Tars in the product gas can be tolerated if the gas is to be used as a fuel and closely coupled to the applications, such as boilers and kilns. For these applications, cooling and condensation of the tars can be avoided, and the energy content of the tars adds to the calorific value of the product gas. However, in more demanding applications tars in the raw product gases, even at low concentrations, can create major handling and disposal problems. Different systems for tar removal are shown in Figure 6.11. As soon as the temperature of the producer gas drops below the dew point, tars will either form aerosols or directly condense on the inner surfaces of the equipment, resulting in plugging and fouling of pipes, tubes, and other components downstream from the gasifier.

Aerosols are especially difficult to remove by filtration or scrubbing systems, causing deposits in the cooler parts. At temperatures above about 400°C, tars can also undergo subsequent dehy­dration reactions to form solid char and coke that further tend to plug up systems. The most important consideration is often to maintain the producer gas above the tar dew point (~400°C), thus avoiding condensation in the pipes. Internal combustion engines and methanol synthesis

applications require that the gas be cooled before final use. However, there are many techni­cal and economic reasons, such as thermal efficiency, environmental emissions compliance and tar-effluent treatment costs, to justify catalytic cracking and reforming of tars before cooling.

IC (internal combustion) applications require that particles and tars be lowered to about 30mg/m3N for particulates and 100mg/m3N for tars. Ideally, gas turbine applications require that the hot gas is completely cleaned and remains hot (and under pressure) before use. It is not practical, or thermodynamically efficient, to cool down the gas after gas generation in the gasifier. The range of the particulate concentration limit for gas turbines is 0.1 to 120mg/m3N, depending on the design and the operating conditions. Alkalis are also critical contaminants, and the reduction of these to acceptable levels (usually below 0.1 mg/m3N) remains a great challenge.

Hydrocarbons also possess potential problems for the methanol synthesis processes. To prevent catalyst poisoning (particularly the copper/zinc-based catalysts), total olefin content should be less than 6 mg/m3N and the ethene concentration should be below 4 mg/m3N. The catalysts are also very intolerant to the presence of sulfur and chlorine, as will be discussed.

Two basic approaches may be identified to remove tars from product gas streams:

• The physical methods are utilized for removing condensed tar aerosols, using technologies similar to those used for particulate removal such as wet scrubbers, electrostatic precipitators, or other technologies. These require that the product gas be cooled to ensure the tars are in a condensed form.

• The catalytic and thermal tar reduction methods have been studied to convert the tars to perma­nent gases. The catalytic approaches can potentially destroy tars in either the vaporized or the condensed state. These two approaches are discussed more in below, and special emphasis will be put on the catalytic tar cracking techniques as these possess the most promising techniques for tar removal.

In these techniques for physical tar removal, tars are removed from the gas stream by cooling the product gas, allowing tar condensation into aerosol droplets. Thereafter, the droplets are removed by systems similar to those used for particulate removal. The two most common techniques for this are wet scrubbing and electrostatic precipitation. As a result of the physical tar removal, a condensate contaminated with tars is generated and this condensate has to be treated.

In the wet scrubbers tars are collected by impinging the material on water droplets. Those tar — containing water droplets lead to a decanter where the bulk tars are separated from the aqueous phase. The use of water in these scrubbers needs the gas temperature at the exit to be in the range of 35-60°C (Stevens, 2001). Generally, particulates and other gas impurities, such as acidic or alkaline compounds, are removed simultaneously with tars by these techniques. It is possible that the condensation of tars on particulate surfaces can lead to plugging and fouling of gas conditioning equipment. Nevertheless, the use of wet scrubbers for small applications has proven to be a less reliable method for tar elimination because of their cost.

Tar removal in the electrostatic precipitators is based on the same principles as particulate removal. The collector surfaces of the electrostatic precipitators are washed continuously to remove the tar material. These collectors can operate up to about 150°C, but preferably at lower temperatures to avoid tar vaporization. The electrostatic precipitators are efficient removing tars and particulates from the product gas stream as they can remove up to 99% of particles less than ~0.1 ^m. However, the use of these systems in large-scale biomass gasifiers is rare because of their high operating and investment casts.

In the wet scrubbers and wet electrostatic precipitators, tars are collected as a tar-water mixture. The biomass tars include a wide variety of organic compounds, and most of them are at least to some extent water-soluble. This implies that, the wet waste cannot be clearly separated into organic and aqueous fractions. An incomplete separation of the effluent into two phases can be accomplished by settling though, but the resulting organic product still contains large amounts of water (typically 50%wt or more). The separated aqueous phase also contains lower molecular weight oxygenates including organic acids, aldehydes, and phenols. Plain wastewater cleanup and tar disposal is not feasible due to environmental concerns. Treatments for the aqueous wastewater are commercially available, but will increase costs. The most common techniques employed are: adsorption of dissolved organics by carbon, wet oxidation of wastewaters, and dilution and biological treatment of wastewaters. The operational costs of these wastewater treatments are, in principle, directly proportional to the contamination level of the condensate.