Trouble free operation of an internal combustion engine using producer gas as fuel requires a fairly clean gas (see section 2.1.3).
As has been mentioned in sections 2.3 and 2.5 well designed downdraught gasifiers are able to meet the criteria for cleanliness at least over a fairly wide capacity range (i.e. from 20% - 100% of full load). Up draught gasifiers in engine applications have to be fitted with bulky and expensive tar separating equipment. It is however possible to get the gas from up draught gasifiers up to specification as is reported by Leuchs (26). Methods are under development to reform the gas in a high temperature zone (secondary gasification), in order either to burn or crack the tars.
When suitable fuels are used, the gasifier and cleaner are well designed and the gasifier is operated above minimum capacity, tar contamination of the gas does not present a major problem.
Gas cooling mainly serves the purpose of increasing the density of the gas in order to maximize the amount of combustible gas entering the cylinder of the engine at each stroke. A ten percent temperature reduction of the gas increases the maximum output of the engine by almost two percent. Cooling also contributes to gas cleaning and makes it possible to avoid condensation of moisture in the gas after it is mixed with air before the engine intake.
The major problem in producing an engine quality gas is that of dust removal.
The amount of dust that is present in the producer gas at the outlet of the gasifier depends on the design of the equipment, the load of the gasifier and the type of fuel used.
In most gasifiers the direction of the gas stream is already reversed over 180° inside the apparatus, and this simple measure removes the coarsest dust.
The amount of dust present in the gas per m³ generally increases with the gasifier load, for the simple reason that higher loads give rise to higher gas velocities and more dust dragging.
Smaller fuel particles generally cause higher dust concentrations in the gas than do the larger fuel blocks. The type of fuel also has an influence: hardwoods generally generate less dust than softwoods. Maize cob gasification leads to severe dust contamination as reported by Zijp et al. (48).
For normal type "Imbert" downdraught gasifiers, the dust leaks when using wood blocks of about 4 x 4 x 4 cm are reported to vary between 0.5 - 5 g/m³ gas (34).
Investigations of the size and size distribution of generator gas dust were undertaken by Nordström (33) and the results are reproduced in Table 2.8. It is possible to separate about 60% - 70% of this dust from the gas steam-by means of a well designed cyclone.
The rest (dust particles of smaller diameter) has to be removed by other means.
Table 2.8 Size distribution of producer gas dust (33)
|
Particle size of dust m.10-6 |
Percentage in the gas % |
|
over 1000 |
1.7 |
|
1000 - 250 |
24.7 |
|
250 - 102 |
23.7 |
|
102 - 75 |
7.1 |
|
75 - 60 |
8.3 |
|
under 60 |
30.3 |
|
losses |
4.2 |
During the Second World War a multitude of dry filters containing wood wool, sisal fibre, glass wool, wood chips soaked in oil, and other types of fibrous or granular material were used for removal of the fine dust (average particle size below 60 micron), but success was very limited.
Wet purifiers such as water and oil scrubbers and bubblers are also effective but only within certain limits.
The best cleaning effect is obtained by employing cloth filters. However, normal cloth filters are very sensitive to the gas temperature. In the case of wood or agricultural waste gasification, the dew-point of the gas will be around 70 C. Below this temperature water will condense in the filters, causing obstruction of the gas flow and an unacceptable pressure drop over the filter section of the gasification system.
At higher temperatures normal cloth filters are likely to char and decompose in the hot gas stream. Another of their disadvantages is that they are subject to a rapid build-up of dust and so need frequent cleaning if not used in conjunction with a pre-filtering step.
The disadvantages of cloth filters can be partly offset by using woven glasswool filter bags as proposed by Nordstrom (33). This material can be used at temperatures up to 300°C. By heating (insulated) filter housing by means of the hot gas stream coming from the gasifier, temperatures above 100°C can be maintained in the filter, thus avoiding condensation and enhanced pressure drop. If a pre-filtering step consisting of a cyclone and/or an impingement filter is employed. It is possible to keep the service and maintenance intervals within reasonable limits, i.e. cleaning each 100-150 h. This combination is probably the most suitable for small and medium-sized systems (up to 150 kW electric power), and experience has shown that engine wear is no greater than with liquid fuels (33).
Electrostatic filters are also known to have very good particle separating properties, and most probably they could also be used to produce a gas of acceptable quality. However, such filters are expensive, and it is for this reason that their use is foreseen only in larger installations, i.e. equipment producing 500 kW electric power and more.
An excellent presentation of generator gas cooling theory is to be found in (43). Major factors to be taken into consideration are the sensible heat in the gas, the water vapour content of the gas and its heat of condensation and the effects of fouling of the cooler.
Generator gas coolers come in three broad categories: natural convection coolers, forced convection coolers and water coolers.
Natural convection coolers consist of a simple length of pipe. They are simple to use and clean and require no additional energy input. They can be rather bulky, though this problem can be partly offset by using fined pipe in order to increase the conductive surface. Forced convection coolers are equipped with a fan which forces the cooling air to flow around the gas pipes. This type of cooler can be much smaller than the natural convection coolers. Its disadvantages are the extra energy input to the fan and the necessity to use gas cooling pipes of small diameters, which can lead to fouling problems. The former can in some cases be offset by using the cooling air supplied by the engine fan.
Water coolers are available in two types, the scrubber and the heat exchanger; where a water scrubber or bubbler is used, the objective is generally to cool and clean the gas in one and the same operation.
Scrubbers of many different types exist, but the principle is always the same: the gas is brought in direct contact with a fluid medium (generally water) which is sprayed into the gas stream by means of a suitable nozzle device. The advantage of this system is its small size. Disadvantages are the need for fresh water, increased complexity of maintenance, and some power consumption resulting from the use of a water pump.
The cleaning of the cooling water from phenols and other tar components is by all probability also a necessary and cumbersome operation. But so far very little experiences or cost calculations for the waste water treatment are available.
It is also possible to cool the gas by means of a water cooled heat exchanger. This is a suitable method in case a source of fresh water is continuously available and the extra investment and power consumption of a suitable water pump can be justified.
