Sara Jones 24NOV08

Background  In most conventional combustion processes, air is used as the source of oxygen  Nitrogen is not necessary for combustion and causes problems by reacting with oxygen at combustion temperature  A high concentration of nitrogen in the flue gas can make CO2 capture unattractive  With the current push for CO2 sequestration to ease global warming, it is imperative to develop cost- effective processes that enable CO2 capture  The use of pure oxygen in the combustion process instead of air eliminates the presence of nitrogen in the flue gas, but combustion with pure oxygen results in very high temperatures

Background  In 1982 oxy-fuel combustion was proposed to produce CO2 for Enhanced Oil Recovery  Recycling of hot flue gas has also been suggested to reduce furnace size and NOx emissions for metal heating furnaces  Lately, interest has been paid to oxy-fuel combustion as a means to reduce pollutant emission control cost and create a CO2 gas stream that can easily be compressed and sequestered

Process Variations  Is the plant to be retrofitted or purpose- built?  What is the optimum O2 proportion in the oxidant gas?  What is the desired proportion of CO2 in the flue gas?  To what extent will the flue gas be cleaned of NOx, SOx, and Hg?  Will CO2 be fully/partially sequestered?

Oxy-fuel combustion flow sheet  Oxygen at greater than 95% purity and recycled flue gas are used for fuel combustion, producing a gas that is mainly CO2 and water  Recycled flue gas is also used to control the flame temperature and replace the volume of the missing nitrogen needed to carry heat through the boiler

Differences from Replacing N2  To have a similar adiabatic flame temperature, oxygen must have a concentration of about 30%  For an oxygen concentration of 30%, ~60% of the flue gases are recycled  Because of high concentrations of carbon dioxide and water, the furnace gas has a higher emissivity  Flue gas volume after recycling is 80% smaller than conventional combustion, and its density is increased  3-5% excess of oxygen is required  Species present in flue gases are in higher concentrations after oxy-fuel combustion  Power must be provided for flue gas compression and air separation

Differences  Oxy-fuel combustion with CO2 sequestration involves oxygen separation, flue gas recycling, CO2 compression, and transport and storage  A number of modifications to conventional pf coal technology must occur  Running more processes leads to a reduction in availability  The use of carbon sequestration increases capital and operating costs

Oxy-fired PF Power Plant  O2 is separated from air and mixed with the recycle stream from the boiler  Fuel is fired into this mixed stream, and a portion of the flue gases is recycled  Water vapor in the flue gas is condensed to form a stream of supercritical CO2 of high purity, 70% by mass CO2 as compared to 17% with air-fired combustion  CO2 can then be cooled and compressed for transportation and storage

Design and Operational Issues  Concentration of tri-atomic flue gas molecules is much higher in oxy-fuel combustion - changes the emissivity  CO2 and H2O also have higher thermal capacities than nitrogen, which leads to a higher heat transfer in the convective section of the boiler  Amount of gas passing through the boiler is lower, and heat transfer is increased in the radiative section of the boiler  Result is lower heat transfer in the convective section of the boiler and a lower gas temperature at the furnace exit

Design and Operational Issues  Effects of flue gas recycling on trace elements emissions and fly ash size distribution have not been experimentally determined  Several studies have shown problems with flame stability and ignition  Currently, no studies have been conducted that assess the impact of oxy-fuel combustion on deposit formation and structure

Pilot Scale Studies-EERC and ANL  10 Million Btu/hr boiler pilot facility  With wet recycle, an oxygen concentration of 23.8% in the burners was needed to match the heat transfer performance of air-fired combustion.  For dry recycle, a 27% oxygen concentration was needed  In-furnace gas temperature profile was similar with oxy-fuel combustion, but NOx and SOx emissions were lower (by 50% for NOx) and the carbon burnout was higher  No operational difficulties were found

Pilot Scale Studies - IFRF  2.5 MW furnace with air-staged swirl burner  oxy-fuel combustion had radiative and convective heat transfer performance, in-flame gas composition trends, combustion performance, and flame length and stability comparable to air-fired combustion  Optimum ratio for recycled flue gas was found to be 0.61  Maximum flue gas CO2 concentration of 91.4% was achieved  NOx emissions were reduced  Low NOx burner technology was demonstrated to be practicable for oxy-fuel combustion

Pilot Scale Studies – IHI  1.2 MW combustion-test furnace with swirl burner  Injection of pure O2 at the center of the burner improves flame stability and decreases the ash’s unburnt carbon content  NOx conversion was lower than with conventional combustion, but increased as oxygen concentration increased  NOx reduction occurs because of rapid reduction of recycled NOx into HCN and NH3  SOx emissions decreased from condensation of sulphates in ducts and adsorption of sulphur in ash

Pilot Scale Studies – Air Liquide  1.5 MW pilot-scale boiler with air staged combustion system  Smooth transition from air to O2 with good flame stability and heat transfer achievable  Less NOx than conventional – below the 0.15 lb/MMBtu standard for units installed after July 1997  Effective removal of SOx with wet FGD equipment and significant reduction of Hg emission (~50%)  Large reduction in unburnt carbon in fly ash, leading to improved boiler efficiency

Pilot Scale Studies – CANMET  0.3 MW vertical combustor research facility  Flue gas CO2 concentration close to theoretical (~92%)  Equivalent flame temperature at 35% O2. O2 purity (<5% N2) had no significant effect on flame temperature  Reduced NOx – dependent on O2 concentration, flame temperature. Difference decreases significantly if 3% N2 present  Increasing O2 concentration decreases CO emission. Decrease in CO concentration along flame is slower than conventional because of high CO2 concentration  Experimental results compared to modeling efforts

Advantages/Disadvantages  Industry is familiar with this type of technology  Viable for near-zero emissions  Can be retrofitted to existing plant as oxy- fuel with CO2 liquefaction or direct flue gas liquefaction  Can be implemented in new plant or put into new plant design for later retrofit  Low NOx emissions  Reduced efficiency  Not demonstrated at commercial-scale – might be unforeseen technical difficulties  SOx removal might be required  Oxygen separation plant needed

Future R&D  Heat transfer performance of new and retrofitted plants  Impact of O2 concentration and CO2 recycle ratio  Assess retrofits for electricity cost and cost of CO2 avoided  Combustion of coal in O2/CO2 – ignition, burn-out, emissions  Development of less costly O2 generation

Conclusion  Oxy-fuel combustion is technically feasible with current technologies  CO2 in flue gas is relatively pure and is sufficient for sequestration  Potential to reduce pollutant emissions, especially NOx