Discussion on thermal efficiency of circulating fluidized bed boilers
at present, two standards are mainly used for performance assessment of circulating fluidized bed boilers in China. One is based on asme-ptc42008 performance test standard, and the other is based on gb10184 combined with dl/t  due to the inconsistency of assessment standards and different agreed boundary conditions, the efficiency indicators have poor reference, and even the same unit is assessed according to different standards, The difference in efficiency value is more than 1%, which is extremely unfavorable to the economic analysis of the power plant
2. Several notes
in order to facilitate the analysis, the following problems are explained:
2.1 positive balance method and negative balance method
efficiency assessment mainly includes positive balance method and negative balance method . Due to the large error of positive balance method, it is not conducive to the analysis of factors affecting boiler efficiency, Therefore, the counter balance method is widely used in the industry at present, that is, boiler thermal efficiency = (1- sum of all losses/input) × 100%
all analyses in this paper are based on the counter balance method. The net efficiency and gross efficiency of
net efficiency refers to taking the heat release of boiler fuel as the only heat input, An efficiency algorithm that takes other heat (such as the heat of fuel sensible heat atomizing medium, etc.) as the intermediate process of the whole energy cycle  gross efficiency is an algorithm that includes the sensible heat of fuel sensible heat air and other heat entering the boiler into the boiler heat input.
using fuel efficiency is very helpful to calculate the coal consumption of power plants, that is, the heat input of fuel multiplied by the boiler efficiency is the heat output, which is very intuitive and concise, Therefore, this paper takes the net efficiency as the benchmark for analysis
2.3 high and low thermal efficiency
asme standard adopts high thermal efficiency for assessment, that is, the input heat of fuel is calculated to contain the latent heat of vaporization , while China's standard adopts the low thermal efficiency of fuel for assessment. It is well known that the latent heat of vaporization in fuel cannot be used in utility boilers at this stage because the exhaust gas temperature of boiler is usually higher than the water dew point temperature, Therefore, it is more intuitive and convenient to adopt low-level thermal efficiency, which is more conducive to economic analysis. Even if the ASME standard is adopted for assessment in China, it should be converted to low-level thermal efficiency, which has little impact on the economic accounting of boilers
as the concept of ASME and the national standard performance assessment test is basically the same, most calculation methods are interlinked, so this paper will not list the efficiency assessment calculation methods one by one, Only the main differences between the two are compared. It is hereby explained that
3. The deviation of CFB thermal efficiency caused by the assessment standard
is different from the pulverized coal boiler. The pressure head of the primary and secondary fans of the fluidized bed boiler is high, resulting in the temperature of the cold air entering the boiler is much higher than the ambient temperature. Taking the 150MW fluidized bed boiler as an example, the temperature of the primary fan usually rises by 20 ℃, and the temperature of the secondary fan rises by about 15 ℃, This is different from the pulverized coal boiler. For the temperature rise of the fan, the efficiency loss calculated by the two standards is very different, which is mainly reflected in the Q2 loss item, that is, the boiler exhaust heat loss
exhaust heat loss is the heat loss taken away by the flue gas generated by combustion . According to asmeptc42008, The exhaust gas loss is obtained by subtracting the exhaust gas enthalpy corresponding to the boiler exhaust gas temperature from the exhaust gas enthalpy corresponding to the air preheater inlet cold air temperature. According to the provisions of gb10184 and dl/t, the exhaust gas loss is the exhaust gas enthalpy corresponding to the boiler exhaust gas temperature minus the ambient temperature exhaust gas enthalpy
for example, when the ambient temperature is 20 ℃, the air preheater inlet cold air temperature is 35 ℃, and the exhaust gas temperature is 130 ℃, According to ASME standard, the exhaust gas loss is calculated as the exhaust gas enthalpy corresponding to the temperature difference of 95 ℃, while according to the national standard, the exhaust gas enthalpy corresponding to the temperature difference of 110 ℃, with a difference of 15 ℃ and an efficiency difference of about 1%
for the problem of temperature rise caused by fans, the national standard and ASME have different ideas. The national standard treats the temperature rise of fans as an external heat source, that is, it is included in the heat input item in the gross efficiency, It is equivalent to increasing the denominator of efficiency. ASME treats the temperature rise of the fan as heat gain and directly deducts this part of heat from the Q2 loss item, which is equivalent to reducing the numerator in the loss item.
for the above two assessment methods, the author believes that the ASME method is more scientific. At the same time, the reasons are as follows:
1) the energy of the temperature rise of the fan comes from the auxiliary power, and in the energy balance of the whole power plant, The auxiliary power has been eliminated as a loss item, and the heat gain of the fan is equivalent to the heat recovered from the loss item of auxiliary power. Since the heat input in the fuel efficiency algorithm only considers the heat release of the fuel, it is reasonable to deduct the recovered heat from some flue gas losses, The energy balance of the whole power plant is closed-loop
2) considering this part of heat as input heat after deceleration by the deceleration system in the national standard is actually inconsistent with the algorithm of net efficiency. In the actual assessment, the treatment method that ignores this part is usually adopted, that is, after deducting the energy consumption of the fan in the auxiliary power, the part of heat converted into heat energy and returned to the boiler is ignored, From the perspective of energy balance, the algorithm has certain loopholes
3) the size of smoke loss is an important indicator of the boiler's ability to cool the flue gas. The theoretical limit value of smoke exhaust temperature can only be the inlet cold air temperature of the air preheater, not the ambient temperature. From this point of view, the ASME calculation method is more referential
4. The influence of the selection of slag discharge temperature
the bottom slag ratio of the fluidized bed boiler is much higher than 10-15% of that of the pulverized coal boiler, Usually about 50%, while domestic fluidized bed boilers mainly use low calorific value and high ash fuel, and the weight of sensible heat loss of bottom slag is greater. The calculation method of bottom slag loss is closely related to the type of slag cooler used. At present, there are mainly three types of slag coolers: air-cooled and water-cooled, and water-cooled drum slag coolers are mainly used for domestic large fluidized bed boilers, Taking the water-cooled drum slag cooler as an example, the following two methods are used to assess the Q6 loss specified in the bidding of large domestic fluidized bed boilers and the boiler technical agreement:
1) the furnace slag discharge temperature is used for calculation. This algorithm defines the performance assessment limit point at the inlet of the slag cooler, That is, part of the slag cooler is isolated outside the system
2) the slag discharge temperature at the outlet of the slag cooler is used for calculation. The algorithm defines the performance assessment limit point at the outlet of the slag cooler, that is, the slag cooler is divided within the system
due to different agreed boundary conditions, the performance assessment efficiency value varies greatly. Of course, this value has a great relationship with fuel ash desulfurization products and fuel calorific value. The higher the converted ash content, the greater the impact, Taking a 300MW gangue fuel fluidized bed boiler as an example, the fuel is gangue with a low calorific value of 3150kcal/kg and an ash content of 46%, and its bottom slag volume is as high as 67t/h. If the slag discharge loss is calculated according to the slag temperature at the outlet of the slag cooler (150 ℃), its Q6 loss is only 0.24%. If the furnace slag discharge temperature (890 ℃), its Q6 loss is as high as 2.14%. The efficiency difference caused by the two algorithms is close to 2%
as for which method is more reasonable, Let's first take a look at the cooling water system of the slag cooler. Generally, the cooling water is mainly from the boiler condensate, and the cooling water is returned to the steam turbine low pressure heater system after passing through the slag cooler. Because the heat recovered by the slag cooler is not directly returned to the boiler, that is, it is not used by the boiler system, and it does not contribute to the superheating system and reheating system, so the boiler coal consumption cannot be directly reduced, Therefore, it is reasonable to calculate the efficiency by using the boiler slag discharge temperature.
the reason why the slag cooler is included in the boiler thermal system is that the heat sent by the slag cooler to the steam turbine regenerative system is considered as the heat output, that is, the boiler heat output includes three parts: superheated steam, reheated steam (if any) and the heat returned by the slag cooler. This algorithm is reasonable in some aspects, but it is not conducive to the energy balance calculation of the whole plant, Because this scheme not only sends some heat back to the system, but also has a certain impact on the cycle efficiency, which will make the energy balance calculation of the whole power plant very complicated, but will not affect the application life of the internal electrical parts and transmission parts of the equipment for a long time. Therefore, it is not recommended to use
except for the above two cases, There are also the following special cases:
1) the heat of slag cooler is used as heat supply or industrial gas. In this case, wumaosen, general manager of Chalco asset management company and deputy leader of Chalco Shanxi enterprise turnaround transformation and upgrading working group, has not contributed to the primary and secondary steam, but the heat has not been lost. The author believes that it is a reasonable algorithm to consider it as the heat gain of the whole plant, That is, it is more reasonable to calculate the loss first and then the income.
2) the heat recovered from the air-cooled part of the air-cooled slag cooler and the air-water slag cooler is directly returned to the furnace, so all algorithms include the heat recovered from the stroke of the air-cooled slag cooler or the air-water combined slag cooler into the boiler efficiency. This algorithm is not controversial at present.
due to different standards and different assessment conditions, At present, the efficiency assessment values of fluidized bed boilers in various domestic projects can not be directly compared, which is not conducive to economic analysis. For the two problems mentioned in this paper, the author prefers ASME standard method to calculate Q6 in combination with furnace slag discharge temperature for performance assessment