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Show The U.S. pulp and paper industry segment includes 547 mills in 42 states, produces 82 million tons of paper and paperboard and 10 million tons of market pulp. The industry has long been a leader in recovery and recycling (recycling, composting, and energy recovery). About 40% of all the paper used in the U.S. is currently being recovered for recycling, and the industry intends to increase this rate to 500/0 by the year 2000 (AFPA 1994). About 73% of all mills use recovered paper as raw material for papennaking, and more than 370/0 rely upon it entirely. The industry faces strong competitive challenges from abroad. More demanding environmental requirements are also a major burden for the future. In order to comply with new regulatory initiatives, the industry is facing increases in capital expenditures, operating costs, and energy use. The pulp and paper industry is the nation's most capital intensive manufacturing industry (on the basis of investment dollar per employee) and is very energy intensive. The resulting economic consequence of the need for major capital equipment replacement tends to limit experimentation, development, and application of large, new core teclmologies. The industry is currently 57% energy self-sufficient through the use of byproduct black liquor, wood, and wood waste fuels . The industry expects that more energy-efficient processes wiIl evolve and that a larger amount of energy will be self-generated, with excess energy marketed as electricity and (perhaps) liquid fuels . The pulp and paper industry uses about three quads of energy per year, or about 31 million Btu/ton of product. Because it relies heavily on byproduct black liquor, bark, and wood wastes to meet its energy needs, it uses two-thirds of all renewable fuels conswned by U.S. industry. It also accounts for 400/0 of all power cogenerated by U.S. manufacturing. Still, it spends $5 .5 biIlion on energy - 4.3% of the value of its shipments. Even with the extensive use of energy byproducts and renewable energy resources, the industry is still the fourth largest user of fossil fuels in the industrial sector. Based on the work conducted in generating the Vision 2020 docwnent, the high-priority research efforts identified by the industry include sustainable forest management, environmental performance, energy perfonnance, improved capital effectiveness, recycling, and sensors and controls. In the area of energy perfonnance, the industry believes that biomass and black liquor gasification will play an increasingly important role as components of advanced cogeneration teclmologies. These teclmologies will be incorporated to optimize the generation of electricity and process heat. Compliance with air regulations will necessitate effective combustion control and accurate continuous monitoring. 4.7.1 Combustion-Related Problems Combustion is the primary means for producing needed process steam and electricity from various fuel resources available to the industry. Besides the traditional fossil fuels (coal, fuel oil, and natural gas), wood, wood waste, bark, byproduct black liquor, sludge, and waste paper are also used as fuels in boilers to generate needed process steam and (by means of a steam turbine) electricity. Combustion is also used for lime calcination and direct heating of air in paper coating dryers. As part of the process for regenerating needed pulping chemicals, limestone is calcined in rotary kilns or fluidized bed calciners. Although the calcination process is combustion related, it is not considered a significant problem area. The chemical recovery boiler is a steam generator that uses byproduct black liquor as its fuel. Combustion of black liquor provides two key benefits. Steam is generated by the combustion of the fuel source, and inorganic chemicals are recovered for reuse in the pulping process. In a modern kraft mill, approximately 95-97% chemical recovery is achieved. Because the byproduct black liquor fuel contains a large quantity of water (about 300/0 by weight) and because endothennic chemical reactions occur in the boiler, these units typically operate at about 66 to 75% efficiency. Black liquor combustion is different from combustion of other low-energy streams. The black liquor is a low value Btu fuel (5,400-6,600 Btu/lb) and is sprayed, rather than atomized, into the combustion zone, forming very coarse droplets (1-5 mm in diameter). The black liquor must be concentrated to above 600/0 solids so that it will burn without supplemental fuel. Typical solids concentrations of black liquor are about 70%, though some new boilers have operated at as high as 800/0 solids. The black liquor solids consists of about two-thirds organic material (dissolved from the wood during pulping) and one-third inorganic pulping chemicals. Once concentrated, the material is sprayed into the combustion chamber, partiaIly combusting in flight, with particles either impacting the furnace walls, or falling to a char bed at the bottom of the wlit. Air inj ection at several stages achieves multilevel stoichiometric control. A reducing atmosphere is provided in the lower furnace to maximize the endothennic reduction of sulfur compounds to the form needed for pulping chemical regeneration. This is follo\\ed by secondary air injection at a higher level, and then a tertiary air rich zone to complete combustion. 21 |