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ADVANCED COAL-BURNING POWER PLANT TECHNOLOGY. Traditional coal-fired power plant suffers from two primary drawbacks: overall thermal efficiency limited major source of pollution There are strategies to reduce levels of pollution immediately in traditional plants.
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ADVANCED COAL-BURNING POWER PLANT TECHNOLOGY • Traditional coal-fired power plant suffers from two primary drawbacks: • overall thermal efficiency limited • major source of pollution • There are strategies to reduce levels of pollution immediately in traditional plants. • However, very little can be done to raise its efficiency, being limited by thermodynamic constraints. • Efficiency of 49-50% feasible within 20 years. Circulating fluidized bed power plant by B&W
Advanced Coal Technology, Its Uses • Advanced Coal-Burning Technologies • Principle of Fluidized Bed Combustion • Fluidized Bed Schematic and Examples • Principle of Integrated Gasification Combined Cycle • IGCC Project – 250 MW Tampa, Florida • Cost of Coal-Fired Plants and SO2 Removal • Applicability, Advantages, Disadvantages • Environmental Impact & Risks Topics – Advanced Coal
Advanced Coal-Burning Technologies • 1) Fluidized bed combustion - a layer of sand and fuel with high pressure air blown through it forms a floating bed - burns variety of coals and poorer fuels like coal-cleaning waste, petroleum coke, wood and other biomass. • 1.a) Atmospheric fluidized bed (AFB): 1 bar (1 atm) - Bubbling bed (BB) – simplest and most widely use - Circulating fluidized bed (CFB) – more complex • 1.b) Pressurized fluidized bed (PFB): 5 to 20 bar - Bubbling bed (BB) - Circulating fluidized bed (CFB)
Advanced Coal-Burning Technologies • 2) Integrated gasification combined cycle (IGCC) - based on gasification of coal and combined cycle - converts coal into a mixture of hydrogen [H2] and carbon monoxide [CO] which are both combustible - heat generated by the gasifier is used to raise steam to drive a turbo-generator - gas produced is cleaned and burned in a gas turbine to produce electricity; exhaust heat is recovered to raise additional steam for power generation.
A. Fluidized Bed Combustion • Layer of sand, finely ground coal or any fine solid material is placed in a container and high pressure is blown though it from below • Small particles become entrained in the air and form a floating or fluidized bed of solid particles that behaves like a fluid that constantly move and collide with one another • Bed contains only 5% coal and the balance are inert materials like ash or sand; low temperature of bed (950 C) significantly lowers NOX formation • Limestone (CaO) may be added to the bed to capture sulfur and form gypsum, thus reducing SO2 emissions: SO2 + ½ O2 + CaO CaSO4 + 6733 btu/lb S • Boiler pipes immersed in the bed captures the heat given off and raises thermal efficiency.
Fluidized Bed Combustion Efficiency • Bubbling bed can achieve 70-90% sulfur removal • Circulating bed can achieve higher removal of 90-95% with C/S mole ratio of 2.0-2.5 • Thermal efficiency similar to traditional pulverized coal plant (47%) • With pressurization, capturing the vented exhaust gases thru a gas turbine will raise efficiency to 50% • Usual capacity of 200 MW; larger 350 MW units begin developed
Fluidized Bed Schematic Circulating fluidized bed Bubbling bed schematic
55 MW Circulating Fluidized-Bed Combustion for low volatile bituminous coal
Advanced Coal-Burning Power Generation Technology Asia’s Largest CFB400 MW Tonghae Plant, Korea
B. Integrated Gasification Combined Cycle (IGCC) • IGCC - advanced coal burning plant based on the gasification of coal, an old technology used to produce town gas until natural gas came • Modern gasifiers convert coal into a mixture of hydrogen [H2] and carbon monoxide [CO], both of which are combustible C + O2 CO2 (complete combustion) C + ½ O2 CO (incomplete combustion) CO + H20 CO2 + H2 (water-gas shift) • Gasification takes place by heating the coal with a mixture of steam [H2O] and oxygen [O2] or air [21% O2, 79% N2]. This can be carried out in a fixed bed, fluidized bed or an entrained flow gasifier.
Integrated Gasification Combined Cycle Efficiency • Partial combustion of coal takes place in the gasifier, releasing considerable amount of heat that is used to generate steam to drive a turbo-generator: C + ½ O2 CO + 4347 btu/lb C • Gas produced is cleaned and burned in a GT to produce more electricity and the heat from GT exhaust is recovered in a waste heat boiler to raise additional steam for power generation (combined cycle). • IGCC can already achieve 45% efficiency and will reach 50-51%. It can remove 99% of sulfur from coal and reduce NOX emission to below 50 ppm.
IGCC Projects (US DOE) • Tampa Power Station in Mulberry, Florida – first “greenfield” (built as brand new) commercial gasification combined cycle power station. Gross capacity 313 MW Net capacity to grid 250 MW Sulfur removal 98% NOX emissions reduced by 90% of PC Total cost $303,288,446 or $1,213 / kW Started 1996 21,000 hrs operation Power generated 6 million MWh On-stream factor 83.5% Availability factor 94% GT power output 192 MWe HRSG output 124 MWe Plant heat rate 9350 Btu/kWh (HHV) Thermal efficiency 38.4 % (LHV)
Tampa Electric IGCC (2) • Coal/water slurry and oxygen are reacted in a high temperature and pressure gasifier to produce a medium-temperature syngas. • Syngas moves from the gasifier to a high-temperature heat-recovery unit, which cools the syngas while generating high-pressure steam. Cooled gases flow to a water wash for particulate removal. • The syngas is further cooled before entering a conventional amine sulfur removal system that keeps SO2 emissions below 0.15 lb/106 Btu (97% capture). • The cleaned gases are then reheated and routed to a combined-cycle system for power generation.
Tampa Electric IGCC (3) • A GE MS 7001FA combustion turbine generates 192 MWe. • Thermal NOx is controlled to below 0.27 lb/106 Btu by injecting nitrogen. • A steam turbine uses steam produced by cooling the syngas and superheated with the combustion turbine exhaust gases in the HRSG to produce an additional 124 MWe. • Plant heat rate is 9,350 Btu/kWh (HHV), which is an efficiency of 38.4% (LHV). • Using IGCC, more of the power comes from the GT. Typically 60-70% of the power comes from the GT with IGCC, compared with about 20% using PFBC.
Applicability of Fluidized Bed • Fuel preparation - fluidized bed accepts crushed solids less than 6.4 mm (between stoker firing and pulverized firing), thus avoiding costly pulverizing system. • Lower temperature – needs less refractory, cheaper unit. • Reduced emissions – with lower temperature, cheap limestone or dolomite can be used as a sorbent to remove SO2 without the need for sulfur removal equipment like FGD; air-staging and post-combustion techniques even lower NOX emissions • Fuel flexibility - variety of fuels from very low-btu coal cleaning tailings, municipal solid wastes, biomass, high-btu solid fuels like coal, and fouling and slagging fuels may be burned efficiently with little difficulty.
Applicability of IGCC • Fuel preparation and flexibility – a new way of utilizing coal, wastes and biomass has been developed – by first gasifying it, then purifying the synthetic gas like natural gas, and using it in a CCGT for the cleanest and most efficient way of generating power • High efficiency – after the coal/water slurry and oxygen have reacted at high temperature and pressure to produce a medium temperature synthetic gas in a gasifier, the gas goes to a heat recovery unit to cool the gas and generate high-pressure steam for power generation. • Reduced emissions – the cooled gas is water-washed for particulate removal, then a COS hydrolysis reactor converts sulfur prior to feeding in a conventional amine sulfur removal system (97% sulfur capture); cleaned gas is reheated and fired in the CCGT
Environmental Impact • Uncontrolled coal combustion is generally a filthy process • Like oil, the obvious contaminants are SO2, NOX, CO, CO2, unburnt HC and particulates (fly ash) • Typical emissions for CFB are low: 100-200 ppm NOX < 200 ppm CO < 20 ppm UHC Fly ash <44 microns (requires bag filters) • Coal generates more CO2 than natural gas which contains less carbon and more hydrogen, aside from being more efficient. • Fluidized bed combustion results in lower temperature, hence lower NOX emission
Environmental Impact (2) • Addition of limestone to capture sulfur before it goes to the stack is a positive benefit that results in 70-90% sulfur removal for bubbling bed and higher 90-95% removal for circulating bed. • IGCC achieves much higher sulfur removal of 99% and attains NOX emissions below 50 ppm compared to traditional coal firing • Because of the higher thermal efficiency of fluidized bed combustion and IGCC, the emission of greenhouse gas CO2 is much lower per unit kWh: CO2 = (heat rate, kJ/kWh) / (LHV, kJ/kg) * (% C/100) * (mw CO2/mw C) = (3600 / efficiency) / (LHV, kJ/kg) * (% C/100) * (mw CO2/mw C)
Risks • Technology risk - advanced coal-burning technologies like fluidized bed combustion and IGCC have only achieve commercial status only recently; some technology risk - atmospheric fluidized bed combustion have been extensively demonstrated in power generation and their reliability is generally proven; technology risk would be minimal • Fuel supply risk (low) - fluidized bed combustion and IGCC have added flexibility of being able to burn different coals with ease, including low-btu wastes and biomass - local coal could be cleaned and blended with imported high quality coal to achieve desired performance and costs
Risks • Technology risk (moderate) - Pressurized fluidized bed combustion (PFBC) and IGCC are still in the demonstration and early commercialization stage; long-term reliability has to be established and some component developments still remain; full-scale commercial implementation has to be monitored to see its performance - The advanced coal-fired systems require a higher level of technological expertise to manufacture and maintain; core components will often have to be imported