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

Topics – Advanced Coal 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

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

Pyroflow System (Foster Wheeler)

Advanced Coal-Burning Power Generation Technology Asia’s Largest CFB 400 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 Process Flow Diagram

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.

Cost of Coal-Fired Plants and SO2 Removal

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