Design and experiment of a heating boiler using straw briquette as fuel

Biomass resources are rich in most rural areas in China, particularly with about 400 million tons of standard coal biomass feedstock available for energy utilization each year. However, a large amount of agricultural waste, such as straw biomass, has not been fully utilized in recent years. The straw can be densified into the briquette for space heating, in order to reduce the emission of greenhouse gases and air pollutants, according to local conditions. However, there are many challenges in the briquetting fuel boiler for the combustion of straw briquettes, including low automation, low efficiency, and high air pollutant emissions. In this study, a straw briquetting boiler was designed with an automatic feeding device using the combustion characteristics of straw briquettes suitable for rural household heating. A double-chain plate transmission mechanism was adopted in the automatic feeder, further to realize the continuous, stable, and small-flow delivery of fuel, according to the size of the briquette. The straw briquette was mainly burned in the gas phase. A dual combustion chamber structure was designed for the zonal combustion of volatile matter and fixed carbon. The combined air supply with air induction was adopted to improve the combustion performance for the full combustion of fuel. The flue gas circulation devices were also added to reduce NOX emissions. A special igniter was adopted to significantly shorten the ignition time for the biomass. The automatic rocking grate system was adopted for the removal of intermittent ash, and the fuel layer was loosened at a fixed time to increase the contact area between fuel and air. A performance test was conducted in the straw briquette fuel boiler. The experimental results show that the automatic feeder continuously delivered the fuel at a stable, small-flow rate for 10 h with a fuel delivery volume of 2.57-4.12 kg/h, fully meeting the heating demands of different users. The rated thermal power of the fuel boiler was 8 kW. The positive balance efficiency and reverse balance efficiency of the fuel boiler were 71.20% and 71.51%, respectively, of which the heat loss was 4.11%, suitable for indoor heating. The power consumption was 3.83 (kW·h)/d. The emission concentrations of dust, CO, and NOx were 35.78 mg/m3, 0.104 %, and 191.1 mg/m3, respectively, under the optimal combustion conditions, which fully met the emission limits of national standards. The test and evaluation of the fuel boiler can be used to verify the boiler structure. The finding can provide a strong reference for the subsequent design in the high-automatic straw briquetting fuel boiler.