lindstromIntroduction_Draft11-06

lindstromIntroduction_Draft11-06 - 1 INTRODUCTION The kraft...

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1. INTRODUCTION The kraft or sulfate process is the dominant chemical pulping technology employed in the paper industry today. The competitive advantage that has lea d to its position is the capability to convert most wood species to high strength pulp combined with an efficient chemical recovery based around the Tomlinson recovery boiler. There are approximately 280 Tomlinson recovery boilers in operation in North America, according to the Black Liquor Recovery Boiler Advisory Committee of the American Forest and Paper Association (AF&PA). Many of these boilers are nearing the end of their useful life, and will either need to be upgraded or replaced in the near future. Black liquor gasification (BLG) is one technology that could be implemented into the kraft process to replace a mature technology, while creating new opportunities in process operations and the potential for enhanced competitiveness. The underlying fundamental for implementation of any new technology is the impact it will have on the overall process economics. Some deciding factors that will influence the implementation of BLG involve the cost-benefits associated with power generation and other high-value products that can be derived from the syngas. Another area of importance is the potential cost-savings that can be realized through process modifications and optimization. The effect on wood, chemical and fuel demand from changes in the pulping process can have a significant effect impact on the variable operating cost, capital investment and maintenance costs. Therefore, research exploring the impact of BLG on pulping technologies will be of great importance for the eventual implementation of this technology. The following effort has explored s the potential benefits realizable through the implementation of BLG integrated with the modified pulping technologies that it enables. Presently, in a typical chemical pulp mill the black liquor is concentrated to greater than 65% dissolved solids and burned in a recovery boiler. The pulping chemicals are recovered in the smelt and the heat energy is converted to steam, which is used in a steam turbine generator to produce electricity. The typical thermal efficiency of a recovery boiler is generally 65-70%, and the thermal efficiency of the Rankine cycle for the conversion of steam to electricity varies from 30-38%, depending on the temperature and pressures of the different streams in the cycle. These values result in an overall system thermal efficiency of about 23% ( 1 ) ( i ). On the other hand, if the black liquor is gasified, the syngas can, after cleanup, be combusted in a combined cycle for production of electricity. Combined cycle power generation entails the sequential utilization of a gas turbine followed by a steam turbine. The fuel gas is first burned in a gas turbine to produce electricity. The hot exhaust gas from the turbine is then passed through a heat exchanger to produce steam which is then used in a power-producing steam turbine.
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