5.3.1 Operating record and observations on the performance
5.3.2 Disturbances of the operation
5.3.3 Desirable modifications
The biomass gasification plant was in operation intermittently for about forty hours in November 1982 on several fuels at Peradeniya University.
Operational data are presented in Table 5.1
Some tendency to slagging was observed when operating on coconut husks, rubber tree wood and coir dust briquettes. Quite possibly this was due to a relatively large amount of sand that was introduced into the gasifier together with the fuel. Regular grid rotation prevented excessive pressure drop across reactor and bunker, and bunker flow could be maintained at the de-sired levels.
At the end of November 1982 the unit was transferred to Giriulla Mills where it has since been operating, fuelled by coconut shells.
Operational data for this period are also summarized in Table 5.1. The system is producing power for six desiccating motors at a load of up to 30 kVA.
The system can follow a step load change of 20 kVA.
Collected condensate appears to be a clear liquid, indicating that only traces of tarry components are present in the producer gas.
The overall system efficiency was between 15 and 20 percent, as a ratio of output electrical energy to input biomass energy.
System efficiency is of course dependent on fuel characteristics, load characteristics and environmental conditions. The highest efficiency was obtained when operating on coir dust briquettes; the slagging tendency of this fuel has to be further tested in an extended run, in order to decide whether automatic and continuous grid rotation is necessary.
At Giriulla Mills the system was continuously operated for about 1000 hours.
From Table 5.1 it is clear that on average the installation was operating at 50 percent of its full capacity. From the more detailed data it may be concluded that operation periods at loads of around 33 percent of full capacity are frequent.
On 19 January 1983 the engine stopped and thereafter it was not possible to operate the starter motor. The motor was replaced and the engine then started easily.
It is not clear whether this problem stemmed from a defective engine ring gear (which may have been faulty from the beginning) or whether the problem has to do with the use of producer gas, which often calls for somewhat prolonged use of the engine's starter motor.
The gasifier operated normally during January and February 1983. However, on several occasions the gasifier outlet temperature rose above 475°C, a phenomena which was not accompanied by higher pressure drops over the gasifier and the filter section. Rotating the ash grate did not result in a reduction of the outlet temperature. The engine was stopped for one to two hours on these occasions.
Later (see below), it was possible to attribute the fault to air leakage into the gasifier.
On 4 February, it was noticed that the frequency of refilling with shells had increased from the normal 15 minutes to 5-10 minutes. Since the weather had been exceptionally hot and dry from late January onwards, the shells were wetted before loading into the gasifier. Thereafter the frequency of filling returned to the original 15 min.
On 11 February, the engine started misfiring (speed equivalent to 47-50 Hz). Upon replacement of the spark plugs with new ones the misfiring of the engine stopped.
On 21 February the temperature of the gasifier outlet rose to 500 C. It was noted that tar was leaking out of the gasifier safety hatch door. Because it wars suspected that air was being sucked in through this safety hatch, thus causing the temperature rise, the door was tightly closed. This brought down the gasifier outlet temperature to the normal value of 430°C.
It was planned to replace the spring-loaded safety hatch door by a metal explosion disc, as in the bag house filter.
On 25 February it was noticed that the inside of the inspection hatch (at the gasifier hearth zone) had deteriorated. Therefore an air leak was suspected through this door. A repair was carried out with mild steel plates as there was a danger of the refractory lining cracking and falling down. The sealing on the door of the inspection hatch was cleaned. Inspection a week later showed that the mild steel plates were holding out.
On 1 March it was noticed that the refractory lining had flaked off at the top and bottom edges. This may have been caused by the frequent cooling/heating cycles. No measures were taken and the situation seemed to remain stable.
Inspection of the ash pit on 23 March showed a semisolid slag as if the ash had burned in the ash pit. Air must therefore have been leaking into the gasifier ash-space either through the ash outlet or through the grate seal. The grate seal was suspected as it could not be tightened. The Giriulla mills-engineer fitted an additional sleeve after which there were no further problems.
In February the average engine inlet temperature rose to 50°C, as compared to 42°C in January. Probably this was due to the high ambient temperature in February (on 11 February it was 37° C in the shade and 39°C inside the gasifier shed).
In May 1983 the engine started misfiring again. A change of spark-plugs had no effect. Upon dismantling of the engine it was found that the pistons were covered with carbon 1 mm thick. Also it was noted that the engine inlet manifold was covered with a 2 mm thick semi-solid layer of either tar or dust/water slurry. The engine's valves appeared to be clean. The misfiring of the engine was attributed to pre-ignition caused by carbon build-up in-the cylinders.
It is not clear whether the tar production results from the rather low loads at which the engine is now and then operated or stems from an incorrect installation of-the glass fibre cloth filter.
The first months of operation revealed the following shortcomings in design and lay-out:
1. The accessibility of the impingement separator is poor and a vacuum cleaner is required to remove the dust.
2. The bag house filter maintenance is difficult. Removal of the top lid requires removal of a flange and the lid is too heavy requiring either three people or a mechanical device to raise it.
3. The grate shaker is not convenient to operate. A lever system which could be operated while standing up would be an improvement.
4. Condensate from the cooling tubes collected in the piping close to the engine inlet. It was necessary to fix and additional drain tank to this pipe.
5. Refractory lining flakes off, probably as a result of the frequent heating/cooling cycle. This could be avoided by supporting the base of the lining with a metal flange.
6. Hot radiator cooling air from the engine is partially directed towards the cooler of the gasifier. This diminishes the effectiveness of the gas cooler.
7. The fuel hopper is too small for coconut shells. It is inconvenient to have to refill the fuel bunker every 15 minutes.
8. The engine's maximum power output is too low. Although the system is designed to deliver 40 kVA, it is possible to get 35 kVA only. This is probably due to the unorthodox system of engine speed control, which controls the gas and the air input to the engine simultaneously, instead of using the normal system where only a gas/air mixture valve is controlled.
Also from general principles it seems possible that the air/gas mixing in the engine's simple gas/air carburettor is not complete, as a result of which some cylinders may get too rich and others too poor mixture.