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close this bookLow Cost Charcoal Gasifiers for Rural Energy Supply (GTZ, 1994, 49 p.)
close this folder7. Derived technical demands for field application of gasifier-engine systems
View the document(introduction...)
View the document7.1 Issues in engine operation
View the document7.2 Typical applications
View the document7.3 Repair and maintenance of the ferrocement gasifier

(introduction...)

The following considerations are limited to stationary gas producer plants for mechanical and electrical energy supply. The use of process heat is a further important application, but is not treated here as not being within the terms of reference of the project.

7.1 Issues in engine operation

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.

7.2 Typical applications

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.


FIGURE

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.


FIGURE

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.

7.3 Repair and maintenance of the ferrocement gasifier

It is a decisive advantage of the ferrocement gasifier that no corrosion of materials is to be expected. Whereas all metal plants-if not made of expensive stainless steel-corrode rapidly and have to be repaired by welding or replacement of components after one or two years, the ferrocement vessels and tubes are basically maintenance-free. This is especially important when the gasifier is not used all year round. A metal gasifier will just corrode away if it is left for some month without having been carefully cleaned and painted. A ferrocement gasifier is not affected at all by moisture and climate.

If there appears any rupture during operation, this may be the result of overheating when not enough cooling water was used. In that case, however, the repair is very easy by just chiselling up the rupture and adding new mortar to it.

A repair which is frequently necessary at classical metal gasifiers is the replacement of sealing ropes, nozzles, threaded tubes, hinges and so on - all these parts are affected by heat or corrosion. These sensitive parts are virtually non-existent at the ferrocement gasifier. The simplicity of the design avoids nearly all of the traditional repair problems.


FIGURE

The only necessary maintenance work is the replacement of the filter bags, when they are too contaminated. It is recommendable to have a second set at hand. The replacement is then done in 10 minutes. The replacement intervalls depend on the average load of the system; as a rule of thumb, every 100 hours (or once a month) is a typical interval. Diagr. 5 shows the average pressure drop across the filter section at various loads.

Diagramm 6 shows the pressure drop across the filter system over a period of 3.5 hours and over two subsequent test runs. It can be seen that a self-cleaning effect takes place: By the sudden collaps of the filter bags, when the suction of the engine stops at the end of a run, the dust layer on the filter clothes is partly removed.

A pressure gauge at the gas outlet is a very recommendable device for control of the filter performance. Even if the operator is experienced enough to know when the filter has to be changed normally, a defect in a filter bag can result in a clogging of the safety filter and a decrease of engine power.

Events of that kind are indicated by a sudden increase of the pressure drop across the filter units.

The ash removal from the ash container (under the reaction cylinder) should be done daily, together with the cleaning of the grate, before the new start. It is recommendable to let a layer of ash or sand continuously at the bottom of the ash box as a thermal protection of the cement.

The removal of deposits in the two settling chambers should be done once a month (together with the servicing of the filter bags).


FIGURE