PRODUCER GAS CONDITIONING
One of the most attractive applications of producer gas is its use in internal combustion engines for power or electricity generation. Since the producer gas is a low calorific value fuel, the producer gas–air mixture compared to diesel or petrol–air mixture is definitely a poorer quality of fuel. Apart from this the producer gas as delivered from the gasifier contains significant quantities of contaminants such as tar and SPM that create problems in smooth operation of the engine. Therefore, it is necessary that tar and SPM of the producer gas are brought down to a reasonable acceptable limits to give smooth and trouble free operation to the engine.
Contaminants in Producer Gas
1. Nitrogen Compounds: Under the normal operating conditions of the gasifier i.e. about 1 atm pressure and temperature varying from 1000 to 1500 oC, a part of the nitrogen in the fuel is converted into NH3 (ammonia) and HCN (hydrogen cyanide). It may be pointed here that the most of the nitrogen of the air used in the gasification process is not converted rather it is only the nitrogen of biomass that form NH3 and HCN. Both these compounds are soluble in water and are removed during the water scrubbing of the producer gas. When the gasification process takes place in presence of steam, the temperature remains low and the formation of the nitrogen compounds is negligible.
2. Sulfur Compounds: The sulfur present in the biomass is converted to H2S (hydrogen sulfide) and sulfur dioxide. Bulk of sulfur contained in biomass is converted to H2S (92 to 97%) in gasification. In dry gasification and at high temperatures CS2 is also formed. The amount of sulfur compounds primarily depends on the sulfur content in the fuel. Sulfur content of several biomass is reported to vary between 0.02 to 0.2%. With this little sulfur, the generation of H2S is not very significant. Both H2S and SO2 are highly soluble in water and are removed during water scrubbing of the producer gas.
3. Tar: It is a mixture of organic compounds formed during biomass pyrolysis. Producer gas generated from biomass invariably contain some tar and solid particulate matter that depend on the type of the gasifier and operating parameters such as fuel moisture content, reactor temperature, air flow rate, etc. Tar content in producer gas varies from a low of 100 mg/Nm3 to 20,000 mg/Nm3 or even higher. Down draft gasifier with throat is known to generate producer gas with minimum of tar, followed by throatless gasifier, fluidized bed gasifier and updraft gasifier, irrespective of fuel. Lower reactor temperatures and high fuel moisture generally leads to high tar in producer gas irrespective of the type of the gasifier.
The due point of tar ranges from 120 to 160 oC. It is a combustible matter with a heating value of 22-24 MJ/kg. The tar is in vapor form in the gas at the exit of the gasifier, but as the temperature drops, it condenses into a highly viscous black paste like material. Condensed tar creates problems such as gumming of the valves due to deposition on engine valves and air intake channels and contamination of lubricating oil. Sticking of the valves may be so severe that the entire system has to be pulled apart and cleaned.
In thermal application of producer gas the problem is not very severe as gas above the tar due point can be transferred to the gas burner by insulating the pipe and the tar burn along with the producer gas.
4. Dust: Solid particulate matter (SPM) in the producer gas is mainly as the inorganic minerals present in the fuel as ash and carbon particles due to unconverted char. Char is also formed due to the conversion of carbon monoxide into carbon and carbon dioxide at low temperature in the gas flow channels. The SiO2 which form a major part of the SPM and some other constituents such as Fe2O3 and MgO are very abrasive and tend to damage the critical parts of the engine.
The amount of SPM in the producer gas depends on many factors such as type of the gasifier, type of fuel, specific gasification rate and the temperature of the oxidation and reduction zone. At higher specific gasification rate, the gas velocity increases and so does the dust carrying capacity of the gas. A gasifier operating at 120% of rated load can result in SPM about 3-4 times more than the SPM in gas when the gasifier is operating at rated load. High temperature also increases the gas velocity in the oxidation and reduction zones and so does the dust carrying capacity.
Amount of SPM that can be tolerated in the producer gas entering an internal combustion engine has been the subject of extensive discussion and testing. It was observed that with a dust concentration upto 10 mg/Nm3 the engine wears is of the same order as obtained with gasoline of diesel. Beyond that concentration of SPM, the engine wear increased rapidly, being upto 5 times as great when the SPM concentration in the gas reached 50 mg/Nm3.
5. Temperature of the Producer Gas: The producer gas temperature at the exit of the gasifier depends on the type of the gasifier and to extent the operating parameters of the gasifier. The gas produced from an updraft gasifier has a temperature ranging from 150 to 250 oC since the gas pass through the low temperature pyrolysis and drying zones before it exit from the gasifier. The temperature of producer gas from a downdraft gasifier range from 200 to 350 oC. If heat economizer is provided in the gasifiers, the producer gas temperature may be a little lower. The gas temperature from cross-draft and fluidized bed gasifier is much higher compared to updraft or a down draft gasifiers. In any gasifier the exit gas temperature is a function of specific gasification rate. In case of a down draft throatless gasifier with the SGR increasing from 100 to 220 kg/h-m2, the exit gas temperature increases from 190 to 300 oC.
The extent of producer gas cooling and cleaning requirement is application specific. If the gas to be used in a furnace boiler or any other thermal application, the cooling of the producer gas is not required. When the gas is used to run internal combustion engines, because the high temperature gas reduces the efficiency of the engine and it also carry large quantity of tar and other condensable organic acids which need to be removed before feeding the gas to the engine, cooling of producer gas to ambient temperature is necessary.
Cleaning of Producer Gas
The cleaning of producer gas is application specific. For engine application the producer gas should be free from tar, dust and other condensable vapors and the gas should be at ambient temperature. On the other hand the gas-cleaning requirement for thermal application is not that stringent. Only coarse dust particles need to be removed from the producer gas and the gas with tar vapors at high temperature cam be fed to the burner. For engine application ideally only the combustible constituents CO, H2, and CH4, C2H4, C2H2 and C2H6 should reach the engine. For removing tar, dust, NH3, H2S that are harmful to engine and cause operational problems, a combination of different cleaning equipment in series or in parallel are necessary. Cleaning of gas for stationary application as gasifier-engine-pumping set or gasifier-engine-alternator is simplified where water is available. For portable units mounted on automobiles trucks etc. the cleaning of gas is not an easy task because of the specific requirement such as compactness and lightweight of the gas-cleaning unit. The cleaning equipment can be classified into two categories:
1 Dry gas cleaning units
2 Wet gas cleaning units
Dry Gas Cleaning Units
Solid particles having diameter larger than 1 mm can be settled by gravity or inertia and can be removed. They follow Stokes' law and can be captured by impaction, gravitational or centrifugal means. This equipment includes cyclones, fabric filters or dry filters and scrubbers. All of them are commercially available, however, can be designed and build to suit the specific requirement of the gasifier.
Cyclone: A cyclone is dust collector without any moving part in which the velocity of an inlet gas stream is transformed into a confined vortex. The dust separation from the gas stream takes place through centrifugal forces. The suspended particles tend to be driven to the wall of the cyclone and are collected in an ash-bin at the bottom. The cyclone is usually located right after the gasifier where the gas velocity is the highest. However, advantage of having the cyclone just after the gasifier is that the gas is cooled through expansion before it reaches subsequent units. Some general design criteria for high and medium efficiency cyclones is given in Figure 1. Efficiency range of medium and high efficiency cyclones is given below.
Particle size Efficiency
(micron) (%)
below 5 < style="mso-tab-count:2"> 50-80
5-20 50-80 80-95
15-40 80-95 95-99
Above 40 95-99 95-99
The parameter that is chosen first is the pipe diameter between the gasifier and the cyclone. All other dimensions are determined accordingly. The diameter selection is on the basis of the gas velocity and the nature of the contaminants. Recommended maximum gas velocity for smoke fumes and dust is 10-15 m/s.
Figure 1 High and low efficiency cyclones
Gravity Settling Chamber: As long as unlimited space and materials are available, a gravity settling chamber theoretically can achieve any level of particle separation down to the Stokes' limit of about 1 mm. In fact, many of the earliest gasworks used gigantic settling chambers. However, though it is effective, but this method tends to be a bit cumbersome.
Bag House Filter: Bag house filters are used widely today to capture fine dust particles and to separate fly ash from combustion gases. A bag house filter consists of one or more fibrous filter bags supported on metal cages enclosed in a chamber through which the gases are forced to pass. A deposit of the separated particles soon builds up on the bag and establishes a dust cake of appropriate pore size through which additional particles cannot pass. As more dust is accumulated, the pressure drop increases. When the cake attains an optimal thickness for removal, the bag is agitated either by gas pressure or by mechanical means, causing the excess cake to drop to the bottom of the housing where it is eventually removed.
Fibrous bag filters have been found to be outstanding in the removal of particles down to sub-micron sizes. The reason for this is that the primary capture element is the dynamic cake that forms on the filter surface. This cake, presents a circuitous path that effectively captures fine particles, while coarser captured particles maintain an open cake structure to promote high gas permeability.
Bag-house filters have been used with success in many successful and reliable engine gasifier systems. The use of fabric filters has virtually eliminated the corrosive ash that otherwise was present in condensate or scrubbed water. The fabric filter is no doubt the most efficient device for fine cleaning; but for wood gas, extensive precautions against condensation of tar or water were necessary.
Electrostatic Precipitators: Electrostatic precipitators have a long history of industrial use to produce exceptionally clean gas. During operation, the gas passes through a chamber containing a central high-voltage (10-30 kV) negative electrode. A corona discharge forms around the central electrode, which imparts a negative charge to all particles and droplets. The negatively charged particles then migrate to the positive electrode, which may be washed by a continuous water stream to remove these particles. The electrostatic precipitator is effective for all droplets and particle sizes.
Figure 2 Electrostatic Filter
Wet Gas Cleaning Units
For particles smaller than 0.1 mm, motion is dominated by molecular collisions. They follow Brownian motion principles and behave more like a gas and may be collected by diffusion onto a liquid surface.
Cyclone Spray Scrubbers: The cyclone spray scrubber combines the virtues of the spray tower and dry cyclone separator. It improves the particle-capture efficiency of the spray droplets in ordinary spray scrubbers by increasing spray-droplet impact. The cyclone spray scrubber also has the advantage, compared with the spray scrubber, of being self cleaning, of collecting more particles regardless of size, and operating at smaller pressure drops. Commercial cyclone scrubbers are better than 97% efficient at removing particles with diameters greater than 1 mm.
Figure 3 Wet Cyclone
Sieve-Plate Scrubbers: A sieve-plate scrubber consists of a vertical tower with a series of horizontal perforated sieve plates. The scrubbing liquid is fed into the top of the column and flows downward via down-comers from plate to plate. The gas to he scrubbed is introduced at the bottom of the column and passes upward through the sieve holes counter to the liquid. Contact between the liquid and gas is enhanced by using plates with bubble caps, impingement plates. or sieve plates.
The sieve-plate scrubber captures large particles by impingement and impaction, and small particles by diffusion. Gas passes upward into the water layer through holes in the sieve plate. The high gas velocity through the sieve holes atomizes the scrubber liquid into fine droplets, and most inertial particle collection takes place just as the bubble is being formed, by impaction .on the inner surface of the bubble. A typical sieve-plate scrubber can attain 90% efficiency for 1-mm Particles using 3/16-in sieve holes, at a specific velocity of 15 m/s.
Figure 4 Sieve Plate Scrubber
Venturi Scrubbers: The venturi scrubber captures large particles by impaction and impingement, and also rinses away any deposits that might otherwise form. Some fine particles are also captured here by diffusion. High-velocity flow through the low-pressure throat area atomizes the droplets. The low pressure at the throat causes condensation, and the high relative velocity of the droplets with respect to the gas captures most larger particles by impaction.
Figure 5 Venturi Scrubber (I)
The atomized droplets present a considerable surface area for fine particles to be captured by diffusion. Furthermore, condensation in the throat improves capture through diffusion because of the phenomenon of Stefan motion. The atomized droplets rapidly agglomerate in the diffuser section, where collection through diffusion continues. Entrained droplets containing captured contaminants are separated inertially from the cleaned gas. Liquid recycle requires cooling and removal of captured materials, or disposal and replenishment.
Figure 6 Venturi Scrubber (II)
The collection efficiency and droplet size are determined by the pressure drop efficiencies may be increased by reducing the throat area to raise the pressure drop.
Packed-Bed Scrubbers: The packed-bed scrubber is simple and open in design, and uses spheres, rings, or saddles as random packing to enhance the gas-liquid contact area. Packed beds are more effective for both gas absorption and liquid-gas heat exchange than they are for particle collection. However, packed beds are excellent for capturing entrained liquids. For entrainment separation, the optimum superficial gas velocity for packed-bed scrubbers using 1/2-in. spheres is 10 to 12 ft/s. Flooding and re-entrainment occur above a gas velocity of 12 ft/s. The pressure drop is 7.5 to 8.5 in water gauge for a 6-in deep bed
Figure 7 Packed bed Column and Packing Materials
Entraininent Separators: Entrained liquids from the wet scrubber must be thoroughly removed from the gas stream because they carry a slurry of captured materials. Entrainment droplets are typically greater than 10 mm and may be captured using a variety of techniques, including a packed bed, a packed fiber bed, a cyclone separator, a spray tower, or a settling chamber. Poor entrainment separation has been a common problem for wet scrubbers in gasifier systems.
Gas Cleaning Unit Developed at
The producer gas from the gasifier contains tar and solid particulate matter and has high temperature (@ 200 °C). For engine application, the producer gas should be free from solids particulate, tar and should be at ambient temperatures. For removing tar and solids particulate and cooling of producer gas, the gas may be passed through a gas cleaning unit comprising of a water scrubber, a dry filter and a fabric filter.
Water Scrubber
Water scrubber cools the producer gas at a temperature close to ambient conditions. Tar vapors are thus condensed and removed from the producer gas. The water scrubber had a charcoal bed with a countercurrent contact of gas and water. The superficial gas velocity in the water scrubber was kept at 3.5 cm/s with a residence time of 15 s. Based on the superficial gas velocity and gas flow rate, the charcoal bed height and diameter of the water scrubber are calculated using following expression.
Substituting the values of producer gas flow rate Fg (25.45 Nm3/h) and superficial gas velocity Vs, the cross-sectional area of the reactor comes out to be 0.1976 m2. Accordingly the diameter of the water scrubber comes out to be about 50 cm. Column height was determined using following expression.
The estimated bed height was 45 cm. The water scrubber was developed having a diameter of 50 cm and charcoal bed height of 50 cm. At the bottom and top 20 cm space on each end was left for providing water inlet and gas exit from the top and gas inlet and water exit at the bottom. Water was sprayed through a coil of perforated copper tube of 6 mm diameter and was fitted at a height of 10 cm above the charcoal bed. Charcoal bed was supported on a wire mesh of one cm opening with wire thickness of 3 mm. The water scrubber used in the experiments is shown in Figure 8.
Figure 8 Line diagram of water scrubber
Dry Filter
The producer gas from the water scrubber contains water and tar droplets and some solid particulate matter. For drying the gas and removing the particulate matter, the gas was passed through a dry filter. Dry filter was developed having dimensions similar to the water scrubber. Dry maize cob pieces were used as packing material for the dry filter. Maize cob pieces were selected as packing material because of their high surface area and easy availability even in the village environment, and at very little or no commercial value. On the top of the maize cobs bed, a 1.5 cm thick sheet of rubber foam was fitted to remove solid particulate material left over, if any, in the producer gas. The dry filter is shown in Figure 9.
Figure 9 Line diagram of dry filter
Fabric Filter
The producer gas after the dry filter was passed through a fabric filter fitted around a chicken wire mesh cage to remove any left over tar droplets and solid particles. The superficial gas velocity through the fabric filter is about 1/3rd of the wet scrubber or dry filter. A line diagram of fabric filter is shown in Figure 10.
Figure 10 Line diagram of fabric filter
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