Effect of Highly Preheated Combustion Air on Fuel Saving in a Municipal Solid Waste Incinerator

摘要

Incineration is an alternative way to dispose of municipal solid waste. It has more advantages, e.g., reduction in volume and weight of waste, destruction the contamination in waste, recovery of energy, etc., but it's drawback is the higher net treatment cost compared to disposed in landfill. Since the combustion processes in a controlled-air incinerator are divided into two-stages in two combustion chambers, the combustion in primary combustion chamber (PCC) is in a sub-stoichiometric condition which releases the combustible gas to be completely re-burned in the secondary combustion chamber (SCC) by using secondary burner to control the desired combustion temperature but increases the treatment cost of the incinerator due to the supplemental fuel fed to the burner. The concept of preheating the combustion air is benefit for fuel saving, but it substantially increases NO emissions as a result of the higher flame temperature; this limits the preheated temperature of combustion air. The objective of this study is to verify this concept to an incinerator for enhancing of fuel saving. The experiments were done on a 17.5-kW lab-scale controlled-air incinerator in order to find the relationship of fuel saving and preheating temperature of secondary air, temperature distribution and emission gases. Sawdust at the feed rate of 3.5 kg/h was burned in PCC with a feeding of a primary air in sub-stoichiometric condition, an incomplete product of combustion flowed into SCC to be re-burned with secondary combustion air which preheated from room temperature to 300, 500, 700 and 900℃. The chamber profile temperature, the concentration of gas at the exit of the chamber were measured to analyse for fuel saving. The results showed that the higher preheat secondary combustion air temperature results in an increasing average flame temperature, NO_x emission, more uniform heat flux, and reduction in fuel consumption up to 56%. Furthermore, the hot combustible gases from PCC can be auto-ignited without an ignitor and maintained a stable temperature inside the SCC with an orange flame colour.