Energy Efficiency of a Steam Boiler
The energy efficiency of a steam boiler is determined by the amount of steam that is actually generated and not lost through heat loss. This is not the same as thermal efficiency, which is the amount of heat that is converted from fuel into usable steam. Fuel-to-steam efficiency is a very important measurement to understand and use because it helps in making sound economic decisions when purchasing equipment or considering retrofits that will save fuel.
Steam is a highly efficient heat carrier and requires very little energy to produce. This is why many processes in industry are powered by steam generators. In general, a Energy Efficient Steam Boiler consumes energy to drive the pump that is required for insertion of the feed water into the combustion chamber and to create high pressure. The more pressure the system operates at, the higher the own power demand (Table 3.3).
This energy is used to generate steam and heat the working fluid. It is also consumed to operate the air pollution control (APC) system and to maintain a sufficient level of steam vapor in the flue gas stream. The steam vapor content is dependent on the combustion conditions and can be improved by proper fuel preparation, sizing, and control.
What is the Energy Efficiency of a Steam Boiler?
In the case of a natural gas boiler, the energy consumed is in addition to the energy required for production of the heat carrier itself and of the combustion products. These include water vapour, carbon dioxide, and nitrogen oxides. The emissions are related to the combustion conditions, the type of fuel used, the combustion equipment design, and the operating parameters.
Another significant factor that influences boiler efficiency is the temperature of the flue gases as they exit the stack. The flue gas temperature is influenced by the air-to-fuel ratio and the burner design. A lower flue gas temperature reduces dry gas loss, water-from-fuel loss and unburned carbon in the residue. A lower exhaust temperature also reduces the energy consumption of the economizer and other systems that recirculate the flue gases.
An economiser is a heat exchanger that transfers heat energy from the flue gas into the boiler feedwater. This enables the system to achieve low stack gas temperatures without reducing the overall combustion efficiency. A typical savings of 5% in energy is achieved by the installation of an economiser.
The last main contributor to boiler efficiency is the combustor performance. The three main combustor losses are the flue-gas exit temperature, excess air and burnout. A good design can achieve low flue-gas exit temperatures for any of the technologies, but the two factors that are specific to the combustor technology are the excess air and burnout. Too much excess air deteriorates efficiency, while too little increases the quantity of unburned carbon in the residue and decreases the oxygen content in the stack gas. The optimum balance between the three must be found for each technology to maximize efficiency.
