Remove Sulfur from the fuel Another alternative is to remove the sulfur from the fuel before it is burned Coal cleaning, Pyritic sulfur can be removed by grinding the coal to a small enough size that the pyrites are mostly present as free pyrite particles. Gravity methods are then used to separate the low-density coal(sg =1. I to 1.3)from the high-density pyrites (s.g. = 5.0). This approach is particularly suited for coals in which a substantial fraction of the sulfur is present as pyrites Unfortunately, pyrite particles are generally quite small, so that very fine grinding is needed to separate them from the rest of the coal Solvent-refined coal. It is also possible to dissolve coal in strong enough solvents and then to treat the solution by the same kind of catalytic hydrogenation processes that are used to remove alfur from petroleum products. The mineral(ash-forming) materials do not dissolve, so they are rejected by filtration or settling When the solvent is then removed for reuse, the remaining product is a very clean-burning combustible solid, free of ash and sulfur, called solvent-refined coal. Considerable development work on this process showed that it can be done, but so far not at a price com rable to"burn and then scrub m pr Modify the Combustion Process The standard way of burning large amounts of coal (pulverized-coal furnace) is to grind the coal to about 50-to 150-H size and blow it with hot air into a large combustion chamber. There the small coal particles decompose and burn in the one to four seconds that they spend in the furnace, transferring most of the heat generated to the walls of the furnace as radiant heat. The furnace walls are made of steel tubes in which fluid(most often water turning to steam)is heated. The hot gases leaving the furnace then pass over banks of tubes and transfer much of their remaining sensible heat to the fluid being heated Fluidized bed combustion Sulfur H2S+O一H2O+S burn coal that is currently Power n the demonstration plant stage. In it, coal is Exhaust burned in gravel-sized Oxygen Coal gasification fuel g C+HO ng them CO+H boiler nto a hot fluidized bed of limestone particles dispersed powder in ai a fluidized bed is a dense suspended in air; such e.g., in fluidized bed catalytic cracking. The furnace,because more time is needed to get complete combustion of the much larger particles o FIGURE IL 10 coal spends much longer Schematic flow diagram of a synthetic fuel gas, gas-turbine steam-turbine power plant(also called a in the bed than it would combined cyele plant). The net power produced is the sum of that from the gas turbine and from the stean n a pulverized In such a fluidized bed combustor SOz is formed in the presence of a large number of limestone particles and has a high probability of reacting with one of them in the combustion bed Here the temperatures are much higher than in the dry processes discussed, and most of the limestone has been converted to CaO, so that the reaction of so2 with Cao is rapid enough to provide adequate SO control. The limestone in the bed is steadily replaced, and the material withdrawn has largely been converted to CaSO4. Here again, a dry powder waste is produced instead of a wet scrubber sludge The fluidized bed has tubes full of water and steam projecting into the bed. The heat transfer between the hot bed in which the coal is burned and the tubes is much better than that between the flames and the walls of an ordinary coal-fired boiler. For this reason fluidized bed combustors are smaller and operate at lower temperatures than ordinary coal-fired boilers This saves on some costs and greatly reduces the formation of nitrogen oxides. These boilers, however, have other problems, so that they are not yet a clear winner over conventional boilers11－13 Remove Sulfur from the Fuel Another alternative is to remove the sulfur from the fuel before it is burned. Coal cleaning. Pyritic sulfur can be removed by grinding the coal to a small enough size that the pyrites are mostly present as free pyrite particles. Gravity methods are then used to separate the low-density coal (s.g. = 1.1 to 1.3) from the high-density pyrites (s.g. = 5.0). This approach is particularly suited for coals in which a substantial fraction of the sulfur is present as pyrites. Unfortunately, pyrite particles are generally quite small, so that very fine grinding is needed to separate them from the rest of the coal. Solvent-refined coal. It is also possible to dissolve coal in strong enough solvents and then to treat the solution by the same kind of catalytic hydrogenation processes that are used to remove sulfur from petroleum products. The mineral (ash-forming) materials do not dissolve, so they are rejected by filtration or settling. When the solvent is then removed for reuse, the remaining product is a very clean-burning combustible solid, free of ash and sulfur, called solvent-refined coal. Considerable development work on this process showed that it can be done, but so far not at a price comparable to "burn and then scrub." Modify the Combustion Process The standard way of burning large amounts of coal (pulverized-coal furnace) is to grind the coal to about 50- to 150-μ size and blow it with hot air into a large combustion chamber. There the small coal particles decompose and burn in the one to four seconds that they spend in the furnace, transferring most of the heat generated to the walls of the furnace as radiant heat. The furnace walls are made of steel tubes in which fluid (most often water turning to steam) is heated. The hot gases leaving the furnace then pass over banks of tubes and transfer much of their remaining sensible heat to the fluid being heated. Fluidized bed combustion is an alternative way to burn coal that is currently in the demonstration plant stage. In it, coal is burned in gravel-sized pieces by injecting them into a hot fluidized bed of limestone particles instead of as a finely dispersed powder in air. A fluidized bed is a dense bed of solid particles suspended in air; such beds are widely used in chemical engineering, e.g., in fluidized bed catalytic cracking. The coal spends much longer in the bed than it would in a pulverized coal furnace, because more time is needed to get complete combustion of the much larger particles. In such a fluidized bed combustor SO2 is formed in the presence of a large number of limestone particles and has a high probability of reacting with one of them in the combustion bed. Here the temperatures are much higher than in the dry processes discussed, and most of the limestone has been converted to CaO, so that the reaction of SO2 with CaO is rapid enough to provide adequate SO2 control. The limestone in the bed is steadily replaced, and the material withdrawn has largely been converted to CaSO4. Here again, a dry powder waste is produced instead of a wet scrubber sludge. The fluidized bed has tubes full of water and steam projecting into the bed. The heat transfer between the hot bed in which the coal is burned and the tubes is much better than that between the flames and the walls of an ordinary coal-fired boiler. For this reason fluidized bed combustors are smaller and operate at lower temperatures than ordinary coal-fired boilers. This saves on some costs and greatly reduces the formation of nitrogen oxides. These boilers, however, have other problems, so that they are not yet a clear winner over conventional boilers