxidizer supplied through the tertiary oxidizer port 188. The secondary oxidizer and the tertiary oxidizers are supplied to the product gas to further complete the reactions of the product gas and increase the temperature of the product gas.
[0050] Since the temperature of the furnace zone is not increased to provide the additional heat to the gasification zone 150 and to the pyrolysis completion zone 140, the ash falling down does not form clinkers. The temperature of the gasification zone 150 may be increased by about 5% to 15% through the additional heat supply from the surroundings through the product gas heat. The temperature of the pyrolysis completion zone 140 may be increased by about 5% to 10% through the additional heat supply from the surroundings through the product gas heat. Further, by regulating the temperature of the product gas in the jacket and supplying heat to the gasification zone 150 and the pyrolysis zone 140, the temperature of the gasification zone 150 is regulated to be in a range from 750ºC - 800ºC, as higher temperature than 800ºC in these zones even in the absence of high temperature in the furnace zone 160 is detrimental to the gasification reactions.
[0051] Additional oxidizer supply through the secondary oxidizer port 186 also aids completion of the gasification reactions of the product gas and make the condition of the product gas suitable for steam reforming of tar and hydrocarbons, when the product gas exits the gasification system 100. In the embodiments having the tertiary port 188 designed to pass the tertiary oxidizer, further aids in the reaction completion.
[0052] Simulation of the performance of the gasifiers of FIG. 1 and FIG. 2B for woody biomass gasification were conducted and compared. Oxygen was used as the oxidizer for the simulation. An example gasification system 100 having a total height of about 1.4 m is used for conducting simulation studies to understand the effect of temperatures, effect of introduction of secondary oxidizer and the effect of introduction of tertiary oxidizer gas in the system 100. The gasification system has a furnace at the bottom in the range 1.2 m to 1.4 m depth from the top of the gasification system. The jacket extends from the bottom up to 0.3 m depth from the top of the gasification system. The product gas port is located at top of the periphery of the jacket 180. Thus, the depth of the product gas port is about 0.3 m from the top gasification system.
[0053] For simulation, it was assumed that the temperature at the entrance to jacketed portion was at least 120º C, hence the solid entering the jacketed portion is essentially dry. The simulation results showed that within 300 seconds (5 minutes), solid at hot end became dry as the temperature increased to 116º C. The solid within 0.1 m of the hot edge was dry and remained dry in spite of the wet feed continuously entering from the top. The content above 0.1 m edge remained fairly wet. By varying the temperature at the hot end, the predicted depth of drying zone changed. However, from the simulation results, it can be concluded that the drying zone is only 100 to 200 mm thick and feed above that essentially remains wet. Drying zone extends up to 200 mm above the entrance to the jacketed section.
[0054] The rate of heat transfer on the bed of solid was modelled in a dynamic model. In the simulation, the hot end was assumed to reach a temperature of 300º C, the typical temperature required for pyrolysis. Conclusion from the dynamic study was that the temperature reaches a steady state after 1 hour. The results indicated rapid heat transfer after temperature reaches above 250º C and significant slowdown of rate of heating at lower temperature rate. Thus, over a bed height of 300 mm, the temperature can rise from 110º C to 250º C, assuming a hot source of about 250º C -300º C is available at the hot end. Distance for drying as well as pyrolysis is in the 300 mm to 500 mm range provided the temperature reached at the start of jacketed zone is 300o C.
[0055] In the jacketed zone, the temperature is expected to rapidly increase as pyrolysis is initiated and if the temperature is high enough and heat transfer from jacket is high enough, gasification reactions will also start. As a first step, the heat transfer from jacket to the solid was determined. For determining the heat transfer, the wall temperature was set at 200º C at the top end of the jacket and was set at 700º C at the bottom end. With this, the temperature of the solid showed a linear trend with radius, with slope increasing with temperature. Heat transfer rate at the wall was calculated from the simulation results of temperature rise as solid drops down the gasifier and the temperature difference between the wall and average solid temperature. The heat transfer coefficient increased from 33 W/m2C at 200º C to 91 W/m2C at 700º C.
[0056] The gasification reactions along the jacketed portion were simulated as average conditions along the axes. The heat transfer coefficients calculated above were used to determine the heat conducted into the solid from the wall. Reaction rates were also calculated. The heats of formation determine the heat demand/supply from each individual reaction. Dry solid was assumed to enter the jacketed portion and oxygen was injected at 0.75 m from start of jacket. Entire jacket length was kept as 0.9 m. Gas composition based on 3 kg/hr DAF crop residue feed and 8 ml/minute oxygen feed were predicted by mathematical simulation. In the furnace zone gas separates from solid and flows up the jacket. As the gas flows up, it transfers heat to the downflowing solid as well as gas released by pyrolysis (reaction (1)).
[0057] FIG. 3 illustrates a graph 300 showing an expected relationship of the temperature of the solid biomass feed 310 moving down the gasification system and the product gas 320 in the jacket with the depth of the gasification system in a gasification system shown in FIG. 1. The primary oxidizer input is provided at a depth of about 1 m from the top of the gasification system. In this case, the additional restriction is also based on the clinker formation tendency of ash. For agro-waste or crop residue materials, the tendency of clinker formation starts as temperature exceeds 800ºC. With this constraint, the maximum solid temperature may reach 800ºC.
[0058] Due to small temperature difference between the jacket and solid temperature the solid only reaches a maximum temperature of about 600ºC prior to introduction of primary oxidizer. Our simulation has shown that at 600ºC, the reaction rate of steam-carbon and steam CO2 reaction (reactions (3) and (4)) is quite low. These conclusions from simulation were confirmed by gasifier material balance during a 20-hour gasifier operation at steady state. The material balance as measured is shown in the Table 1 below. Any calculated values are indicated as calculation.
[0059] Table 1
Material Weight kg
Agro-waste Feed (12 % moisture, 9.09 % Ash) 48
Dry Ash Free Material (calculation) 37.9
Oxygen Feed 10.4
Carbon Burnt by oxygen (calculation) 3.9
Unburnt Carbon in discharged Solid 5.7
Total Carbon (Burnt + Unburnt) 9.6
Total carbon accounted for as % of DAF feed (Calculation) 25%
[0060] It can be seen that the total carbon accounted for matches exactly the expected amount of char from reaction 1. This result implies that, in this case, very little carbon gasification reaction (reactions (3) and (4)) took place. Therefore, it can be concluded that, to be effective as higher efficiency gasifier, the jacket gas temperatures need to be much higher.
[0061] To verify the above conclusion, simulation runs were made by simply specifying the wall separating jacket from the solid reached higher temperature and pyrolysis took place in the top 300 mm of the gasification system. The resulting char as well as pyrolysis gases flowed down towards the furnace while wall temperature increased rapidly from 300 ºC at 0.3 m depth from the top of the gasification system to 900 ºC by 0.8 m depth. Initially, no primary oxidizer (oxygen) supply was considered for the simulation. The simulation results showed zero carbon in the furnace, even in the absence of supply of primary oxidizer, when the shell wall temperature was raised as described above. Thus, simulation showed that all carbon formed during pyrolysis was consumed by gasification reactions (3) and (4) before reaching the furnace zone. The average solid temperature reaching the furnace was 775 ºC, just below the maximum allowable temperature to prevent clinkering. The temperature profile implied by this simulation is shown in FIG. 4. In FIG. 4, graph 400 shows an expected relationship of the temperature of the product gas in the jacket 180 (a) 420- under the influence of secondary oxidizer (b) 430-under the influence of secondary oxidizer and tertiary oxidizer, in the jacket in a gasification system shown in FIG. 2A and FIG. 2B, respectively.
[0062] Although the simulation showed full carbon gasification even in the absence of any primary air, it may not be practically possible as the final small quantity of carbon remaining unreacted can only be reduced to near zero value by oxygen. The secondary oxygen to be provided as per simulation results will raise the temperature of gas entering jacket to 1000ºC, which is not practical to contain in the Jacket. Hence simulation was carried out by introducing a small amount of primary oxidizer at the depth of 1 m from the top of the gasification system, about 150 mm to 200 mm before reaching the furnace zone. Secondary oxidizer is supplied at the entrance of the product gas to the jacket and tertiary oxidizer in the jacket at about 1 m to 1.1 m from the top. Table 2 shows simulation results of the product gas composition of the gasification system of FIG. 1 and FIG. 2.
Table 2. Simulation Results- Gas Composition
Sr. No. Gasifier Gas Flow Mole/hr Gas Composition (Mole Percent) Unreacted Char
CO H2 CO2 H2O Tar CH4
1 Gasification system without addition of secondary and tertiary oxidizer 168.1 26.3 12.1 12.0 43.2 1.9 4.5 4%
2 Gasification system with the addition of secondary and tertiary oxidizer 228.7 32.7 19.6 15.9 31.7 -- -- --

[0063] It can be seen from the results and predicted temperature profile that most of the tar produced by pyrolysis will be unchanged and simply pass through the jacketed portion in case of no addition of secondary and/or tertiary oxidizer. A secondary oxygen injection at the entrance to jacket in the furnace zone and a tertiary oxygen after the injection of the secondary oxidizer burns the tar and raises the product gas temperature. The results of the simulation have led to the modification of the design of the gasification system from that showed in FIG. 1 to that showed in FIG. 2B. Physical trials with the gasification system of FIG. 1 and gasification system of FIG. 2B were conducted and the results obtained are shown in the Table 3. Trials were conducted using various primary oxygen flow rate and run duration. Tar formation was indirectly measured using the amount of caustic consumption. Tar contains mostly aromatic hydrocarbons with significant presence of phenolic species. Raw product gas was washed with caustic added water. Phenols form sodium phenate neutralize the caustic. Wash water pH was monitored and as the pH dropped below 8.5, additional caustic was added to the wash water. The amount of caustic consumed by wash water directly correlates with the phenolic species present in tar and thus indirectly with quantum of tar produced.
Table 3. Summary of FEMA Gasifier Trials
Oxygen Flow Rate
lpm Run Duration
Hr Caustic Consumption
Lit/hr Oxygen Flow Rate
lpm Run Duration
Hr Caustic Consumption
Lit/hr
Without Secondary / Tertiary oxygen After Addition of Secondary Oxygen
8 6 0.4 8 9 0.2
10 3 0.6 8.4 9 0.1
10 5 0.5 8.8 16 -
10 4 0.8 8.8 10 -
8 6 0.6 8.2 46 -
8 8 0.4 8.5 11 -
10 7 1 5.6 32 0.06

[0064] It was observed that operating the gasification system of FIG. 1 produced large quantum of tar. The 7 trials of Table 3 show caustic consumption ranging from 0.4 to 1 lit/hr of operation at gas generation rate corresponding to 8 to 10 lpm oxygen feed. After addition of secondary oxygen, caustic consumption was maximum 0.2 lph in the initial trial. The caustic consumption was reduced to zero with more operating experience.
[0065] Further experiments included a bi-carbonate and hydroxide analysis of wash water after 24 hours of operation. The color of water was clear to whitish and not dark brown to black as was common during initial trials with arrangement like figure 1 (without secondary oxygen). The wash water contained 13200 ppm bi-carbonate and 300 ppm hydroxide. In this trial the wall temperature at the entrance to the jacket ranged from 950-1000oC; the measured temperature implied actual gas temperature in the range of 1000-1050oC. The chemical analysis clearly and conclusively proved total elimination of tar; taking tar and methane reactions (5), (6), (7) and (8) to completion.
[0066] The table 3 clearly shows a reduction in tar formation as secondary oxygen was added. Since the initial experiment described here was a simple modification of the standard design, benefits of the secondary oxygen addition were not realized completely in this experiment. In these experiments, heaviest components of tar were eliminated, and small amount of tar formation still occurred. However, by increasing the jacketed volume to allow gas phase reactions to proceed to completion, by adding a fire cement layer to protect the shell metal, and by lowering the peak temperature to which the waste material and char is exposed, the benefits of secondary oxidizer addition can be fully realized, and it is possible to conduct gasification without producing any tar. With these additional changes, the expected flame temperature at the secondary air addition will increase to 1000º C, taking the tar and methane reactions (5), (6), (7) and (8) to completion. With the introduction of secondary and tertiary oxidizer, the hydrogen concentration in syngas will rise from 12 % to around 20 %. Cooling this gas to reduce the moisture content will again raise the hydrogen content of syngas to 28% as indicated in Table 2.
[0067] Oxidant addition for gasification is often expressed as equivalence %; where 100% corresponds to 100% oxidation of feed. Value for primary oxidant used in the gasification system of FIG. 1 was about 23.2%. Initial simulation results were obtained for zero primary oxidant, 12% secondary oxidant and 8.6% tertiary oxidant. By conducting further practical simulation, the range of primary, secondary and tertiary oxidizer values are determined. In some embodiments of the process of gasification in the gasification system, the total oxidizer supplied for the gasification process is in a range from 18-30% of an equivalence oxidizer percentage. In some embodiments, the primary oxidizer is supplied in a range from 5% to 10%, the secondary oxidizer is supplied in a range from 10% to 20%, as equivalence oxidizer percentage.
[0068] The modified design of the gasification system along with the addition of secondary and tertiary oxidizer during the gasification process significantly reduces the tar formation. optimized design and /or process variations are found to be having the potential of completely eliminating the tar formation.
,CLAIMS:1. A gasification system (100) for a waste material gasification, the gasification system (100) comprising:
a buffer and drying zone (120) at the entry of the waste material feed;
a pyrolysis start zone (130) downstream of the buffer and drying zone (120);
a pyrolysis completion zone (140) downstream of the pyrolysis start zone (130);
a gasification zone (150) downstream of the pyrolysis completion zone (140);
a furnace zone (160) downstream of the gasification zone (150);
a jacket (180) surrounding the pyrolysis completion zone (140), and the gasification zone (150);
a primary oxidizer port (182) configured to supply a primary oxidizer to the gasification zone (150) for the combustion of the waste material feed and produce a product gas, wherein the furnace zone (160) is configured to move the product gas from the gasification zone (150) to the jacket (180); and
a secondary oxidizer port (186) configured to supply a secondary oxidizer to the product gas at an entrance to the jacket (180) from the furnace zone (160) to increase a temperature of the product gas.

2. The gasification system (100) as claimed in claim 1, wherein the jacket (180) extends to a height span in a range from 65% to 85% of the total depth of the gasification system (100) and starts at a distance in a range from 15% to 35% of the total depth of the gasification system (100) from the top.
3. The gasification system (100) as claimed in claim 1, wherein the primary oxidizer port (182) is located at a distance in a range from 70% to 85% of the total depth of the gasification system (100) from the top.

4. The gasification system (100) as claimed in claim 1, wherein a product gas port (184) is located in the jacket (180) near the top of the jacket (180) at a distance in a range from 20% to 40% of the total depth of the gasification system (100) from the top.

5. The gasification system (100) as claimed in claim 1, wherein the secondary oxidizer port (186) is located in the jacket at the entrance from the furnace zone (160), at a distance of nearly 85% of the total depth of the gasification system (100) from the top.

6. The gasification system (100) as claimed in claim 1, comprising a tertiary oxidizer port (188) in the jacket (180) configured to supply a tertiary oxidizer to the product gas to further increase the temperature of the product gas.

7. The gasification system (100) as claimed in claim 6, wherein the gasification system (100) comprises a plurality of primary oxidizer ports (182), a plurality of secondary oxidizer ports (186), a plurality of tertiary oxidizer ports (188), or any combinations thereof.

8. A process for gasification of a waste material using a gasification system (100), the process comprising:
supplying a primary oxidizer to a gasification zone (150) of the gasification system (100) during gasification for combusting the waste material feed and producing a product gas; and
supplying a secondary oxidizer to the product gas escaping from the gasification zone (150) of the gasification system (100) through a furnace zone (160) to a jacket, for raising the temperature of the product gas for making the product gas conditions suitable for steam reforming of tar and hydrocarbons.

9. The process as claimed in claim 8, wherein a total oxidizer supplied for the gasification process is in a range from 18-35% of an equivalence oxidizer percentage, in which the primary oxidizer is supplied in a range from 8% to 12%, the secondary oxidizer is supplied in a range from 10% to 20%, as the equivalence oxidizer percentage.

10. The process as claimed in claim 8, comprising sustaining the temperature of the gasification zone (150) in a range from 750ºC - 800ºC by increasing the temperature of the product gas at entrance to the jacket to a temperature in the range of 950ºC -1050oC.