A multiyear DOE-supported study was completed to identify, determine, and understand fundamental burn characteristics and properties of alternative fuel sources to replace coal for energy generation, with emphasis on impacts in cement processing. Portland cement manufacturing involves the combustion of solid fuels along with raw materials to produce clinker. Temperatures exceeding 1,400°C are needed to carry out reactions for cement chemistry. Energy needs for a typical manufacturing plant are 12,000 MMBTU/day. Potential alternatives to fossil fuels must provide adequate and economical energy but they must also result in products of combustion that have negligible impact on cement quality and emissions.
An operating Portland cement manufacturing plant (Lafarge North America’s Roberta Plant in Calera, Alabama) served as facility for a series of trial burns to replace coal at various substitution rates with bio-based and waste fuels. More than 20 three-day trial burns were conducted from 2007 to 2010 replacing a portion of the coal with scrap tires, waste plastics, woodchips, switch grass, poultry litter, forests trimmings, scrap railroad ties, soy waste products, or waste liquids. Each trial burn contained one or more of the alternative fuels. An elaborate sampling and analysis plan was carried out for each trial burn with close cooperation between the Lafarge plant, the Lafarge testing laboratory, Auburn University, and outside laboratories.
Results will be presented from a comprehensive evaluation of the raw materials, fuels, clinker, cement, emissions, and plant operations from the test burns. The technically viability for each fuel as supplements for coal for direct burn in a manufacturing plant will be discussed. Alternative Solid Fuel (ASF) handling and process operations were established and evaluated for the use of each of the fuels. Replacement ratios (on a BTU basis) were up to 18% for the waste and bio-based fuels and the synergistic burns where two or more of the alternative materials were used together. Impacts on product quality and emissions were negligible and in some cases advantageous.
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