THE ENERGY INDUSTRY TIMES - OCTOBER 2016 SCR (if needed) FGD are not needed for NOx and SOx control, dramatically reducing plant construction and operating cost and water consumption, while improving plant reliability and efficiency. Since the fuel ash does not soften or melt in a CFB, the size of the furnace does not grow as much as PC boilers when firing lower quality fuels. In order to control fouling, slagging and corrosion, a PC furnace height typically increases by 45 per cent and its footprint by more than 60 per cent when firing low quality fuels such as Special Technology Supplement Reheater instead of an open flame, circulating solids are used to achieve high combustion and heat transfer efficiency to burn a wide range of fuels. The fuel’s ash does not melt or soften, which allows the CFB to avoid the fouling and corrosion problems encountered in conventional boilers. From an environmental aspect, the low temperature CFB combustion process minimises NOx formation and allows limestone to be fed directly into the furnace to capture SOx as the fuel burns. In most cases, a SCR and high sodium lignite. With a CFB, boiler height increases by only 8 per cent and its footprint by only 20 per cent. This results in a smaller and lower cost CFB boiler as compared to the PC boiler for lower quality fuels. Further, unlike a PC, a CFB does not need soot blowers to control the build-up of deposits and slag in the furnace since the circulating solids keep the furnace walls, panels and steam coils clean for efficient heat transfer. Another important advantage of CFB boilers is their ability to withstand the corrosion that can occur when certain fuels are burned under high temperature and pressure. In a boiler, final superheat and reheat steam coils operate at the highest metal temperatures in the boiler making them the most vulnerable to corrosion and fouling attack. In a PC or oil/gas boiler, these coils are hung from the furnace ceiling and are directly exposed to the slagging ash and corrosive gases (sodium and potassium chlorides) in the hot furnace flue gas. To cope with this undesirable situation, boiler designers use expensive high-grade alloys and recommend a high level of cleaning and maintenance for these coils. This is avoided in Amec Foster Wheeler’s CFBs by submerging these coils in hot solids, fluidised by clean air in heat exchanger compartments called INTREXs, protecting them from the corrosive flue gas. The bubbling hot solids efficiently conduct their heat to the steam contained in the coils and since the solids never melt or soften, fouling and corrosion of these coils are minimal. Further, due to the high heat transfer rate of the solids (via conduction heat transfer), the final superheat and reheat coil sizes are many times smaller than the pendent and convective coils in PCs saving more capital and operating cost. Looking forward, Giglio believes utility-scale CFBs will continue to operate with supercritical steam conditions and will be increasingly used in different parts of the world. While some argue that countries should be moving away from burning coal, lignites and other high carbon fossil fuels, Giglio favours a more pragmatic approach. “I think it’s an important policy for countries that can afford to do that. “Electricity demand in mature economies like the US and Europe is not growing so strongly. They are seeing a green revolution, where old coal plant is being replaced with green renewables, which is good. But these countries can afford it. “Different countries, however, have different goals and different levels of development. Going to countries like India and Vietnam, where coal is their lowest-cost option for largescale power, and telling them they have to build more expensive and smaller scale renewables or use gas that they don’t have, and then telling the people that as a result they won’t get the power because they can’t afford it, is very unfair. “It’s like forcing you to go into a car dealership where the only car offered for sale is an electric vehicle that you have to pay an extra €20 000 for… it’s all about giving people choices that make sense to them.” By all predictions, coal will continue to be burned for some time to come. In this scenario, operators will be looking for technologies that can burn all types of coal while providing the type of environmental futureproofing that CFBs can provide without operators needing to make any changes to their plant. Having reached a size where they offer a real alternative to PC boilers, equipment suppliers are likely to continue to look at how CFBs can bring more value to a power sector where flexibility is becoming increasingly important. As Giglio summarised: “The technology is at a point where it has reached its apex in terms of advancement. Continuing to improve reliability and expanding the fuel range is really what our focus is.” PC vs. CFB boiler design feature comparison Once-through supercritical CFB technology was first demonstrated at the 460 MWe Łagisza plant in Poland Finishing Superheater Sootblowers Milling and Fuel Transport Systems SCR Low NOx Burners & OFA PC Boiler Secondary Superheater Primary Superheater Vertical Tube Evaporator Super/Reheaters Protected in INTREX CFB Boiler SNCR NH3 injection Gravity Feed Fuel System Primary Superheater Cyclones with Super/Evap Surface CFB Process Video PC Process Video Cross-Over Duct with Superheat Surface Economizer Economizer Air Heater Air Heater Technology value points (M$) PC CFB Comment Boiler cost savings 60 Installed boiler cost saving of PC without SCR compared to CFB boiler, with both firing the same 4500 kcal/kg sub-bituminous Indonesian coal.Savings based on average $100/kWe discount for PC seen in market pricing over 2015-2016 period. Fuel flexibility 156 NPV of fuel cost savings for CFB assuming average $10/tonne fuel cost discount over 30-year plant life due to CFBs ability to fire lower quality fuels. Based on $50/40/tonne (4500/4000 kcal/kg) coals. SCR cost savings 36 NPV cost saving for avoiding SCR in CFB to achieve 200 mg/Nm3 stack NOx emission. Based on a $40/kWe installed SCR cost for PC plus 30-year NPV of SCR operating expenses. Post boiler FGD savings 39 NPV cost savings for avoiding post boiler FGD for CFB plant to achieve 400 mg/Nm3 stack SO2 emission. Based on $60/kWe installed seawater scrubber cost for PC plus 30 year NPV of FGD operating expenses. Biomass co-firing 35 NPV of future CO2 credits. Based on 10 per cent biomass co-firing in CFB and average $10/tonne for future CO2 credit value over 30-year plant life. Total value (M$) 60 266
Energy Industry Times October 2016
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