Fluidized Bed Combustion (FBC) power plants

Fluidised bed combustion (FBC) is a method of burning coal in a bed of heated particles suspended in an upward gas flow (WCI 2005a). At sufficient flow rates, the bed performs as a fluid. The continuous and fast mixing of the particles promotes nearly complete combustion at lower temperatures than in PC combustion. The maximum gas temperature available from the FBC is limited by ash fusion characteristics, therefore the gas turbines differ from those used in IGCCs and GCCs (IEA Clean Coal 2005b). The primary driving force for the development of fluidized-bed combustion was the reduction in SO2 and NOx emissions at the combustor (Bernero 2002). The relatively low combustion temperature (800-900°C) reduces the production of NOx in the outlet gas compared to PC, but increases the amount of the greenhouse gas N2O. FBCs produce dramatically less SOx when limestone or dolomite is continuously added to the coal feed. FBCs can also use a wider range of fuels than PCs (WCI 2005a). Relatively coarse particles at around 3 mm size are fed into the combustion chamber (IEA Clean Coal 2005b). The efficiency of most fluidised beds used for power generation is similar to that of conventional plants (WCI 2005a). FBC technologies include: atmospheric pressure fluidized bed combustion (AFBC) and pressurized fluidized bed combustion (PFBC). The development of this technology has been stimulated by its better environmental performance than PCs with lower grade fuels, in particular high ash coals, and/or those with variable characteristics, although PFBC has also been used on a commercial scale in Sweden and Japan with traded coals of higher quality (IEA Clean Coal 2005b). Between 1985 and 1995, installation of FBCs was rapidly growing, but at present they represent less than 2% of the world total coal capacity (IEA Clean Coal 2005b).

3.1.3.1 Atmospheric fluidised bed combustion (AFBC)

Atmospheric-pressure fluidised bed plants are commercially available in two types:

bubbling-bed (Bubbling-bubbling-bed Fluidised Bed Combustion – BFBC) and circulating-bubbling-bed (CFBC). Both technologies use mainly subcritical steam turbines, together with sorbent injection of limestone or dolomite into the bed for SO2 reduction and particulates (ash together with reacted sorbent) removal from flue gases. Carbon-in-ash losses are higher in FBC residues that in those from PC. Combustion temperatures of AFBC are between 800°C and 900°C. Air staging can further reduce NOx formation. (IEA Clean Coal 2005b) AFBC are particularly suited for high ash coals, coals with variable characteristics and co-combustion of biomass/waste/coal slurries, and any type or size unit can be repowered, which are all advantages on PC (Decon 2003).

3.1.3.1.1 Bubbling fluidised bed combustion (BFBC) at atmospheric pressure

Bubbling beds use a lower fluidizing velocity than circulating beds. The bed has a depth of about 1 m (IEA Clean Coal 2005b). Sand is often used to improve bed stability, together with limestone for SO2 absorption. As the coal particles are burned away and become smaller, they are elutriated with the gases, and ultimately removed as fly ash (IEA Clean Coal 2005b). In-bed tubes are used to control the In-bed temperature and generate steam. The flue gases are normally cleaned using a cyclone, and then pass through further heat exchangers to generate further steam (IEA Clean Coal 2005b).

Atmospheric BFBC is mainly used for boilers up to about 25 MWe, although there are a few larger plants where it has been used to retrofit an existing unit. There are hundreds of such small BFBC units in China. Overall thermal efficiency is around 30% (IEA Clean Coal 2005b). Low capacity units are of lesser interest for European conditions.

The residues consist of the inert material originally in the coal, most of which does not melt at the combustion temperatures used. Where sorbent is added for SO2 removal, there will be additional CaO/MgO, CaSO4 and CaCO3 present. In BFBCs, a much higher Ca/S ratio is needed than in atmospheric CFBC in order to remove SO2. This increases costs, and in particular the cost of residues disposal (IEA Clean Coal 2005b).

3.1.3.1.2 Circulating fluidised bed combustion (CFBC) at atmospheric pressure

Combustion temperatures and NOx/N2O formation are similar to BFBC. Reduced NOx emissions by 60% when compared with conventional PC technology are reported for CFBC (Decon 2003). SO2 emissions can be reduced by the injection of sorbent (limestone or dolomite) into the bed, and the subsequent removal of ash together with reacted sorbent, again similarly to BFBCs.

Circulating beds use a higher fluidizing velocity than bubbling beds. The coal particles are constantly held in the flue gases, and pass through the main combustion chamber with very short residence time and vigorous mixing (IEA Clean Coal 2005b). Immediately following, a cyclone, operating at a temperature near that of the exhaust gas, separates the larger particles (unburned coal and ash) to return them back to the combustion chamber. Individual particles may undergo this process anything from 10 to 50 times, depending on their size, and how quickly the char burns away, with residence times in the bed on the order of tens of seconds (IEA Clean Coal 2005b).

CFBCs are designed for the particular coal to be used; the design must take into account ash quantities and properties.²⁹ Circulating beds are suited for low grade, high ash coals which are

29 “Fuel flexibility often mentioned in connection with FBC units can be misleading.” It does refer to the

difficult to pulverise, and which may have variable combustion characteristics. CFBCs are also appropriate for co-firing coal with low grade fuels, including some waste materials (IEA Clean Coal 2005b).

The finest fly-ash leaves the cyclone with the flue gases, and is normally separated by using an ESP. The fly-ash can contain quite high proportions of carbon, possibly up to 15% (IEA Clean Coal 2005b)

Atmospheric CFBC is used in a number of units around 250-260 MWe size, and there are a number of commercially operating plants (IEA Clean Coal 2005b). New units are being built up to 300 MW_e size, and there are designs for units up to 600 MW_e size (IEA Clean Coal 2005b). However, CFBC boilers are used more extensively by industrial and commercial operators in smaller sizes, both for the production of process heat, and for on-site power supply. A few are used by independent power producers, mainly in sizes in the 50 MW_e to 100 MWe range (IEA Clean Coal 2005b).

In the 100-200 MWe range, the thermal efficiency of FBC units is commonly lower than that for equivalent size PC units by 3 to 4 percentage points (IEA Clean Coal 2005b). The reasons are manifold. In CFBCs, the heat losses from the cyclone are considerable. High heat losses are associated with the removal of both ash and spent sorbent from the system, in spite of the ash heat recovery systems. The use of a low grade coal with variable characteristics tends to result in lower efficiency (IEA Clean Coal 2005b).

The residues consist of the original mineral matter, most of which does not melt at the combustion temperatures used, like in BFBCs. Where sorbent is added for SO2 removal, there will be additional CaO/MgO, CaSO₄ and CaCO₃ present, although in less amounts than in BFBCs (IEA Clean Coal 2005b).

3.1.3.2 Pressurised fluidised bed combustion (PFBC)

Pressurized fluidized bed combustion (PFBC) is a FBC technology where the combustor and hot gas cyclones are all enclosed in a pressure vessel. Both coal and sorbent for sulphur removal have to be fed across the pressure boundary, and similar condition applies for ash removal, which introduces some significant operating complications compared to AFBC designs. With hard coal as fuel, the coal and limestone can be crushed together, and then fed as a paste, with 25% water.

Figure 3.8 shows a simplified flow diagram of a PFBC power plant.

capability to burn different coal qualities but in different appropriate units. “Once the unit is built, it will operate most efficiently with whatever design fuel is specified.” (IEA Clean Coal 2005b)

Figure 3.8 Simplified scheme of a PFBC power plant (WCI 2005a).

As with AFBC, bubbling beds as well as circulating beds are possible. However, currently commercial-scale operating units all use bubbling beds (first generation of PFBC), and hence the name PFBC is normally used to refer to the latter technology (IEA Clean Coal 2005b).

Gas and steam are produced which are driving a combined cycle. The combustion air is pressurized in the compressor section of the gas turbine. PFBC is a combined cycle technology. The proportion of electricity produced at the steam:gas turbines is approximately 80%:20% (IEA Clean Coal 2005b).

Considerable efforts have been made for the development of PFBC during the 1990s especially in Sweden and Japan, with traded coals of high quality, but also in Germany, Spain, and the USA (IEA Clean Coal 2005b). PFBC has been deployed at commercial scale.

However, the number of installations is still small and it is likely to remain a niche technology (PF 03-05 2003).

The pulverized coal is burned at 1-1.6 MPa and at relatively low temperature, approximately 800°C to 900°C (IEA Clean Coal 2005b; Dones et al. 1996). The maximum gas temperature must be kept around 900°C in order to prevent ash softening and alkali metals vaporisation, otherwise they will re-condense elsewhere in the system. As a result, a high pressure ratio gas turbine with compression inter-cooling is used (IEA Clean Coal 2005b).

Limestone is added into the combustion chamber (approximately 6500 kg/GWh_th (PFBC, 1991)), reacting with sulphur to yield calcium sulphate (gypsum). The efficiency of this process depends on the Ca/S ratio and can reach 97% (PFBC, 1991). If an excellent coal quality is chosen, the SOx production may decrease to about 18 kg/GWh_th, whereas for lower fuel quality, the double can be assumed. Conditioned by the low combustion temperature, only very low NOx is produced 36 kg/GWhth, but N2O emissions increase to 72-216 kg/GWhth compared to 1.8 kg/GWh_th generated in PCs (Dones et al. 1996).

From the combustion chamber the flue gas is routed to cyclones and other dust removal

systems. The cleaned flue gas drives gas turbines that contribute 25% to 30% to the total electricity generated by the plant. Moreover, the turbine drives an air compressor to keep the combustion chamber under pressure. Before reaching the stack, the flue gas is further used to preheat the feedwater for the steam cycle.

Heat release per unit bed area is much greater in pressurized systems than in AFBCs. Bed depths of 3-4 m are required in order to accommodate the heat exchange area necessary for the control of bed temperature (IEA Clean Coal 2005b). At reduced load, bed material is extracted (IEA Clean Coal 2005b).

PFBC units are intended to give an efficiency value of over 40%, and low emissions. Current commercial PFBC achieve efficiencies of up to 45% (WCI 2005a).

The current PFBC demonstration units are all of about 80 MWe capacity, but two larger units have started commercial operation in Japan at Karita (New Unit 1, owned by Kyushu Electric Power, July 2001 (WCI 2005b)) and Osaki (IEA Clean Coal 2005b). These are of 360 MWe

and 250 MWe capacity, respectively. The Karita unit uses supercritical steam with 241 bar/565°C/593°C (IEA Clean Coal 2005b; Bernero 2002). The Karita facility uses in-furnace desulphurisation, denitrification equipment, and two-stage cyclones and an electrostatic precipitator to reduce dust emissions. The plant achieves net efficiency levels of around 41% (WCI 2005b).

From the environmental point of view, the technology has several advantages. Practically no thermal NOx is produced due to the relatively low combustion temperatures. However, about 10% of the nitrogen in the fuel is converted to NOx. Conversely, substantial emissions of N2O occur. The in-bed sulfur absorption by dolomite or limestone injection is amplified by the elevated operation pressure. Within the bed operation temperature range, H2S and SO2

generated from fuel-sulfur can be absorbed within the beds. Sulfur exits the PFBC system as solid sulphate with ash, allowing easy handling. Thus, 98-99% of SO₂ can be removed. The result is SO2 emissions of 0.19 g/kWh at 3.65% sulphur content in the coal and 99% SO2

removal (Bernero 2002). Another advantage is that NOx, SOx, and CO are quite independent in a pressurized process, therefore very low emissions of all three pollutants can be achieved at the same time. Since excess oxygen is available in the fluidized bed, H₂S emissions are at or below detectable amounts and carbonyl-sulfide cannot be detected as well. Also the carbon monoxide emissions are negligible (less than 20ppm) under the pressurized operation (Bernero 2002).

As for other FBCs, the residues consist of the original mineral matter contained in the coal feed and additional CaO/MgO, CaSO4 and CaCO3 due to the used sorbent for sulphur removal. For the construction of the plant, a higher amount of high quality steel is required for a PFBC in comparison to a PC because of the higher pressures involved.