Abstract
Abstract: A large amount of straw has been widely returning in China. For example, 89 million tons with a comprehensive utilization rate of 86.79% in 2017. The rest of the unutilized straw can be treated via abandonment or burning in the field, even though it can cause heavy pollution. Alternatively, the crop straws can be utilized as fuels for hot water boilers in cleaning heating. By contrast, there are 400 million tons of standard coal consumed annually for heating in North China. A large amount of coal burning has led to a substantial increase in the emission of pollutants, especially in rural areas in winter. But, there are still so many challenges in the combustion system of straw. In this study, a continuously baled straw combustion boiler was designed to improve the thermal efficiency and emission with the rated thermal power of 1.4 MW suitable for small-scale heating. Some parameters were also determined for the furnace combustion chamber, grate, air distribution, and heat exchange system. A reciprocating grate was designed to increase the gap of the fuel, particularly for the full contact between fuel and oxygen. A state-of-the-art air staging was utilized to design the combustion system for the best distribution of primary and second air volume. The total air excess coefficient was set as 1.4. The reason was that the pollute emission was the largest when the air excess coefficients were around 1.0 and 1.8 in the primary and second combustion chamber, respectively. The heat transfer surfaces were assumed as the radiate and convection heating exchanging system, which were mainly absorbed the heat in the process of fuel combustion, thereby maintaining the furnace in the process at a suitable temperature. The temperature values of the combustion chamber, flue gas at the combustion chamber outlet, flue gas at the chimney outlet were about 1 314, 972,and 147℃, respectively. The heat and emission tests were conducted under the national standard after the boiler was completed. The field tests were carried out in the Tieling County, Liaoning province of China. The fuel was taken as the square bales of corn straw, where the section size was 450 mm×350 mm, the density was 110 kg/m3, and the moisture content was 18%. The results show that excellent performance of combustion boiler was achieved, where the average thermal efficiency was 80.37%, the average emission concentration of particulate matter was 48 mg/m3, the mass concentration of NOx was 197 mg/m3, and the mass concentration of SO2 was 7 mg/m3. The environmental protection and energy efficiency indexes were fully met the design requirements and national standards. Furthermore, the total fuel consumption was calculated in a demonstration project using the whole cycle assessment. Specifically, straw heating was utilized to effectively reduce the use of standard coal by about 657.9 t and green gas emissions (CO2 eq.) by 1 654 t. This finding can provide promising technical guidance to launch the large-scale heating project for the low-emission straw burning boiler.Abstract: A large amount of straw has been widely returning in China. For example, 89 million tons with a comprehensive utilization rate of 86.79% in 2017. The rest of the unutilized straw can be treated via abandonment or burning in the field, even though it can cause heavy pollution. Alternatively, the crop straws can be utilized as fuels for hot water boilers in cleaning heating. By contrast, there are 400 million tons of standard coal consumed annually for heating in North China. A large amount of coal burning has led to a substantial increase in the emission of pollutants, especially in rural areas in winter. But, there are still so many challenges in the combustion system of straw. In this study, a continuously baled straw combustion boiler was designed to improve the thermal efficiency and emission with the rated thermal power of 1.4 MW suitable for small-scale heating. Some parameters were also determined for the furnace combustion chamber, grate, air distribution, and heat exchange system. A reciprocating grate was designed to increase the gap of the fuel, particularly for the full contact between fuel and oxygen. A state-of-the-art air staging was utilized to design the combustion system for the best distribution of primary and second air volume. The total air excess coefficient was set as 1.4. The reason was that the pollute emission was the largest when the air excess coefficients were around 1.0 and 1.8 in the primary and second combustion chamber, respectively. The heat transfer surfaces were assumed as the radiate and convection heating exchanging system, which were mainly absorbed the heat in the process of fuel combustion, thereby maintaining the furnace in the process at a suitable temperature. The temperature values of the combustion chamber, flue gas at the combustion chamber outlet, flue gas at the chimney outlet were about 1 314, 972,and 147℃, respectively. The heat and emission tests were conducted under the national standard after the boiler was completed. The field tests were carried out in the Tieling County, Liaoning province of China. The fuel was taken as the square bales of corn straw, where the section size was 450 mm×350 mm, the density was 110 kg/m3, and the moisture content was 18%. The results show that excellent performance of combustion boiler was achieved, where the average thermal efficiency was 80.37%, the average emission concentration of particulate matter was 48 mg/m3, the mass concentration of NOx was 197 mg/m3, and the mass concentration of SO2 was 7 mg/m3. The environmental protection and energy efficiency indexes were fully met the design requirements and national standards. Furthermore, the total fuel consumption was calculated in a demonstration project using the whole cycle assessment. Specifically, straw heating was utilized to effectively reduce the use of standard coal by about 657.9 t and green gas emissions (CO2 eq.) by 1 654 t. This finding can provide promising technical guidance to launch the large-scale heating project for the low-emission straw burning boiler.
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