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.
tar compounds condense or dissolve from the gas stream into the heavy gas oil utilized in the oil scrubber. The oil scrubber also removes residual particulate matter missed by the upstream cyclones. Conventional techniques for removing tar and particulate matter use water as the scrubbing liquid, which requires treatment and disposal. Using oil enables further processing of the waste stream for applications such as heat and power generation, in which the tars and particulate matter serve as additional carbon feedstock rather than contamination.
Water is the most common choice of absorption medium selected in many gasification systems. Because of poor solubility of tar in water, hydrophobic absorbents (diesel fuel, biodiesel fuel, vegetable oil, and engine oil) were studied on their absorption efficiency of biomass tar and compared with water. The results showed that only 31.8% of gravimetric tar was removed by the water scrubber, whereas the highest removal of gravimetric tar was obtained by a vegetable oil scrubber with a removal efficiency of 60.4%. When focusing on light PAH tar removal, the absorption efficiency can be ranked in the following order; diesel fuel>vegetable oil>biodiesel fuel>engine oil>water. On the other hand, an increase in gravimetric tar was observed for diesel fuel and biodiesel fuel scrubbers because of their easy evaporation. Therefore, the vegetable oil is recommended as the best absorbent to be used in gasification systems.
Gas produced from 1kg of wood: 2.185 standard cubic metres
Gas produced from 1lb of wood: 35 standard cubic feet