|Low Cost Charcoal Gasifiers for Rural Energy Supply (GTZ, 1994, 49 p.)|
|7. Derived technical demands for field application of gasifier-engine systems|
As soon as a hot charcoal bed is established in the gasifier and the hot combustion gases are driven by suction or pressure through the charcoal layer next to the combustion zone, gas production begins. As soon as the pipes and vessels between gas producer tank and engine inlet are filled with gas and the gas is mixed with the adequate amount of secondary air, the engine should start.
The performance of the gasifier depends first of all on the engine characteristics. The adaptation of the engine to the dimensions and performance of the gasifier (or vice versa) is essential to the performance of the complete system. The engine has the correct size, if its gas consumption (dependent on cylinder volume and revolutions per minute) in nominal operating conditions corresponds to the nominal gas production of the gasifier. Details of this correlation are often not sufficiently understood by operating personnel and even involved technicians. It is well known that gas production can vary within a certain range, depending on the suction of the engine. The relation between maximum and minimum gas production is often called "turn down ratio". But it is not clearly defined which numeric values of gas flow are to be expected at a given gasifier and a given engine.
It is obvious that the lower and upper limit of gas production is defined by the involved amount of charcoal. Less obvious is the dependence on the geometry of the reaction chamber (diameter and height). This paper is not the adequate place to go into too many scientific details, let us take it as an empiric result that a height of the reaction cylinder of 20 cm and a diameter between 25 and 30 cm is most suitable for an engine shaft power of 10 kW.
If an engine of 2 litres cylinder volume is used, the minimum number of revolutions is approximately 1400 rpm, which corresponds to a gas-air-mixture of 60.42 Nm³/h entering into the engine and a gas production of 28.7 Nm³/h in the gasifier. It was found empirically that less gas production than the above given value result in too low temperatures in the reaction zones. This freezes the kinetic reaction and results in very poor gas quality. The upper limit of gas production of the 25- 30 cm diameter hearth should theoretically be indicated by a decrease of the gas heating value, when the involved charcoal is just burnt by an excessive oxygen supply, leaving no charcoal bed for reduction of the combustion gases. In practice, this could not be observed in test runs of the FLEUS group, as gradual overheating of the materials (refractory, cement walls, cooling water) began when 12 kW shaft power were exceeded.
The practical consequence of these considerations is:
(1) A reasonable engine speed is 2500 rpm. This is a good working point for the combustion engines, where good torque can be expected. Too high speed can be critical in terms of insufficient lubrication and cooling.
(2) A cylinder volume between 1.8 and 3 liter is adequate for the standard open core gasifier. This will render between 8 and 15 kW at 2500 rpm at full load operation. For loads higher than approximately 6 kW, good cooling by a continuous water flow through the system is necessary (depending on ambient temperatures). For a 3 liter engine, a reactor diameter of 300 mm is adequate.