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1. Introduction With economic development, a growing population, and rising living standards, the amount of urban waste generated is also increasing steadily. In today’s world, large amounts of waste have become a persistent source of pollution in cities. Improper disposal of waste can lead to severe air pollution, water pollution, and soil contamination, as well as the occupation of large areas of land. The serious incidents that occurred in Spain in the mid-1990s, involving the collapse and sliding of garbage piles and severe pollution of coastal areas, served as a warning to humanity. It can be said that waste pollution of the environment has become an increasingly serious problem. How to handle waste in an economical and efficient manner is an urgent problem facing environmental protection workers and the industry as a whole. Waste incineration is one of the effective methods for dealing with solid waste at present, aiming at the harmless treatment and utilization of waste. In developed Western countries, the use of waste incineration technology dates back nearly 130 years, and it is still considered one of the most effective and economical waste treatment methods today. At present, China mainly relies on open-air stacking and landfilling for waste disposal, and it started relatively late in the research, development, and application of waste incineration technology. In contrast, the level and scale of design, production, and application of waste incineration equipment in our country still fall far short of those in developed countries. Therefore, it is extremely urgent and important for environmental protection workers and manufacturing enterprises in our country to understand the development trends of combustion technologies and equipment for waste incinerators, and thus to learn and master advanced design and manufacturing techniques for such incinerators. This article reviews the history of development in combustion technologies and equipment for waste incinerators, analyzes the characteristics of various combustion devices, and offers suggestions for the advancement of waste incineration technologies and equipment in our country. 2. Main types and characteristics of waste incinerators in their early stages of development Starting from the second half of the 19th century, developed countries in the West began to design and develop waste incineration equipment. The main purposes of applying waste incineration technology and equipment at that time were: (1) to carry out harmless treatment of waste at high temperatures, in order to eliminate bacteria and pathogens ； (2) Produce usable ash and slag ； (3) Avoid smoke and odors generated by combustion ； (4) Convert the energy contained in waste into steam, electricity, or hot water for utilization ； (5) Carry out waste incineration at the lowest possible cost, with reasonable equipment operation and working conditions ； (6) Burn all combustible waste that cannot be utilized. The world’s first solid waste incineration equipment was developed in Europe during the Second Industrial Revolution. In the second half of the 19th century, Paddington in Britain had developed into a densely populated industrial city. In 1870, a waste incinerator was put into operation in the city of Paddington. At that time, the waste contained high levels of moisture and ash, resulting in a low calorific value and making it difficult to burn it; as a result, this incinerator did not operate properly and stopped functioning after a short time. To address the issues of poor quality waste and difficulties in burning it, a double grate system was first employed (with a layer of coal burning intensely on the lower grate), and in 1884 attempts were made to burn waste together with coal in order to improve the combustion properties of the waste fuel. However, neither attempt yielded satisfactory results, and the low height of the chimneys caused the surrounding area to be polluted by irritating smoke. To address the problems of irritating smoke and carbon black pollution, the first measure taken was to raise the incineration temperature to 700°C, which was later increased further to 800–1100°C. By that time, it was already known that the amount of air used for combustion and the way in which it was supplied affected the temperature of the flue gases; therefore, measures such as constructing taller chimneys and installing supply fans and exhaust fans were adopted to increase ventilation and meet the requirements of the combustion process for air. By raising the chimney, the issue of the dispersion of irritating and harmful substances in the flue gas was also resolved. Since the types and composition of waste can vary significantly depending on region and season, waste incineration equipment must have good fuel adaptability. In this regard, the technical measures taken at that time were to add a waste drying area to the incinerator and to use preheated combustion air. 2.1. Box-type waste incinerators In 1876, a waste incineration plant was put into operation in Manchester, UK. The plant is equipped with several box-type waste incinerators, which share a single exhaust duct, and the furnaces utilize fixed inclined ladder grates. The operation results show that this box-type incinerator is suitable for waste incineration. During operation, waste is manually fed from the furnace door into the combustion chamber for burning. In addition to being used for adding waste, the furnace door is also used for removing slag and stoking the fire. The furnace arch above the grate reaches a white-hot state during combustion, and the intense radiation generated thereby dries the new fuel. Due to the success of this type of waste incinerator in Manchester at that time, by the end of the 19th century a total of 210 such waste incineration units were built and put into operation in the UK, with 14 of them located alone in London. The main drawback of this box-type incinerator is the high workload for the operators and the poor working conditions. Furthermore, manual feeding and slag removal operations cause discontinuity in the incineration process. The box-type waste incinerator shown in Figure 1 has separate slag removal and stoking holes added to its rear wall. Thereafter, many types of waste incinerators emerged on the basis of the aforementioned box-type incinerators, such as double-box incinerators and series-fired grate furnaces. A double-chamber incinerator is composed of two sets of combustion equipment for chamber incinerators arranged back to back, with a rear wall connecting the two sets of combustion equipment and a waste feeding port. During operation, the processes of slag removal, feeding, and incineration are carried out alternately in the two combustion units; that is, while one unit is performing slag removal and feeding operations, waste incineration is taking place in the other unit. The newly added waste can also be dried by coming into contact with the scorching back wall. Therefore, even when the calorific value of the waste is low, the combustion temperature in the furnace can still reach 700–1000°C. The door of a box-type incinerator can be up to 1 m wide, while the grate area is usually between 1.0 and 1.5 m2, with a maximum of 3 m2. Due to frequent slag removal operations and the discontinuous nature of the combustion process, the incineration capacity of box-type incinerators is only 6–10 t per day (on a 24-hour basis); this figure can reach 20 t per day when the waste has been preheated. 2.2. Vertical Waste Incinerators The city of Wiesbaden in Germany built a waste incineration plant using vertical incinerators in 1902. During operation of this type of incinerator, waste is fed into the furnace from above through an inverted funnel, while combustion air enters via gaps in the horizontal slag well; this process preheats the air while also cooling the slag. At the bell mouth, the flue gas is directed out from the side, and after secondary air is added, it enters the combustion chamber to ensure that the combustible components contained in the flue gas (especially combustible gases) are completely burned. This incinerator is 3 m in height, with a rectangular cross-section for its furnace chamber; the dimensions of its furnace bottom are 0.8×0.8 m. Compared to box-type incinerators, vertical incinerators can burn more waste and achieve better combustion results, but they still have the drawback of requiring significant manual effort for slag removal. After improvements were made to the aforementioned type of incinerator, vertical incinerators with cylindrical chambers were developed. The combustion air is now supplied from the sides of the furnace, while slag removal is carried out using a special slag cleaning blade. When slag removal is required, the bottom cover of the cylindrical furnace chamber is opened, and a slag removal tool is inserted from the bottom into the furnace chamber to carry out the slag removal process ； After it completes the slag removal operation and exits the furnace chamber, the bottom cover of the furnace chamber is closed again; the waste falls onto this cover automatically due to its own weight, and the space above the furnace chamber is filled with newly added waste. This type of incinerator can burn 2.6 t/h per m2 of grate area, with a furnace temperature of around 900°C. A waste incineration plant can consist of several vertical incinerators, with a common ash chamber and fly ash collection system. Since then, various improved vertical incinerators have appeared in European countries, with new technologies applied including the use of water-cooled walls to cool the vertical furnace walls, the use of spiral continuous feeding devices, and mechanical slag removal. It is particularly worth mentioning that vertical waste incinerators similar to those with boiling combustion already existed in Europe at the beginning of the 20th century. Waste was fed into the furnace from above, while combustion air was injected from below. Under an air pressure of 40–50 mbar, the debris in the waste with a lower calorific value is separated and carried away by the flue gas to the ash collection chamber together with the fly ash, while the larger pieces of waste with a higher calorific value burn on a grate with an area of about 1.2 m2. This type of incinerator is equipped with a mechanical feeding system; the burning rate per m2 of grate area is 1.1 t/h, and the furnace temperature ranges from about 1000 to 1100°C. 2.3. Other types of waste incinerators In order to mechanize the main operations in the waste incineration process, such as feeding, stoking, slag removal, and ash removal, stepped grates, inclined grates, chain grates, as well as rotary drum waste incinerators have been developed and applied over time. Similar to modern inclined reciprocating grates, the stepped grates of the 1920s were also composed of fixed and movable grate bars, with the entire grate having a stepped structure and a horizontal inclination of about 10–13°. Waste entered the furnace through a spiral feeder and, after preheating, was thrown to the lowest part of the grate. Through the back-and-forth movement of the grate bars, the waste moves continuously and evenly from bottom to top on the grate, which facilitates effective stirring of the flames and helps to break up hardened layers of ash. A grate inclination of 10–13° allows the molten ash to flow back into the newly added waste, which helps to accelerate the drying and ignition of the waste. Therefore, the stepped grate can not only better adapt to fluctuations in waste composition but is also more suitable for situations with high burning and evaporation rates compared to box-type and vertical incinerators. Depending on the amount of waste to be burned, the stepped grate can be designed with a single row or two rows. At that time, the length of the stepped grate could reach 3.5 m, the width of a single row was 1.3 m, and the average burning rate could reach 3.5 t/h. Subsequently, waste incinerators using inclined grates were further developed. This type of inclined grate is characterized by a slope of 15–25° from the feed inlet toward the ash hopper (i.e., roughly the same slope as that of modern inclined reciprocating grates). As the grate bars move back and forth, the waste moves in the direction of the ash hopper while being loosened at the same time; therefore, it also serves an excellent function of stirring the fire. The grate area of the waste incinerator equipped with double rows of inclined grates amounts to 16 m2; it can incinerate up to 8 t/h of waste, with a furnace temperature of around 900–1000°C. The development and application of rotary drum waste incinerators represent an important step toward the mechanization of waste incineration processes. Typically, the combustion apparatus of such incinerators consists of a rotating drum and an inclined grate. After entering the furnace, the waste is first dried by partial high-temperature flue gas on the inclined grate ； Before entering the drum, the waste is already on fire or has begun to burn. For waste with a low calorific value, three sets of inclined grates are arranged throughout the furnace, each designed for waste drying, ignition, and combustion. Since the grills used for drying and igniting waste are located above both the incineration grills and the rotary drum, this results in a larger furnace height and volume, as well as higher equipment costs. Therefore, from the perspective of technical and economic comparison, the incineration capacity of a rotary drum waste incinerator should be no less than 30,000 t/year. Stirring operations have a significant impact on the burning process of waste, especially waste with low calorific value and high ash content. One of the main disadvantages of chain grates is their lack of automatic disturbance of the fuel layer; stirring still requires manual effort, which is why using chain grates for burning waste with low calorific value is not very suitable. By the 1960s, foreign companies were using chain grates in waste incinerators. However, due to various technical shortcomings in the combustion process of chain grates—such as poor ignition conditions, weak stirring effect, and limited ability to adjust air supply—by the end of the 1980s they were replaced by reciprocating pusher grates. 3. Main types and characteristics of modern waste incinerators Waste incineration technology has undergone nearly 130 years of development; it has become increasingly sophisticated, and the related equipment is now widely used. The commonly used waste incineration systems in developed Western countries mainly include the following types: (1) Layer combustion waste 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 layer combustion method is that waste does not require strict pretreatment. Rolling grates and reciprocating grates have a strong stirring effect, 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 consumed; 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) Emission standards for waste incineration are becoming increasingly stringent, particularly requiring effective control of the emission of harmful substances in flue gases. In addition to 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 the incinerator, its 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 stirring effect to ensure uniform and thorough combustion of the waste across the entire grate surface, as well as 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 feed 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 for the 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 slag and fly ash (1–3%), resulting in good combustion. One of the main types of grates in modern layered waste incinerators is the reciprocating push-fed grate, among which the most widely used type is the straight-push inclined reciprocating grate arranged in a single stage or multiple stages. The waste is automatically fed into the furnace by a mechanical feeding device, and it passes through the drying and ignition zone, the main combustion zone, and the burning-out zone on the grate, thereby completing the entire combustion process. The residence time of the waste in the furnace is generally 1 hour. Thanks to the inclination of the grate and its reciprocating motion, the waste is continuously stirred as it moves toward the ash hopper, resulting in strong stirring effects. To adapt to changes in the amount of waste incinerated, as well as the types and composition of the waste, the amount of combustion air and its distribution can be adjusted, and it can be supplied separately as primary air, secondary air, or tertiary air. The waste incineration system of the German company EVT is a typical example of a pusher-type inclined reciprocating grate. It is characterized by the use of a chain grate to ensure uniform and continuous conveyance of waste. By enabling stepless adjustment of the conveyor speed of the chain grate, the incinerator can respond flexibly to fluctuations in the calorific value of waste, which facilitates the regulation of combustion conditions. The rotary grate is also a type of push-type grate, typically consisting of multiple rollers arranged at an angle. Driven by the hydraulic system, the drum rotates, causing the waste on it to move in a wave-like manner during combustion; this ensures thorough mixing of the waste, resulting in effective stirring and complete combustion. The design of the furnace chamber in this type of incinerator combines effectively the characteristics of a rotary grate with those of waste incineration. The initial rollers serve as areas for drying and burning the waste, enabling waste with high moisture content and low calorific value to be dried quickly and ignited promptly. Under the effect of the high-temperature radiation from the front arch, waste with a low calorific value creates a high-temperature zone necessary for waste incineration, thereby ensuring complete combustion of the waste and reducing the generation and emission of harmful substances. Under the action of the rear arch, the flame and hot flue gas directly strike the waste on the combustion section of the rear drum, thereby promoting further combustion of the waste. A typical example of a reverse-tilting reciprocating grate is the grate produced by the German company Martin. Its difference from a forward-tilting reciprocating grate lies in the fact that the movement direction of the grate bars is opposite to that of the waste material (Figure 5). Therefore, the use of a retrograde inclined reciprocating grate allows the hot ash and slag from the main combustion area to mix more thoroughly with the waste in the drying ignition area, which facilitates the ignition of the waste. It can be seen that this type of grate is more suitable for the incineration of waste with high moisture content and low calorific value. 4. Characteristics of municipal solid waste in China and the development of incineration equipment With the rapid development of production and the swift growth of the economy, China has become one of the countries with high levels of municipal solid waste generation. According to statistics, in 1990 the total amount of municipal waste generated in China’s cities was 69 million tons, of which Shanghai produced 2.7 million tons of waste (about 7,500 tons per day), ranking first in the country. In 1995, the total amount of municipal waste generated across the country reached 100 million tons. According to surveys of 418 large and medium-sized cities, the amount of waste produced in Chinese cities is increasing at a rate of 10% per year; it is estimated that by 2010, the annual waste generation will reach around 200 million tons. To date, the cumulative amount of waste stored nationwide has exceeded 6 billion tons, occupying an area of 500 million square meters (equivalent to 750,000 mu). Of the more than 600 cities across the country, over 200 are surrounded by garbage piles. Within the 1,260 km2 area of Shanghai’s urban zone, there are nearly 2,000 waste piles larger than 50 m2, covering an area of about 7,900 mu. The long-term open-air dumping of waste has already posed significant threats and hazards to the atmospheric environment, groundwater, and soil. Therefore, it is imperative to vigorously advance the research on urban waste incineration technology and the development and application of related equipment in our country. Urban household waste consists of two main categories: combustible and non-combustible waste. The combustible portion includes discarded paper, rags, bamboo and wood, leather, plastics, as well as residues from animals and plants. The non-flammable components include various types of waste metals, sand and gravel, glass and ceramic fragments, etc. The level of urban consumption in our country is low, the proportion of non-combustible components in waste is high, and its calorific value is much lower than that of developed countries. However, the standard of urban living in our country is continuously rising, and urban waste is showing a trend of decreasing moisture content and an increasing proportion of combustible components. The calorific value of waste from cities of medium size and above is generally between 2512 and 4605 kJ/kg; in some areas it reaches 3349 to 6280 kJ/kg, which meets or is close to the requirements for waste incineration (a calorific value of not less than 3350 kJ/kg). It should be noted that urban waste in our country contains a high proportion of non-combustible components, has a low calorific value and a high moisture content. Moreover, the composition of the waste varies depending on region, season, the consumption level of cities, and the year. Therefore, waste incineration equipment needs to be highly adaptable to these changes in waste composition – particularly changes in moisture and calorific value – so as to be able to make timely and effective adjustments to the combustion process, thereby ensuring prompt ignition and stable combustion of the waste. Since Shenzhen introduced Martin-type waste incinerators from Japan in 1985, cities in China such as Zhuhai and Guangzhou have also adopted foreign layered combustion waste incineration systems. The domestic waste incineration plant in Pudong New Area, Shanghai, will likewise introduce inclined reciprocating grate incinerators provided by a French company. By introducing advanced foreign equipment and accumulating operational experience, we can gradually incorporate these advanced foreign technologies, and then, in light of China’s actual conditions, develop waste incineration equipment suitable for our national circumstances. By analyzing the characteristics of various urban waste incineration equipment, it can be seen that developing inclined reciprocating feed grate incinerators in line with China’s national conditions is reasonable and feasible. In the design, in addition to considering the heat transfer efficiency of the heating surfaces, factors such as corrosion and wear of the heating surfaces and furnace walls, flue gas purification, and automatic control also need to be taken into account. In the design of the furnace arch and furnace chamber, as well as in the arrangement and distribution of combustion air, it is necessary to fully take into account the characteristics of urban waste in our country, namely its low calorific value and high moisture content. Given the actual conditions in our country, while developing large-scale waste boilers and constructing large waste incineration plants, efforts should be made to encourage the development of medium and small-sized waste incineration equipment. Regarding incineration emission standards, it is neither appropriate to copy those of developed countries nor to set them too loosely; instead, emission standards suitable for China’s national conditions should be gradually established on the basis of referring to relevant foreign standards. This post was last edited by johncom on 2009-4-18 16:06.]