The U. S. pulp and paper industry consumes about 2.6 EJ (2.5 quad) of energy per year, or about 2.9% of total annual U. S. energy consumption (U. S. Dept, of Energy, 1995). Energy consumption per tonne of paper produced is about 36 GJ or 6 BOE, a disproportionately large amount, especially when consid­ered in terms of the energy content of conventional paper products, about 17.6 GJ/dry t. In the mid-1990s, U. S. paper production capacity was about 30% of the world’s total capacity and accounted for 40% of all electric power cogenerated by U. S. manufacturing. Yet the paper industry still spent $5.5 billion on energy in 1991, or about 4.3% of the value of its shipments (U. S. Dept, of Energy, 1995). It is not unexpected, therefore, that the paper industry has made a great effort to become as energy self-sufficient as possible.

Black liquor is a major waste biomass resource and a by-product of the paper industry. In the pulping step of the paper manufacturing process, cellu — Iosic fibers are separated from the debarked, chipped wood. The dominant process used for pulping in the United States is the kraft process, which involves cooking the chips at elevated temperature and pressure with a solution of sodium hydroxide and sodium sulfide. Most of the lignins are dissolved in this process, and the resulting pulp is washed to remove the chemicals before further processing of the pulp into paper. The mixture of dissolved wood components and used pulping chemicals in the extract is called black or spent liquor. It is currently burned in recovery boilers to recover the pulping chemicals and to generate steam. The wood residues serve as boiler fuel and the spent chemicals are the bottoms that are processed for reuse in the pulping step.

In 1993, the U. S. paper industry manufactured about 77.1 million tonnes of paper and paperboard products. The average usable energy yield of black liquor for the industry then corresponds to about 14 GJ/t of paper manufactured (2.6 EJ/[77.1 X 106 t]), or about 2.3 BOE/t. This clearly illustrates the need to recover energy from the black liquor, if only to minimize production costs. Black liquor can almost be considered to be a co-product rather than a waste by-product. If boiler efficiencies were included in this calculation, the need to recover and use the energy in the paper manufacturing process would be even more apparent. However, these figures also suggest that there is consider­

able room for improving the efficiency of the chemical pulping process. The fact remains that approximately one-half of the annual primary energy consumed as biomass in the United States in the 1990s is attributed to the paper industry. Black liquor is available in large quantities and is the mainstay of this consump­tion pattern.