Experimental Study on the Characteristics of Self-Desulfurization

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Experimental Study on the Characteristics of Self-Desulfurization during Sugarcane Leaf Combustion in a Circulating Fluidized Bed Chunjiang Yu,* Jisong Bai, Hu Nie, Lianming Li, Qinhui Wang, and Zhongyang Luo State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: During biomass combustion, the SO2 emission would still be a problem in some cases, although the average sulfur content is relatively lower than that of coal and other fossil fuels. In this work, the characteristics of self-desulfurization during sugarcane leaf combustion in a circulating fluidized bed were experimentally investigated. First, a laboratory fixed-bed reactor was used to study how the char particles take part in the self-desulfurization process. It was observed that SO2 can be largely captured by char in the temperature range of 700
900 °C. Most of the captured sulfur was incorporated with organic char matrix rather than directly retained by inherent alkali and alkaline-earth matters in the form of inorganic salts. During char combustion, substantial amounts of the captured sulfur could be retained in the ash, which was mainly limited by the alkali and alkali-earth matters available. At higher temperatures (>800 °C), the sulfur retention reduced, mainly because the alkali and alkali-earth matters would give priority to form silicate compounds rather than the occurrence of sulfate reactions. Then, a pilot-scale circulating fluidized-bed experiment was performed to study SO2 emission behaviors during sugarcane leaf combustion. It showed that there was nearly no SO2 emission when the combustion temperature was controlled below 800 °C. The sulfur retention calculation based on ash balance showed that about 87% of fuel S was retained in fly ash. This was mainly attributed to the intense gas
solid contact within the combustor that could largely enhance the sulfur retention reactions.

1. INTRODUCTION As one of the major renewable energy sources, biomass fuels attract an increasing interest throughout the world. Of the primary thermochemical conversion technologies available for biomass, combustion is the most developed and widely applied because of its low costs and high reliability, which accounts for over 97% of the world’s bioenergy production.1,2 Although the average sulfur content of biomass is considerably lower than that of coal and other fossil fuels, with increasingly stringent environmental regulations, SO2 emission would still be a challenge in some cases.3,4 An additional flue gas desulfurization system is the least attractive choice because of its high costs. Some researchers5
7 focus on using additives, such as Ca-based sorbents, to reduce SO2 emissions. For developing low-cost primary SO2 reduction measures, it is important to obtain a deep understanding on the sulfur transformation during the biomass thermal conversion process. There have already been several experimental investigations in this area.8
13 It is generally accepted that sulfur in biomass mainly exists in two forms: organically bound (proteins, sulfate esters, and sulfur lipids) and inorganic sulfate. In the combustion process, the organic sulfur is easy to release to gas phase through devolatilization at low temperatures (800 °C), not only 3889

dx.doi.org/10.1021/ef200781n |Energy Fuels 2011, 25, 3885–3891

Energy & Fuels the SO2 capture capacity of char reduces but also calcium and potassium in the ash are preferably incorporated into silicates instead of sulfates, both of which could lead to the increase of SO2 emission. From Figure 8, it is also observed that the SO2 concentration in the freeboard of test 2 is lower that of test 3. This is probably attributed to the higher secondary air ratio in test 2, which favors the sulfur retention reactions by increasing the residence time and enhancing the gas
solid mixing. Moreover, for test 2, the calculation of sulfur retention based on ash balance was also carried out. Because of the lighter mass property of biomass ash, there was no bottom slag discharging during the whole test and it could be assumed that the retained sulfur was entirely present in the fly ash. The mass flow of pure fly ash was theoretically calculated by the fuel feeding rate and the ash content. Considering the influence of bed material loss and carbon content in the ash, a necessary amendment was made to obtain the actual mass flow of fly ash. Therefore, sulfur retention can be simply calculated by the amount of sulfur that was retained in fly ash and the total sulfur input from fuel. The result showed that about 87% of fuel S was retained in fly ash, which was much higher than the supposed value derived from the fixed-bed experiment using the same fuel in our previous study.31 This further verifies the above conclusion that the self-desulfurization reactions are largely enhanced during CFB combustion.

4. CONCLUSION In the temperature range of 700
900 °C, SO2 could be largely captured by sugarcane leaf char. During the capture process, most of the captured sulfur was incorporated with organic char matrix. Further, XPS analysis showed that the captured sulfur mainly existed as thiophenic sulfur and sulfonic sulfur, and the former accounts for a greater share, which is more temperature-stable. Moreover, at higher temperatures, the SO2 capture capacity of sugarcane leaf char decreases. In the char combustion stage, the captured organic sulfur can be more easily retained in the ash, which is mainly limited by the alkali and alkali-earth matters available. However, when the combustion temperature exceeds 800 °C, the alkali and alkali-earth matters will give priority to form silicate compounds rather than the occurrence of sulfate reactions, leading to the sulfur retention reduction. During the sugarcane leaf combustion in a pilot-scale CFB combustor, there was nearly no SO2 emission when the combustion temperature was controlled below 800 °C. The sulfur retention calculation based on the ash balance showed that about 87% of fuel S was retained in fly ash. It is mainly attributed to the intense gas
solid contact within the combustor that could largely enhance the sulfur retention reactions. Above all, it is concluded that the CFB combustion mode is beneficial to control SOx emission during sugarcane leaf combustion. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: +86-571-87952801. Fax: +86-571-87951616. E-mail: [email protected].

’ ACKNOWLEDGMENT The research on self-desulfurization characteristics during sugarcane leaf combustion in a fluidized bed was financially supported by the National Natural Science Foundation of China (50976102).

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