and their importance for optimizing efficiency and emissions
Boiler systems provide the necessary thermal energy for power generation, the heating of buildings, cement and glass manufacture and many other industrial applications. With fuels such as coal, oil or gas, they are capable of generating large quantities of energy with good overall efficiency.
Because large quantities of fuel are used for heat generation and large amounts of flue gases arise from the combustion, setting boiler systems involves achieving a high level of efficiency along with the lowest possible pollutant emissions – especially as the statutory emission limit levels for pollutants such as NOX, CO and CO2 are becoming stricter and stricter.
For this reason, the emission values are determined during the commissioning and maintenance of boilers and burners, as well as for official measurements. The cost-effectiveness of the system and the setting of the burner can be comprehensively assessed using these data. It is important to know the basic principles of the combustion process and to understand the influence of the individual measurement and control parameters on performance and pollutant emissions in order to optimize the efficiency of the boiler system and to be able to adjust the emissions to the legal requirements.
Carbon or hydrocarbon compounds are burned with the oxygen in the air to generate heat in boiler systems. Combustion takes place in a sealed combustion chamber. The thermal energy that is created is transferred to a heat transfer medium by means of a heat exchanger and taken to its destination point. Solid fuels are burned either in a fixed bed, a fluidized bed or in an entrained dust cloud, liquid fuels are sprayed via a burner into the combustion chamber as a mist and gaseous fuels are already mixed with the combustion air in the burner.
The other system components ensure the supply and distribution of the fuel, the transfer and dissipation of the heat and the discharge of combustion gases and combustion residues, such as ash and slag.
Combustion gives rise to numerous substances which are discharged from the combustion chamber as flue gas. Water vapour and carbon dioxide (CO2) constitute the largest proportion of the flue gas or exhaust gas. These arise as reaction products from fuel and combustion air. Depending on the air supply, the flue gas also contains nitrogen oxides (NOX) or carbon monoxide (CO) and incompletely combusted fuel components. Impurities in the fuels may also mean the flue gas contains hydrogen sulphide, sulphur oxides, hydrofluoric acid (HF) and hydrochloric acid (HCI), and in addition soot, heavy metals and particulate matter are often to be found.
The combustion air itself is made up of several substances. It above all contains nitrogen (N2) and oxygen (O2), a variable proportion of water vapour and traces of carbon dioxide (CO2), hydrogen (H2) and noble gases. With the exception of the oxygen and small amounts of the nitrogen, these components are also to be found again in the flue gas.