Method and system for producing a producer gas The present invention relates to production of a producer gas from a carbonaceous material.

Gasification is a process that converts carbonaceous

materials, such as, biomass, coal or any other solid fossil fuel under sub-stoichiometric condition into carbon monoxide, hydrogen, carbon dioxide, methane and a few inerts. The resulting gas mixture is called producer gas or syngas and itself is a fuel. The primary combustible products of a producer gas are carbon monoxide and hydrogen. Agricultural residues or agro-wastes are significant biomass sources produced as a consequence of harvesting of crop. They are largely disposed off by burning them in the fields except for a small component being used as fodder for domestic animals and for cooking application in rural areas. Thermal

processing (gasification and combustion) of these agro-wastes has provided great potential for various end applications including energy production. Agro-residue is typically characterized by low density, high moisture and ash content and is high in inorganic such as calcium and potassium. These characteristic make them one of the difficult fuels from handling, transportation, storage and end application point of view.

Few examples of agro-residue are rice or paddy husk, rice or paddy straw, sugar cane trash, bagasse, coir pith, groundnut shells, and cotton stalks etc. Among the agro-residue, rice husk is a waste generated from paddy milling operation. It has been proved to be the most difficult fuel for thermo- chemical conversion process (combustion or gasification) because of its inherent high ash content (20% ash or silica) , resulting in poor carbon conversion efficiencies. Even though inefficient, rice husk has been successfully deployed as a fuel in large power plant for power generation application using conventional combustion technology. These are typically in the range of 1 to 10 MWe . The low efficiency is usually offset by lower fuel cost as compared to fossil fuels.

However, the combustion technology becomes unviable in the sub-megawatt power range. The gasification technology is proven to be viable at such low power levels. However, the existing technologies for processing high ash content fuels such as rice husk is beset with problems such as poor gas quality in terms of low calorific value and high contaminant level, for example, tar, making them highly unsuitable for power generation via combustion engine route. Moreover, the carbon conversion being low makes the whole process highly energy inefficient. The object of the present invention is to improve the

calorific value of a producer gas produced from a

carbonaceous material.

The above object is achieved by a system for producing a producer gas according to claim 1, wherein the system

comprises a first reactor comprising an inlet for receiving a carbonaceous material and air, and for inducing a first swirl to the carbonaceous material and air, a heating module adapted to provide heat such that the first swirl induced carbonaceous material is pyrolysed and subsequently undergo oxidation and reduction to produce a raw producer gas and a char, and a second reactor adapted to receive the raw

producer gas and the char and adapted to combust a portion of the raw producer gas to produce an amount of heat for

inducing a reduction reaction of the char to produce a resultant producer gas.

The raw producer gas is laden with tar and is of lower calorific value. The resultant producer gas is of high calorific value and comprises reduced amount of tar than the raw producer gas. The first swirl induced to the carbonaceous material and air increases the residence time of the

carbonaceous material inside the first reactor and thus achieves efficient production of char. The amount of heat produced by the combustion of the portion of the raw producer gas in the second reactor is the required heat for inducing the reduction reactions of the char as the reduction

reactions are endothermic. This enhances the carbon

conversion and thus increases the calorific value of the resultant producer gas and also makes the resultant producer gas less tar laden. According to an embodiment, the second reactor is further adapted to mix raw producer gas and the char. The mixing of producer gas and the char induces a tortuous path of flow of the producer gas and the char and thus, increases the

residence time of the char inside the second reactor.

Additionally, the mixing of the producer gas and char is enhanced which further enhances the carbon conversion. As the residence time of the raw producer gas and the char inside the second reactor is increased, the removal or cracking of tar contained in the raw producer gas is increased.

According to another embodiment, the second reactor comprises an inlet port for receiving the raw producer gas and the char proximate to a first side end and a gas outlet for

discharging the resultant producer gas from the second reactor and an ash outlet for discharging an ash generated from the reaction of the raw producer gas and the char proximate to a second side end, the second side end being opposite to the first side end. This enables in utilizing the entire length of the second reactor for the reduction

reactions.

According to yet another embodiment, the second reactor comprises a radially inwardly disposed rotatable shaft comprising at least one member extending from the rotatable shaft towards a wall of the second reactor for mixing the raw producer gas and the char. As the mixing is induced using the rotatable shaft, the second reactor is not required to be rotated. This enables in preventing leakage of the producer gas from the second reactor and thus increases the safety.

According to yet another embodiment, a wall of the second reactor is rotatable for mixing the raw producer gas and the char .

According to yet another embodiment, wherein the inlet port is at the first side end of the second reactor and the gas outlet and the ash outlet are at the second side end of the second reactor. In aspects where the wall of the second reactor is rotatable, the inlet can be positioned at the first side end and the gas outlet and the ash outlet are at the second side end of the second reactor.

According to yet another embodiment, the second reactor comprises at least one oxidizer inlet for providing an oxidizer for combustion of the portion of the raw producer gas. The oxidant could be either ambient air, hot air, elevated temperature steam or a combination thereof.

According to yet another embodiment, the at least one

oxidizer inlet (29) is adapted to provide a sub-stoichometric amount of the oxidizer. The flow rate of the oxidant through the oxidizer inlet can be predetermined so that a sub- stoichometric amount of oxidant is supplied into the second reactor .

According to yet another embodiment, the second reactor comprises a plurality of said oxidizer inlets positioned such that a plurality of reaction zones are created inside the second reactor producing the respective said amount of heat for inducing the reduction reaction of the char. Providing the oxidant at plurality of locations inside the second reactor combusts the producer gas at multiple locations creating a plurality of reaction zones. This increases the temperature inside the second reactor. The increased temperature results in efficient gas phase reduction

reactions using the char which is hot.

According to yet another embodiment, the carbonaceous material is an agricultural residue. As the calorific value of the resultant producer gas is increased, agricultural residues can be efficiency used for producing the producer gas . Another embodiment includes, a method of producing a producer gas, the method comprising inducing a first swirl to a carbonaceous material and air, pyrolysing and subsequently oxidizing and reducing the first swirl-induced carbonaceous material to produce a raw producer gas and a char and

combusting a portion of the raw producer gas to produce an amount of heat for inducing a reduction reaction of the char to produce a resultant producer gas.

The present invention is further described hereinafter with reference to illustrated embodiments shown in the

accompanying drawings, in which:

FIG la is a schematic diagram of a system for producing a producer gas using a carbonaceous material fuel according to an embodiment herein,

FIG lb is a schematic diagram of a system for producing a producer gas using a carbonaceous material as a fuel according to another embodiment herein,

FIG 2 is a schematic diagram of a system for producing a producer gas using a carbonaceous material as a fuel according to yet another embodiment herein, FIG 3 is a flow diagram illustrating a method of

producing a method for producing a producer gas according to an embodiment herein, FIG 4 depicts a graphical representation of validation results for wood stalks,

FIG 5 depicts a graphical representation of validation results for coconut shells, and

FIG 6 depicts a graphical representation of validation results for rice husk. Referring now to FIG la is illustrated a schematic diagram of a system 1 for producing a producer gas using a carbonaceous material as a fuel according to an embodiment herein. The system 1 uses a two stage approach comprising for producing the producer gas using the carbonaceous material. According to aspects, herein the two stage approach is deployed in such a way so that the residence time is increased resulting in increased carbon conversion with reduced contaminants in the producer gas. The system 1 comprises a first reactor 3 comprising an inlet 5 for receiving a carbonaceous material and air, designated as 7 in FIG la. In the embodiments described herein, the carbonaceous material for producing the producer gas is a biomass. The biomass can include

agricultural residues, including, but not limited to, rice or paddy husk, rice or paddy straw, sugar cane trash, bagasse, coir pith, groundnut shells, cotton stalks and the like. Some of the biomass listed above could be used in the form they are available and for some pre-processing such as drying, sizing or pulverizing may be required. However, the

carbonaceous material can include other kinds of biomass, coal or any other solid fossil fuel.

The inlet 5 is designed to induce a first swirl to the incoming carbonaceous material and air as it is introduced tangentially into the reactor 3. For example, the first reactor 3 can be deployed using a cyclone reactor. The swirl induced carbonaceous material and air is pyrolysed or

devolatalized and subsequently undergoes oxidation and reduction inside the reactor 3 resulting in the production of a raw producer gas and a char. The raw producer gas produced by pyrolysis and subsequent oxidation and reduction of the carbonaceous material comprises contaminants, such as, tar. The first reactor 3 can be preheated to appropriate

temperatures for inducing the pyrolysis, oxidation and reduction reactions. For example, the first reactor 3 can be preheated to temperatures between 350-500 °C. The preheating of the reactor 3 can be achieved using a fossil liquid fuel burner or external heating of a wall 9 reactor 3. For

example, the external heating may be achieved by electrical means. Accordingly, the system 1 can comprise a heating module for generating the temperature between 350-500 °C for heating the carbonaceous material and air. The heating module can be located inside the reactor or external to the reactor based on the means of heating used.

The swirling of the carbonaceous material and air inside the reactor 3 enhances the mixing of the carbonaceous material and air. Additionally, the residence time of the carbonaceous material inside the reactor 3 is also increased as the carbonaceous material rotates in a swirl along with air, and does not flow down vertically. The enhanced mixing and increased residence time results in efficient conversion of the carbonaceous material into the raw producer gas and the char.

According to an aspect, the reactor 3 can be heated or operated at higher gasification temperatures for at least partially removing contaminants from the raw producer gas by reaction with the char. For example, a portion of the tar contained in the raw producer gas can be cracked thermally within the reactor 3 using the char produced in the reactor 3. Referring still to FIG la, the system 1 further comprises a second reactor 11 for receiving the raw producer gas and the char produced in the first reactor 3. The char can be

completely un-reacted char or a portion of the char may have partially reacted or converted if the portion of char has been used in the first reactor 3 for reduction process to generate the raw producer gas. According to an aspect herein, the second reactor 11 is adapted to increase the residence time of the char inside the second reactor 11 and enhance the mixing of the char and the producer gas. This results in enrichment of the raw producer gas such that the char further reacts with raw producer gas and forms a enriched or high calorific value resultant producer gas. Additionally, the contaminants in the raw producer gas, such as tar, are reduced so that the resultant producer gas produced comprises reduced contaminants and is of high calorific value. To achieve this, the second reactor 11 is adapted to mix the raw producer gas and the char so that the residence time of char inside the reactor is increased and the mixing of the

producer gas and the char is enhanced. The mixing of the raw producer gas and the char induces the raw producer gas and the char to flow in a tortuous path and thus increasing the residence time.

Referring still to FIG la, the second reactor 11 comprises an inlet port 13 adapted to receive the raw producer gas and the char produced in the first reactor 3. To achieve this, in the shown example of FIG la, the outlet 8 of the first reactor 3 is connected to the inlet port 13 of the second reactor 11 so that the second reactor 11 can receive the raw producer gas and the char from the first reactor 3. The second reactor 11 also comprises a gas outlet 15 for discharging the resultant producer gas produced, as designated by the arrow 16 and an ash outlet 17 for discharging an ash generated by the

reaction of the raw producer gas and the char, designated by the arrow 18. The inlet port 13, the gas outlet 15 and the ash outlet 17 are arranged such that substantially the entire length of the reactor 11 is used for the reaction of the raw producer gas and the char. To achieve this, for example, the inlet port 13 can be arranged proximate to a first side end 19 of the second reactor 11 and the gas outlet 15 and the ash discharge can be arranged at a second side end 21 of the second reactor 11. The side ends 19, 21 are opposite sides of the second reactor 11.

In the shown example of FIG la, the reactor 11 comprises a wall 23 substantially cylindrical in shape. In the shown example of FIG la, for mixing the raw producer gas and the char, a rotatable shaft 25 is radially disposed inwardly into the second reactor 11. In the shown example of FIG la, the rotatable shaft 25 is introduced into the second reactor 11 from the first side end 19 and the shaft 25 extends towards the second side end 21. The rotatable shaft 25 comprises a plurality of members 27 extending from the rotatable shaft 25 towards the wall 23 of the reactor 11. The members 27 are rotated with the rotation of the rotatable shaft 25 for mixing the raw producer gas and the char. In the present example, the members 27 are arranged on the shaft 25 at an angle such that the rotation of the members 27 induces migration of the char in the reactor towards the second side end 21 from the first side end 19. Alternatively, the

migration of the char inside the reactor 11 can be induced by having the second reactor 11 tilted such that the end of the reactor 11 at the first side end 19 is higher than the end of the reactor 11 at the second side end 21. The shaft 25 can be rotated using a drive unit which can be located external to the reactor 11.

Referring still to FIG la, the reactor 11 is adapted to combust a portion of the raw producer gas received from the first reactor 3. To achieve this, the reactor 11 comprises one or more oxidizer inlets 29 for providing either one or combination of oxidant/s, so that the portion of the raw producer gas can be combusted and also participate in char reaction. The oxidant could be ambient temperature air, hot air or steam at elevated temperature. According to an aspect, the flow rate of the oxidant from the one or more oxidizer inlets 29 can be predetermined so that sub-stoichometric amount of oxidizer is provided for the combustion. For example, the oxidizer inlets 29 can be in the form of nozzles which are adapted to provide the sub-stoichometric amount of oxidant. The combustion of the portion of the producer gas produces an amount of heat which induces a gas phase

reduction reaction with the un-reacted char inside the reactor 11 to produce the resultant producer gas. The amount of heat produced by the combustion is the heat required for the reduction reactions of the char. In the shown example of FIG la, a plurality of oxidizer inlets 29 are provided such that the oxidant provided by the oxidizer inlets 29,

designated by the arrows 30, creates a plurality of reaction zones inside the second reactor 11. The plurality of reaction zones increases the temperature of the second reactor 11 which enables in inducing reduction reactions efficiently. Additionally, the mixing of the char and the producer gas enables in efficient mixing of the char and air for the reduction reactions. The reduction reactions occurring inside the reactor 11 can be expressed as follows:

C +C0 ₂ ^2CO (1)

C +H ₂ O^CO +H ₂ (2)

C +2H ₂ ^ CH, ( 3 )

CH +2H ₂ 0 ^CO +3H ₂ (4)

Referring to equation (1), carbon contained in the char reacts with carbon dioxide produced due to the combustion of the portion of the producer gas and is reduced to carbon monoxide. Referring to equation (2), carbon contained in the char reacts with water and is reduced to carbon monoxide and hydrogen. Referring to equation (3), carbon contained in the char reacts with hydrogen to form methane. Referring now to equation (4), the methane produced as per equation (3) reacts with water and is reduced to carbon monoxide and hydrogen. Referring now to equation (1), (2) and (3), it can be seen that the carbon from the char is reduced to carbon monoxide and hydrogen which are constituents of a producer gas. Thus, the carbon monoxide and hydrogen produced by the gas phase reduction reaction of the char and the raw producer gas produces the resultant producer gas with increased concentration of carbon monoxide and hydrogen. Thus, the calorific value of the resultant producer gas is increased due to the enhancement in the carbon conversion. Additionally, the tar contained in the raw producer gas is also thermally cracked using the amount of heat produced by the combustion of the portion of the producer gas. Thus, the resultant producer gas exiting the second reactor 11 via the gas outlet 15 will be of high calorific value with reduced amount of tar. The mixing of the raw producer gas and the char in the second reactor 11 increases the residence time of the char inside the reactor 11 and also enhances the mixing of the char with air resulting in enhanced carbon conversion and thus, increasing the calorific value of the resultant producer gas. The increase in the residence time of the char in the reactor 11 also enhances the thermal cracking of tar and thus achieves in reducing the contaminants contained in the resultant producer gas. The ash generated due to the reaction of the char can be discharged from the ash outlet 17.

The resultant producer gas having enhanced calorific value can advantageously be combusted in any form of prime mover for power generation. The primer mover could be either an internal combustion engine such as diesel engine, gas engine or gas turbine or external combustion engine such as Stirling engine, steam engine/turbine . The enhanced calorific value of the resultant producer gas increases the efficiency of power generation. Additionally, as the contaminants of the

resultant producer gas are reduced, it can be combusted in an internal combustion engine without any additional cleaning. Tar being one of the major contaminants of producer gas is required to be removed before combustion of the producer gas in an internal combustion engine. The embodiment described herein achieves in efficient removal of tar contained in the producer gas . FIG lb illustrates the system 1 according to another

embodiment herein. In the shown example of FIG lb, the raw producer gas and the char is discharged from the top of the reactor 3 and not from the bottom of the reactor as

illustrated in FIG la. An outlet 9, also know as a vortex finder, is disposed axially inwardly into the first reactor 3. Advantageously, the outlet 9 can be in the form of a duct. A conduit 10 in fluid communication with the outlet 9 of the first reactor 3 and the inlet port 13 of the second reactor 11 enables in transportation of the raw producer gas and the char from the first reactor 3 to the second reactor 11. Large sized particles of the char may tend to concentrate towards the wall of the reactor 3 and thereafter migrate to the bottom of the reactor 3. Therefore, to provide the char from the bottom of the first reactor 3 to the second reactor 11, the outlet 8 of the first reactor 3 is connected to the second reactor 11 for providing the char which has migrated to the bottom of the reactor 3. For example, the outlet 8 can be connected to the second reactor 11 using a duct 12 and advantageously the arrangement can be such that the char can slide easily from the first reactor 3 to the second reactor 11 through the duct 12.

FIG 2 illustrates the system 1 according to another

embodiment herein. In the shown example of FIG 2, the wall 23 of the reactor 11 is rotatable for mixing the raw producer gas and the char. Accordingly, in the present embodiment of FIG 2, the wall 23 of the reactor 11 is cylindrical in shape. The wall 23 is rotated using rings 31 attached to the wall 23 of the reactor 11. The rings 31 can be rotated using a drive (not shown) . Openings are provided on the inside surface of the side ends 19, 21 to receive the wall 23. Seals 33 can be used to seal the openings in the first side end 19 and the second side end 21 in which the wall 23 is received. Thus, in the shown example of FIG 2, only the wall 23 of the reactor 11 is rotated and the side ends 19, 21 are not rotated. As the wall 23 is rotated for mixing the raw producer gas and the char, the inlet port 13 through which the reactor 11 receives the raw producer gas is positioned on the first side end 19 and the gas outlet 15 and the ash outlet 17 are positioned on the second side end 21. In the shown example of FIG 2, the outlet 8 of the first reactor 3 is connected to the inlet port 13 of the second reactor 11 so that the second reactor 11 can receive the raw producer gas and the char. The ash outlet 17 is advantageously positioned substantially at the bottom of the second side end 21 for easy discharge of the ash from the reactor 11. In the present embodiment, migration of the char towards the second side end 21 is achieved by having the reactor 11 tilted such that the end of the reactor 11 at the first side end 19 is higher than the end of the reactor 11 at the second side end 21. Referring still to FIG 2, as the wall 23 of the reactor 11 is rotated, advantageously, the oxidizer inlet 29 can be

provided at either one of the side ends 19, 21 or at both the side ends 19, 21. In the shown example of FIG 2, the oxidizer inlet 29 is provided at the first side end 19. Thus, the oxidant for the reaction of the portion of the raw producer gas and the char will be introduced into the second reactor 11 through the side end 19. In the shown example of FIG 2, the oxidant is transported within the reactor 11 such that multiple reaction zones are created. To achieve this, a conduit 37 in fluid communication with the oxidizer inlet 29 is provided to transport the oxidant within the reactor 11. The conduit 37 comprises a plurality of holes 39 so that the oxidant can be supplied such that multiple reaction zones are created inside the reactor 11. The supply of the oxidant through the holes 39 is designated by the arrows 30.

Advantageously, the conduit 37 can extend till the second side end 21 so that the reaction zones are created along the complete length of the reactor 11, i.e., the distance from the first side end 19 to the second side end 21. As mentioned in the example of FIG 1, the flow rate of oxidant through the oxidizer inlet 29 can be predetermined so that sub- stoichometric amount of air is provided for the combustion. FIG 3 is a flow diagram illustrating a method of producing a method for producing a producer gas according to an

embodiment herein. At block, 40 a first swirl is induced to a carbonaceous material and air. Next, at block 45, the first swirl-induced carbonaceous material is pyrolysed and

subsequently oxidized and reduced to produce a raw producer gas and a char. Moving next to block 50, a portion of the raw producer gas is combusted to produce an amount of heat for inducing a reduction reaction of the char to produce a resultant producer gas.

As the calorific value of the producer gas is enhanced and the amount of contaminants reduced, the embodiments described herein can be used efficiently used for the production of producer gas using fuels having high ash content, such as rice or paddy husk, rice or paddy straw, sugar can trash, bagasse, coir pith, ground shells, cotton stalks and the like. The high ash content of these fuels make them highly unsuitable for power generation via internal or external combustion engine as the producer gas produced is of low calorific value and comprises high amount of contaminants.

Example : A kinetic lumped model for gasification of rice husk and air for a proposed entrained bed system was prepared. In the present model, mixing of the raw producer gas and the char is not included in this modeling. From this lumped model, the reduction zone was modeled and the equations (1), (2), (3) and (4) were considered in the reduction zone. The products of combustion of these gases were calculated using sub- stoichometry oxidation of the same. These products were provided as input to the reduction zone model. The combustion zone output temperatures were assumed to be 1400K and

pressure was assumed to be 101.325 kPa. With mass, energy and momentum balance equations, the model was thus framed and developed to estimate the variation of pressure, velocity, temperature and gas compositions along the reduction zone length .

The challenge was to develop a lumped model that could consider the catalytic effects of char and the gas phase kinetics. The char kinetics was incorporated in the model by using a correlation as provided in, Dynamic behavior of stratified downdraft gasifiers, Di Blasi, Chemical

Engineering Science, 2000, for char reactivity that is a function of particle diameter, bed porosity and solid

concentration .

The model was validated for wood stalks. Experimental results from Computer Simulation of a downdraft wood gasifier for tea drying, Jayah et.al., Biomass and Bioenergy, 2003, were used for validation. The validation results are explained with reference to FIGS 4 to 6.

FIG 4 depicts a graphical representation of validation results for wood stalks. The residence time for wood char calculated from the above model was 20 min. The optimum gasification zone length for the above process was found near to 0.2m. The conversion levels obtained for the process was 83%.

FIG 5 depicts a graphical representation of validation results for coconut shells. The residence time for coconut char calculated from the above model was 27 min. The optimum gasification zone length for the above process was found near to 0.2m. The conversion levels obtained for the process was 89%.

The validations results represented in FIG 4 and FIG 5 establishes a basic model for a downdraft gasification system. The process inputs for the model were then modified for an entrained bed gasifier system with concurrent air and rice husk feed that best replicates the gasification process of the embodiments described herein. The results were validated with an entrained bed rice husk and air system described in Characteristics of rice husk gasification in an entrained flow reactor, Biosource Technology, Zhao et.al,

2009.

FIG 6 depicts a graphical representation of validation results for rice husk. The residence time for rice husk char calculated from the above model was 20 min. The optimum gasification zone length for the above process was found near to 0.2m. The conversion levels obtained for the process was 83% Thus for the rice husk, the minimum residence time required for the suggested entrained bed system (without considering the rotation effect) was calculated to be 20 min. This was compared with residence time values obtained for a rotary system (without the chemical reaction or gasification being considered), using the same feedstock, rice husk char. An empirical equation used for rotary dryers was used to estimate the residence time. (Since drying operation is a mass transfer phenomenon analogous to pyrolysis, the

empirical equation gave a rough estimate of the minimum residence time required in the suggested rotary unit) . The minimum residence time as per the equation was calculated to be 31 min. This is thus higher than the minimum calculated by the model for the gasification process, without the rotation effect being considered, (20 min) . The additional of the second reactor 11 having means for enhanced mixing to the entrained bed system, provides sufficient residence time to the agro biomass resulting in improved gasification

efficiencies and high CV producer gas.

The following equation suggested in Experimental Study of esidence Time in a Direct Rotary Dryer, Drying Technology, Alvarez et.al, 1994, is used for estimating residence time for rotary dryers : 1 Where, d is the diameter of particles of the carbonaceous material in m, p is the density of the particles of the carbonaceous material in kg/m ³ , F is the flow rate of the carbonaceous material in kg/s, N is the speed of rotation in rpm, s is the slope in m/m and G is the gas (air) flux rate in kg/m ² s.

While this invention has been described in detail with reference to certain preferred embodiments, it should be appreciated that the present invention is not limited to those precise embodiments. Rather, in view of the present disclosure which describes the current best mode for

practicing the invention, many modifications and variations would present themselves, to those of skilled in the art without departing from the scope and spirit of this

invention. The scope of the invention is, therefore,

indicated by the following claims rather than by the

foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.