Текст 14. Heat recovery steam generators (hrsg)

As the name implies, heat recovery steam generators (HRSGs) are boilers where heat, generated in different processes, is recovered and used to generate steam or boil water. The main purpose of these boilers are to cool down flue gases produced by metallurgical or chemical processes, so that the flue gases can be ei- ther further processed or released without causing harm. The steam generated is only a useful by-product. Therefore extra burners are seldom used in ordinary HRSGs. HRSGs are usually a link in a long process chain, which puts extremely high demands on the reliability and adaptability of these boilers. Already a small leakage can cause the loss of the production for a week. Problems occurring in the- se boilers are more diverse and more difficult to control than problems in an ordi- nary direct heated boiler.

HRSGs in power plants

Gas turbines and diesel engines are nowadays commonly used in generating electricity in power plants. The temperature of the flue gases from gas turbines is usually over 400ºC, which means that a lot of heat is released into the environment and the gas turbine plant works on a low efficiency. The efficiency of the power plant can be improved significantly by connecting a heat recovery boiler (HRSG) to it, which uses the heat in the flue gases to generate steam. This type of combina- tion power generation processes is called a combined cycle.

Since the flue gases of a gas turbine are very clean, tubes can be tightly seat- ed or rib tubes can be used to improve the heat transfer coefficient. These boilers are usually natural circulation boilers. If the life span of the power plant is long enough, the boiler is usually fitted with an economizer. If more electrical power output is wanted, but the temperature of the flue gas is insufficient, the boiler can be equipped with an extra burner (that burns the same fuel as the gas turbine) in

order to increase the flue gas temperature and thus generate steam with a higher temperature.

Текст 15. Refuse boilers

The standard refuse (or waste) recovery boiler incinerates solid or liquid waste products. This boiler type is not to be mixed with the recovery boilers used in pulp and paper industry. Therefore, we will always refer to refuse boilers when talking about waste and recovery boilers when we mean the specific chemicalre- covery process used in the pulp and paper industry.

The combustion of waste differs radically compared to other fuels mostly due to the varying properties of waste. Also, the goal when combusting waste is not to produce energy, but to reduce the volume and weight of the waste and to make it more inert before dumping it on a refuse tip.

Waste is burned in many ways, but the main method is to combust it in a grate boiler with a mechanical grate. Other ways to burn waste is to use a fixed grate furnace, a fluidized bed for sludge or rotary kilns for chemical and problem- atic waste. Waste is usually “mass burned”, i.e. it is burned in the shape it was de- livered with minimal preparation and separation. The main preparation processes are grinding and crushing of the waste and removal of large objects (like refrigera- tors). Waste has to be thoroughly combusted, so that harmful and toxic compo- nents are degraded and dissolved.

Waste can be refined into fuel, by separating as much of the inert and inor- ganic material as possible. This is called refuse derived fuel (RDF) and can be used as the primary fuel in fluidized bed boilers or burned as a secondary fuel with other fuels. RDF is becoming more common nowadays.