Boiler efficiency has a direct impact on overall plant economics. It can be measured in terms of the combustion process or by the transfer of heat to the boiler water.
Thermal efficiency is a measure of the heat exchange between the combustion process and the water in the boiler. Expressed as a percentage, it is a straightforward ratio between the energy transferred to the water during the heating process and the energy provided by the heat source.This measure of boiler efficiency has its limitations, as it does not take into account heat losses via other boiler components. It is therefore not an accurate measure of the true efficiency of heat exchange.
Combustion efficiency measures the degree to which a boiler converts the latent energy in fuel into usable heat. Optimal combustion efficiency is achieved when all fuel is burned without the oversupply of air in the combustion chamber.A shortage of oxygen will result in some fuel left unburned and the extent will be evident in the level of carbon dioxide in the flue gas. An oversupply of oxygen will result in too much heat being wasted, with a corresponding drop in combustion efficiency. The correct amount of air needed for optimum combustion is determined by measuring stack temperature, and the concentration of both oxygen and carbon dioxide in the flue gas.
This measure attempts to correct the inaccuracy of the thermal efficiency calculation. It accounts for heat loss through radiation and convection when measuring the efficiency of heat exchange.There are two ways to measure fuel-to-steam efficiency. The first is known as the Input-Output Method. This method expresses the efficiency as a percentage of the boiler thermal energy output to the input. The second is the Energy Balance Method. When using this method, overall boiler efficiency is calculated by subtracting the loss in efficiency from stack, radiation and convection heat losses from an ideal efficiency. Stack heat loss is directly proportional to the stack temperature. Radiation and convection heat losses are established as a constant for any particular boiler configuration.