The fouling of heat exchanger surfaces can be a major problem with solid biomass fuels, especially straws and herbaceous residues. Fouling occurs be­cause of formation in the conversion zone of low-fusion point alkali metal salt eutectics such as the alkali metal silicates. If the temperature is above the fusion point of the salts, particulates form in the combustion gases that can stick to heat exchanger surfaces when the gases leave the zone. The problem can be severe in biomass combustion systems, but is usually not severe in biomass gasifiers (Chapter 9). Furnace-boiler systems for solid biomass fuels are often designed to keep the temperature in the combustors below about 900°C to reduce slagging and formation of molten agglomerates. Careful design of the internals is necessary to avoid contact of the hot gases that may contain low-fusion point particulates with higher temperature surfaces.

Another method of eliminating this problem with solid biomass fuels when the combustors and gasifiers are operated above slagging temperatures is to remove the ash from the bottom of the units as molten slag. This technique is well established with coal fuels, which often have higher ash contents than biomass, and seems to be quite effective. It is important to note that some biomass, although high in mineral matter, may be low in alkali metals. Fouling by sticky particulates should therefore be far less with this type of biomass. An obvious approach to the reduction of alkali metal fouling is to remove the alkali metals from the fuel before conversion. Extraction of the water-soluble salts has been evaluated, but unless it is effective and low in cost, it adds unnecessary complexity and expense to the process.

A slagging index developed by the coal industry has been used to rate solid fuels for fouling. This index corresponds to the mass of alkali metal as oxides (K20 + Na20) per energy unit in the fuel and is useful for rating biomass feedstocks too. The calculation is made by

0.1[(% ash)(% alkali in ash)](MJ/dry kg)"1 = kg alkali/GJ

An index range of 0 to 0.17 kg/GJ (0 to 0.4 lb/MBtu) is a low slagging risk; 0.17 to 0.34 kg/GJ (0.4 to 0.8 lb/MBtu) indicates the material will probably slag; and an index greater than 0.34 kg/GJ indicates virtual certainty of slagging (Miles et al, 1993). When applied to hybrid poplar, pine, and switchgrass by use of the data in Tables 3.3 and 3.4, the corresponding indexes are 0.11, 0.009, and 0.85 kg alkali/GJ, respectively.

Another fouling mechanism that can occur is corrosion of boiler tubing and erosion of refractories due to formation of acids and their buildup in the combustion units from conversion of sulfur and chlorine present in the fuel. Fortunately, the amounts of these elements in most biomass are nil to small. The addition of small amounts of limestone to the media in fluidized-bed units or the blending of limestone with the fuel in the case of moving-bed systems are effective methods of eliminating this problem. Other sorbents such as dolomite, kaolin, and custom blends of aluminum and magnesium compounds are also effective (Coe, 1993).