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8. Gasification of selected crop residues

Authors: Meach Phalla, 2005.

Summary: The experiment was carried out at the Center for Livestock and Agriculture Development (CelAgrid), located about 26 km from Phnom Penh . The gasifier was imported from Ankur Scientific Energy Technology Pvt. Ltd, India . The gasifier (WBG model) is specifically designed for woody feedstock. The Ankur biomass gasifier model “Ankur GAS-9” consists of a biomass gasifier (model WBG-15) connected to a 25 HP Field Marshal gas engine coupled with a 15 KVA alternator (duly modified to operate in 100% producer gas mode) to give a gross output of about 9 kWh with a net output of 8 kWh. The whole system is divided into three main units, i) gasifier, ii) filter and iii) engine. The four treatments were different sources of fibrous biomass: Coconut husk, cassava woody stem, mulberry stem and branches from Cassia stamea . Each source of fibrous biomass was evaluated as feedstock in the gasifier on three separate occasions. The coconut husk was collected from the sale of the coconut juice in Phnom Penh , the cassava woody, mulberry stems and Cassia stamea branches were harvested at CelAgrid. The different sources of biomass were cut into small pieces, of about 2 to 4 cm length and 1 to 3 cm width, and then sun-dried in the open air until the moisture content was less than 20%. Small branches (diameter below 2 cm) were cut by hand with a knife. Larger branches (diameter >2 cm) were cut by an electrical woody biomass cutter.

The first step for gasifiern operation was to fill the reduction zone with charcoal (about 20 kg) with a particle size of 2 to 3 cm. A weighed quantity (about 40 kg) of feedstock was then added to the hopper. Air was forced through the gasifier by the action of the water pump and the venturi valve. The charcoal inside the gasifier was then lit with a flare and after 5 to 10 minutes, the producer gas was tested for quality. When the gas was seen to burn with a colourless f lam e and that the flow was constant, the engine was started and the parameters of the system were recorded until all the feedstock was used up and the engine halted. The system was allowed to cool, after which the hopper was removed and the quantities of residual feedstock and charcoal were measured. The quantity of char (mixture of charcoal and ash) removed from the grate was weighed. The following day the procedure was repeated with another source of feedstock. The moisture content of the biomass was determined with an electronic meter with precision of ±1 unit. Bulk density (kg/m3) was measured by putting the cut pieces of feedstock in a wooden box (previously weighed) of known volume. The box was then weighed to determine the weight of each biomass.

Abstract: The density of the feedstock was highest for the Cassia stamea and lowest for cassava and coconut, with an intermediate value for the mulberry. Despite these differences in feedstock density, there were no differences among sources of feedstock in the operating parameters of the gasifier (Yield and conversion of feedstock to electricity, and energetic efficiency). The conversion of feedstock to electricity was in the range of 1.11 to 1.23 (kg dry biomass/kwh); energetic efficiency (MJ of electricity/MJ of biomass) was in the range of 0.19 to 0.22, compared with 0.29 for a diesel engine-generator. Fibrous woody residues in the form of coconut husks and stems of cassava, mulberry and Cassia stamea were efficient and effective sources of feedstock in a downdraft gasifier.

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