How to calculate the total thermal efficiency of boiler?

The percentage of effective heat utilization per unit time to the input heat of the boiler, or the percentage of effective heat utilization per kilogram of fuel (solid and liquid fuel) or per standard cubic meter of fuel (gas fuel) to the input heat, that is, the thermal efficiency of the boiler, is an important technical and economic index of the boiler, which marks the perfection and operation management level of the boiler equipment. There are usually two methods to measure and calculate the thermal efficiency of boilers:

1. positive equilibrium method

The method of calculating thermal efficiency by the ratio of the heat used by the boiler to the total heat released by the fuel is called positive balance method, also called direct measurement method. The calculation formula of positive equilibrium thermal efficiency can be expressed as follows:

Thermal efficiency = total heat released by effective utilization of heat/fuel * 100% = boiler evaporation * (steam enthalpy-feed water enthalpy)/fuel consumption * low calorific value of fuel * 100%.

Where the boiler evaporation is the actual measured value, kg/h;

Steam enthalpy-obtained from enthalpy-entropy diagram, kj/kg;

Enthalpy of feed water-obtained from enthalpy-entropy diagram, kj/kg;

Fuel consumption-measured, kg/h;

Low calorific value of fuel-actually measured, kj/kg.

The above thermal efficiency formula does not consider the influence of steam humidity, sewage discharge and steam consumption, and is suitable for the rough calculation of thermal efficiency of small steam boilers.

As can be seen from the above thermal efficiency calculation formula, the positive balance test can only get the thermal efficiency of the boiler, but not the total heat loss. Therefore, through the positive balance test, we can only know the evaporation and thermal efficiency of the boiler, but can't find out the reasons and put forward improvement measures.

2. Anti-equilibrium method

By measuring and calculating the heat loss of boiler, the method of obtaining thermal efficiency is called balance method, also known as indirect measurement method. This method is conducive to a comprehensive analysis of the boiler, find out various factors affecting the thermal efficiency, and put forward methods to improve the thermal efficiency. The anti-equilibrium thermal efficiency can be calculated by the following formula.

Thermal efficiency = 100%- sum of various heat loss percentages = 100%-Q2-Q3-Q4-Q5-Q6.

Where Q2—— is the heat loss of exhaust gas,%;

Q3-heat loss due to incomplete combustion of gas,%;

Q4—— Heat loss of incomplete combustion of solid,%

Q5—— heat dissipation loss,%;

Q6—— Physical heat loss of ash,%.

Many times, anti-equilibrium calculation is used to find out the main factors affecting thermal efficiency.

Extended data:

The thermal efficiency of boiler refers to the percentage of effective heat in the heat emitted by fuel.

The thermal efficiency of coal-fired boilers is 70-85%, and the thermal efficiency of oil, gas and electric boilers is 90-99%.

Improving the thermal efficiency of boiler is to improve the effective utilization rate of heat and reduce various heat losses of boiler, with the focus on reducing the heat loss of boiler flue gas and mechanical incomplete combustion loss.

use

(1) Reduce the heat loss of boiler exhaust gas.

1) Reduce the air leakage rate of air preheater, especially the air leakage rate of rotary air preheater.

2) Strictly control the boiler water quality index. When the scaling amount of water-cooled wall pipe reaches 400mg/m3, it should be pickled in time.

3) Try to burn high-quality coal with low sulfur and reduce the inlet air temperature of air preheater. Modern large-capacity power generation boilers are equipped with air preheaters to prevent condensation on the heating surface at the cold end of the air preheaters, resulting in low-temperature corrosion of the air preheaters. The service life of the air preheater is prolonged by increasing the inlet air temperature of the air preheater and the exhaust gas temperature of the boiler (increasing the heat loss of exhaust gas).

(2) Reduce the heat loss of mechanical incomplete combustion.

1) Adjust the combustion condition according to the boiler load and time, distribute air reasonably, reduce the flame center position in the furnace as much as possible, and let the coal burn fully in the furnace.

2) Adjust the coal feeding amount according to the volatile matter and time of raw coal to keep the best value of coal.

(3) Reduce the heat loss of the boiler, and mainly strengthen the maintenance and overhaul of the boiler pipeline and the thermal insulation layer of the body.

Take a 1t boiler as an example, the calorific value of the boiler is 600,000 kcal/hour, and that of natural gas is 8600 kcal/hour.

(1) 600,000 kcal/hour ÷ 8,600 kcal/standard cubic meter ÷ 100%≈69.76 standard cubic meter

That is to say, when the boiler energy is converted into 100% thermal efficiency, the burner can generate 600,000 kcal/hour of heat when burning 69.76 standard cubic meters of natural gas, which can meet the operation requirements of 1 ton boiler.

(2) 600,000 kcal/hour ÷ 8,600 kcal/standard cubic meter ÷92%≈75.83 standard cubic meter

That is to say, when the energy of the boiler is converted into 92% thermal efficiency, the burner can generate 600,000 calories/hour by burning 75.83 standard cubic meters of natural gas, which can meet the operation requirements of 1 ton boiler.

(3) 600,000 kcal/hour ÷ 8,600 kcal/standard cubic meter ÷96%≈72.67 standard cubic meter

That is to say, when the energy of the boiler is converted into 96% thermal efficiency, the burner can generate 600,000 calories/hour by burning 72.67 standard cubic meters of natural gas, which can meet the operation requirements of 1 ton boiler.

The higher the thermal efficiency of the boiler, the lower the gas consumption.

References:

Baidu encyclopedia-boiler thermal efficiency