Study on the Effect of Waste Bed Thickness on Automatic Combustion of Waste Incinerators
Abstract:Waste-to-energy incineration, as the most effective method for achieving the “harmless, reduced, and resource-utilized” treatment of waste, has become the primary approach for waste disposal. In waste-to-energy plants, automatic combustion control is generally adopted. However, due to the complex composition of waste, the stability of automatic control is reduced. This paper investigates the methods for measuring waste bed thickness and the strategies for adjusting bed thickness, aiming to maintain ideal stability of automatic control in waste incinerators when handling waste with varying calorific values.
Introduction
At present, there are three main waste treatment and disposal methods commonly used domestically and internationally: sanitary landfill, composting, and waste-to-energy incineration. With the increasing amount of municipal solid waste, traditional methods such as landfill and composting can no longer keep pace with waste generation, leading to “waste siege” situations in many cities. Moreover, the drawbacks of traditional waste treatment have become evident: sanitary landfill requires a large amount of land and takes 10–30 years for complete degradation, which can easily pollute groundwater and soil; composting cannot decompose inorganic substances in waste, and over time generates large amounts of foul odors while occupying significant space. By contrast, waste-to-energy incineration has gradually become the main method for municipal solid waste disposal, with the following advantages over traditional waste treatment modes:
1.More thorough harmless treatment:Through high-temperature incineration at 850–1200 °C, harmful components in waste are fully decomposed, bacteria and viruses are completely destroyed, and various malodorous gases are decomposed at high temperature.
2.Significant waste reduction:Municipal solid waste contains a large amount of combustible material; incineration can reduce its volume by about 90% and its weight by 75%–80%.
3.Resource recovery and utilization:The heat generated by waste incineration can be recovered for heating or power generation. The resulting ash can be used as raw material for cement production or brick-making.
4.Lower environmental impact:Modern waste incineration technology further enhances the treatment of harmful gases produced during combustion. By reasonably selecting advanced incineration equipment, organizing combustion conditions appropriately, and using proper flue gas purification processes, harmful gas emissions can be greatly reduced. Leachate can be sprayed into the furnace for high-temperature decomposition, avoiding groundwater pollution.
Compared with manual stoking, waste incinerators with automatic combustion control can reduce the labor intensity of operators, stabilize the oxygen content at the furnace outlet, and prevent large system fluctuations. Stable combustion conditions in the furnace help extend the boiler’s service life, reduce grate wear, boiler corrosion, and tube burst incidents, and bring considerable economic benefits. Therefore, research on combustion control in waste incinerators has high practical significance.
Currently, most domestic waste incinerators are equipped with matching ACC (Automatic Combustion Control) systems, but ACC control strategies vary among manufacturers, resulting in differences in automatic control performance. Automatic control of waste incinerators generally includes two aspects: automatic air supply and automatic feeding.
Automatic air supply is usually achieved by adjusting dampers to increase or decrease the air volume in the combustion zone based on the deviation between the actual main steam flow and the set main steam flow. Feeding adjustment is mainly based on giving the feeding system a basic speed according to the set main steam flow, then comparing the actual bed thickness with the set bed thickness, and making speed corrections to the feeding system accordingly.
Therefore, the accuracy of waste bed thickness measurement is particularly important. The measurement result directly reflects the waste distribution on the grate at that moment, and this data plays a key regulatory role in automatic combustion control. Thus, accurate bed thickness monitoring is crucial for the long-term stable automatic operation of waste incinerators.
1. Methods for Measuring Bed Thickness
In waste bed thickness measurement, a common approach is to calculate bed thickness by measuring the pressure difference between the inside of the furnace and beneath the grate. However, this method cannot eliminate the influence of changes in under-grate air volume on the pressure difference.
Instead, the thickness can be determined by calculating the ratio of the pressure difference between the inside of the furnace and the hopper below the grate to the airflow through the hopper. The resulting percentage value, multiplied by a corresponding coefficient, gives the waste bed thickness. In practical applications, the pressure in the main air duct can be kept constant, while the under-grate airflow is controlled by damper openings and fan frequency adjustments.
This method of displaying thickness as a percentage effectively reflects the real situation inside the incinerator. Compared with other thickness-measuring devices, it is more conducive to combustion control, because it eliminates the influence of air volume on pressure difference while using airflow to represent thickness, which better corresponds to the actual combustion state of waste.
2.Setting and Adjusting Bed Thickness
Once an accurate bed thickness value — that is, the ventilation performance of the grate — is obtained, it can be used to correct the speed of the feeder, the drying-zone grate, and the combustion-zone grate. When permeability increases, indicating a thinner bed on the grate, the feeder and grate speeds can be increased to replenish fuel promptly, preventing deterioration of boiler operating conditions. When permeability decreases, indicating a thicker bed, the feeder and grate speeds can be reduced to decrease fuel input, preventing overfeeding that could result in poor combustion and the presence of unburned waste.
By adjusting different set values for bed thickness, the system can adapt to waste with different calorific values, thus achieving more stable automatic combustion. In China, waste calorific value is strongly affected by seasonal changes — higher in summer and lower in winter — making bed thickness adjustment necessary. In summer, when calorific value is high, bed thickness should be reduced to avoid incomplete combustion in the combustion zone, which could lead to a large amount of unburned waste in the burnout zone and shorten grate life. Excessive bed thickness at high calorific values may also cause boiler overload and affect the continuous service life of the incinerator. In winter, when calorific value is low, bed thickness should be increased to ensure sufficient waste in the furnace, increasing heat storage and allowing the incinerator to maintain stable furnace temperature and steam output even when waste calorific value fluctuates.
Conclusion
Accurate measurement and adjustment of waste bed thickness are crucial for the automatic operation of waste incinerators. Accurate thickness measurement reflects the actual condition of waste inside the furnace and lays the foundation for stable combustion. Although boiler combustion is affected by many factors, timely adjustment of bed thickness settings according to seasonal calorific value changes will positively contribute to combustion stability, reduce load fluctuations, and improve automatic control stability.
Source:https://mp.weixin.qq.com/s/0XzQ_ZMh71mewovwmFob6g