Source: Gao Jianqiang Circulating fluidized bed power generation
China is a major producer and consumer of coal, and its annual carbon emissions are the largest in the world. The coal-fired power generation industry is one of the main sources of carbon emissions in the mainland, accounting for more than 50% of the country's total coal consumption. At present, the mainland is gradually carrying out carbon market research, and the implementation of carbon trading and carbon tax policies is the general trend. In order to reduce the carbon emission cost of the coal-fired power generation industry and improve its competitiveness, it is necessary to accelerate the research and application of carbon emission reduction technologies for coal-fired power plants.
The use of circulating fluidized bed (CFB) boiler units is an important way to solve the problem of the utilization of a large number of inferior coal in mainland China, and it is also an important way to achieve clean coal combustion. CFB boilers can not only burn low-quality coal with low calorific value, but also achieve low-cost emission reductions in NOx and SO2 by means of low-temperature combustion in the furnace and calcination with limestone. In view of the above advantages, CFB boilers are widely used in mainland China, especially in low-quality coal-fired production areas such as Shanxi, Shaanxi and Inner Mongolia. By the end of 2018, the installed capacity of CFB boiler turbines in China exceeded 82.3 million kW, accounting for about 8.16% of the total installed capacity of coal-fired power plants.
In the research on carbon emissions of coal-fired power plants, there are many research results on the carbon emission characteristics, carbon emission intensity calculation methods and main influencing factors and carbon emission reduction measures of pulverized coal boiler units, and the research on carbon emissions of CFB boiler units mainly focuses on the technical route of carbon emission reduction from a macro perspective and the application of carbon capture technology in CFB boiler units. For the carbon emission sources of CFB boiler units, in addition to conventional carbon emission sources such as coal combustion process, the use of limestone as a desulfurizer for desulfurization in the furnace will also produce a certain amount of CO2 emissions, and the high power consumption of the CFB boiler supporting auxiliary system will lead to an increase in power supply coal consumption, which will ultimately affect the carbon emission intensity of the unit. According to the characteristics of CFB boilers, the author analyzes the carbon emission characteristics of CFB boiler units in the coal-burning process and the desulfurization process inside and outside the furnace, establishes a carbon emission intensity calculation model for CFB boiler units, and analyzes the main factors affecting their carbon emission intensity according to the calculation model, so as to provide ideas for on-site operators to reduce the carbon emission intensity of the unit from the perspective of operation.
1. Analysis of carbon emission characteristics
According to the characteristics of CFB boiler system structure, combustion mode and desulfurization mode, the mechanism of CO2 production in the coal combustion process and desulfurization process, as well as the influence of electricity consumption of boiler supporting auxiliary equipment on the carbon emission intensity of the unit, were studied.
1.1 Carbon emissions from the coal-burning process
The chemical equation for the production of CO2 from the combustion of carbon in coal combustion is:
From equation (1), it can be seen that the mass ratio of C to CO2 is 1:1, and the mass fraction ratio is 12:44. CFB boilers are generally designed to be low-quality coal with low carbon content, and the quality of CO2 produced by burning coal per unit mass depends on the carbon content of the coal and the degree of oxidation of the carbon when burned in the boiler. The fossil fuels of CFB boiler units include coal burning and various types of fuel oil, but considering that the fuel consumption is negligible compared to the coal consumption, the author regards the carbon emissions generated by the unit's coal combustion as the carbon emissions generated by fossil fuel consumption.
1.2 Carbon emissions from the desulfurization process
1.2.1 Desulfurization in the furnace
The CFB boiler can achieve desulfurization in the furnace by virtue of the special combustion method and low combustion temperature in the furnace, which is also the unique advantage of CFB boiler in achieving ultra-low SO2 emissions. During the normal operation of the unit, a large amount of limestone is also injected into the CFB boiler as a desulfurizer for in-furnace desulfurization. Limestone, which is mainly composed of CaCO3, is calcined at high temperature to produce CaO and CO2, so the desulfurization process in the furnace is one of the carbon emission sources of CFB boiler units. The chemical equation involved in the desulfurization of the furnace using limestone as the desulfurizer is as follows:
From Eq. (2) ~ Eq. (4), it can be seen that the quantity ratio of CaCO3 to S (hereinafter referred to as the calcium-sulfur ratio, that is, n (Ca)/n (S)) and the quantity ratio of CO2 to S (hereinafter referred to as the carbon-sulfur ratio, that is, n (C)/n (S)) are the same, that is, n (Ca)/n (S) = n (C)/n (S), The mass fraction ratio of S to CO2 was 32∶44. Equation (2) and Equation (3) can be understood as the "preparatory process" for desulfurization in the furnace, and under certain conditions, these two chemical reactions can be carried out completely without being affected by the products. Equation (4) is the most important reaction of desulfurization in the furnace, CaO reacts with SO2 and O2 in the flue gas under an oxidizing atmosphere to form CaSO4, but at the beginning of the reaction, a dense CaSO4 thin layer will be formed on the surface of CaO, and the pores of the thin layer are smaller than the SO2 molecules, which hinders the further diffusion of SO2 into the CaO particles for reaction. The calcium-sulfur ratio in the furnace is affected by a variety of factors, and under normal circumstances, the calcium-sulfur ratio is greater than 2 to achieve a higher desulfurization efficiency.
1.2.2 Desulfurization of flue gas outside the furnace
With the increasing emission standards of air pollutants in coal-fired power plants, CFB boiler units can no longer meet the requirements of ultra-low SO 2 emissions by desulfurization in the furnace alone, so the technical route combining desulfurization in the furnace and flue gas desulfurization outside the furnace is mostly adopted. According to the type of desulfurizer, the flue gas desulfurization outside the furnace can be divided into five desulfurization methods, including the calcium method based on CaCO 3 and the ammonia method based on NH 3, among which the calcium flue gas desulfurization can be divided into limestone-gypsum flue gas desulfurization and semi-dry flue gas desulfurization, among which the calcium flue gas desulfurization process will produce CO2 through different chemical reactions when consuming CaCO 3, so the calcium flue gas desulfurization process outside the furnace is also one of the carbon emission sources of CFB boiler units.
1.3 The impact of energy consumption of auxiliary equipment on carbon emission intensity
The difference between the power consumption of CFB boiler unit and conventional pulverized coal boiler unit is mainly reflected in the difference in the power consumption of auxiliary systems such as air smoke system and coal treatment system, and the comparison of the main auxiliary equipment configuration of the two types of boilers is shown in Table 1. In order to maintain the fluidized combustion and material circulation in the furnace, the flue gas resistance of the CFB boiler is very large, and it is necessary to be equipped with multiple high-power high-pressure head fans, and the fans consume huge energy. At the same time, due to the low calorific value of coal, under the same heat load, more coal is needed into the furnace and the corresponding amount of slag is discharged, which will increase the power consumption of the auxiliary equipment. Under normal circumstances, the high power consumption of the CFB boiler auxiliary system will lead to the power consumption rate of the unit being 2%~3% higher than that of the pulverized coal boiler unit of the same capacity, and the high power consumption rate of the plant will directly lead to the increase of coal consumption for power supply, so that the carbon emission intensity of the CFB boiler unit is at a high level.
2 Carbon emission intensity calculation model
The carbon intensity is numerically equal to the CO2 emissions generated by the unit supplying 1kW ·h of electrical energy to the grid, which can be expressed in M CO2 in g / ( kW · ｈ ）。 There is a linear relationship between the carbon emission intensity of coal-fired units and the coal consumption of power supply, and the proportional coefficient is the CO2 generation coefficient of standard coal. The following is a calculation method for the CO2 formation coefficient per unit of standard coal for CFB boiler units.
2.1 CO2 generation factor per unit of standard coal
The carbon emission sources of CFB boiler units include the coal-fired process, the dry desulfurization process in the furnace and the calcium flue gas desulfurization process outside the furnace, so the sum of the CO2 generation coefficients corresponding to the unit standard coal in these three processes is the unit standard coal CO2 generation coefficient of the CFB boiler unit.
2.1.1 CO2 generation coefficient of coal burning process
From equation (1), it can be seen that when calculating the amount of CO2 generated in the coal combustion process of unit standard coal, it is necessary to determine the converted carbon content w c per unit of standard coal and the burnout of carbon in the furnace, which can be expressed by the carbon oxidation rate O m. The formula for calculating the CO2 formation coefficient K 1 in the coal-burning process per unit of standard coal is as follows:
where: Q net, ar, Q′ net, ar are the low-level calorific value of standard coal and actual coal, kJ/kg, respectively; w ( Crar ) is the received base carbon content of actual coal burning, %.
2.1.2 CO2 generation coefficient of dry desulfurization process in furnace
The CO2 emissions from desulfurization in a unit standard coal furnace are mainly determined by the converted sulfur mass fraction ws and the carbon-sulfur ratio per unit of standard coal, which can be expressed as the calcium-sulfur ratio of dry desulfurization in the furnace. Analogous to the calculation principle of Eq. (5), the formula for calculating the CO 2 generation coefficient K 2 in the dry desulfurization process in a standard coal furnace is as follows:
where: O s is the sulfur oxidation rate, %, which indicates the degree of burnout of sulfur in the furnace in the pulverized coal; w (Sar) is the actual harvested base sulphur fraction of coal, %. Among them, the relationship between the calcium-sulfur ratio and the desulfurization efficiency in the furnace is as follows:
where: η SO2 is the % of desulfurization efficiency in the furnace; A is the self-desulfurization capacity coefficient of coal; B is the desulfurization performance coefficient of limestone.
2.1.3 CO2 in the flue gas desulfurization process of calcium method outside the furnace
The principle of CO2 production is different for different calcium flue gas desulfurization methods with different generation coefficients, so it is necessary to choose the CO 2 emission calculation method according to the specific desulfurization method. When semi-dry flue gas desulfurization is used, CO2 comes from the preparation process of desulfurizer Ca(OH)2, and the CO2 emission is affected by the amount of desulfurizer input, which depends on the sulfur content and calcium-sulfur ratio in the flue gas. At this time, the formula for calculating the CO2 generation coefficient K 3 in the flue gas desulfurization process outside the unit standard coal furnace is as follows:
where: w′s is the remaining sulfur content of standard coal after desulfurization in the furnace; (n(C)/n(S))′ and (n(Ca)/n(S)′ are the carbon-sulfur ratio and calcium-sulfur ratio of flue gas desulfurization by calcium method outside the furnace, respectively. When the limestone-gypsum flue gas desulfurization method is adopted, the amount of CO2 from CaCO3 participating in the desulfurization reaction is 1:1, and the amount of CO2 substances is 1:1 to the amount of SO2 substances absorbed in the desulfurization process, and the magnitude depends on the sulfur content in the flue gas and the desulfurization efficiency. At this time, the formula for calculating the CO2 generation coefficient of the flue gas desulfurization process outside the standard coal furnace is as follows:
where: η′ SO2 is the desulfurization efficiency outside the furnace, %. After obtaining the CO2 formation coefficients corresponding to the unit standard coal combustion process and the two-stage desulfurization process, the CO2 generation coefficient K per unit standard coal of CFB boiler unit is:
It is worth pointing out that when the flue gas desulfurization outside the furnace of the CFB boiler unit does not adopt the calcium flue gas desulfurization method, the flue gas desulfurization process outside the furnace does not produce CO2 emissions, and the CO2 generation coefficient K per unit of standard coal is:
2.2 Carbon emission intensity
The meaning of the CO2 generation factor K per unit of standard coal is the amount of CO2 produced by the unit consuming unit of standard coal, so the carbon emission intensity of CFB boiler units is:
Among them, the power generation efficiency of the unit and the power consumption rate of the plant mainly depend on the load and operation level of the unit.
3 Analysis of influencing factors of carbon emission intensity
The carbon emission intensity of CFB boiler units is affected by a series of factors, such as coal-fired coal quality, desulfurization efficiency, calcium-sulfur ratio, desulfurizer performance, unit load, operating level and power consumption rate, and there is a complex coupling relationship between these factors.
3.1 Coal quality
The design coal type of CFB boiler is low-quality coal with low calorific value, low carbon content and high sulfur content, and the impact of coal quality on the carbon emission intensity of the unit is mainly reflected in: for K 1, because the low calorific value of coal burning is caused by low carbon content, the impact of coal quality difference on K 1 is relatively small; For K2 and K3, the lower the calorific value and the higher the sulfur content, the greater K2 and K3. The deterioration of coal quality will reduce the efficiency of the boiler and increase the output of the pulverizing and desulfurization systems, which will increase the coal consumption of the unit for power supply and increase the carbon emission intensity.
3.2 Desulfurization efficiency
The CFB boiler unit is equipped with a two-stage combined desulfurization system inside and outside the furnace, so there is a problem of desulfurization task allocation, that is, the problem of setting the size of the two-stage desulfurization efficiency. According to equation (7), the desulfurization efficiency inside and outside the furnace is closely related to the corresponding calcium-sulfur ratio, and the dry desulfurization in the furnace has strict requirements for the furnace temperature, and the high-efficiency desulfurization interval is located in the middle and low load stage of the boiler, and the high-efficiency desulfurization interval of the flue gas outside the furnace is located in the medium and high load stage of the boiler. Under the condition that the coal quality and desulfurizer properties remain unchanged, the change of the value of the two-stage desulfurization efficiency under different loads will directly lead to the change of K 2 and K 3, and will also cause the change of the electricity consumption of the corresponding auxiliary equipment, which will ultimately have a certain impact on the carbon emission intensity.
3.3 Calcium-sulfur ratio
When other conditions remain constant, the calcium-sulfur ratio of desulfurization in the furnace is linearly related to K2 (see Eq. (6)), and the calcium-sulfur ratio is an important factor affecting the carbon emission of the desulfurization process. Under normal circumstances, the larger the calcium-sulfur ratio, the higher the desulfurization efficiency, so in the actual operation of some domestic units, in order to ensure that the flue gas emission meets the environmental protection requirements, the calcium-sulfur ratio in the furnace is often set very high, resulting in a large amount of CO2 emissions. When the desulfurization efficiency is constant, the determination of the calcium-sulfur ratio is more complicated, taking the calcium-sulfur ratio in the furnace as an example, the temperature fluctuation in the furnace caused by the change of the unit load or the calorific value of coal, the sulfur content of pulverized coal, the reaction performance and particle size of the desulfurizer, the boiler fluidization rate and the cycle rate and other factors will have a certain impact on the calcium-sulfur ratio.
3.4 Unit load
Unit load is closely related to power generation efficiency, and is the most important influencing factor of unit carbon emission intensity. When the unit is in a low-load state, the efficiency of the steam turbine system is low, the operating level of the unit is low, the power generation efficiency is low, and the carbon emission intensity is high. When the unit is in a high load state, the working fluid parameters are higher, the unit operation level is high, the power generation efficiency is high, and the carbon emission intensity is low. It is worth pointing out that when the load of CFB boiler unit increases, the amount of coal and limestone put into the boiler increases, the amount of ash generated by combustion also increases, the physical heat loss of ash Q 6 of the boiler increases, and the efficiency of the boiler system decreases.
3.5 Electricity consumption of CFB boiler auxiliary system and plant electricity consumption rate
The CFB boiler auxiliary system consumes more electricity and the unit plant uses more electricity, which has a direct impact on carbon emission intensity. At low load, in order to ensure the normal fluidization and sealing of all parts of the boiler, the output of some fans does not decrease with the decrease of load, so the power consumption rate of the plant is very high under low load; In order to maintain the normal fluidized combustion state in the furnace, the output of various fans is larger, and the power consumption of the pulverizing and slag discharge related systems increases, so the power consumption rate of the plant under high load is also at a high level. The reliability of CFB boilers is low, and the frequency of shutdown and maintenance is high, and the shutdown process will consume a lot of electrical energy, which will increase the power consumption rate of the plant.
4 Instance Computing
Taking a 300MWCFB boiler unit in Shanxi Province as an example, the carbon emission intensity calculation model is used to analyze its carbon emission characteristics. The boiler is a 1060t/h subcritical, intermediate reheat, natural circulation CFB boiler, and the data of the boiler design coal type and check coal type are shown in Table 2. The unit adopts a two-stage desulfurization scheme, calcined limestone dry desulfurization is used in the furnace, semi-dry flue gas desulfurization is used outside the furnace, and the chemical composition of the desulfurizer limestone is shown in Table 3.
In order to analyze the impact of different coal types on the carbon emission intensity of the unit, the corresponding unit standard coal CO2 generation coefficient K and other related data were calculated under 100% load. The desulfurization efficiency and calcium-sulfur ratio of different coal types under 100% load are shown in Table 4.
According to Table 2 and Table 4, the corresponding K 1, K 2, K 3 and K of the four coal types are calculated by the carbon emission intensity calculation model, and shown in Figure 1. It can be seen from Figure 1 that the K corresponding to high-quality coal with high calorific value and low sulfur content (check coal type 3) is smaller than that corresponding to low-quality coal with low calorific value and high sulfur content (design coal type). K 1, K 2 and K 3 have certain changes with different coal quality, among which K 1 is always the largest, accounting for 93%~98.6% of the sum of the three, followed by K 2, accounting for 1.27%~6.59% of the sum of the three, and K 3 is the smallest, which indicates that the main carbon emission source of the unit is the coal combustion process, and the carbon emissions generated by desulfurization in the furnace are much greater than those produced by desulfurization outside the furnace. K 2 and K 3 were positively correlated with the sulfur content of coal types, that is, the higher the sulfur content of coal types, the higher the carbon emissions produced by the desulfurization process.
Taking the check coal type 2 as an example, when other conditions remain unchanged, the K corresponding to the average desulfurization efficiency under different calcium-sulfur ratios in different furnaces is shown in Figure 2. As can be seen from Figure 2, when the calcium-sulfur ratio in the furnace increases, the average desulfurization efficiency in the furnace increases, and when the calcium-sulfur ratio is at a low level, the desulfurization efficiency increases greatly, and with the increase of the calcium-sulfur ratio, its influence on the average desulfurization efficiency in the furnace decreases. The change of calcium-sulfur ratio and the average desulfurization efficiency in the furnace will lead to the change of K 2 and K 3, because the increase of the calcium-sulfur ratio leads to the increase of K2 is greater than the decrease of K 3 caused by the increase of the average desulfurization efficiency in the furnace, and finally K increases with the increase of the calcium-sulfurization ratio in the furnace, so the carbon emission intensity of the unit is positively correlated with the calcium-sulfur ratio in the furnace.
The operating data of the load and 50% load are used to calculate the thermal economy and the CO2 generation factor K per unit of standard coal. Table 5 shows the economic indexes of the unit under different loads, and the comparison of the CO2 generation coefficient K and carbon emission intensity M CO2 per unit of standard coal is shown in Figure 3.
As can be seen from Table 5, with the decrease of the load of the unit, the power generation efficiency of the unit decreases, and the power consumption rate of the plant increases, resulting in a significant increase in the coal consumption of the power supply of the unit. As can be seen from Figure 3, K is positively correlated with the load of the unit, which is mainly due to the fact that the desulfurization efficiency in the furnace will decrease due to the increase of furnace temperature at high load, and more limestone desulfurizer needs to be put in order to ensure a certain desulfurization efficiency, so K2 is larger. M CO2 increases with the decrease of unit load, and for every 1% decrease in unit load, M CO2 increases by 2.41g /(kW · h ), especially at low load, M CO2 increases more with the decrease of unit load, which is mainly due to the rapid decline of power generation efficiency of the unit under low load and the large increase in coal consumption for power supply.
Conclusions
(1) The CO2 generation coefficient of unit standard coal corresponding to low-calorific value and high-sulfur inferior coal is greater than that of high-quality coal with high calorific value and low-sulfur content. When the coal quality changes, the CO2 generation coefficient corresponding to the unit standard coal combustion process, the dry desulfurization process in the furnace and the calcium flue gas desulfurization process outside the furnace changes to a certain extent, and the coal-fired process is always the main carbon emission source of the CFB boiler unit, accounting for 93%~98.6% of the total carbon emission intensity, followed by the dry desulfurization process in the furnace, accounting for 1.27% ~6.59% of the total carbon emission intensity, and the carbon emission intensity generated by the calcium flue gas desulfurization process outside the furnace is the smallest.
(2) When the coal quality is unchanged, the CO2 formation coefficient per unit of standard coal increases with the increase of the calcium-sulfur ratio in the furnace, and the carbon emission intensity of CFB boiler units is positively correlated with the calcium-sulfur ratio in the furnace. When the coal quality is unchanged, the CO2 generation coefficient of unit standard coal is positively correlated with the unit load, and the carbon emission intensity of CFB boiler units is negatively correlated with the load, and the carbon emission intensity increases by 2.41g/(kW ·) for every 1% decrease in unit load h ), and the increase of carbon emission intensity with the decrease of unit load is greater at low load.
Bibliographic information
Gao Jianqiang,Song Tongtong,Zhang Xue. Analysis and calculation of carbon emission characteristics of circulating fluidized bed boiler unit[J].Journal of Power Engineering,2021,41(01):14-21.DOI:10.19805/j.cnki.jcspe.2021.01.003.