Meitan xuebao (Oct 2023)
Experimental study on preheating combustion characteristics and NOx emission of pulverized coal based on an entrained-flow gasifier
Abstract
Pulverized coal preheating combustion technology has been proven to be a clean and efficient combustion technology. In view of the limited space and load-bearing capacity around the large coal-fired boiler, a novel technology and system with a compact entrained-flow gasifier to preheat pulverized coal is proposed for the first time. The preheating characteristics in the gasifier and the combustion characteristics of the preheated fuel in the down-fire combustor (DFC) are studied on the novel self-built preheating combustion test rig. The migration and transformation of coal nitrogen during the preheating combustion are investigated. The results show that the experiment system can operate continuously and steadily with small fluctuations in pressure and temperature. The temperature gradient in the gasifier is large, and the high temperature zone is located near the burner plane. The maximum temperature can reach 1 115 ℃, while the outlet temperature of the gasifier decreases to 850 ℃. The high temperature coal gas and char produced by the entrained-flow gasifier are provided to the DFC continuously and steadily. The volume fractions (dry basis) of CO, H2 and CH4 in high temperature coal gas are 13.15%, 8.72% and 0.78%, respectively. Compared to the raw coal, the size of the preheated char decreases. The 50% cut particle size of raw coal is 43 μm, while the preheated semi-coke is 24 μm. The specific surface area increases from 4.05 m2/g to 216.44 m2/g after preheating. At the same time, the pore volume of the char particles increases, and the combustion characteristics are improved. During the preheating process, 96.33% of volatile matter and 40.23% of fixed carbon are released into high temperature coal gas. Also, 69.74% of coal nitrogen is transformed in the gas phase, and 47.67% is converted into N2, the rest into NH3 and HCN. Stable combustion could be achieved with preheated fuels in the DFC with no ignition delay and a uniform temperature distribution. Most of the NH3 and HCN and the nitrogen released from char are converted into N2 in the main combustion zone. There is no NO generation in the main combustion zone. The CO and NOx emissions at the outlet of the DFC are 8.17 mg/Nm3 (6% O2) and 143.02 mg/Nm3 (6% O2), respectively. The combustion efficiency is 99.75%. After the pulverized coal is preheated by the new entrained-flow gasifier and burn in the DFC, only 4.69% of coal N is converted into NO.
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