microbial metabolism. Water vapor is also released, particularly if the lignin coproduct is dried prior to being shipped from the plant for use as boiler fuel at an off-site power-generation facility. The sulfur and nitrogen content of the fermentation residues is typically low, unless chemicals are used in the pretreatment in the biomass materials. Such chemicals, however, can be recovered.

Thermochemical Conversion

Coal-to-liquid fuel plants can be configured to minimize their impacts on the environment, given that the clean-coal technologies that have been developed for the electric power industry can also be used in coal-to-liquid applications. Coal-to-liquid fuel plants need to produce clean synthesis gas from coal using gasification and gas cleaning technologies. As a result, emissions of criteria pollutants and toxics such as sulfur oxides, nitrogen oxides, particulates, and mercury will be low.

The sulfur compounds in the coal are converted into elemental sulfur, which can be sold as a byproduct. The ammonia in the synthesis gas can either be recovered and sold as a fertilizer or sent to wastewater treatment, where it is absorbed by bacteria. All of the mercury, arsenic, and other heavy metals in the syngas are adsorbed on activated charcoal. The coal’s mineral matter (ash), which is exposed to extremely high temperatures during gasification, becomes vitrified into slag. This slag is nonleachable and finds use in cement or concrete. Nitrogenoxide emissions from existing conversion technologies are only about 3 parts per million.

Water usage in thermochemical conversion plants depends primarily on the water-use philosophy implicit in the plant design. For the conversion of coal and combined coal and biomass to transportation fuels with all water streams recycled or reused, the major consumptive use of water would generally be for cooling, hydrogen, and solids handling. If water availability were not limited—say, because of ready access to rivers—conventional forced or natural draft cooling towers would be used. In arid areas, where water is indeed limited, air-cooling would be used to the maximum degree possible. Depending on the degree of air-cooling, water consumption could range from about 1 to 8 barrels of water per barrel of product. For coal-to-liquid fuel plants, additional environmental impacts will be associated with the mining of coal, as discussed in the reports Evolutionary and Revolutionary Technologies for Mining (NRC, 2002) and Coal Research and Development to Support National Energy Policy (NRC, 2007).

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