Field of the invention

The present invention relates to the field of cleaning and cooling of producer gas by wet scrubbing method with heat recovery from the hot gas for better gasification efficiency.

Background of the invention

Cleaning of various gases is carried out with different types of dry and wet cleaning systems. A significant research and development work has been done for the development of various types of gas cleaning systems for better efficiency. Various technologies are available for the cleaning and cooling of producer gas like Wet scrubber, ESP, Hot gas cleaning and packed bed scrubber, dry media etc.

The technology of scrubbing with water is commonly used for the cleaning and cooling of gas. But all the developed technologies have their own limitations. In case of wet scrubber and packed bed scrubber, large quantity of pressurized water is required for the cooling and cleaning of the gas. Large power input is required for the large quantity pressurized water. In the wet scrubber and packed bed scrubber, large quantity of water directly come in the contact with raw gas which results in its contamination with carbon particulate and water soluble polycyclic aromatic hydrocarbons (PAHs). Hence, large quantity of wastewater is generated which needs to be treated for reuse/disposal. Frequent chocking of the bed occurs in case of packed bed scrubber. Also, no heat recovery from the cooling water is feasible thereby reducing the overall system efficiency. The ESP technology can only remove the tar from the gas. For removal of tar it requires large amount of energy. The hot filtration technology removes only particulate. The element used for the hot filtration is the very expensive. Various filter media have been used for the cleaning of producer gas but after some times they choke and need replacement. The carbon particulate and tar loaded media makes its disposal a problem and has many environment issues.

CN 201148417 discloses a utility model belonging to spray evaporation cooling dust apparatus of converter gas purification system by providing the separate cooling tower and scrubbing tower. The cooling tower discloses at least two upper nozzles for spraying the water and a layer of spray gun. A non-metal high-temperature expansion joint is arranged between the cooling tower and the scrubbing tower. The inside of the scrubbing tower is provided with a plurality of layers of lower nozzles. The water can contact the smoke gas early, the temperature of which is reduced in the cooling tower.

US 4008056 discloses scrubber system for removing gaseous pollutants from a moving stream by condensation. The gas is condensed through the plurality of scrubber stages and using cooling spray water until the temperature of the gas is cooled down. A centralized, skimmer-type water treatment facility connected to each of the scrubbers is

) provided to receive the spray water and separate the condensed non-aqueous pollutants therefrom, thereby permitting the spray water to be recirculated back through the scrubbers for reuse and the pollutants to be reclaimed if desired.

JP 08-281049 discloses purification of gas by spraying the fresh water into gas in washing stage and then the gas water is brought into contact with a circulating aq. Salt solution. Thus, all the available technologies have many limitations and their operation and maintenance cost is very high.

The present invention mainly used for the cleaning and cooling of producer gas requires only small quantities of water. The very hot gas cooled in the promiser from initial 400-450 ⁰ C to 40-45 ⁰ C with the help of spray water and circulating ambient temperature clean water. It reduces the carbon particulate and tar loading to a large extent.

Object of the invention

It is an object of the present invention to provide an improved wet cleaning and cooling of producer gas with substantially reduced quantities of process water. The producer gas generated through the gasification process contains fine carbon particulate and PAHs. These impurities in the gas should be removed prior to its use for gas engines and many other applications and equipments. Among various methods used for the cleaning and cooling of producer gas, the present invention objects to utilize very less quantity of water and in - built heat recovery for improved efficiency. Summary of the invention

The present invention provides a unique promiser for cleaning and cooling of producer gas with heat recovery system. This invention overcomes the problem of use of large quantity of water by using a unique water spray mechanism for increased water gas contact, removing the heat from gas through vaporization of water. The invention also significantly reduces the contamination of water as only small quantity of water comes in direct contact with gas whereas clean water used in recirculation in the outer shell of the Promiser does not come in contact with gas and remains clean. As the large water quantities are of clean water and through a system of minimum pressure drop, the power input is very low.

Detailed description of the figures

Figure 1 shows a schematic complete process diagram for the Promiser.

Figure 2 is a schematic enlarged diagram of the promiser.

Detailed description of the invention

Biomass gasification is well known process for the conversion of solid biomass to combustible gaseous fuel called producer gas. The gasifier generated producer gas contains carbon particulates and PAH in gaseous form. Before using gas in the engine for power production or as fuel for thermal application it should be free or cleaned. Commercially various technologies are available for the cleaning and cooling of producer gas. The wet scrubber requires huge quantity of water for cleaning and cooling of PG that require very high flow of water and high power consumption. ESP requires large amount of power. On account of various disadvantages of the conventional technologies the present invention provides a unique promiser for cleaning and cooling of producer gas with heat recovery system. In conventional wet scrubbing systems, the large quantity of water is sprayed on the gas making a limited water-gas contact. Thus heat is removed only through liquid phase of water and large quantity of water gets contaminated.

The present water treatment system handles much smaller quantities of water. Water inventory as well as water ponds all get reduced drastically in size. The water circulation rates can be adjusted for generation of higher temperature water, facilitating easy heat recovery and thereby increasing overall efficiency. The tubes in the promiser are self-cleaned with the velocity of gas and water jet flow. Design of the spray nozzles is for the generating fine water droplets for establish the better contact with the gas. Tube diameter is optimized for the self cleaning of tubes by velocity of gas and water vapor during continuous operation. Significant reduction in power requirement is achieved as only small quantity of water needs to be pumped for gas cleaning. Large quantity of clean water used for indirect cooling of gas minimizes the pressure drop resulting in power saving. The foregoing features and advantages of the invention will become more apparent from a reading of the following description in connection with the accompanying drawings.

The conversion of biomass solid fuels to gaseous fuels takes place in the downdraft gasifier. As shown in Figure - 1, Biomass passes through various zones like drying, pyrolysis, combustion and reduction within the gasifier (1), (2). Moisture (<20%) present in the biomass escapes in the drying zone and reacts further in the combustion and reduction zone. The biomass dried in the drying zone passes through the pyrolysis zone, where biomass gets heated in the absence of air and starts a sequence of pyrolitic reactions leading to formation of various poly cyclic aromatic hydrocarbons (PAHs) called pyrolitic gases. These pyrolitic gases escaped from biomass may further crake in the combustion and reduction zone. The pyrolysed biomass converts to carbon in presence of limited air when it passes through combustion zone. The water vapor and pyrolitic gases react with carbon at combustion and reduction zones and convert in the various combustible gases like H ₂ , CO, CH ₄ and non combustible gases like CO _2; N ₂ .

During the thermo-chemical reactions small quantity of carbon particulate <500 microns and light tar produced also carry with the gas from the gasifier line (5). This tar and carbon particulate loaded producer gas passes through the cyclone (6). In cyclone >100-500 micron size carbon particulates get separated and collected in the ash collection box. After cyclone the gas passes through the line (7) and enters into the tube (39) of the promiser (8). The gas temperature at the inlet of the promiser (8) is 400-450 ⁰ C. The hot gas comes in contact with the water (26) sprayed through carefully designed spray nozzle (37, Fig - 2) and, instantaneously, the vaporization of the water takes place and as a result quick absorption of the heat occurs.

Now, the water vapor and producer gas passes through the tube (39) and ambient temperature water from the cooling tower (29) circulates for cooling at the shell side (27). The ambient temperature water cools the gas and water soluble tars as well as the water vapor get condensed in the tubes (39, 38) and flows to the cooling pond (31) through the line (25). Carbon particulates present in gas also get wet, separate from the gas and collect in the cooling pond (31) though the line (25). The gas temperature at the outlet of the tube (38) of the promiser (8) is within the few degrees of ambient temperature. All the heat contained by the hot gas gets transferred to the circulating water through the line (9 ) from the tube (39) to tube (38) of the promiser (8).

The heat absorbed by the circulating water can be used for heating application/for the energy applications. After the promiser (8) the gas passes through the line (10) to wet blower (11), Wet blower is used for the gas suction from the gasifier and pushes the pressurized gas to down stream. From blower to separation tank it passes through the line (12). The separation tank (13) separates the water and gas. The water separates in the separation tank (13) gets collected in the cooling pond (31) through the line (34). The water separated gas passes through the heat exchanger (15) through the line (14). In the heat exchanger (15) the temperature of the gas reduces from 45 ⁰ C to 20 ⁰ C using the chilled water from the clean water chiller (30) using the line (35)(36). It condenses the water vapour and tar present in gas stream.

The cooled gas passes through the line (16) to mist eliminator (17). The mist eliminator (17) removes the fine water droplets in form of mist which collects and flows through the line (34) and goes to cooling pond (31). The moisture free gas passes through the line (18) to fine filter (19). The fine filter (19) loaded with fine sawdust which removes the fine carbon particulate and make the gas ultra clean. The filtered gas goes to safety filter (21) through line (20). After the safety filter the gas goes to Gas Engine (23) for the Power generation or thermal applications.

Figure - 2 shows the schematic diagram of the Promiser. The hot producer gas contaminated with carbon particulates and PAHs enters the tube (39) of the promiser (8) through inlet line (7). The water jet spray (37) at the top of the promiser (8) sprays water (26) at a pre-determined rate. The spray water directly comes in contact with hot gases at a temperature of 400-450 ⁰ C and water get vaporized quickly by absorbing the heat from the hot gas. Now the water vapor and gas enters in the tubes (39, 38) of the promiser (8) and comes in contact with ambient temperature water circulating in the outer shell (27) from the cooling tower (29).

The producer gas, water vapor and carbon particulate pass through tubes (39, 38). The clean water is circulating through the outer shell (27) of the promiser (8) and returns to the cooling tower (29) through line (28). The partially cleaned producer gas passes through line (10) to the wet blower (11). The carbon particulate and tar loaded water passes through line (25) to the cooling pond (31). Thus, the controlled process and equipment design results in substantial reduction in the particulates and tar content of the gas because of unique evaporation and condensation.

The Promiser (8) is applicable to all sizes of the gasifiers and easy to operate and maintain. The Promiser (8) is applicable to the all types of biomass gasification like wood/woody biomass gasification, coal gasification, lignite gasification, fine and coarse biomass gasification. The Promiser (8) is applicable for the all types of gasifiers like down draft with throat, down draft throatless, updraft gasifier, fluidized bed gasifier.

While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment and the drawings, but, on the contrary, it is intended to cover various modifications and variations within the spirit and scope of the appended claims.