Main types and characteristics of modern waste incinerators

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Waste incineration technology has undergone nearly 130 years of development; such technology and equipment have become increasingly sophisticated and are now widely used. The commonly used waste incineration systems in developed Western countries currently fall into the following categories: (1) Layer combustion incineration systems, such as those using rotary grates, horizontal reciprocating feed grates, and inclined reciprocating grates (including both forward- and reverse-acting inclined reciprocating grates). The main feature of the stratified combustion method is that waste does not require strict pretreatment. Rolling grates and reciprocating grates have strong stoking capabilities, making them suitable for the incineration of municipal waste with low calorific value and high ash content ； (2) Fluidized bed combustion system, which is characterized by the suspended combustion of waste, with thorough contact between air and waste, resulting in good combustion efficiency. However, fluidized bed combustion requires fuel with a relatively uniform particle size, as well as a consistent fuel feed rate; therefore, it is generally difficult to burn large pieces of waste. As a result, fluidized bed combustion systems have strict requirements for waste pre-treatment, which limits their use in the incineration of industrial waste and municipal waste ； (3) Rotary drum incinerators, which are characterized by feeding waste into a continuously and slowly rotating drum for burning until it is completely burned out; this allows for good contact between the waste and air, as well as uniform and thorough combustion. In the West, such incinerators are often used for the treatment of toxic and hazardous industrial waste. 　　In today’s highly industrialized era, urban waste incineration technology is faced with many new situations and problems: 1. In economically developed regions, urban waste has a low bulk density, high calorific value, as well as lower levels of ash and moisture ； 2. The emission standards for waste incineration are becoming increasingly stringent, with particular emphasis on effectively controlling the emission of harmful substances in the flue gases. Apart from smoke and dust, the main harmful substances in waste incineration flue gas include CO, SOx, NOx, organic carbon, as well as dioxins and furanes. Through improvements in combustion technology and adjustments to the incineration process, the generation and emission of these substances can be controlled to a certain extent. In comparison, before the 1950s of this century, there were only restrictions on the dust emissions from waste incinerators and the minimum incineration temperature. The specified minimum burning temperature (such as 800°C) is intended to ensure that harmful substances that produce irritating odors are completely burned out in the furnace ； 3. From the perspective of the investment and operational economics of incinerators, the minimum burning capacity should be between 3 t/h and 20–25 t/h. Therefore, modern layered combustion waste incineration systems should meet the following requirements: (1) A strong stoking effect to ensure uniform and complete combustion of the waste across the entire grate surface and to prevent slag formation. The main factors affecting the fire-stirring function of the grate are: ① the type of grate ； ② The manner and intensity of grate movement ； ③ The grate inclination and the direction of movement of the waste on the grate surface, etc ； (2) To ensure the timely ignition, complete combustion, and burnout of waste, the grate should be divided into three zones: a drying and ignition zone, a main combustion zone, and an ash burnout zone ； (3) The combustion equipment should have the ability to adapt to sudden fluctuations in the components of waste that occur frequently, such as moisture or calorific value. When the composition of the waste changes, the waste feeding rate to the incinerator, as well as the volume and distribution of the primary air and its temperature, must be adjusted promptly and accurately ； (4) Preheat the combustion air (primary air and secondary air) ； (5) There is the possibility of adding certain additives to reduce the emissions of harmful substances such as dioxins, NOx, and SOx ； (6) The entire combustion process is divided into a waste incineration stage and a combustion stage of combustible and harmful substances in the flue gas; sufficient air is required for the combustion of the flue gas in the latter stage. During the waste incineration phase, it is necessary to limit the amount of combustion air in order to avoid severe fluctuations in furnace temperature and the generation of excessive fly ash ； (7) Ensures a low carbon content in ash and fly ash (1–3%), resulting in good combustion.