Discussion on Stable Combustion in Grate-Type Waste Incinerators
In recent years, China's municipal solid waste incineration power generation technology has rapidly developed, forming a waste-to-energy industry. Within this industry, the primary focus lies on controlling secondary pollution and the efficient utilization of thermal energy. Stable combustion of waste within the incinerator is key to addressing these issues. Currently, most MSW in China is collected without sorting, leading to highly variable waste composition, unstable and time-varying incineration processes. Achieving stable combustion purely through real-time automatic control technology is difficult. This paper takes the Martin grate-type waste incinerator as the research subject, explores factors affecting stable combustion from the perspectives of waste quality and incineration operation, proposes countermeasures, and aims to provide reference for the stable operation of grate-type incinerators.
1 Improving Waste Quality
1.1 Waste Quality and Stable Combustion
Waste treatment via incineration reduces harmful substances through high-temperature oxidation, simultaneously emitting flue gas and solid residues. For waste-to-energy processes, high-quality waste primarily refers to waste with high calorific value, stable combustibility, and high degrees of harmlessness and volume reduction. Stable combustion and economical utilization of thermal energy can only be achieved when the average calorific value of waste exceeds 7000 kJ/kg. Improving the waste’s calorific value is key to enhancing its quality, with composition and moisture content being the main influencing factors.
The dry-basis higher heating values (HHV) of common substances in MSW decrease in the following order: plastics, rubber, wood/bamboo, textiles, paper, ash/soil/ceramics, kitchen waste, metals, glass. Waste high in kitchen waste but low in plastics like these inevitably has a low calorific value. As the moisture content of waste increases, its effective calorific value decreases rapidly and proportionally.
1.2 Main Measures to Improve Waste Quality
(1)Separate Collection and Transportation of Biomass Waste.Biomass waste mainly includes kitchen waste from rural and urban residents, containing high levels of degradable biological components. Therefore, setting up dedicated biomass waste bins in communities and deploying specialized biomass waste collection vehicles can effectively increase the waste’s calorific value and reduce the generation of harmful substances.
(2)Physical Dehydration Method.Establishing a centralized compression and dehydration treatment model for MSW in urban residential areas not only increases the waste’s calorific value but also solves problems of leakage during transportation and corrosion of transport equipment.
(3)Biological Stabilization Technology.This technology involves forced aeration before waste incineration, controlling parameters such as oxygen content and temperature. Easily degradable organic matter ferments, generating heat that causes rapid evaporation of moisture in the waste. This significantly reduces moisture content and substantially increases waste calorific value. Currently, utilizing existing waste storage pits in waste-to-energy plants and controlling fermentation time according to seasons is an economical and effective method to increase the calorific value of waste fed into the furnace. For instance, in colder regions during winter, boiler blow down water or steam trap condensate can be used as a heat source to directly heat the waste, facilitating fermentation and the discharge of free water, thereby effectively increasing calorific value.
2 Controlling Fuel Bed Thickness and Primary Air Volume Based on Waste Fermentation Degree
According to current processes in waste-to-energy plants, waste is typically stored and fermented for 3-7 days after delivery before being fed into the incinerator. Due to the uncertain composition and collection volume of municipal solid waste, and the impact of seasons and weather on fermentation effectiveness, the calorific value of waste entering the furnace is unpredictable. This necessitates close coordination between the crane operator and boiler operators. Based on the degree of waste fermentation, they must reasonably control the fuel bed thickness and primary air volume to ensure stable combustion in the incinerator.
For normally fermented waste, primary air temperature, grate fuel bed thickness, grate movement speed, and primary air volume are typically controlled according to design values. If the waste has a high calorific value, the fuel bed thickness should be appropriately thinner, and the air volume appropriately reduced to avoid excessive furnace temperatures causing slagging on heating surfaces. Conversely, if calorific value is low, the fuel bed thickness should be appropriately increased, and the primary air volume increased to ensure stable furnace temperatures.
Waste with sufficient fermentation time and high effective calorific value burns easily. Its burnout time per unit mass is short, making it prone to flame detachment. In this case, the fuel bed thickness must be increased, and the primary air volume controlled to just penetrate the bed suitably. This slows down combustion, controls furnace temperature, and prevents flame detachment. Simultaneously, feeding time and grate movement should be accelerated, with particular attention paid to observing changes in furnace temperature. When the waste contains a significant amount of high-calorific-value waste like rubber, secondary air or blending in poorly fermented low-calorific-value waste can be used to assist in controlling furnace temperature. Additionally, the fire bed can be controlled to be shorter, reducing the amount of burning waste and heat release.
Poorly fermented waste has low effective calorific value, making stable combustion difficult. Matching fuel bed thickness and primary air volume is crucial: Insufficient fermentation time makes drying difficult. To dry new waste quickly, a large air volume is necessary. However, under high air volume, large amounts of waste dry almost simultaneously and burn out rapidly in a short time. Before the newly fed waste can dry adequately, flame detachment occurs. Conversely, low primary air volume prevents normal drying and combustion, leading to unstable furnace temperatures.
3 Conclusion
The combustion conditions in Martin grate-type incinerators are highly susceptible to fluctuations in waste quality. Since most MSW in China is mixed-collected, its calorific value is low and highly variable. Adopting separate biomass waste collection/transportation, physical dehydration, and biological stabilization techniques to improve waste quality;prioritizing prevention,implementing strict production management,flexibly applying DCS (Distributed Control System),and strengthening incineration operational skills are effective measures to achieve stable combustion and economical operation of incinerators.