|Low Cost Charcoal Gasifiers for Rural Energy Supply (GTZ, 1994, 49 p.)|
|7. Derived technical demands for field application of gasifier-engine systems|
At stationary engines, a mechanical or electro-mechanical speed control device (governor) has to adjust the gas valve in order to maintain the nominal engine speed under the varying load situation (whereas in vehicle driving the engine revolutions are controlled by the driver with the accelerator pedal). Stationary diesel engines are always equipped with such a governor. In practical application of gasifiers however, very often a car engine for gasoline operation is used. These engines can run completely on gas (whereas a diesel engine requires dual fuel operation) and are much cheaper than a diesel engine, but the absence of a governor for speed control may present a problem. This is especially true in applications where the plant is running at constant load and the presence of the operator is only needed for refilling the fuel bunker. In the following it will be discussed in which cases a governor is absolutely necessary and in which cases it is not. There is a wide range of possible applications of gasifier-engine-systems, but it is possible to define a few typical situations. They are typical with respect to the capability of the engine to follow the load and retain a constant speed.
Case 1: Constant mechanical load, no need for stable engine speed.
A typical example is the water pump. A pump, working on nominal revolutions, delivers a certain quantity of water over a certain height within a certain time. If the revolutions of the pump are fluctuating, the quantity of delivered water will fluctuate accordingly, but the pump will still work. The acceptable range of fluctuation will be approximately + / - 20 % of nominal rpm. If the gas production and the gas quality of the gasifier is constant enough to meet the energy demand of the engine-pump-set within these limits, a governor is not necessary.
The derived technical demand for water pumping application is:
Even without a governor, the gas production by the gasifier has to be sufficiently constant to maintain nominal engine speed +/- 20 % over the time interval between two refuellings (4 hours).
Diagramme 3 shows the variation of engine speed of the 2 liter Ford engine of CHAR, connected with the standard ferrocement gasifier. The constant load is here simulated by electric heaters, the engine revolutions are directly proportional to the indicated electric voltage. It can be seen that the demand for constant rpm + / - 20 % is met without problems.
Case 2: Variable load, no need for stable engine speed
The power demand of working machines is in most cases characterized by a steep increase from no-load to full load. Neither the no-load situation nor the full load situation is bound to an exactly defined engine speed: No-load condition can be maintained by idling of the engine, but as well by rather high revolutions. For example, a circular saw is running at high rpm without load and is decreasing the speed when the load is put on. The engine speed under load should be sufficiently high to get the necessary power output, but it is not necessary to keep the number of revolutions stable. The situation gets additionally complicated when electric motors are switched on under load (e.g.: Compressors in refrigeration units), as in these cases the initial electric current through the electric coils is very high, resulting in a power drop in the electric line and reduced starting torque.
The wide range of necessary adjustment of the gas valve can normally not be covered by a governor, it requires manual adjustments. But, as these applications require personnel for the working process anyway, it should not be a problem to have a hand on the adjustment valve at the moment when the load is added.
The derived technical demand for working machinery application is:
The load following capacity of the gasifier must be sufficient to react on load changes between 20 % and 70 % of maximum load. Manual adjustment of the gas-air-mixture supply' valve is acceptable.
Applications of that kind, tested successfully with the standard ferrocement gasifier and a 2 liter engine, were:
- circular saw, 40 cm diameter, cutting of hardwood 10 cm thickness (power demand approx. 4 kW)
- a small mulcher for straw and branches (simultaneously to the circular saw, two engines running on one gasifier)
- electric welding (power demand 7 kW on 125 Amperes welding current)
- electric load (stoves and heaters) varying in steps between 2 and 10 kW.
In all these cases, very short load following frequencies were possible, as long as the load did not exceed approximately 70% of the maximum power.
Case 3: Low fluctuations in electrical load, but need for constant engine speed.
This is the case of electricity supply for small local grids (village electrification). Electronic instruments like radio, television, light bulbs etc. require constant voltage and frequency, and this means constant rpm of the engine within + / - 5 % of the nominal rpm.
A gasifier engine system will normally not provide a constancy of gas quality, sufficient for this demand. Furthermore, minor fluctuations of the load cannot be avoided. Therefore, a governor is essential. Even then, good performance of the gasifier is required to guarantee trouble-free operation over a period of a few hours.
The derived technical demand for application of a gasifier for small scale electrification is:
Constant voltage + / - 5 % deviation must be provided by the gasifier-engine system over the time interval between two refuellings of the bunker (2-4 hours).
Diagramm 4 shows the performance of the small metal gasifier, operating an engine with electric generator. The engine speed is controlled by a mechanical governor. The electric voltage is constant over the measuring period (2.5 hours) within + / - 5 % of 220 Volts. The electric load consists of a heater, different bulbs (incandescent and fluorescent), a radio and a television set.
No corresponding diagramm is available for the ferrocement gasifier, as a governor for the applied engine was not yet installed. The good constancy even without governor, shown in diagr. 3, assures the satisfying performance with governor for electricity application.
The conclusion of the results of a large number of test runs is:
(1) The standard ferrocement gasifier, coupled to a 2 liter engine, can be used for mechanical or electrical power applications between 2 and l2 kW. l0 kW is the recommended nominal power, which guarantees the best ratio between costs and output as well as the best overall efficiency.
(2) In water pumping applications, a governor for engine speed control is not necessary. The gas production is stable enough to maintain constant engine speed within two filling periods of the fuel bunker (2 - 4 hours).
(3) In applications for working machinery (grain mills, saw mills, grinder, cutter; electric welding, compressors the load can be varied between 20 and 70% of nominal power without relevant delay, if the gas demand is adjusted manually. A governor may, make the adjustment easier, but will not be sufficient for complete automatic control in many cases.
(4) For electricity supply for household applications, a governor is recommendable to guarantee stable voltage within two filling periods of the bunker.
It must be emphasized that the laboratory results in Bremen were obtained with charcoal of constant quality (carbon content 87.7 %, heating value 31 380 kJ/kg, moisture content d.b. 5 %, ash content d.b. 1.2 %). The behaviour of the large variety of charcoal species from tropical wood is still subject of investigation. Generally, it can be concluded that the charcoal has to be reasonably dry and selected to sizes of 1-6 cm length.