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Preferably, any fluidized bed boiler designed for biomass fuels includes an “open bottom” design.
This allows the bed material to be removed, even if the fuel contains significant quantities of large inert material, such as rocks and metal pieces.
Details of bubbling fluidized bed (BFB) technology
Figures 1 and 2 illustrate typical BFB designs used for biomass fuels. Large bed material inventory results in significant thermal inertia and evens out temperature fluctuations – which result from the rapid variations of the fuel’s moisture content.
Staged combustion in the furnace promotes good performance. In the fluidized bed area, the oxygen supply is limited and thus, fuel is gasified. Typically, only 40-50 percent of the theoretically required combustion air is supplied to the bed area and is dependent on the heating value of the fuel.
To ensure proper fluidization, attractive emissions and robust operation of the plant, it is better to operate the bed at a constant fluidizing velocity and temperature, regardless
The gasification mode of operation in the bed allows the control of the bed temperature; typical operating temperature is 1,400°F (760°C). Therefore, biomass fuels with low ash melting temperature can be burned without sintering problems. The gases rising from the bed are combusted by adding secondary air to the furnace. The turbulence, in conjunction with an adequate freeboard, results in low carbon monoxide concentration in the flue gas. CO emission of 0.06 lb/mmBtu (26 mg/MJ HHV) is usually guaranteed. For high calorific fuels, recirculation gas is injected below the secondary air to control the combustion process. This staged combustion concept results in a moderate temperature profile in the furnace and ensures low NOX emissions. NOX emissions can be further reduced by installing a SNCR system.
Steam parameters and boiler design are selected based on the corrosiveness of the fuel. For low chlorine, clean biomass fuel steam conditions for up to the standard 2,400 psig/1,000°F (170 barg/540°C) cycle can be provided. For difficult, corrosive fuels, more conservative values are recommended, and an appropriate boiler design needs to be selected, such as shown in Figure 1.
Typical steam parameters range from 600 psig/850°F (41 barg/450°C) to 1,500 psig/950°F (100 barg/510°C).
Details of circulating fluidized bed (CFB) technology
In CFB boilers, fuel is combusted at a moderate temperature of around 1,560°F (850°C). The result is low emissions: NOX generation is low, and by using SNCR, the NOX emissions can be reduced by 60 percent. With woody biomass, NOX emissions in the range from 0.10-0.14 lb/mmBtu (43-60 mg/MJ HHV) were measured without the use of SNCR. In cases where fuels with significant sulfur content are combusted, the higher operating temperature of CFBs compared to BFBs, can capture more than 90 percent of sulfur by simply adding limestone to the furnace. However, biomass fuels typically have very low sulfur content and sulfur emissions are not an issue. CO emissions also are low; emission values of 0.06 lb/mmBtu (26 mg/MJ HHV), or less, can be achieved with biofuels.