Evaluation of Biomass and Torrefied Coal Co-Firing in Large Utilities Boilers

Abstract

Renewable energy targets and CO2 emissions markets drive the transition to a cleaner and renewable energy production system. In this manner, utilities are looking for cost effective options with a minimum impact on unit performance and reliability. Co-firing biomass, in comparison with other renewable sources, is the main contributor to meeting the world’s renewable energy target. It avoids the destruction of capital, by making coal-fired power plants cleaner without having to replace them. Biomass co-firing provides a relatively low cost means of increasing renewables capacity and an effective way of taking advantage of the high thermal efficiency of large coal fired boilers. The direct displacement of coal when co-firing plus the higher conversion efficiencies generally achieved also contribute to achieving higher CO2 reduction benefits from each co-fired tone of biomass. However, coal–fired power plants are not designed to co-fire large amounts of biomass. This means that not more than 5–10% of biomass can be co-fired. In order to increase this amount, utilities have to make significant investments in dedicated biomass handling and processing equipment. Even when these investments are made, the co-firing percentage is often limited to 20% thermal fraction, because the chemical and physical properties of bio-fuels. Another possibility, to increase biomass fraction in co-firing is torrefied fuel burning. Co-firing torrefied biomass could increase considerably co-firing percentages, while saving investment and transport cost compared to biomass co-firing. However, it should be concerned regarding the ability of generators involved in coal and biomass co-firing that this alternative may impact on boiler reliability due to specific biomass properties and it this issue should be carefully evaluated during design stage. In order to prevent such an undesirable effect we initiated a study to understand the influence of using co-firing on the capacity, limitations of furnace size, heat transfer surfaces, firing systems, pulverizers, fans, airheaters and equipment for post combustion emission treatment. This paper discusses the technical and commercial application of coal and biomass/ torrefied coal co-firing in large utility boilers. In the present study we used a series of simulation using computer codes; the latter are CFD codes suitable for simulation of the performance and emissions of co-fired utility boilers and an expert system that aided in issues like boiler and furnace performance, pulverizing capabilities, post combustion treatment equipment performance, sootblowing optimization, boiler Fans operation and performance.