Fluidized bed combustion (FBC) is a combustion technology used in power plants. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer. FBC plants are more flexible than conventional plants in that they can be fired on coal and biomass, among other fuels.
Combustion systems for solid fuels
FBC reduces the amount of sulfur emitted in the form of SOx emissions. Limestone is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water tubes). The heated precipitate coming in direct contact with the tubes(heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C / 1500 °F ) have other added benefits as well.
Contents [hide]
1 Benefits
2 Types
2.1 FBC
2.2 PFBC
3 See also
4 References
[edit]Benefits
There are two reasons for the rapid increase of fluidized bed combustion (FBC) in combustors. First, the liberty of choice in respect of fuels in general, not only the possibility of using fuels which are difficult to burn using other technologies, is an important advantage of fluidized bed combustion. The second reason, which has become increasingly important, is the possibility of achieving, during combustion, a low emission of nitric oxides and the possibility of removing sulfur in a simple manner by using limestone as bed material.
Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers-flue gas desulfurization). The technology burns fuel at temperatures of 1,400 to 1,700 °F (750-900 °C), well below the threshold where nitrogen oxides form (at approximately 2,500 °F / 1400 °C, the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants; it also avoids the ash melting problems related to high combustion temperature. The mixing action of the fluidized bed brings the flue gases into contact with a sulfur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in coal can be captured inside the boiler by the sorbent. The reductions may be less substantial than they seem, however, as they coincide with dramatic increases in carbon (monoxide?) and polycyclic aromatic hydrocarbons emissions.[citation needed]
Commercial FBC units operate at competitive efficiencies, cost less than today's units, and have NO2 and SO2 emissions below levels mandated by Federal standards. Although, it has some disadvantages such as erosion on the tubes inside the boiler, uneven temperature distribution caused by clogs on the air inlet of the bed, long starting times reaching up to 48 hours in some cases.
[edit]Types
FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling (BFB) and circulating fluidized bed (CFB).
[edit]FBC
Atmospheric fluidized beds use limestone or dolomite to capture sulfur released by the combustion of coal. Jets of air suspend the mixture of sorbent and burning coal during combustion, converting the mixture into a suspension of red-hot particles that flow like a fluid. These boilers operate at atmospheric pressure.
[edit]PFBC
The first-generation PFBC system also uses a sorbent and jets of air to suspend the mixture of sorbent and burning coal during combustion. However, these systems operate at elevated pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine. Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly efficient combined cycle system.
Advanced PFBC
A 1½ generation PFBC system increases the gas turbine firing temperature by using natural gas in addition to the vitiated air from the PFB combustor. This mixture is burned in a topping combustor to provide higher inlet temperatures for greater combined cycle efficiency. However, this uses natural gas, usually a higher priced fuel than coal.
APFBC. In more advanced second-generation PFBC systems, a pressurized carbonizer is incorporated to process the feed coal into fuel gas and char. The PFBC burns the char to produce steam and to heat combustion air for the gas turbine. The fuel gas from the carbonizer burns in a topping combustor linked to a gas turbine, heating the gases to the combustion turbine's rated firing temperature. Heat is recovered from the gas turbine exhaust in order to produce steam, which is used to drive a conventional steam turbine, resulting in a higher overall efficiency for the combined cycle power output. These systems are also called APFBC, or advanced circulating pressurized fluidized-bed combustion combined cycle systems. An APFBC system is entirely coal-fueled.
GFBCC. Gasification fluidized-bed combustion combined cycle systems, GFBCC, have a pressurized circulating fluidized-bed (PCFB) partial gasifier feeding fuel syngas to the gas turbine topping combustor. The gas turbine exhaust supplies combustion air for the atmospheric circulating fluidized-bed combustor that burns the char from the PCFB partial gasifier.
CHIPPS. A CHIPPS system is similar, but uses a furnace instead of an atmospheric fluidized-bed combustor. It also has gas turbine air preheater tubes to increase gas turbine cycle efficiency. CHIPPS stands for combustion-based high performance power system.
Sunday, November 28, 2010
First Aid
First aid treatment for carbon monoxide poisoning
• Move the affected person to the open air and prevent from exerting. Loosen clothing around neck and throat and check for the presence of adequate airway in the throat.
• If the person is unconsciousness, administer artificial respiration immediately
• Administer oxygen or mixed resuscitation gas or Karbogen gas (7% CO2 in Oxygen) as soon as possible. The Karbogen gas removes Carbon Monoxide as much as three times as fast as pure oxygen alone since CO2 in Karbogen gas stimulates the vagus nerve, causing more rapid breathing and hence, faster CO removal
• Summon medical aid but not at the expense of leaving the victim unattended
• Keep the victim warm
• Keep the victim under surveillance, as relapses often occur
• Move the affected person to the open air and prevent from exerting. Loosen clothing around neck and throat and check for the presence of adequate airway in the throat.
• If the person is unconsciousness, administer artificial respiration immediately
• Administer oxygen or mixed resuscitation gas or Karbogen gas (7% CO2 in Oxygen) as soon as possible. The Karbogen gas removes Carbon Monoxide as much as three times as fast as pure oxygen alone since CO2 in Karbogen gas stimulates the vagus nerve, causing more rapid breathing and hence, faster CO removal
• Summon medical aid but not at the expense of leaving the victim unattended
• Keep the victim warm
• Keep the victim under surveillance, as relapses often occur
producer gas
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
GASIFIER DESIGN
Gasifier Design Capacity
The first point in the gasifier design is to identify the application and the size/capacity of the gasifier in term of the producer gas requirement and the fuel to be gasified. The main applications of the gasifier considered are as under:
- Use of Producer Gas as supplementary fuel in diesel engines. In an unaltered diesel engine (compression ignition), the producer gas can only be used as a supplementary fuel replacing about 70% diesel and the engine can not be operated on producer gas alone.
- Use of producer gas as fuel in SI engines. The engine can be operated on producer gas alone, dual fueling is not necessary.
- Thermal application of producer gas. This includes the combustion of producer gas in an appropriate burner and use of sensible energy of gas for hot water, steam and hot air production.
Material and energy balance over the above systems will allow to determine the producer gas requirement for the engine and the fuel/wood requirement for the gasifier.
Material and Energy Balance
- Engine application
During the computations on material and energy balance on different systems, the following assumptions based on the reported information, are made:
| 70% |
| 80% |
| 4.5-5 MJ/Nm3 |
| 22% |
| 18% |
| 90% |
| 70% |
| 15-18 MJ/kg |
| 52% (Vol) |
| 0.3% (Wt) |
During all calculations air is assumed to have a volumetric/mole composition of 79% Nitrogen and 21% Oxygen. All the quantities of air and gaseous products are at normal conditions of temperature and pressure i.e. 0 oC or 273 oK and 1 atm pressure. The calculations are on the basis of one-hour operation.
- Gasifier Diesel Engine Alternator System
A material and energy balance was carried out over a 10 kW capacity gasifier and engine (diesel engine operating in dual fuel mode) system.
The gasifier system comprises of a gasifier reactor, gas-cleaning unit consisting of a water scrubber, a dry filter and a fabric filter and diesel engine-alternator. The material and energy flow diagram for the system as whole is shown in Figure 1. The system was divided into two sub systems; the diesel engine-alternator constituting the first sub system whereas the gasifier and the gas cleaning unit as the second sub system as shown in the figure.
By writing an energy and material balance over the first sub system, the diesel and producer gas requirement was calculated as follows.
Out put energy from the sub system 1 = 10 kW
Energy contribution of PG to engine = (10x3.6x0.7)/0.22 = 114.54 MJ
Amount of PG required = 114.54/4.5 or 25.45 Nm3
Now, consider the second sub-system, comprising of gasifier reactor and gas cleaning unit. In the output energy of the second sub system, only the chemical energy of producer gas (useful energy as far as the engine operation is concerned) is considered. Fuel wood and air used in the gasification process were calculated from an energy balance over the second sub system.
Input energy to the gasifier = 114.54/0.7 or 163 MJ
Figure 1. Gasifier Diesel Engine Alternator System
Air requirement for the gasifier is determined by a nitrogen balance over the second sub system. Following relation represents the nitrogen balance over the process. Fa in the equation below is flow rate of air into the gasifier.
The air flow/consumption rate comes out to be 16.75 Nm3 h-1. Thus the gasifier should be capable of generating 25 Nm3 of gas per hour using about 11 kg dry fuel wood in an hour.
b) Energy and Material Balance over Gasifier-SI Engine System
Le the capacity of the system is 10 kW. The gasifier and the SI engine alternator system is shown in Figure 2. The system was further divided into two sub systems; the SI engine-alternator as the first sub system and the gasifier and the gas cleaning unit as the second sub system as shown in the figure. Writing an energy and material balance over the first sub system, the producer gas requirement can be calculated as under.
Output energy from the sub system 1. = 10 kW
Input energy to engine = (10x3.6)/0.18 = 200 MJ
Amount of PG required = 200/4.5 or 44.4 Nm3
Figure 2. Gasifier SI Engine Alternator System
Now, consider the second sub-system, and writing energy balance equation to evaluate the fuel wood requirement:
Input energy to the gasifier = 200/0.7 or 285 MJ
Quantity of fuel wood used = 285/15 or 19 kg h-1 (dry weight basis)
Air requirement for the gasifier is determined by a nitrogen balance over the second sub system. Following relation represents the nitrogen balance over the process. Fa in the equation is flow rate of air into the gasifier.
The air flow/consumption rate comes out to be 28.65 Nm3 h-1. Thus the gasifier should be capable of generating 44.4 Nm3 of gas using about 19-kg fuel wood in an hour.
- Thermal Application
Now consider gasifier-producer gas burner system capable of generating gas having sensible energy equivalent to say 100 MJ. Le the whole system be divided into two sub systems as in case of engine applications. The first sub system consisting of producer gas burner and the second sub system comprising of gasifier as shown in Figure 3. The out put energy from the sub-system 1, is in the form of sensible energy of flue gases.
Figure 3. Gasifier producer gas burner system
Writing energy and material balance over the sub system 1.
Output energy from the sub system 1. = 100 MJ
Input energy to engine = 100/0.9 = 112 MJ
Amount of PG required = 112/4.5 or 24.7 Nm3
Now, consider the second sub-system, and writing energy balance equation:
Input energy to the gasifier = 112/0.8 or 138 MJ
Quantity of fuel wood used = 138/15 or 9.25 kg h-1 (dry weight basis)
Thus the gasifier should be capable of generating 25 Nm3 of gas per hour using about 9.25 kg fuel wood in an hour.
Turn Down Ratio
The turn down ratio varies from 3 to 18 in down draft gasifiers with throat, depending on the insulation of the hearth of the gasifier. It is defined by the following relation. For vehiel requirement a turn down ratio of 8:1 is essentially required
DESIGN OF DOWN DRAFT GASIFIER WITH THROAT
Once the size/capacity of the gasifier in terms of capability of the gasifier to produce certain amount of producer gas, and a decision is made on the type of the gasifier to used depending on the fuel available, the critical dimensions of the gasifier can be worked out using the techniques given below.
We are considering the design of Imbert type or down draft gasifier with throat in the present exercise. In a down draft gasifier with throat the gas is forced to pass through a constriction or a narrow part called throat of the gasifier. Due to the reduction in the area, the gas velocity increases and a temperature rises and become uniform in this zone. Since most of the combustion takes place around or just above the throat, the oxidation zone is formed at the throat. The high and uniform temperatures help in better quality of producer gas from the gasifier in terms of higher carbon monoxide and hydrogen and also help in thermal cracking and combustion of tar and the resultant gas has low tar and high calorific value. With the introduction of throat the gasifier becomes highly fuel specific and only wood pieces of specific size can be used for gasification.
Hearth Load
It is volume of gas produced in m3 at NTP per unit cross-sectional area at the constriction/ throat (cm2) per hour. The maximum value of the hearth load is about 0.9 Nm3/cm2-h. It is also referred as specific gasification rate (SGR) in the literature.
Superficial Gas Velocity
It is the velocity of gas at the throat of the gasifier in the gasification zone considering the throat to be empty. The superficial velocity corresponding to a hearth load of 0.9 Nm3/cm2-h is 2.5 m/s. The actual gas velocity at the throat of the gasifier is much more compared to this value due to presence of char/wood particle and high temperature in the oxidation zone of the gasifier.
Throat Diameter
Once the size of the gasifier is known, the hearth load value can be used to calculate the throat diameter. Corresponding to an electrical output of 10 kW using SI engine:
Gas production rate: 44 Nm3/h
Cross-sectional area of the throat = 44/0.9 cm2 = 48.9 cm2
Throat diameter = 79 mm say 80 mm
Superficial gas velocity at the throat = 2.5 m/s
Air Tuyers
Air enters into the gasifier through a number of nozzles arranged along the periphery of the gasifier in the oxidation zone. The diameter of the air tuyers taken together is generally small compared to the flow duct or pipe and the air velocity becomes very high as air enters the gasifier which helps the flame to penetrate in to the fuel bed as shown in Figure 4. This ensures the proper and uniform temperature distribution in the oxidation zone of the gasifier. Thus oxidation and pyrolysis of the fuel is completed before the material enters the reduction zone. The higher gas velocity results in higher temperatures in the biomass bed leading to reduced tar content and higher carbon monoxide content in the producer gas.
Figure 4 Air entering the gasifier through the air tuyers
For the capacity of gasifier reactor under reference, the recommended number of air tuyers in Table 1 is 5. The air tuyers diameter can be calculated in the following way.
Corresponding to a gas production rate of 44 Nm3/h and the throat diameter of 80 mm. The number of air tuyers is 5 and (Am*100)/Ah is 6.4 (Reference Table 1). From this the air tuyers diameter can be computed as under.
(Am*100)/Ah = 6.4 with Ah equal to 48.9 cm2 Am can be calculated
Am = 2.95 cm2
Cross-sectional area of one tuyer = 2.95/5 Þ 0.59 cm2
Diameter of the air tuyer (dm) = 0.87 cm
Other dimensions of the hearth
Once the hearth or throat diameter is calculated, the other dimensions of the hearth such as the distance between the throat and the air tuyers, reactor diameter at the air tuyers and other dimensions as shown in the diagram below can be calculated from the tables. The corresponding values for the gasifier under reference are given below.
h 95 mm
dr 268 mm
dr` 150 mm
H 256 mm
R 100 mm
Figure 5 Dimensions of the hearth of the gasifier
Table 1. Design parameters for the Imbert type gasifier (No. 1)
| dr/dh | dh | dr | dr` | H | H | R | A | (Am*100)/Ah |
| 268/60 | 60 | 268 | 150 | 80 | 256 | 100 | 5 | 7.8 |
| 268/80 | 80 | 268 | 176 | 95 | 256 | 100 | 5 | 6.4 |
| 268/100 | 100 | 268 | 202 | 100 | 256 | 100 | 5 | 5.5 |
| 268/120 | 120 | 268 | 216 | 110 | 256 | 100 | 5 | 5.0 |
| 300/100 | 100 | 300 | 208 | 100 | 275 | 115 | 5 | 5.5 |
| 300/115 | 115 | 300 | 228 | 105 | 275 | 115 | 5 | 5.0 |
| 300/130 | 130 | 300 | 248 | 110 | 275 | 115 | 5 | 4.6 |
| 300/150 | 150 | 300 | 258 | 120 | 275 | 115 | 5 | 4.4 |
| 400/130 | 130 | 400 | 258 | 110 | 370 | 155 | 7 | 4.6 |
| 400/150 | 135 | 400 | 258 | 120 | 370 | 155 | 7 | 4.5 |
| 400/175 | 175 | 400 | 308 | 130 | 370 | 155 | 7 | 4.2 |
| 400/200 | 200 | 400 | 318 | 145 | 370 | 155 | 7 | 3.9 |
Fuel Hopper
In batch operation of the gasifier the fuel holding capacity determines the maximum operating hours of the gasifier once the fuel is charged to the gasifier. A reasonable continuous operating period for a batch is 6 to 8 hours, thus the fuel holding hopper should hold fuel for 6-8 hours. Knowing the bulk density of the fuel to be used in the gasifier, the volume can be determined.
Table 2. Design parameters for the Imbert type gasifier (No. 2)
| dr/dh | h/dh | Range of gas output (Nm3/h) Max Min | Wood Consumption (kg/h) |
| 268/60 | 1.33 | 30 4 | 14 |
| 268/80 | 1.19 | 44 5 | 21 |
| 268/100 | 1.00 | 63 8 | 30 |
| 268/120 | 0.92 | 90 12 | 42 |
| 300/100 | 1.00 | 77 10 | 36 |
| 300/115 | 0.92 | 95 12 | 45 |
| 300/130 | 0.85 | 115 15 | 55 |
| 300/150 | 0.80 | 140 18 | 67 |
| 400/130 | 0.85 | 120 17 | 57 |
| 400/150 | 0.80 | 150 21 | 71 |
| 400/175 | 0.74 | 190 26 | 90 |
| 400/200 | 0.73 | 230 33 | 110 |
For 6 hours operation the fuel required 19*6 = 114 or say 120 kg
Bulk density of the wood pieces 350 kg/m3
Fuel hopper volume 120/350 = 0.34 m3
Gasifier diameter is generally 2 to 3 times the dr (let us take it 2.5 times). Accordingly the gasifier reactor diameter comes out to be 26.8*2.5 = 67 cm
If h is the height above the air tuyers
0.34 = P/4 (0.67)2*h Þ h is 1 m
Thus the height is 1m.
Considering 30% extra the actual height above the air tuyers is 1.3m
Total height of the gasifier = 1.3 +0.256 +0.1 = 1.656 m or say 1.75 m. Here the ash holding space i.e. R may be increased to 0.194 m to facilitate the installation of ash removing gate.
Note: Use simplified design as Explained in the Class
A 330 MJ/h capacity gasifier along with producer gas burner system designed at PAU is shown in the diagram below.
Figure 6 Line diagram of a down draft gasifier burner system
DESIGN OF A DOWN DRAFT THROATLESS GASIFIER
Description of the gasifier unit
Throatless gasifier system is a double walled constant diameter reactor containment tube unit. The inner shell is reactor having grate at the bottom. The outer shell acts a containment tube. The top of the gasifier remains open at the time of operation. The gasifier is a special gasifier designed for gasification of agricultural crop residues such as rice husk, groundnut shell etc. Rice husk and air enter the gasifier from the top whereas the producer gas exit is at the bottom. To start this gasifier, a small quantity of husk is burned just above the grate followed by a small charge of rice husk with gas sucked though the system by means of a hand blower. When fir spreads well in whole cross-section of the reactor, the reactor is filled with husk upto top and the gasifier is connected to gas application unit. In this gasifier ignition is started over the grate and the reactor is filled with rice husk. During the gasifier operation, the oxidation/fire zone keeps on moving up and so does the reduction zone and char accumulation. The oxidation zone is formed at the interface of the rice husk and char, where oxygen comes in contact with the husk at high temperature. Reduction zone is formed just below the oxidation zone. The pyrolysis reactions take place above the oxidation zone. A line diagram of the gasifier is shown in Figure 7.
Figure 7 Throatless down draft rice husk gasifier
Depending upon the requirement, the size of the gasifier in terms of the fuel requirement and gas production can be determined by making an energy balance over the system as discussed earlier. During this analysis the following assumptions may be used.
| 65% |
| 75% |
| 4.5 MJ/Nm3 |
| 22% |
| 70% |
| 14.35 MJ/kg |
| 52.5% |
| 0.3% |
| 0.36 |
| 37 MJ/l |
Once the size/capacity of the gasifier in terms of capability of the gasifier to produce certain amount of producer gas, and a decision is made on the type of the gasifier to used depending on the fuel available, the critical dimensions of the gasifier can be worked out using the techniques given below.
Specific Gasification Rate:
Specific gasification rate is the quantity of dry rice husk consumed per unit time and cross-sectional area of the gasifier (kg/h-m2). SGR was determined using the following relation.
A study on the determination of reactor scaling parameters, performance of four gasifier reactor with SGR as a variable parameter was estimated. The SGR and gasification efficiency values for the four reactors is in Figure 8. In all cases the best gasification efficiency correspond to a SGR of about 190 kg/h-m2.
This value of SGR has been selected and used as a design parameter for the rice husk operated throatless gasifier. For a 10 kW capacity rice husk gasifier-diesel engine-alternator, the energy balance indicates a that the hourly rice husk requirement will be about 12.25 kg. The producer gas requirement comes out to be 25.5 Nm3/h
Specific gasification rate kg/h-m2
Figure 8 Effect of SGR on gasification efficiency in different capacity gasifiers
Cross-sectional area = (12.25/190) m2
Diameter = 0.3 m
Continuous gas supply to the engine can be ensured with the installation of two gasifier reactors in parallel to a single gas conditioning unit.
Diameter of containment tube
Containment tube checks the quantity of solid particulate in the producer gas and also prevents excessive heat loss from the reactor to the atmosphere. The recommended superficial gas velocity in the containment tube is 0.15 m/s against the superficial gas velocity of about 6 m/s in the gasifier reactor. Using the following relation the diameter of the containment tube can be determined. In the equation d1 and d2 are the outer reactor diameter and inner diameter of the containment tube respectively.
Substituting the values of producer gas flow rate (25.5 Nm3/h) and outer diameter of reactor as 30.6 cm, the inner diameter of the containment tube comes out to be 36 cm. It is assumed here that the gasifier reactor is made of 3 mm thick MS sheet. The length of the containment tube was kept about 10 cm more than the reactor. This was to allow the producer gas after its exit through the grate to enter the containment tube.
Grate
The grate in the gasifier reactor supports the fuel bed and at the same time allows the producer gas to flow through it. Grate of the gasifier was made from a 15 ASTM stainless steal wire mesh. Wire mesh was fitted on a circular frame/ring of 6 mm thick MS rod and was supported on three legs. The grate was positioned at a distance of 5 cm above the bottom end of the gasifier reactor. The grate was a removable one, which could be taken out for cleaning. The gasifier reactor was rested on an MS angle frame on two hinges provided on the outer surface of the containment tube. The reactor could be tilted along the hinges for ash removal.
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