Title of Invention

THERMO-CHEMICAL TAR CONVERTER

Abstract

In conventional methods of gasification of solid fuels like biomass produced on pyrolysis, fuel gases generated are usually contaminated with tar. The gas when used as a fuel for running an engine or turbine may cause considerable difficulties due to condensation of tar on different engine components, like intake manifold, causing corrosion and jamming of moving parts. This invention aims at overcoming the foregoing difficulties and provides an improved thermo-chemical converter for producing a gas mixture having enhanced calorific value but very little or no tar content, comprising an insulated column of uniform cross-section or a column of lower cross-sectional area at the bottom with an enlarged portion at the top (1), and a feeder (3) for introducing a high-tar gas, a gasifying medium (2) and char particles, characterized in that the gas depleted of tar content is made to leave the converter through a passage (6) to enter a cyclone to effect separation of most of the solid particles in the gas and return the particles (8) through a recirculation passage to the converter (1) and the product gas having low or no tar content goes out through the outlet (9). The converter may be connected either to spouted-fluidized bed or circulating fluidized bed gasifler for supply of raw material, i.e. tar laden gas.

Full Text

1. Introduction : The present invention relates to an improved themo-chemical tour comverter for More particularly this invention pertains to a thermo-chemical gas conversion device which converts tar associated with gas from any gasifier to mostly non-condensible gases so that it can be used to upgrade a gas of high tar content to a gas of low tar content. Gasification is the process of converting a solid fuel to a combustible gas by supplying a restricted amount of oxygen, either pure or from air. In some cases, a limited amount of steam may also be added to oxygen or air used for gasification. Solid fuels commonly used are biomass (i.e. matter originating from living things, such as wood, agricultural residues, dung etc.), coal and char (i.e. solid carbonaceous residue produced from incomplete combustion of organic material, such as wood or coal, char produced from wood being normally called charcoal). Heat evolved from exothermic reaction of oxygen with the fuel serves to maintain the temperature level inside the gasifier above that of the surrounding and derives certain endothermic reactions taking place inside it. Steam alone can also be used as the gasification medium if heat necessary for the endothermic gasification reactions can be supplied indirectly. Gasification of solid fuels with air as the gasifying medium produces a low heating value gas, which contains about 50% nitrogen and can be used as a fuel for engines and furnaces. Gasification with pure oxygen or steam results in a medium heating value gas free of nitrogen. 2. Types of Gasifiers used in industry : Fixed Beds Fixed bed reactors contain a fixed or packed bed of solid fuel (blocks or particles) inside it. There are three broad types of fixed bed gasifiers used in industry, i.e. updraft, downdraft and crossdraft. Fluidized Beds Fluidization is a mode of contacting granular solids with gases or liquids, fluidization with gases being particularly important for several industrial applications. Fluidized bed gasifiers constitute an important class of gasifiers. -2- If a gas flows with increasing velocity upwards through a pipe containing a bed of particles, supported on a suitable grid, then a point will be reached at which pressure drop of the gas in passing through the bed is equal to the weight of the bed per unit cross-sectional area. The bed is then said to be at minimum fluidization. At velocities greater than that required for minimum fluidization, a portion of the gas passes through the bed in the form of bubbles. Such a bed is called a bubbling fluidized bed and can be regarded as consisting of two phases: (1) the discontinuous phase or the bubbles and (2) the continuous phase or the emulsion (also called the particulate phase) of particles and interstitial gas through which bubbles rise. In a circulating fluidized bed reactor, most of the reactions take place in the riser column; gas velocity through the riser column is relatively high and a stream of particles are carried out of the column with the gas. These particles are separated from the gas stream by means of a cyclone and returned to the riser column. Spouted Beds While in bubbling fluidized beds, the gaseous fluidizing medium is introduced into the bed as uniformly as possible through the entire cross-section of the bottom of the bed, in a spouted bed reactor, a gas stream enters it through a small orifice at the center of a conical or flat base. The high velocity jet causes a stream of solid particles to rise rapidly in a hollow central core or spout within the bed. After reaching a level somewhat above the bed level, the particles fail back in the annular space between the spout and the wall of the reactor ; this results in a systematic cyclic pattern of movement of solid particles inside the reactor. Spouted-fluidized bed The spouted-fluidized bed reactor is a combination of a bubbling fluidized bed and a spouted bed. The spouted-fluidized bed combines the fluid flow through a single central inlet opening, as in spouted bed, with an auxiliary fluid flow through a distributor, as in fluidized bed. The distributor of a spouted-fluidized bed can be incorporated into either a flat or conical base of the spouted-fluidised bed column. 3- 3. Theory : The following processes take place inside a gasifier : Drying, Pyrolysis, Reduction and Combustion. The first three of these processes are endothermic in nature and are driven by heat produced in the exothermic combustion process. In the drying process, the moisture content of the solid fuel evaporates. Pyrolysis converts the dried solid fuel particles into char, vapours of water and organic liquids and non-condensible gases. Oxygen supplied to the gasifier reacts with combustible substances present inside it, resulting in formation of products of complete oxidation, namely CO2 and H2O ; these undergo reduction upon contact with char produced from pyrolysis. Chemical Reactions A solid fuel, upon exposure to high temperature inside a gasifier, undergoes drying and pyroysis. The pyrolysis process can be represented by the following general reaction : Coal (or biomass) + Heat -> Char + gases + vapours (1) The vapours produced on pyrolysis are those of water and a large number of organic liquids. Some of the vapours condense to produce a black, viscous and corrosive liquid, called tar. One of the most important chemical reactions taking place inside a gasifier is combustion of char : (2) This reaction provides practically all the thermal energy needed for the endothermic reactions taking place inside the gasifier. Some steam is always present inside a gasifier; it can come from different sources, e.g. water vapour associated with the incoming air, and vapour produced from evaporation of water from, and pyrolysis of the solid fuel. Steam reacts with the carbon content of hot char particles according to the heterogeneous water-gas reaction. (3) _4_ In some gasifiers, steam is supplied to the gasifier along with the main gasifying medium - air or oxygen. CO2 present inside the gasifier endothermically reacts with char to produce CO according to the Boudouard reaction. (4) Another important reaction taking place inside the gasifier is the water-gas shift reaction. (5) This endothermic reaction results in an increase in the ratio of hydrogen to carbon monoxide in the gas, and is employed in manufacture of synthesis gas. Some methane formation also takes place in the gasifier. This can be represented by the overall reaction : (6) Tar produced during gasification As mentioned earlier, one product of the pyrolysis process that takes place inside a gasifier is tar. The tar content of the gas obtained from a gasifier causes a number of problems if the gas is used as fuel for running an internal combustion engine or gas turbine. For example, the tar may condense on different components of the engine intake manifold, cause corrosion of engine parts etc. Many gasifier-engine systems failed in the past due to problems created by tar. Therefore a gas of low tar content is desirable for running internal combustion engines and gas turbines. 4. Conventional methods of tar cleaning and reduction : A number of techniques are available to reduce tar content of the gas for using it to run engines. The most common technique is cooling and water scrubbing of the gas so that most of the tar is removed from the gas by condensing the tar vapours. This technique has a limitation in that the tar content cannot be sufficiently reduced and the residual tar is enough to create unacceptable problems in engine operation. -5- Another technique of reducing tar content of the gas is to use special gasifier designs ; the tar initially produced during pyrolysis is mostly burned or cracked inside the gasifier to produce non-condensible gases. A third technique of reducing tar content of the gas is to treat the gas catalytically outside the main gasifier section to crack the tar to non-condensible gases. None of the prior art techniques is capable of reducing the tar-content of the gases emanating from a gasifier to a level which would not cause problems when used for running an engine. In the case of providing a cracking unit outside the main gasifier section, cost escalation poses a difficulty. The principal object of this invention is to provide an improved thermo-chemical tar converter capable of converting a stream of gas having high tar content to a gas stream of very low or no tar content. A further object of this invention is to provide a thermo-chemical tar converter which is made from the indigenously available raw materials and the reactor may be of any geometrical shape, namely, cylindrical with circular cross-section or cylindrical with square or rectangular section. A still further object of this invention is to provide a thermo-chemical tar converter which is cost-effective and is provided with a means like cyclone for separating most of the solid particles including tar and returning the same to the converter, giving rise to a substantially char-free gas mixture of high heating value. The present invention aims at achieving the foregoing objects and relates to an improved thermo-chemical converter for producing a gas mixtureJiaving enhanced calorific value but very little or no tar content, comprising an insulated column of uniform cross-section or a column of lower cross-sectional area at the bottom with an enlarged portion at the top, and a feeder for introducing a high tar gas, a gasifying medium and char particles, characterised in that the gas depleted of tar content is made to leave the converter through a passage to enter a cyclone to effect separation of most of the solid particles in the gas and return the particles through a recirculation passage to the converter and the product gas having low or no tar content goes out through the outlet. -6- 5. The Thermo-chemical tar converter of the present invention with reference to the accompanying drawings : As mentioned earlier, tar present in the gas obtained from a gasifier causes a lot of difficulties and creates problems if the gas is used for running an engine. To overcome the problems created by the presence of tar the present invention has evolved a tar conversion device of improved design which effectively converts a stream of gas having high tar content to a gas stream of very low or no tar content. This invention will now be illustrated by means of the accompanying drawings in which - Fig. 1 illustrates conventional types of fixed bed gasifiers, namely, updraft, downdraft and cross-draft gasifiers ; Fig.2 illustrates different fluidized bed configurations ; Fig.3 shows a thermochemical converter of this invention with uniform cross section ; Fig.4 depicts a thermochemical converter with enlarged top ; Fig.5 shows a thermochemical tar converter mounted on a spouted- fluidized bed gasifier, Fig.6 shows a tar converter mounted on a spouted-fluidized bed gasifier with enlarged top, and Fig.7 shows an integrated circulating fluidized bed gasifier and tar converter. As indicated in Fig. 2, fluidized beds may be of different types, suchas , for instance, a bubbling fluidized bed reactor as illustrated in Fig. 2A, a circulating fluidized bed reactor as shown in Fig. 2b, a spouted bed reactor as shown in Fig. 2c and a spouted-fluidized bed reactor, which is a combination of a bubbling fluidized bed and a spouted bed, as illustrated in Fig. 2d. -7 - The improved design of the thermo-chemical tar converter of the present invention is illustrated in Figs. 3 and 4 of the drawings. Raw high tar gas enters the converter at its inlet (4) [at the bottom in Figs. 3 and 4] and flows through it. A stream of gasifying medium is also introduced into the tar converter (1) near its inlet (2). A stream of a gasifying medium is also introduced into the reactor column (1) of the tar converter. The gasifying medium is normally air but it can also be pure oxygen, or air mixed with steam, or air mixed with pure oxygen or any other gas mixture containing oxygen; the oxygen present in the gasifying medium serves to burn up most of the tar and organic vapours present in the incoming combustible gas ; some tar molecules may also break down to smaller molecules of non-condensible gases due to high temperature created as a result of the gasifying medium supply. The oxygen present in the supply of the gasifying medium also partly burns up some of the combustible components, such as CO, H2 present in the entering high-tar gas ; which causes a tendency of reduction in the calorific value of the gas. To overcome the problem of reduction in calorific value of the gas on contact with the gasifying medium supplied to the converter, the gas is next exposed to a stream of char particles suitably introduced inside the converter by means of a hopper (3). The said char is gasified on contact with the gasifying medium supplied to the converter with the help of a feeder (10) or the non-condensible gases and vapours produced from combustion of the combustible components of the high-tar gas on contact with the oxygen present in the gasifying medium supply ; the combustible gases produced as a result of gasification of the char particles serves to improve the calorific value of the final gas leaving the converter through an outlet (6). 6. Details of the proposed gasifier : i) Design The invented Thermo-Chemical Tar Converter (1), as illustrated in Figure 3, is normally of circular cross-section. It may have other shapes as well, for example, square, rectangular or elliptical sections. The supply of gasifying medium to the converter enters the converter near its bottom (4) and results in burning or cracking of most of the tar vapours in the incoming high-tar gas from the gasifier ; therefore, the gas leaving the tar converter (1) has very low or no tar content. -8- Char particles are also fed into the converter near its bottom (4). These particles as well as any solid particle entering the converter along with the high-tar gas flow are gasified on contact with the gasifying medium (2) mentioned above or the products of combustion resulting from introduction of the gasifying medium into the converter ; gasification of the char particles serves to improve the calorific value of the final gas obtained from the gasifier. It is possible to introduce optional and additional streams of the gasifying medium and char particles into the converter above the char and gasifying medium streams mentioned above. Some char particles will be normally carried out of the converter with the high velocity exit gas stream going out through the exit pipe or outlet (6). The gas from the converter and particles suspended in it enter into a cyclone (5). The purpose of using the cyclone is to separate the solid particles from the gas and return them to the converter through recirculation pipe (8) if needed. The cyclone (5) and the piping connecting it to the converter (8), are also suitably insulated to prevent excessive heat loss from it. It is also possible to have as a further feature of this invention an enlarged section near the top of the converter as illustrated in Figure 4 ; the enlarged cross-sectional area of this part (1) results in lower gas velocity and allows the solid particles to stay indise the converter for a longer time and promotes gas-solid reactions. The low-tar gas produced in the converter goes out of it (9) through the cyclone (5). The gas contains very little or no tar, and would need cooling and cleaning for running engines. The converter (1) is normally made of metal (normally mild steel) and is thermally insulated (7) inside with firebricks or other suitable materials, such as castable refractory cement. The wall may also be made of any material or combination of materials to provide necessary thermal resistance to heat flow and structural strength. ii) Process highlight -9- A stream of a gasifying medium supplied to the converter serves to burn up most of the tar and organic vapours present in the incoming high-tar gas; some tar molecules may also break down to smaller molecules of non-condensible gases due to high temperature created as a result of the gasifying medium supply. The gasifying medium supplied also partly burns up some of the combustible gases, such as CO, H2present in the in-coming high-tar gas; this causes a reduction in the calorific value of the gas. The gas is next exposed to a stream of char particles suitably introduced inside the converter through its wall; the char particles can be obtained from an external source or may be simply particles separated from the gas stream in the cyclone and returned to the converter. The said char particles get gasified on contact with the gasifying medium supplied to the converter or the non-condensible gases produced from combustion of the gases and vapours on contact with the gasifying medium supply ; the combustible gases produced as a result of gasification of the char particles serve to improve the calorific value of the final gas leaving the tar converter. The invention mainly lies in the treating of high-tar gas from a gasifier to one or more stages of supplies of a gasifying medium and char particles to reduce its tar content while nearly maintaining its heating value. iii) Physical location with respect to gasifier The tar converter can be suitably connected to a gasifier producing the high-tar gas to be processed using insulated passages, such as pipes. It can also be directly connected to the exit of the gasifier. Figures 5 and 6 illustrate a converter directly mounted above a spouted-fluidized bed gasifier. In both Figs 5 and 6 of the drawings, the latter showing a reactor with enlarged top, numerals (1) to (10) have the same significance as in Figs. 3 and 4 of the drawings discussed earlier in this specification. A spouted-fluidized bed gasifier is shown in Figs. 5 and 6 as (11), spouting air inlet is marked as (12) and fluidizing air inlet is (13). The bottom section (14) is insulated, the second section or connector is shown with (15) and the third section (16) attaches the spouted part to the reactor (1). The fluidized bed is fed with biomass from a hopper (17) by means of a feeding screw (10) similar to the one used for char hopper. - 10- In case of a circulating fiuidized bed gasifier, the tar-converter can be integrated with the riser column of the gasifier. The upper part of the circulating fiuidized bed gasifier consists of the gas exit and cyclone (not shown), and the solid return piping is replaced by the tar converter, as indicated by (16). A tar converter integrated with a circulating bed gasifier is shown in Fig. 7 of the drawings. In this case steam may optionally be mixed with air or oxygen entering the reactor from the bottom. It is to be understood that selection of the type of gasifier will depend on the conditions prevailing for obtaining raw materials and equipments for gasification. While the invention has been described in detail and with reference to the specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without deviating or departing from the spirit and scope of the invention. Thus the disclosure contained herein includes within its ambit the obvious equivalents and substitutes as well. Having described the invention in detail with particular reference to the illustrative drawings, it will now be more specifically defined by means of claims appended hereafter. -11- I Claim : 1. An improved thermo-chemical converter for producing a gas mixture having enhanced calorific value but very little or no tar content, comprising an insulated column of uniform cross-section or a column of lower cross-sectional area at the bottom with an enlarged portion at the top (1), and a feeder (3) for introducing a high tar gas, a gasifying medium (2) and char particles, characterised in that the gas depleted of tar content is made to leave the converter through a passage (6) to enter a cyclone to effect separation of most of the solid particles in the gas and return the particles (8) through a recirculation passage to the converter (1) and the product gas having low or no tar content goes out through the outlet (9). 2. A tar converter as claimed in Claim 1, wherein char particles stored in a char-hopper (3) are fed into the reactor with the help of a screw-feeder (10). 3. A tar converter as claimed in Claim 1, wherein the said converter is mounted on a spouted-fluidised bed gasifier consisting of a bottom section (11) which is an insulated spout-fluid bed, a second section (12) being a connection and a third section (13), the said bottom section (11) having inlets for fluidizing air (14) and spouting air (15) and in turn fitted with a biomass hopper (16) wherein the said biomass is fed into fluidizing bed by means of a feeding screw (10). 4. A tar converter as claimed in Claim 1, wherein the gasifying medium and char are fed to the converter one at a time. 5. A tar converter as claimed in Claims 1 and 4, wherein the gasifying medium is air or oxygen and is supplied with char through additional inlets. 6. A tar converter as claimed in any of the preceding claims, wherein the reactor (1) is normally cylindrical in shape but may have other geometrical configuration like cylindrical with square or rectangular section. - 12 - - 13- 7. A tar converter as claimed in Claims 1 to 6, wherein the walls of the reactor/converter (1) may be made of material(s) to provide thermal resistance to heat flow and structural strength, such as metal with inside thermal insulation, or high temperature resistant metal with external insulation. 8. A tar converter as claimed in Claim 1, wherein the said converter is integrated with a circulating fluidized bed gasifier for supply of starting material. 9. A tar converter as claimed in Claims 1 and 8, wherein additional stream of air or oxygen is supplied along with the char through the bottom (4) of the said converter, optionally fortified with steam. 10. An improved thermo-chemical tar converter substantially as hereinbefore described with particular reference to the accompanying drawings. In conventional methods of gasification of solid fuels like biomass produced on pyrolysis, fuel gases generated are usually contaminated with tar. The gas when used as a fuel for running an engine or turbine may cause considerable difficulties due to condensation of tar on different engine components, like intake manifold, causing corrosion and jamming of moving parts. This invention aims at overcoming the foregoing difficulties and provides an improved thermo-chemical converter for producing a gas mixture having enhanced calorific value but very little or no tar content, comprising an insulated column of uniform cross-section or a column of lower cross-sectional area at the bottom with an enlarged portion at the top (1), and a feeder (3) for introducing a high-tar gas, a gasifying medium (2) and char particles, characterized in that the gas depleted of tar content is made to leave the converter through a passage (6) to enter a cyclone to effect separation of most of the solid particles in the gas and return the particles (8) through a recirculation passage to the converter (1) and the product gas having low or no tar content goes out through the outlet (9). The converter may be connected either to spouted-fluidized bed or circulating fluidized bed gasifler for supply of raw material, i.e. tar laden gas.

Documents:

00380-kol-2004-abstract.pdf

00380-kol-2004-claims.pdf

00380-kol-2004-correspondence.pdf

00380-kol-2004-description(complete).pdf

00380-kol-2004-drawings.pdf

00380-kol-2004-form-1.pdf

00380-kol-2004-form-18.pdf

00380-kol-2004-form-2.pdf

00380-kol-2004-form-3.pdf

00380-kol-2004-form-5.pdf

00380-kol-2004-letters patent.pdf

00380-kol-2004-p.a.pdf

00380-kol-2004-reply f.e.r.pdf

380-KOL-2004-(08-02-2012)-FORM 27.pdf

380-KOL-2004-(17-11-2011)- EXAMINATION REPORT REPLY RECIEVED.pdf

380-KOL-2004-(17-11-2011)-OTHERS.pdf