Текст 13. Fluidized bed boilers

Fluidized bed combustion (сжигание в кипящем слое) was not used for en- ergy production until the 1970’s, although it had been used before in many other industrial applications. Fluidized bed combustion has become very common during the last decades. One of the reasons is that a boiler using this type of combustion allows many different types of fuels, also lower quality fuels, to be used in the same boiler with high combustion efficiency. Furthermore, the combustion tem- perature in a fluidized bed boiler is low, which directly induce lower NOx emis- sions. Fluidized bed combustion also allows a cheap SOx reduction method by al- lowing injection of lime directly into the furnace.

Principles. The principle of a fluidized bed boiler is based on a layer of sand or a sand-like media, where the fuel is introduced into and combusted. The com- bustion air blows through the sand layer from an opening in the bottom of the boil- er. Depending on the velocity of the combustion air, the layer gets different types fluid-like behaviour. This type of combustion has the following merits:

a) fuel flexibility; even low-grade coal such as sludge or refuse can be burned; b) high combustion efficiency; c) low NOx emission; d) control of SOx emission by desulfurization during combustion; this is achieved by employing limestone as a bed material or injecting limestone into the bed; e) wide range of acceptable fuel particle sizes; pulverizing the fuel is unnecessary; f) relatively

small installation, because flue gas desulfurization and pulverizing facilities are not required.

Main types: there are two main types of fluidized bed combustion boilers: Bubbling fluidized bed (BFB) and Circulating fluidized bed (CFB) boilers.

In the bubbling type, because the velocity of the air is low, the medium par- ticles are not carried above the bed. The combustion of this type of boiler is gener- ated in the bed.

The CFB mode of fluidization is characterized by a high slip velocity be- tween the gas and solids and by intensive solid mixing. High slip velocity between the gas and solids, encourages high mass transfer rates that enhance the rates of the oxidation (combustion) and desulfurization reactions, critical to the application of CFBs to power generation. The intensive mixing of solids insures adequate mixing of fuel and combustion products with combustion air and flue gas emissions reduc- tion reagents.

In the circulating type, the velocity of air is high, so the medium sized parti- cles are carried out of the combustor. The carried particles are captured by a cy- clone installed in the outlet of combustor.

Combustion is generated in the whole combustor with intensive movement of particles. Particles are continuously captured by the cyclone and sent back to the bottom part of the combustor to combust unburned particles. This contributes to full combustion.

The CFB boiler has the following advantages over the BFB Boiler:

˗ High combustion efficiency;

˗ Lower consumption of limestone as a bed material;

˗ Lower NOx emission;

˗ Quicker response to load changes

The main advantage of BFB boilers is a much larger flexibility in fuel quali- ty than CFB boilers. BFB boilers have typically a power output lower than 100 MW and CFB boilers range from 100 MW to 500 MW. In recent years, many CFB

boilers have been installed because of the need for highly efficient, environmental- friendly facilities.

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 400º C, 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.