Designing considerations for 1MW DOWNDRAFT GASIFIER WITH TAR CRACKING

This write up gives complete considerations for designing and setting up a biomass gasifier based power plant of 1 MW to meet the energy needs of an industry.
1. Need for fixed bed gasifiers
As gasifiers up to 1MW capacity may be of the fixed bed type. Due to its simplicity, this type is recommended. Fluidized bed gasifiers can be built and operated only at higher capacities. Two down draft gasifiers of Imbert type are recommended. Two gasifiers each of 500 kW capacities will give the following advantages.
i. If one unit is shut down for maintenance, the other will deliver power without interrupting the operation of the FC power plant.
ii.Both 500 kW units can work near full load taking advantage of the higher efficiency of the gasifiers at full load.
iii. Flexibility in meeting the fuel wood inventory even in lean periods of supply.
2. Components of the gasifier plant
The gasifier plant is proposed to have the following components in order: a) Fixed bed gasifier (down/up draft), b) Cyclone, c) Hot gas cleaning unit, d) Tar cracking unit , e) High temp. tar trap (filter), f) Secondary gas cleaning unit, g) Shift converter, h) CO2 Scrubber (if needed), I) Boiler, j) Combustor and k) Heat exchangers.
The fuel handling, communition and drying equipments will include: a) Chain conveyors for handling fuel logs to crushers, b) Two stage toothed roll crushers to produce chips of required sizes, c) Chain conveyor for taking chips form the roll crushers to the storage bin, which is assisted by a hot air blower for drying chips, d) Storage bin, e) Screw conveyors with lock hoppers for handling the chips and feeding to the gasifiers, f) Gas sampling ports, g) Temperature sensors, h) CO alarm and oxygen masks and I) Flame arrestors.
3. The gasifier
3.1 Dimensions
The key dimensions of a typical down draft gasifier can be worked out based on a hearth load of 1 Nm3 /cm2. h as follows: Throat diameter: 450mm; No of nozzles: 10; Air nozzle diameter: 18mm ; Fuel container diameter: 1.5 m, Height: 2.5m; Overall dimensions: 2.5 m; Height : 3.0m.
3.2 Insulation of the gasifier
The thickness of insulation with kaolin is 500 mm for the assumed heat load.
3.3 Biomass storage
The requirement of biomass is about 18 tonnes per day. The storage space is required for an inventory of 600 tonnes to meet the demand for one month.
The wood logs can be stored as a stack facilitating natural drying. An area of 40 m by 30m is needed for the installation of two stage roll crushers and chain conveyors to feed the logs into the crushers. An additional area of a minimum of 2m by 7m is needed for the trucks to unload the logs into the storage space.
3.4 Chips handling
In order to feed the gasifiers with chips of say 30 by 40 by 70 mm size, roll crushers are needed. In the first stage logs are broken down to small pieces. In the second stage chips of desired size are prepared. The power consumption of the crushers and chain conveyors will be 30 kW with 225 mm serrated rolls to handle logs of up to 150 mm diameter.
The chips shall be dried by hot air generated by the waste heat available from the combustor exhaust gas through a heat exchanger or by dilution with air.
It is proposed that the dry chips shall be fed by screw conveyors to the gasifiers from the top. The chip storage bunker may contain a scraper for feeding the horizontal screws fitted at the end of the storage space.
The screw conveyor may consist of a horizontal section to collect chips from the bunker, an upright section to the top of the gasifier and a short section to feed the gasifiers. Lock hoppers may be provided at appropriate locations to prevent gas leakage during feeding. The diameter of the screw is 225 mm in this case.
3.5 Ash handling
A short screw section can be used followed by pneumatic handling after collecting from the bottom of the gasifiers. The grate needs to be kept cool in order to avoid warping. For collecting ash from the bottom a rotating grate with a scraper can be used.
3.6 Air handling
A continuous supply of air at about 30 % of the stoichiometric requirement is needed. This shall be supplied by a blower of 600 Nm3 per hour capacity. The hot air blower for chip drying shall have a capacity of 200 Nm3 per hour.
4. Tar cracking
Particulates, volatiles and tar have to be removed from the producer gas prior to the reformer in case power is to be produced by internal combustion engine. Particulates are removed in a cyclone and hot gas filters. For tar cracking a temperature of 800 to 900 deg C is necessary. Several studies indicate that dolomite is an efficient catalyst. Glanshammar dolomite has been shown to yield 2 g tar per kg DS. The hydrogen content of the gas is increased at the same time. It is possible to have the tar cracking unit either integrated with the gasifier taking advantage of the heat loss at high temperature zone or to set up the unit outside the gasifier. The dust load and fluctuation in temperature coupled with gas leakage problems favour a separate tar cracker .
4.1 T.C. Unit specifications
The amount of tar contained in the producer gas ranges as follows. Downdraft: 0.001 to 0.01 kg per kg dry feed.
Studies conducted on tar cracking using pyrolysis gas indicate that a temperature of 800 to 900 deg.C is required to minimize the tar content in the exit gas. A catalyst load of 0.25 to 0.30 kg per kg DS may be adopted. The detailed dimensions is to be worked out based on the component fractions, amount of tar, regeneration possibilities of catalyst, catalyst granulometry, temperature profile, etc.
4.2 Combustor
A combustor is needed to produce hot gas to heat the reforming catalyst bed to around 900 deg. C. A swirl gas combustor of 300 mm diameter and 280 mm shroud length can be tentatively suggested. The fuel gas can be taken from the producer gas plant. The requirement shall be calculated based on the differential temperature, catalyst bed temperature, catalyst bed temperature and heat capacity of the bed.
4.3 Drying of chips
It is wise to dry chips rather than logs. Hot air at 60 to 70 deg C is recommended. Hot air can be obtained by mixing about 10 times air with the combustor exhaust gas.
5. Steam boiler
Steam has to be supplied to the reformer and the shift converter to get hydrogen rich gas in case of running a fuel cell. The hot water from the fuel cell generator and waste heat from the gasifier can be used for steam generation. The steam capacity of the boiler is estimated to be 800 kg per hour at a pressure of 20 bar giving super heated steam at 850 deg.C.
6. Housing and instrumentation
The housing requirement is estimated as follows. Biomass storage, crushers and dryer: 50 m by 40 m. Gasifiers, tar cracking units and reformer: 10 m by 4.5m, Steam boiler, combustor and heat exchanger: 2.5 m by 3 m. Safety instruments such as CO monitor & alarm (150ppm), oxygen masks, fire protection machineries will also be housed. Flame arresters are to be provided in the gasifiers and combustors. Data monitoring and acquisition systems for chips level in bunker and gasifier, temperature and gas flow need to be incorporated.