Cleaning dust from the gas
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 Nordstrom (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 т. Ю"6 |
Percentage in the gas % |
|
over iOOu |
i./ |
|
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.