Key learnings:
- Pulverized Fuel Firing Definition: Pulverized fuel firing involves grinding coal into fine particles and spraying them into the combustion chamber for efficient burning.
- Improved Thermal Efficiency: Pulverized coal increases the surface area for combustion, leading to higher thermal efficiency and faster combustion rates.
- Fuel Versatility: Lower grade coal can be effectively used once pulverized, making the system flexible.
- Quick Load Response: The system can quickly adjust to changing loads, providing stability to the electrical grid.
- Higher Costs and Safety Risks: Although efficient, the system has higher initial and running costs, and requires careful handling to prevent fire hazards and explosions.
Grate or other lump-coal firing is slow to follow load changes and is rarely used for large thermal power plants. Incomplete burnout also leaves more carbon in the ash and lowers thermal efficiency.
Pulverized fuel firing grinds coal to a fine powder, then blows that powder into the furnace with dry hot primary air. Secondary air is added to finish combustion. Finer coal raises the burning surface and usually raises furnace efficiency, so this method can burn coals that do not suit grate firing. It remains a common utility-boiler method; other firing systems still exist.
Advantages of Pulverized Fuel Firing
- Pulverizing coal increases its surface area, so the coal burns faster and more completely. That can cut the excess air needed and ease the duty on the forced-draught fans.
- Lower-grade coal can often be burned more completely once it is pulverized, provided the mill and burners are designed for that fuel.
- The faster burn rate lets the operator change firing rate more quickly when the load changes.
- A dry-bottom pulverized-coal furnace avoids the large clinker beds of a grate, but slagging and fouling can still occur on furnace walls and heat-transfer surfaces.
- A pulverized-coal furnace of a given size can release more heat than a grate-fired furnace of the same size, because combustion occurs in suspension through the furnace volume.
- A pulverized-coal boiler can be brought from cold to load faster than a comparable grate-fired unit. That faster start helps a plant follow grid demand.
- Another feature of the pulverized firing system is that the furnace itself has no grate or stoker moving parts, which reduces in-furnace mechanical wear.
- Ash handling is different rather than simpler: most ash leaves as fly ash in the flue gas, and a smaller share falls as bottom ash.
Disadvantages of Pulverized Coal Firing
- The first cost of a pulverized-coal firing plant is higher than that of a simple grate-fired plant of similar steam duty, because mills, burners and fans must be added.
- Running cost is also higher, mainly from mill power, wear parts and the extra draught plant.
- Pulverized coal produces a large share of fly ash.
- Removing that fly ash from the flue gas needs a dust collector such as an electrostatic precipitator or a fabric filter, which adds plant cost.
- Fine dry coal dust can ignite or explode if the mill or duct mixture is not kept inside the safe air-to-fuel range.
- Pulverized coal should not be stored in bulk for long periods. Mills and hoppers need fire and explosion protection.
