Throat less gasifier design

Gasification
Thermo chemical gasification is the conversion of carbonaceous feedstock such as biomass or coal by partial oxidation at elevated temperature a into a gaseous energy carrier. Gasification occurs in sequential steps: drying to evaporate moisture, pyrolysis to give gas, vaporized tars or oils and a solid char residue, followed by gasification or partial oxidation of the solid char, pyrolysis tars and pyrolysis gases.
The gas obtained on gasification contains carbon monoxide, carbon dioxide, hydrogen, methane, trace amounts of higher hydrocarbons such as ethane and ethene, water, nitrogen (if air is used as the oxidizing agent) and various contaminants such as small char particles, ash, tars and oils. The partial oxidation can be carried out using air, oxygen, steam or a mixture of these.
Air gasification produces a poor-quality, low energy density gas (4-7 MJ/ cu.m, higher heating value) which is suitable for boiler, engine and turbine operation, but not for pipeline transportation. Oxygen gasification produces a better-quality gas (10-18 MJ/ cu.m, higher heating value) which is suitable for limited pipeline distribution and for use as sythesis gas for conversion to methanol and gasoline.
Open-core downdraft gasifier
This type of gasifier was first devised by the Chinese for rice husk gasification and further developed by Syngas Inc. from work carried out at the Solar Energy Research Institute (now the National Renewable Energy Laboratory - NREL).
This type of gasifier is called as static bed or open core or throat less gasifier and is a simple reactor technology developed principally for small-scale or remote applications requiring fuel gas for heat or power. This type of gasifier has been developed with no throat and the bed is supported on a grate.
The reactor generally consists of two concentric cylinders (one may be sufficient), in which a stationary fuel bed is converted by a reaction front propagated through the fuel bed. When under suction created at the intake of an engine, the reactor top can be left open for refueling without venting producer gas.
Specifc gasifcation rate
Specifc gasfication rate (SGR) is an important parameter which expresses the rate of fuel consumption per unit cross-sectional reactor area. An optimum value of this parameter is used for designing different capacity range of throatless gasifiers.
Calculation of air to be supplied
The equivalence ratio (ER) is defned as the ratio of actual air used in a run to stoichiometric air requirement for the run where ER= (Amount of air used in a run)/(Amount of stoichiometric air). Knowing the elemental composition of raw material, the stoichiometric air requirement can be estimated (for dry rice husk the value is 3.35 cu.m / kg). Optimum value of equivalence ratio for gasification can be taken as 0.40. The air fuel ratio can be estimated using the expression, A/F=Amount of air used (kg)/Amount of dry material (kg). This calculation helps to find the amount of air to be supplied for gasification (through engine suction or an external blower).
Determination of gas flow rate
The gas flow rate can be determined by installing a calibrated orifice meter in the producer gas line. But due to the presence of tar in the gas the orifice meter tend to get foulded and soon will result faulty gas flow readings. It is therefore preferred to measure the air flow to the gasifier and make a nitrogen balance to estimate the gas flow rate. For this, install a pre-calibrated orifice meter in the upstream section of the gasifier (just before the gasifier) to measure the air flow rate. Knowing the producer gas composition, elemental analysis of biomass, feed rate and air flow rate, nitrogen balance over the gasifier may be carried out using the following procedure for gas flow rate determination. For this computation assumptions are made that air has a molar composition of 0.79 Nitrogen and 0.21 Oxygen and all the nitrogen entering the gasifier leaves it in producer gas.
Nitrogen input = Flow rate of air (Nm3 / h)* 0.79 + Fuel feed rate into the gasifier (kg/h)* Weight fraction of nitrogen in the biomass * (22.416 / 28)
Nitrogen output = Flow rate of producer gas (Nm3 / h)*mole / volume fraction of nitrogen in the producer gas
By equating the above two statements we can solve for flow rate of producer gas
Reactor sizing
Previous works report a specific gasification rate of approximating 170 kg/sq.m.h, which is an intermediate value between two levels and a maximum thermal efficiency exists at this value. Using an optimal value the size of the reactor can be readily computed from the energy demand on the system.
Theoretical modeling and experimental works have been done for moving bed open-core rice hull gasifier using a 45 cm diameter reactor. Results of their work showed an optimum gasification load or specific gasification rate of 125-175 kg/sq.m h, depending on bed height. According to another report the cold gas efficiency of 26 cm. reactor diameter gasifier reached a peak of between 50 and 60% at specific gasification rate of 200 kg/sq. m h.
Another report indicates an optimum value of specifc gasifcation rate for gasifcation of rice husk in throatless open core gasfier reactor as 192.5 kg/sq.m h. Optimum value of equivalence ratio was 0.40, the gas lower heating value of producer gas was about 4 MJ/N cu.m.and the cold gas efficiency was around 65%.
For determining the reactor diameter for a downdraft stratified gasifier , Reed has indicated an optimum value of specific heat rate as 390 kg/sq.m h for 8 to 75 cm diameter reactors. It is further reported that the maximum specific heat rate can go up to 580 kg/sq.m h with gasifier having mechanical ash removal unit. However it is reported that these figures were derived through experiments with a particular gasifier type and mode of operation. From the fore going it is clear that a preliminary value of 200 kg/sq.m h can be taken as a value for SGR for determining the reactor diameter.
Reactor construction
The reactor can be a batch fed type having constant diameter. It can be made from a minimum of 3 mm thick steel sheet. The gasifier consists of an inner reactor and a concentric containment tube. The containment tube and the reactor can be flanged together at the top. The top end of the reactor can remain open during the operation. Air entry into the reactor will be from the top and gas exit through preferably a stainless steel wire mesh grate, fitted at the bottom of the reactor. The bottom of the containment tube should be water sealed. The diameter of the containment tube can be selected in such a way that the producer gas velocity in the space between the reactor and the containment tube is around 0.6 m/s.
Starting a small gasifier
A small amount of char is placed over the grate followed by feedstock. The purpose of adding char over the grate was to protect the grate from high temperature damage. A suction blower can be connected in the down stream section of gasifier after the first filter to start the initial establishment of ignited charcoal. Then the feedstock is filled and the blower operated for some time to start the gasification. A flare burner may help find that combustible gas is generated from the gasifier.
Chinese gasifier design for rice husk
The gasifier consists of an inner tubular steel shell reactor of 25 cm diameter, open at the top and closed at the lower end by a stainless steel mesh screen. It was housed within a concentric 35 cm gas collector. The lengths of reactor and collector are 168 and 183 cm respectively for one hour continuous operation. Air enters the reactor at the open top and passes downward through the fuel column to the reaction zone when under suction from engine intake system. The gas from the reaction zone flows in the reverse direction to the hot outlet.
Raw gas is passed through a wet sieve plate (scrubber and particulates). The twin reactor design enables the engine to draw from one reactor while the other is being serviced.
The gasifier generates a nearly uniform reaction front propagating upwards at a velocity of 0.77-0.87m/h. With a temperature of the reaction front maintained at 950-1050°C.
Gas composition is as follows (Vol%): CO13.4%, H2 11.1%, CH422%, 0221.4%, N258.9%, H2O4.13%, lower heating value (LHV) 40223.8KJ/Nm3, Gas flow is 18.44 Nm³/h. Specific gas output is 2.39Nm³/kg rice husk. Specific gasification rate is 185 kg/m-h. Cold and raw gas efficiency was 52.4% and 72.2%.
Static bed 25 cm rice husk gasifier yielded an optimal value of specific gasification rate in the vicinity of 195 kg/m² -h. Cold gas efficiency (52.4%) and gas flow (18.44NM3/h) are favorable for selected duel-fuel engines.
Indian gasifier design for sugarcane leaf and bagasse
This is a low-density biomass gasification system for thermal applications. The gasifier can handle fuels like sugarcane leaves and bagasse, bajra stalks, sweet sorghum stalks and bagasse etc. The system delivered under laboratory conditions at 288-1080 MJ/h output levels. The HHV of the gas was 3.56-4.82 MJ/N cu.m. The system also produced char of about 24% by weight of the original fuel. It can be briquetted to form an excellent fuel for wood stoves or can be used as a soil conditioner. The system was retrofitted to a specialty ceramics baking LDO-fired furnace in a metallurgical company.