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J O U R N A L A R T I C L E S B Y F L U E N T S O F T WA R E U S E R S JA147

CFD Guides Design of Biomass Boiler Retrofit to Increase Capacity by 25% and Decrease Ash Carryover by 60%
Computational fluid dynamics (CFD) analysis supported the design optimization of a wood-fired furnace that was tested to operate with a 25% capacity increase with a 60% decrease in particulate emissions. The CFDmodeling studies illustrated the potential operational improvements and increased the confidence levels of a successful set of boiler modifications. The combined use of CFD modeling with traditional boiler design methods were particularly effective for assessing the retrofit potential of an older stoker furnace. Proven retrofit design experience and a detailed baseline testing program thatexamined all aspects of the boiler equipment were key attributes to the award of a contract to ALSTOM Power Inc. (ALSTOM) for the modification of a 1975 vintage travelling grate stoker boiler located at a paper mill in Louisiana. When considering investing in upgrading an older boiler, the condition of the different components in the boiler system, combined with the fuel and mode of operation made itdifficult to accurately quantify the performance to be gained by a retrofit. ALSTOM solved this problem by using CFD analysis to simulate the performance of the existing boiler, and then modeled the performance of

Figure 1: Arrangement of the Stoker (left) and computational mesh (right)

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the new system. The results helped identifylimitations of the baseline system and how the modifications would improve the performance. “With CFD, ALSTOM was able to illustrate to the mill that the new design represented a significant improvement and was a wise investment. When both the customer and the supplier team up to gather all the CFD model inputs, everyone benefits. Things that people assumed might be happening suddenly become moretangible. The goal was to satisfy the customer and help them to be able to operate their equipment more efficiently,” noted Paul Chapman of ALSTOM's Power Plant Laboratories. ALSTOM's parent is a global specialist in energy and transport infrastructure. It serves the energy market through its activities in the field of power generation, and the transport market through its activities in rail and marine.In fiscal year 2002/2003, it had annual sales in excess of 20 billion Euros and employed approximately 100,000 people in more than 70 countries. It has supplied almost 20 percent of the world's total installed capacity in power generation equipment.

Upgrading wood and biomass-fueled boilers
There are hundreds of wood and wood waste boilers in operation in the US that serve the important role ofproviding process steam by burning low-grade wood wastes associated with wood and paper processing. These boilers are required to burn a changing mix of wood, wood wastes, hog fuel, and other various solid and gaseous waste streams. With the rising cost of energy, and the desire to reduce gaseous and solid emissions, previously overlooked power boilers are receiving more attention. Many of theseolder units are now potential candidates for revitalization. Modernization can provide many benefits, including increased steam capacity, higher boiler efficiency, lower gas and solids emissions, and decreased operating and maintenance costs. While these objectives are clear, the task of quantifying a significant renovation that often involves modification of pressure components boils down toeconomic justification. Given the range of different style boilers and the unique conditions at each site, prediction of performance and calculation of investment payback remains difficult. “In the initial phases, an engineering assessment is made to develop a proposal,” according to Dave Gadai, director of pulp and paper engineering for ALSTOM's Windsor, CT. operations. “While the upgrade usually...