I. Introduction
As we all know, energy consumption is a major cause of environmental deterioration, especially in the process of burning coal directly as energy, there are problems of low efficiency and serious pollution. Statistics show that 80% of the pollutants discharged into the atmosphere every year in China are soot, 87% are SO2 and 67% are nitrogen oxides, all of which come from coal burning. The air pollution in China is mainly soot-type pollution caused by flue gas produced by burning coal in boilers and kilns. At present, China's energy is still dominated by coal. Changing the energy structure and using clean energy such as oil, gas and electricity are not suitable for China's national conditions. For a long time to come, the dominant position of coal in China's primary energy structure will not change, which has become an indisputable reality. Therefore, vigorously developing and applying clean coal combustion technology and equipment is an important measure to solve and control air pollution.
In recent years, people have done a lot of research and practice on clean coal combustion technology, but the comprehensive effect still needs to be improved. On the basis of summarizing, learning from, perfecting and developing related technologies at home and abroad for many years, we have done a lot of research on raw material gasification and split-phase combustion technology. Through a large number of experiments and working practices in recent years, we have solved more than ten technical problems, mastered a clean combustion technology of boilers-coal gasification split-phase combustion technology, and developed an integrated boiler that converts coal into gas by using this technology, which we call a coal gasification split-phase combustion boiler. Its outstanding feature is that it does not need a dust removal system outside the furnace, and it realizes "smoke elimination and dust removal in the furnace" through a brand-new combustion, gas-solid separation and heat exchange mechanism in the furnace, making its flue gas colorless-commonly known as smokeless. The emission concentrations of soot, SO2 and NOX meet the requirements of national environmental protection standards, and the thermal efficiency is as high as 80 ~ 85%. According to the theory of gas-solid split-phase combustion, the boiler integrates complementary control technology and gas-solid split-phase combustion technology in one furnace, integrates coal gasification and combustion, and realizes continuous combustion and clean combustion of raw coal.
Second, coal gasification split-phase combustion technology
The main pollutant of soot is carbon black, which is the product of incomplete combustion. The main reason for the formation of black smoke is the formation of combustible light hydrocarbons, refractory heavy hydrocarbons and free carbon particles during coal combustion. These refractory heavy hydrocarbons and free carbon particles are discharged with the smoke, and thick black smoke can be seen.
Generally speaking, coal combustion belongs to multiphase mixed combustion. Volatiles are separated from coal during combustion, which restricts the combustion of coal char and makes the combustion process of solid carbon complicated and difficult. The side reactions in the oxidation reaction of solid fuel, that is, the formation of carbon monoxide and carbon dioxide, the oxidation reaction of carbon monoxide and the reduction reaction of carbon dioxide, are not conducive to the combustion of solid carbon and natural mineral coal, and the gas-solid phase separation combustion can effectively solve the above problems.
Gas-solid split-phase combustion is to decompose solid fuel into gas fuel and solid fuel in the same device, and make them burn in the same device in a related way according to their respective combustion characteristics and corresponding combustion methods, which are interdependent and promote each other, so as to achieve the goal of complete combustion or near complete combustion.
According to the theory of gas-solid phase separation combustion, coal gasification is combined with gas-solid phase separation combustion. Using coal as raw material and air and water vapor as gasifying agents, hydrocarbons in combustible volatiles which are easy to produce black smoke are first moderately gasified into gas at low temperature, and then burned in a combustion chamber together with coal char from which volatiles are removed. In this way, in the same combustion chamber, gas fuel and solid fuel burn separately according to their respective combustion laws and characteristics, which are interdependent and promote each other, thus eliminating black smoke and improving combustion efficiency. In the whole combustion process, it is beneficial to reduce the generation of nitrogen oxides and sulfur dioxide, thus achieving the dual effects of clean combustion and improving the thermal efficiency of the boiler.
The application of coal gasification and split-phase combustion technology in boilers enables the drying, dry distillation and gasification of solid fuels, as well as the resulting gas phase and solid coal char to be burned simultaneously in the same boiler. The boiler has realized two integrations in structure, namely, the integration of gas producer and layered boiler, and the integration of layered boiler and dust collector, so coal gasification and combustion can be realized without setting up another gas producer; There is no dust collector outside the furnace, so the smoke can be eliminated in the furnace, and the flue gas discharged from the boiler is colorless. The combustion mechanism is shown in figure 1. The two-dot chain line box represents the combustion process of solid coal and coal char, the one-dot chain line box represents the combustion process of gas, the solid line box represents the dry distillation process of coal, and the dotted line box represents the gasification process of coal char.
Firstly, raw coal is burned, gasified and pyrolyzed in a gasification chamber under anoxic conditions. Coal is added from the top, and the coal seam is ignited from the bottom, forming a layered structure of oxidation layer, reduction layer, dry distillation layer and dry layer from bottom to top. Among them, the oxidation layer and the reduction layer constitute the gasification layer, and the main reaction in the gasification process takes place here. The gasification agent is mainly air, which enters from the bottom of the gasification chamber, so that the coal seam at the bottom is oxidized and burned, and the generated blowing air contains a certain amount of carbon monoxide. This high-temperature blast air flow passes through the dry distillation layer to dry, preheat and dry coal. Coal is added from the upper part of the gasification chamber. As the coal descends and absorbs heat, the low-temperature dry distillation process proceeds slowly, and volatiles are gradually separated out to form dry distillation gas. Its components are mainly water, light oil and volatile matter in coal.
After dry distillation of raw coal, hot coal char enters the reduction layer, and gasification reaction is carried out by using the oxidation reaction heat of lower coal char. At the same time, a proper amount of water vapor can be injected to generate water gas reaction, so that the mixture of air and water vapor can be used as gasification agent and react with hot carbon in the gasification chamber to generate gasification gas. Its main components are carbon monoxide and carbon dioxide, carbon and water in solid fuel, hydrogen and methane generated by the reaction between products, and more than 50% nitrogen. In this way, the dry distillation gas generated by the dry distillation layer is mixed with the gasification gas entering the dry distillation layer and discharged from the gas outlet. See table 1 for the functions and main chemical reactions of each layer in the gasification chamber.
Table 1: Functions and main chemical reactions of various layers in gasification chamber
The function of regional names and the main chemical reactions in the working process
The gasification agent is distributed in the ash layer, and the gasification agent is preheated by the sensible heat of the ash.
The carbon in the oxide layer reacts with the oxygen in the gasifying agent to release heat, which is required by the endothermic reaction of the reducing layer and C+O2=CO2.
2C+O2=2CO exothermic
In the reduction layer, CO2 is reduced to CO, water vapor and carbon are decomposed into hydrogen, and CO2+C=2CO gives off heat.
H2O+C = carbon monoxide +H2 exotherm
Carbon monoxide +H2O = carbon dioxide +H2 absorbs heat.
The coal in the dry distillation layer exchanges heat with hot gas for thermal decomposition, and the dry distillation gas including water, light oil and volatiles in coal are separated.
The drying layer dries the coal material.
In the gasification chamber of the boiler, the coal is added from top to bottom, and gradually moves down during gasification, while the gasifying agent enters from the bottom, and the generated gas is led out from the top of the fuel layer through the grate from bottom to top. This process belongs to countercurrent process, which can make full use of the sensible heat of gas to preheat the gasification agent, thus improving the thermal efficiency of the boiler, and because the dry distillation gas does not pyrolyze in the high temperature area, the calorific value of the gasification gas is also improved to some extent.
After passing through the upper dry distillation layer, the gas generated by light gasification and low-temperature pyrolysis of raw coal enters the combustion chamber from the gas outlet of the gasification chamber, is fully mixed with sufficient secondary air, spontaneously ignites under the high temperature condition of the combustion chamber, and intersects with the upward flame of coal char entering the grate of the combustion chamber. In this way, coal gas and coal char burn in the combustion chamber according to the combustion characteristics and combustion mode of gas phase and solid phase respectively, which are interrelated and promote each other, so that carbon monoxide and soot can be completely burned, reaching or approaching complete combustion.
Third, the structural characteristics and application of coal gasification split-phase combustion boiler
In the process of boiler development, we have been paying attention to two major issues: improving boiler thermal efficiency and reaching the standard of smoke and dust emission. The traditional boiler basically solves these two problems by strengthening combustion and heat transfer, improving the thermal efficiency of the boiler and setting a dust collector outside the boiler. Intensified combustion often leads to the increase of initial emission concentration of boiler smoke and dust, which increases the burden of dust collector. In developed countries, electrostatic precipitator or bag precipitator with dust removal efficiency above 99% can be used to control the dust emission concentration below 50mg/Nm3, while in China, due to economic constraints, only mechanical or wet precipitator with relatively low price can be used, and the dust removal efficiency is generally lower than 95%, which makes the dust emission concentration greater than 100-200 mg/Nm3. This method, which relies on the dust collector outside the furnace, not only increases the floor space and capital investment of the boiler room, but also increases the power consumption of the induced draft fan, resulting in secondary pollution. Because the coal gasification split-phase combustion boiler has completely changed the combustion principle of the traditional boiler, the hydrocarbon in the combustible volatile matter which is easy to produce black smoke in the coal combustion process is first converted into combustible gas, and then burned in the combustion chamber together with the coal char from which the volatile matter is removed. Because the temperature of the combustion chamber is as high as 1000℃, the flue gas can be fully decomposed, which solves the problem of black smoke produced by direct combustion of coal. This kind of boiler not only makes the raw coal burn fully and use efficiently as far as possible, but also has high thermal efficiency, and reduces the emission of smoke and dust, SO2, NOX and other harmful gases as much as possible, thus achieving the function of smoke elimination and dust removal, and making the environmental protection and energy saving index of the boiler greatly superior to the national standard.
The application of coal gasification and split-phase combustion technology in boiler breaks the traditional mode of adding dust collector to boiler and creates an integrated mode without external dust collector. This integration is not to mechanically add a dust collector to the boiler. Compared with ordinary gas-fired boilers and layer-fired boilers, coal gasification split-phase combustion boilers have their own unique structure, which organically combines the latter two, and mainly consists of three parts: front coal gasification chamber, middle combustion chamber and tail convection heating surface. (See Figure 2: Schematic Diagram of Boiler Structure and Combustion)
Gasification chamber is the technical core of boiler, which looks like an open gas producer. Its main functions are as follows: firstly, combustible volatiles in coal react with coal gasification to generate gas, which is discharged into the combustion chamber for combustion in the form of gas; The second is to transport semi-coking coal with volatile matter to the combustion chamber for further combustion; The third is to control the reaction temperature and the thickness of coal char layer in the gasification chamber. The key to realize the above functions is: first, to ensure a certain original coal seam; Second, it is necessary to rationally allocate gas supply and gasification agent to improve the gasification rate of coal and the gasification intensity of gasification chamber; Thirdly, the gas outlet and the coal coke outlet should be reasonably arranged at the joint of the coal gasification chamber and the combustion chamber. The gasification chamber is mainly composed of a furnace body, a coal feeding device, a grate, a gasifying agent inlet, a gas outlet and a coal coke outlet.
In the gasification chamber, coal is used as raw material, air and water vapor are used as gasification agents, and the mild gasification reaction of coal is carried out under normal pressure, so that the volatile generated by low-temperature pyrolysis of coal is driven out of coal. When the temperature in the gasification chamber reaches the set conditions, the devolatilized high-temperature coal char in the gasification chamber is transported to the grate of the combustion chamber for intensified combustion.
The main functions of the combustion chamber are: first, to make the gas and coal char fully burn and improve the combustion efficiency; The second is to reduce the initial smoke exhaust volume and smoke blackness. The coal gas generated in the gasification chamber is injected into the combustion chamber through the coal gas outlet, and rotates downward under the disturbance of controllable secondary air, and intersects with the coal char flame entering the combustion chamber from the gasification chamber for mixed combustion. Coal gas is combined with fixed carbon (coal char) combustion to strengthen combustion, achieve the purpose of full combustion and clean combustion, and improve combustion efficiency. Moreover, because semi-coking coal coke is burned on the grate, the amount of fly ash produced is small, and the smoke concentration and smoke blackness are relatively low. At the same time, an explosion-proof door is set above the combustion chamber to ensure the safe operation of the boiler.
The main function of convection heating surface is to complete the heat exchange with flue gas, achieve the rated output of the boiler and improve the heat exchange efficiency of the boiler. There are many structural forms, which are not much different from ordinary boilers, so most boilers can be transformed into coal gasification split-phase combustion boilers. And the boiler does not need a dust collector, which greatly saves the total investment and occupied area of the boiler room.
Several problems that should be paid attention to in the design of coal gasification split-phase combustion boiler;
1, reasonably arrange the position and size of gas outlet and coal coke outlet;
2. Temperature control of coal char;
3. Gasification agent inlet and coal inlet;
4. Set the secondary air door and explosion-proof door reasonably;
5, gasification chamber and combustion chamber water circulation should be reasonable.
As can be seen from the above, the structure of coal gasification split-phase combustion boiler is not complicated. Just add a gasification chamber in the front of the traditional boiler, set secondary air and explosion-proof door on the original furnace, and combine some control technologies. Using this principle, various types of boilers can be designed, mainly boilers with various parameters ranging from 0.2t/h to10t/h. At present, there are dozens of boilers of this type in operation in Northeast China alone, which are widely used in bathing, heating, medical and health fields, and many industrial boilers have been reformed by this technology, and the effect is very good.
Take a DZL2t/h boiler as an example. See Table 2 for the comparison before and after the transformation.
Table 2: Comparison of DZL2t/h boiler before and after transformation
Comparison before and after transformation
The thermal efficiency is 73%, 78% and 5% higher respectively.
The coal consumption (AII) is 380kg/h and 356kg/h, saving 6.3% coal.
Wide adaptability of coal, wide adaptability of lignite, stone coal and anthracite coal.
The external volume of the boiler is 5.4×2×3.2m 5.9×2×3.2m, and the length of the boiler is increased by about one meter.
The environmental protection performance emits black smoke, and the smoke emitted when the environmental protection is not up to standard is colorless, which meets the environmental protection requirements.
The new boiler comprehensively applies modern high-tech and high-efficiency heat transfer technology, organically combines the gas producer and the layer-fired boiler, realizes clean combustion, and can eliminate the smoke and dust in the boiler by itself. During the operation of the boiler, the flue gas is colorless, the flue gas concentration is ≤ 100 mg/nm3, which is 30-50% lower than that of the traditional boiler, and the SO2 concentration is ≤ 1200. The nitrogen oxides are less than 400 mg/nm3, which meets the requirements of Class I areas in national environmental protection standard GB13271-2001,and the thermal efficiency is above 82%. The cost is only less than 10 thousand yuan higher than that of the traditional boiler, but it saves a dust collector. The coal feeding frequency is only 2 ~ 3 times per hour, and mechanized coal feeding and slag discharge can be realized, which greatly reduces the labor intensity of firefighters.
Four, the characteristics of coal gasification split-phase combustion boiler
The traditional way of burning coal will produce a lot of pollutants in the process of burning coal, causing serious environmental pollution. The main reasons are:
(1) coal is not easy to fully contact with oxygen to form incomplete combustion, and the combustion efficiency is low, which relatively increases the pollution discharge;
(2) The combustion process is not easy to control. For example, when a large number of volatiles are precipitated, oxygen supply is often insufficient, resulting in smoke precipitation and black smoke;
(3) The temperature of solid fuel is difficult to be uniform, forming a local high-temperature zone, which promotes the formation of a large number of nitrogen oxides;
(4) During the combustion period when most sulfur in raw coal is oxidized into SO2;
(5) When untreated solid coal is directly burned, a large amount of dust will be discharged with flue gas, resulting in a large amount of dust pollution.
The coal gasification split-phase combustion boiler integrates coal gasification and gas-solid split-phase combustion, which effectively solves the problem of environmental pollution. Compared with the traditional coal-fired boiler, it has the following advantages:
1, low smoke concentration and smoke blackness, and good environmental protection performance.
Gasification gas generated by gasification layer and dry distillation gas generated by dry distillation layer are finally mixed together and fully mixed with secondary air in the combustion chamber. Because it is a gaseous fuel, the oxygen supply is sufficient, and it is easy to achieve complete combustion, so that carbon monoxide and soot burn out. However, the hot coal char entering the combustion chamber from the gasification chamber is separated, which avoids the adverse effect of volatile matter on the combustion of fixed carbon. The remaining volatiles are further oxidized inside the coal char, and the generated combustible substances such as carbon monoxide and soot are burned out when passing through the surface of the coal char layer. In addition, the amount of fly ash produced during coal char combustion is small, and the boiler adopts dust removal technology, which fundamentally eliminates "carbon black" and effectively removes fly ash from flue gas.
2, energy saving and high thermal efficiency.
Coal is fully gasified and pyrolyzed in the gasification chamber before combustion, which not only avoids the adverse effects of volatiles, carbon monoxide and carbon dioxide. It can promote the combustion of coal char, and the hot gas entering the combustion chamber from the gasification chamber is easier to burn, which has a certain promotion effect on the combustion of coal char. The hot coal char entering the combustion chamber removes most volatiles, which is not only high in temperature, but also porous inside, which can enhance the internal and external diffusion oxidation reaction and enhance the combustion of coal char, thus making carbon monoxide and carbon black burn out and burn more fully under the condition of reducing the excess air coefficient, thus reducing the heat loss of chemical and mechanical incomplete combustion and improving the combustion thermal efficiency of coal, saving coal by 5- 10 compared with direct coal combustion.
3. Low emission of nitrogen oxides.
In the gasification chamber, the coal seam is ignited from the lower part and burned at the lower part. The temperature in the gasification chamber is generally low, which belongs to low-temperature combustion. Moreover, the excess air coefficient in the gasification chamber is very small, about 0.7- 1.0, which belongs to low oxygen combustion. This provides favorable conditions for reducing the emission of nitrogen oxides. The chemical dose of organic nitrogen in coal is small, and it is in a reducing atmosphere, and it is only converted into non-toxic nitrogen molecules that do not participate in combustion. Part of the nitrogen oxides contained in coal react to generate nitrogen, water vapor and carbon monoxide under the catalysis of coal seam semi-coke, and part of them are reduced to nitrogen when passing through the upper reduction layer. However, the high-temperature coal char from which most of the volatiles have been removed in the gasification chamber will get sufficient oxygen supply and intensified combustion after entering the combustion chamber, and the remaining small amount of volatiles will be further pyrolyzed and oxidized in the semi-coke, and the nitrogen oxides in the coal char will be further reduced, and the generated soot combustible will be burned out when passing through the surface of the coke layer, thus controlling and reducing the generation and emission of nitrogen oxides.
4. It has a certain desulfurization effect.
Sulfur in coal mainly exists in the form of inorganic sulfur (FeS2 and sulfate) and organic sulfur, while almost all sulfate remains in ash, which will not cause coal pollution. In a coal gasification split-phase combustion boiler, FeS2 reacts with organic sulfur in coal in a gasification chamber, and reacts with hydrogen in coal gas for reduction, so that sulfur in coal is removed and released as hydrogen sulfide gas. Moreover, in the lower part of the gasification chamber, the temperature is generally around 800℃, which is just the best reaction temperature for the desulfurizer to play its role. If coal with high sulfur content is burned, a good desulfurization effect can be obtained by adding a proper amount of limestone or dolomite into the broken coal particles, thus greatly reducing the sulfur dioxide content in the flue gas.
5. Simple operation and control.
Gas is generated and burned in two devices of the same equipment, and there is no need to set up a separate gas ignition device. The gas is automatically ignited by the high-temperature open flame in the combustion chamber, which is easy to operate and control, simplifies the operation management and is convenient to operate, reduces the labor intensity of firefighters, improves the sanitary conditions of the boiler room, and realizes civilized production.
6. Stable combustion and strong adaptability of coal.
Coal is ignited in the lower part of the boiler gasification chamber, so the combustion is stable. Combustible inferior coal mines and coal with high ignition point have strong adaptability, and are suitable for coal in fire-resistant area or medium slagging range. Among them, lignite, long flame coal, unbonded or weakly bonded bituminous coal and small spherical briquette are ideal fuels.
Verb (abbreviation of verb) conclusion
Practice has proved that the new combustion theory and integrated technology composed of many patents ensure the stability and advancement of high efficiency and environmental protection of coal gasification split-phase combustion boiler, overcome the waste and pollution problems that the old technology can not solve, and obtain obvious economic and environmental benefits, which are favored by users. China is rich in coal resources. With the increasingly high energy policy and environmental requirements, the coal gasification split-phase combustion boiler has a very broad market prospect in China.