Application of gasification phase-separated combustion technology in boilers

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Giving points to posts is a virtue. . . . . I. Introduction As is well known, energy consumption is one of the main causes of environmental degradation today. In particular, when coal is burned directly as an energy source, it presents problems such as low efficiency and severe pollution. Statistics show that 80% of the pollutants released into the atmosphere in our country each year, 87% of SO2, and 67% of NOx stem from the combustion of coal. Air pollution in our country is mainly of the soot-type pollution caused by the smoke generated from burning coal in furnaces and kilns. At present, coal still constitutes the main source of energy in our country. Changing the energy structure to use cleaner sources such as oil, gas, and electricity is not very suitable given China’s national conditions. For a considerable period of time to come, coal will remain the dominant source of primary energy in our country, and this has become an undeniable reality. Therefore, the vigorous development and application of clean coal combustion technologies and devices are an important measure to address and control air pollution. In recent years, extensive research and experimentation have been carried out on clean coal combustion technologies, but the overall effectiveness still needs to be improved. Over the years, by summarizing, drawing on, improving, and developing relevant technologies from home and abroad, we have conducted extensive research on original coal gasification and phase-separated combustion technologies. Through numerous experiments and practical efforts over several years, we have solved more than a dozen technical challenges, mastered a technology for clean combustion in boilers – namely coal gasification phase-separated combustion technology – and utilized this technology to develop an integrated boiler that converts coal into gas for combustion; we call this boiler a coal gasification phase-separated combustion boiler. Its key feature is the absence of a separate dust removal system outside the furnace; through a new combustion, gas-solid separation, and heat exchange mechanism within the furnace, smoke and dust are removed there, resulting in smoke emissions that are colorless – commonly referred to as smoke-free. The emission concentrations of dust, SO2, and NOX meet the requirements of **environmental protection standards**, and the thermal efficiency is as high as 80–85%. Based on the theory of gas-solid phase-separated combustion, this type of boiler combines complementary control technologies with gas-solid phase-separated combustion technology; it integrates coal gasification and combustion into one unit to form a gasification-phase-separated combustion boiler, thereby enabling the continuous and clean combustion of raw coal. II. The main pollutant in the dust generated by the gasification phase-separated combustion technology is carbon black, which is a product of incomplete combustion. The main reason for the formation of black smoke is that, during the combustion of coal, flammable light hydrocarbons, difficult-to-burn heavy hydrocarbons, and free carbon particles are generated. As these flame-retardant heavy hydrocarbons and free carbon particles are emitted with the smoke, thick black smoke can be seen. Under normal circumstances, the combustion of coal is a multiphase mixed combustion process; volatiles are released from the coal during combustion, and the combustion of these volatiles restricts the burning of the coal coke, thereby complicating and making more difficult the combustion process of solid carbon. The side reactions that occur during the oxidation of solid fuels, namely the formation of carbon monoxide and carbon dioxide, as well as the oxidation of carbon monoxide and the reduction of carbon dioxide, are detrimental to the combustion of solid carbon and natural mineral coal. Gas-solid phase combustion can effectively address these issues. Gas-solid phase combustion involves decomposing solid fuel within the same device into gaseous and solid forms of fuel, allowing them to burn in that same device in a coordinated, interdependent, and mutually reinforcing manner, in accordance with their respective combustion characteristics and appropriate combustion methods, thereby achieving complete or near-complete combustion. The gasification phase-separated combustion technology is based on the theory of gas-solid phase-separated combustion; it combines coal gasification with gas-solid phase-separated combustion. Using coal as the raw material and air and water vapor as gasifying agents, it first carries out mild gasification through low-temperature pyrolysis to convert the hydrocarbons in the combustible volatile components of coal, which are responsible for the formation of black smoke, into gas. This gas is then burned together with the coal char from which the volatile components have been removed, in a combustion chamber. In this way, within the same combustion chamber, the gaseous fuel and the solid fuel burn separately according to their own combustion laws and characteristics, while being interconnected, dependent on each other, and reinforcing one another. This eliminates soot, improves combustion efficiency, and throughout the entire combustion process it helps to reduce the formation of nitrogen oxides and sulfur dioxide, thereby achieving both clean combustion and higher thermal efficiency of the boiler. The application of gasification phase-separated combustion technology in boilers enables the drying, carbonization, and gasification of solid fuels, as well as the simultaneous combustion within the same furnace of the resulting gaseous coal gas and solid coal coke. This also achieves two integrations in the structure of the boiler: the integration of the gas generator and the stratified combustion boiler, and the integration of the stratified combustion boiler and the dust collector. As a result, coal gasification and combustion can be achieved without the need for a separate gas generator ； There is also no need for an external dust collector; smoke and dust can be removed inside the furnace, resulting in colorless exhaust gas from the boiler. Its combustion mechanism is shown in Figure 1. The process of combustion of solid coal and coal coke is indicated by the double-dashed box, the combustion process of gaseous coal gas is indicated by the single-dashed box, the dry distillation process of coal is indicated by the solid-line box, and the gasification process of coal coke is indicated by the dashed line box. The raw coal is first burned and gasified through pyrolysis under oxygen-deficient conditions in the gasification chamber; the coal material is fed from the top, while the coal layer is ignited from the bottom, resulting in a layered structure consisting of an oxidation zone, a reduction zone, a carbonization zone, and a drying zone. The oxide layer and the reduction layer together form the vaporization layer, where the main reactions of the vaporization process take place. The air-based gasifying agent enters from the bottom of the gasification chamber, causing the coal layer at the bottom to burn oxidatively. The resulting blast air contains a certain amount of carbon monoxide; this high-temperature blast air flows through the carbonization layer, where it dries, preheats, and carbonizes the coal. Coal is fed from the upper part of the gasification chamber; as the coal descends and heat is absorbed, a low-temperature carbonization process takes place slowly, during which volatiles are gradually released to form carbonized gas. Its components are mainly water, light oil, and volatiles from coal. After dry distillation, the raw coal forms hot coal coke which enters the reduction zone, where the gasification reaction takes place thanks to the heat generated by the oxidation of the coal coke in the lower layers. At the same time, an appropriate amount of water vapor can be injected to initiate a water-gas reaction; thus, using a mixture of air and water vapor as the gasification agent, it reacts with hot carbon in the gasification chamber to produce gasified coal gas. Its components are mainly carbon monoxide and carbon dioxide, as well as hydrogen and methane generated by reactions between the carbon in solid fuels and water vapor, and between various products; more than 50% of it is nitrogen. In this way, the carbonization gas generated in the carbonization layer mixes with the gasification gas that enters the carbonization layer, and is discharged through the gas outlet. The functions of each layer in the gasification chamber and the main chemical reactions are shown in Table 1. Table 1: Functions of various layers in the gasification chamber and main chemical reactions
Layer name, Function and working process, Main chemical reactions
Ash layer: Distributes the gasifying agent; preheats the gasifying agent using the heat stored in the ash.
Oxidation layer: Carbon reacts with oxygen in the gasifying agent, releasing heat that is used for the endothermic reactions in the reduction layer.
C + O2 = CO2 (exothermic)
2C + O2 = 2CO (exothermic)
Reduction layer: CO2 is reduced to CO; water vapor and carbon decompose to produce hydrogen.
CO2 + C = 2CO (exothermic)
H2O + C = CO + H2 (exothermic)
CO + H2O = CO2 + H2 (endothermic)
Dry distillation layer: Coal material exchanges heat with hot gas, resulting in thermal decomposition and the production of dry distillation gas, which includes moisture, light oil, and volatile substances from the coal. The drying layer is used to dry the coal material. In the gasification chamber of the boiler, the coal is added from top to bottom and moves downward gradually during the gasification process; the gasifying agent enters from the bottom and moves upward through the grate, while the generated gas is extracted above the fuel layer. This process is a counter-current one; it makes full use of the sensible heat of the gas to preheat the gasifying agent, thereby improving the thermal efficiency of the boiler. Moreover, since the dry-distilled gas does not undergo pyrolysis in high-temperature areas, the calorific value of the gasified gas increases. The gas produced by the mild gasification and low-temperature pyrolysis of raw coal passes through the upper dry distillation layer before entering the combustion chamber via the gas outlet of the gasification chamber. There it mixes thoroughly with an adequate amount of secondary air, ignites on its own under the high temperatures in the combustion chamber, and intersects with the flames rising from the coal coke on the grate in the combustion chamber. In this way, within the combustion chamber, the gas and the coal coke burn separately according to their respective gas-phase and solid-phase combustion characteristics and mechanisms, while also interacting with each other and reinforcing one another, thereby ensuring that carbon monoxide and soot are completely burned, achieving or approaching complete combustion. III. Structural features and applications of gasification phase-separated combustion boilers. Throughout its development, the boiler industry has focused on improving both the thermal efficiency of boilers and ensuring that dust emissions meet regulatory standards. Traditional boilers address these two major issues primarily by enhancing combustion and heat transfer to improve thermal efficiency, as well as by installing external dust collectors. Intensified combustion often leads to an increase in the initial emission concentration of dust from boilers, thereby increasing the burden on dust collectors. In developed countries, electrostatic precipitators or bag filters with a dust removal efficiency of over 99% can be used to keep the dust emission concentration below 50 mg/Nm3. However, in China, due to economic constraints, only relatively inexpensive mechanical or wet dust collectors are available; their dust removal efficiency is generally below 95%, resulting in dust emission concentrations of 100–200 mg/Nm3, which do not meet the environmental standards set by those countries. This approach of relying on external dust collectors for dust removal not only increases the floor space required in the boiler room and the capital investment needed for construction, but it also raises the power consumption of the exhaust fans, while simultaneously causing secondary pollution. Since the gasification phase-separated combustion boiler completely changes the combustion principle of traditional boilers, it utilizes the theory of gas-solid phase-separated combustion to convert the hydrocarbons in the combustible volatiles of coal, which cause black smoke during combustion, into combustible gas. This gas then burns together with the coal coke from which the volatiles have been removed, in the combustion chamber. Due to the combustion chamber temperature exceeding 1000°C, the smoke is fully decomposed, solving the problem of black smoke generated by direct coal combustion. This type of boiler not only ensures the most complete and efficient combustion of raw coal, resulting in high thermal efficiency, but also minimizes the emission of dust as well as harmful gases such as SO2 and NOX, thereby achieving smoke and dust reduction; as a result, its environmental protection and energy-saving performance exceeds the standard requirements. The application of gasification phase-separated combustion technology in boilers breaks away from the traditional boiler-with-dust remover setup, creating an integrated system that eliminates the need for a dust remover outside the boiler. And this integration is not a mechanical addition of a dust collector to the boiler. Compared with ordinary gas boilers and stratified combustion boilers, the gasification phase-separated combustion boiler has its own unique structure; it combines the latter two in an organic way and is mainly composed of three major parts: a gasification chamber at the front, a combustion chamber in the middle, and convective heating surfaces at the rear. (See Figure 2: Schematic diagram of the boiler structure and combustion process.) The gasification chamber* is the core technical component of the furnace; it resembles an open-type gas generator. Its main functions are to convert the combustible volatile components in coal, as well as the gases produced by the coal gasification reaction, into gas, which is then sent to the combustion chamber for burning ； Second, the semi-coke coal that releases volatile substances is transported to the combustion chamber for further combustion ； Third is to control the reaction temperature and the thickness of the coal char layer in the gasification chamber. The key to achieving the above functions is, first, to ensure a certain thickness of the raw coal layer ； Secondly, it is necessary to allocate air supply and gasifying agents reasonably in order to improve the coal gasification rate and the gasification efficiency of the gasification chamber ； Third, the gas outlet and coal coke outlet should be properly arranged at the connection point between the gasification chamber and the combustion chamber. The gasification chamber consists of components such as the furnace body, coal feeding device, grate, gasifier inlet, gas outlet, and coal coke outlet. In the gasification chamber, coal is used as the raw material, with air and water vapor serving as the gasifying agents; a mild gasification reaction of coal takes place at atmospheric pressure, thereby driving out the volatile substances produced by the low-temperature thermal decomposition of the coal. When the temperature inside the gasification chamber reaches the set condition, the high-temperature coal coke free of volatiles from the gasification chamber is transferred to the grate in the combustion chamber for enhanced combustion. The main functions of the combustion chamber are: first, to ensure complete combustion of gas and coal coke, thereby improving combustion efficiency ； Second, reduce the initial emission of smoke and dust as well as the sootiness of the flue gas. The gas generated in the gasification chamber is ejected through the gas outlet into the combustion chamber, where it swirls downward under the influence of controlled secondary air, and mixes with the upward-moving flame of coal coke that enters the combustion chamber from the gasification chamber to burn together. The combination of gas combustion with fixed carbon (coal coke) enhances combustion, achieving complete burning and clean combustion, thereby improving combustion efficiency. Furthermore, since the combustion on the grate involves semi-coked coal coke, the amount of fly ash generated is low, and both the dust concentration and the smoke darkness are relatively low. At the same time, an explosion-proof door is installed above the combustion chamber to ensure the safe operation of the boiler. The main function of the convective heating surface is to carry out heat exchange with the flue gases, thereby achieving the boiler’s rated output and improving its heat transfer efficiency. It can come in various structural forms and differs little from ordinary boilers; therefore, most boilers can be modified into gasification phase-separated combustion boilers. Moreover, the boiler does not require a dust collector, **saving the total investment and floor space required for the boiler room. When designing a gasification phase-separated combustion boiler, several points should be taken into consideration: 1. Reasonably arrange the location and size of the gas outlet and the coal coke outlet ； 2. Temperature control of coal coke ； 3. Gasifier inlet and coal inlet ； 4. Reasonably configure secondary air and explosion-proof doors ； 5. The water circulation in the gasification chamber and combustion chamber should be reasonable. As can be seen from the above, the structure of a gasification phase-separated combustion boiler is not complex; it merely requires adding a gasification chamber at the front of a conventional boiler, installing secondary air and explosion-proof doors on the original furnace, along with the use of certain control technologies. Using this principle, boilers of various specifications can be designed, mainly including boilers with capacities ranging from 0.2 t/h to 10 t/h and different parameters. Currently, dozens of boilers of this type are in operation only in the Northeast region; they are widely used in areas such as bathing, heating, and medical care. This technology has also been employed to upgrade many industrial boilers, with very good results. Taking a DZL2t/h boiler as an example, the comparison before and after the renovation is shown in Table 2. Table 2: Comparison before and after the renovation of the DZL2t/h boiler
Comparison before and after renovation:
Heat efficiency: 73% vs. 78%, an increase of 5%
Coal consumption (AII): 380 kg/h vs. 356 kg/h, a reduction of 6.3%
Suitable coal types: AII, AIII, lignite, bituminous coal, AI, AII, AIII; anthracite. The boiler has wide compatibility with different coal types.
Boiler dimensions: 5.4×2×3.2 m vs. 5.9×2×3.2 m; the length increases by approximately one meter.
Environmental performance: The traditional boiler emits black smoke and does not meet environmental standards, while this new boiler produces colorless exhaust gas that meets environmental requirements. This modern boiler incorporates advanced technologies and efficient heat transfer methods, combining a gas generator with a layer-fired boiler to achieve clean combustion. It eliminates smoke and dust internally, so that during operation there is no need for an external dust collector; the exhaust gas is colorless, with a dust concentration of ≤100 mg/Nm3, which is 30-50% lower than that of traditional boilers. The SO2 concentration is ≤1200 mg/Nm3, and NOx levels are <400 mg/Nm3, meeting the requirements for Class I areas as specified in the environmental standard GB13271-2001. Additionally, its heat efficiency is over 82%. The cost is less than 10,000 yuan higher than that of traditional boilers, yet it eliminates the need for a dust collector. The number of coal additions per hour is low, only 2–3 times, and mechanical coal feeding as well as slag removal are possible, thereby **reducing the workload of the boiler operator. IV. Characteristics of gasification phase-separated combustion boilers: Traditional coal combustion methods generate large amounts of pollutants during the burning process of coal, causing severe environmental pollution. The main reasons are: (1) Coal does not come into full contact with oxygen, resulting in incomplete combustion, low combustion efficiency, and an increased level of pollution emissions ； (2) The combustion process is difficult to control; for example, when a large amount of volatiles are released, oxygen supply is often insufficient, resulting in the formation of smoke and black fumes ； (3) When solid fuel burns, it is difficult to maintain a uniform temperature, resulting in localized high-temperature areas that promote the formation of large amounts of NOx ； (4) Most of the sulfur in raw coal is oxidized to SO2 during combustion ； (5) When untreated solid coal is burned directly, large amounts of dust are emitted along with the flue gases, causing severe dust pollution. The gasification-phase-separated combustion boiler integrates coal gasification and gas-solid phase-separated combustion, effectively addressing environmental pollution issues. Compared with traditional coal-fired boilers, it has the following advantages: 1. Low dust concentration and low smoke darkness, resulting in excellent environmental performance. The gasified coal gas produced in the gasification zone and the carbonized coal gas produced in the carbonization zone are eventually mixed together, and then thoroughly mixed with secondary air in the combustion chamber. As it is a gaseous fuel, adequate oxygen supply ensures complete combustion, allowing carbon monoxide and soot to be burned out. The hot coal coke that enters the combustion chamber from the gasification chamber has had most of its volatile components removed, which prevents these volatiles from having an adverse effect on the combustion of the fixed carbon. The remaining volatile components are further oxidized within the coal coke, and combustible substances such as carbon monoxide and soot are burned up as they pass through the surface of the coal coke layer. In addition, coal coke produces little fly ash during combustion, and dust removal technologies are used in the boiler, thereby eliminating \"carbon black\" altogether and efficiently removing the fly ash from the smoke. 2. Energy-saving with high thermal efficiency. The coal material is fully gasified and pyrolyzed in the gasification chamber before being burned, which not only avoids the adverse effects of volatiles, carbon monoxide, carbon dioxide, etc. on the combustion of coal coke, but also makes the hot gas entering the combustion chamber easier to burn, thereby facilitating the combustion of coal coke to a certain extent. The hot coal coke that enters the combustion chamber has had most of its volatile components removed; it not only possesses a high temperature but also has internal pores that facilitate both internal and external diffusion of oxidation reactions, thereby enhancing the combustion of the coal coke. This allows carbon monoxide and soot to be burned more completely even with a lower excess air coefficient, reducing heat losses due to chemical and mechanical incomplete combustion. As a result, the thermal efficiency of coal combustion is improved, and coal usage can be reduced by 5–10% compared to burning coal directly. 3. Low nitrogen oxide emissions are achieved because the coal layer is ignited from the bottom within the gasification chamber and burns there; as a result, the temperature in the gasification chamber remains relatively low, indicating low-temperature combustion. Moreover, the excess air coefficient in the gasification chamber is very low, ranging around 0.7–1.0, indicating low-oxygen combustion. This creates favorable conditions for reducing nitrogen oxide emissions. The chemical amount of organic nitrogen in coal is small, and in a reducing atmosphere it transforms only into non-toxic nitrogen molecules that do not participate in combustion. The nitrogen oxides contained in coal react, under the catalysis of the semi-coke in the coal seam, to produce nitrogen gas, water vapor, and carbon monoxide; another portion is reduced to nitrogen gas as it passes through the upper reduction layer. The high-temperature coal coke, from which the vast majority of volatile substances have been removed in the gasification chamber, enters the combustion chamber where it receives an adequate supply of oxygen to facilitate intensified combustion. The remaining small amount of volatile substances undergo further pyrolysis and oxidation within the semi-coke, while nitrogen oxides are further reduced inside the coal coke. The combustible soot particles generated are burned as they pass over the surface of the coke layer, thereby controlling and reducing the formation and emission of nitrogen oxides. 4. It has a certain desulfurization effect. Sulfur in coal exists mainly in the form of inorganic sulfur (FeS2 and sulfates), with almost all of the sulfates remaining in the ash, thus not causing pollution from coal combustion. In gasification phase-separated combustion boilers, FeS2 and organic sulfur in coal undergo thermal decomposition reactions within the gasification chamber, as well as reduction reactions with hydrogen in the gas, thereby removing the sulfur from the coal in the form of hydrogen sulfide gas. Moreover, at the lower part of the gasification chamber, the temperature is generally around 800°C, which is precisely the optimal reaction temperature for the desulfurizer to be effective. When burning coal with a high sulfur content, adding an appropriate amount of limestone or dolomite to the coal particles is sufficient to achieve good desulfurization effects, thereby **reducing the sulfur dioxide content in the flue gases. 5. Simple and easy operation and control: The generation and combustion of gas take place in two units within the same device, eliminating the need for a separate gas ignition device. The gas is ignited by a high-temperature flame inside the combustion chamber, which makes operation and control straightforward. This simplifies operational management, facilitates use, reduces the workload on boiler operators, improves the hygiene conditions in the boiler room, and promotes civilized production practices. 6. Stable combustion with strong adaptability to different coal types. The coal is ignited at the lower part of the boiler’s gasification chamber, resulting in stable combustion. Combustible low-quality coal and coal with a high ignition point have strong adaptability to different coal types; coal types within the refractory range or within the moderate slagging range are all suitable. Among them, lignite, long-flame coal, non-caking or weakly caking bituminous coal, and pelletized coal are relatively ideal fuels. V. Conclusion Practice has shown that the new combustion theory, together with the integrated technologies based on various patents, ensures the high efficiency, environmental friendliness, and reliability of gasification phase-separated combustion boilers. It overcomes the problems of waste and pollution that traditional technologies could not resolve, resulting in significant economic and environmental benefits, and thus it is well received by users. China is rich in coal resources, and as energy policies and environmental requirements become increasingly stringent, gasification and phase-separated combustion boilers hold great prospects in the Chinese market.