Ultra-Supercritical Oxyfuel Power Generation for CO2 Capture

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1 Ultra-Supercritical Oxyfuel Power Generation for CO2 Capture Andrew Seltzer Zhen Fan Horst Hack Foster Wheeler North America Corp. Perryville Corporate Park, Clinton, NJ Presented at 36 th International Technical Conference on Coal Utilization & Fuel Systems Clearwater, Fl, USA June 5 June 9, 2011

2 Ultra-Supercritical Oxyfuel Power Generation for CO 2 Capture Andrew Seltzer (andrew_seltzer@fwc.com) Zhen Fan (zhen_fan@fwc.com) Horst Hack (horst_hack@fwc.com) Foster Wheeler North America Corp., R&D Perryville Corporate Park, Clinton, NJ ABSTRACT Advanced high efficiency power plants will play a vital role in the reduction of carbon dioxide emissions associated with fossil fuel-based power generation. Foster Wheeler is developing advanced ultra-supercritical oxyfuel power plants, which combined with the Flexi-burn approach, will significantly reduce greenhouse gas and pollutant emissions through a substantial increase in power plant cycle efficiency and the capture of CO 2. The combination of ultrasupercritical and oxyfuel technologies raises new technical challenges, especially in the area of advanced high-temperature boiler materials. In oxy-combustion, the combustion air is replaced by oxygen, and flue gas is recycled back to the boiler to limit the flame temperature and to avoid excessive slagging and fouling. The quantity of the recycle flow rate is governed by the material selection and whether the boiler is new or a retrofit. Process simulations were performed on a 750 MW pulverized coal boiler with outlet steam conditions of 5000 psig/1350ºf/1400ºf to identify the compositions of the combustion gases that will exist in the furnace during oxy-combustion. This paper presents ultra-supercritical oxyfuel conceptual designs for boilers designed to operate at two different inlet net mixed oxygen concentration levels ( 25%, vol. and 40%, vol.) and burning high sulfur bituminous and low-sulfur PRB coals. Proposed boiler heat transfer surface layout and materials are identified. ASPEN-Plus system simulation is used to predict overall power plant performance and average gas compositions. A detailed 3-D FW-FIRE CFD model is utilized to predict localized micro-climates at the furnace walls. INTRODUCTION Efficiency and environmental performance are key issues when considering either repowering existing power plants or building new power plants with high efficiency steam cycles. High cycle efficiency requires less fuel consumption per unit of plant output and results in lower levels of ash discharge and gas emissions, including CO 2. CO 2 emissions can be reduced by 10-15% by replacing modern subcritical steam plants with supercritical plants and can be reduced by an additional 10-15% by replacement by ultra-supercritical (USC) steam plants. Oxyfuel combustion is one of the most promising methods of removing carbon dioxide from the exhaust gases of a coal power plant. Oxyfuel combustion is based on combusting coal with oxygen and recycled flue gas, to produce carbon dioxide and water vapor as the main components of the exhaust gas. This allows the carbon dioxide to be much more easily captured from the exhaust gas than in air combustion where nitrogen is the dominant flue gas component.

3 However, since flue gas must be recycled to avoid extreme temperatures in the furnace, the potential exists for significant increases in pollutants and contaminants within the boiler. Within pulverized coal-fired (PC) boilers, localized regions can exist with flue gas compositions significantly different from that of the bulk gas flow. The location and magnitude of these localized microclimates are influenced by air/gas flow distribution, fuel type, burner design, burner arrangement, particle burning trajectories, etc. These microclimates can lead to damaging waterwall corrosion/wastage. CFD modeling is used to identify representative microclimate conditions so that candidate boiler materials can be subjected to simulated boiler environments to identify which materials are best suited for oxy-fired boilers. Presented herein are the design of an ultra-supercritical oxyfuel boiler and the prediction of the associated furnace microclimates. Two ultra-supercritical oxyfuel boiler designs are produced: 1) a low-o 2 design, suitable for retrofit, capable of running either on air or O 2 and 2) a high-o 2 design new plant. Firing of both high sulfur bituminous and low sulfur PRB coals were simulated. PC BOILER DESIGN The reference power plant selected employs a once-through sliding pressure dry hopper boiler, selective catalytic reduction (SCR) for reducing NOx in air-firing, a wet flue gas desulfurization (FGD) for capturing SOx, and a electrostatic precipitator to remove particulates. The furnace has opposed wall-fired low NOx burners and over-fire air (OFA) ports. The furnace heat transfer surfaces consist of waterwalls, constructed from optimized vertical rifled tubes, radiant wall and platen superheaters (SH), and convective finishing superheater and reheater (RH) tube banks. After leaving the furnace, flue gas energy is recovered in a parallel-pass convective heat recovery area (HRA) and a regenerative rotary air heater. Final main steam temperature is controlled by spray water attemperation, while reheat steam temperature is controlled by a HRA gas flow proportioning damper or a cold RH bypass (for high O 2 -firing). Primary air and secondary air are heated in the air heater after which the former is sent to vertical spindle MBF pulverizers to convey the coal and the latter is sent to the furnace windbox. For the oxyfuel design the main additional equipment added to the air design are flue gas recirculation ducts, oxygen distribution piping, increased size of HRA lower economizer, a low pressure economizer downstream of the air heater, and a quench tower. The reference power plant is an ultra-supercritical 750 MWe plant with turbine inlet conditions, shown in Table 1. Coal properties are shown in Table 2. Table 1 Turbine Inlet Conditions Table 2 Coal Analysis Main Steam Pressure psia 5015 Temperature F 1350 Flow Rate klb/h 4035 Reheat Steam Pressure psia 1128 Temperature F 1400 Flow Rate klb/h 3212 Ultimate Analysis Illinois 6 Eagle Butte Ash % 9.99% 5.70% S % 2.51% 0.29% H % 4.50% 3.45% C % 63.75% 54.09% H2O % 11.12% 23.57% N % 1.25% 0.72% O % 6.88% 12.18% Total % % % Volatile Matter (daf) % 44.35% 44.54% HHV, as received Btu/lb 11,666 9,074

4 Figure 1 compares the thermodynamics of the ultra-supercritical boiler with typical subcritical and supercritical boilers. Figure 1 - Boiler Pressure-Enthalpy Diagram 212 F 257 F 302 F 347 F 392 F 437 F 482 F 527 F 572 F 617 F 662 F 707 F 752 F 797 F 842 F 887 F 932 F 1022 F 1112 F 1202 F 1292 F 1382 F 1472 F Subcritical Supercritical Ultra-supercritical Economizer Inlet Furnace Inlet Separator SH outlet Pressure (psia) 1000 Enthalpy (Btu/lb) BOILER DESIGN The ultra-supercritical boiler design was based on a current supercritical design. In addition to material upgrades due to higher temperatures and pressure, additional platen and radiant wall SH surface is incorporated into the upper section of the furnace due to the greater superheat duty and lower evaporator duty (Figure 1). A low-o 2 oxyfuel conceptual design was developed to be capable of either running on air or oxygen (Flexi-Burn 1 ). Consequently, such a design is adaptable to a new boiler or a retrofit of an existing boiler. By properly selecting the flue gas recirculation flow rate, the same boiler geometry, materials, and burners can be used in both air-fired and oxygen-fired modes. As shown in Figure 2, in the oxygen-firing mode, the coal is combusted in the furnace where the oxidizer consists of a mixture of O 2 and recycled flue gas (i.e % by wt.), which contains primarily CO 2 gas. The net oxygen concentration in the furnace (before combustion) is similar to the air-fired operation (i.e. 23%-27%, vol. for O 2 -firing vs. 21%, vol. for air-firing). 1 Flexi-Burn is a trademark of Foster Wheeler, registered in the US, EU, and Finland.

5 A high-o 2 oxyfuel design was developed to decrease the size of the boiler and reduce auxiliary power consumption. However, due to the higher flame temperature material upgrades were required. Figure 3 presents the low-o 2 and high-o 2 boiler design layouts and selected materials. Figure 2 - Process Schematic in Air-Firing and O 2 -Firing Modes Flue gas (CO2, H2O, O2, N2, Ar, SO2) To stack H 2O & soluble impurities Inert Gases Boiler Furnace HRA SCR FW Heater Gas Preheater Ash Removal FGD Recycled flue gas CO2 Processing CO 2 Coal Primary Gas Mill Air Transport Storage Secondary Gas Oxygen (97% O2, 2% Ar, 1% N2) Legend Combustion Reactant Combustion Product Water/Steam Added Oxyfuel component O2 Preheater ASU N2 Air Figure 3 Boiler Designs Attemp. 1355F 1400F 900F Sep. Finishing Finishing Reheater Super- Platen Platen Superheateheater Reheater I740 I740 I740 H230 Radiant SH I 617 Evaporator I 617 S c r e e n HR3C/TP347 Reheater T91/T23 Economizer 210C 875F 629F From FWH Burners 690F Low O 2 -Fired High O 2 -Fired

6 POWER PLANT SIMULATION A power plant design and analysis was performed using the Aspen-Plus computer program. The scope of the model covers the majority of the power plant, exclusive of the air separation unit (ASU) and CO 2 processing unit (CPU). Heat released from combustion is absorbed in the furnace by waterwalls, platens, and roof and is absorbed by the superheater, reheater, and economizer convective tube bundles in the heat recovery area. After exiting the economizer, flue gas passes through the SCR for NOx control and then through a regenerative air heater to heat up both primary air (PA) and secondary air (SA). Downstream of the air heater, an electrostatic precipitator (ESP) removes particulates and a flue gas desulfurizer captures SOx. An air ingress of 5%, vol. is assumed and lumped into the ESP in the model. Main controlled operational variables are main steam temperature by two water attemperators, reheat steam temperature by an HRA parallel pass gas damper, pulverizer PA outlet temperature by tempering air, and steam generation by coal firing rate. The mole fraction of O 2 in the furnace flue gas is maintained at 3.0%, vol. for both air-firing and oxy-firing. Modifications to adapt the plant to accommodate oxyfuel firing include the addition of flue gas recirculation ducts, oxygen distribution piping, increased size of HRA economizer, a low pressure economizer downstream of the air-heater, and a quench tower. An additional low pressure economizer is required to cool the gas temperature down in oxy-firing because when the air heater functions as a recuperator, the air heater duty is reduced since recycled flue gas flow is less than the air-firing air flow and the secondary gas inlet temperature is significantly greater than the air inlet temperature. Recycled gas is taken after the FGD and quench tower. Ammonia is injected into the SCR only during air-firing. POWER PLANT PERFORMANCE Plant simulations were run at full load. Table 3 presents the power plant performance in air-fired and O 2 -fired modes burning either bituminous or PRB coal. The Foster Wheeler CFD computer program, FW-FIRE, was used to determine the furnace performance. The furnace model is shown in Figure 4. The upper section of the waterwalls is a radiant superheater and the platen surface is used as SH surface for the low-o 2 design and for SH and RH surface for high-o 2 design. Figure 6 shows that although the furnace gas temperature is approximately 100ºF lower in O 2 - firing than air firing, the maximum heat flux is about the same due to the higher emissivity of the oxycombustion flue gas. Figure 6 shows that the maximum flame temperature for the high O 2 - fired case is approximately 600ºF higher than for the low O 2 -fired case and the maximum heat flux is nearly double of that of the air-fired and low O 2 -fired cases requiring upgrading the furnace waterwall materials from T23/TP347 to Inconel 617 (Figure 3). This may increase the potential for slagging in the furnace depending on the ash fusion characteristics. Consequently, for a dry bottom furnace design, a minimum flue gas recycling flow may be required to avoid slagging depending on fuel type. Because of the significantly higher flame temperature of the high O 2 -fired design and lower flue gas mass flow rate, more heat is shifted to the furnace (from

7 the HRA) as shown in Figure 5. The high O 2 -fired boiler has approximately 25% less tube weight, with most of the weight reduction coming from the low and moderate temperature tube materials. Table 3 - Performance Summary in Air-Fired and O 2 -Fired Modes High S Bituminous Low S PRB Air-fired Low O2-fired High O2-fired Air-fired Low O2-fired High O2-fired Fuel Flow klb/h Air Flow klb/h Oxygen Flow klb/h Recirc. Flue Gas % 0.0% 72.0% 55.2% 0.0% 69.7% 49.2% Inlet Oxygen, wt % 22.8% 22.7% 36.3% 22.3% 22.5% 38.2% Inlet Oxygen, vol % 20.3% 26.4% 40.9% 19.7% 25.6% 41.3% Boiler efficiency % Gross Power MWe Net Power MWe Net efficiency % Furnace Outlet Comp. O2 %, vol. 2.99% 3.02% 3.02% 2.48% 2.47% 2.48% N2 %, vol % 14.75% 8.99% 71.65% 13.31% 7.35% CO2 %, vol % 69.05% 69.59% 14.91% 68.32% 66.98% H2O %, vol. 8.52% 12.90% 17.96% 10.92% 15.87% 23.13% SO2 %, vol. 0.21% 0.28% 0.44% 0.03% 0.04% 0.06% Total %, vol % % % % % % WATER WALL GAS CONDITIONS Figure 7 presents the CO concentration at the wall under high sulfur bituminous firing, which peaks at approximately 5.0% for the low O 2 -fired case without OFA (compared to 4.6% for airfired). Since it is not known whether combustion staging is required in the low O 2 -fired furnace or whether NOx control will be handled in the CPU, a case was run with 20% OFA. Maximum CO concentration for the 20% OFA low O 2 -fired case is approximately 9.4%. The CO concentration at the wall peaks at approximately 8.8% for the high O 2 -fired case. Since the high O 2 -fired design will be a greenfield design (and not capable of running on air), it is assumed combustion staging will not be installed and that NOx control will be handled in the CPU. Figure 8 presents the H 2 S concentration at the wall under high sulfur bituminous firing, which peaks at about 650 ppm for the low O 2 -fired case with no OFA and 1280 ppm for the low O 2 - fired case with 20% OFA (compared to 850 ppm for the air-fired case). The H 2 S concentration at the wall peaks at about 885 ppm for the high O 2 -fired case.

8 Figure 4 - Furnace Models (with right side wall removed) Figure 5 USC Boiler Heat Absorption Distribution Heat Absorption 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Air Low O2 High O2 Evaporator Platen SH/RH Wall SH HRA Figure 6 Furnace Gas Temperature and Wall Heat Flux for Bituminous Firing

9 Figure 7 Furnace Wall CO for Bituminous Firing Figure 8 Furnace Wall H 2 S for Bituminous Firing (ppm)

10 CLOSING REMARKS To ensure continued U.S. power generation from the country s abundant domestic coal resources, new coal combustion technologies must be developed to meet future emissions standards, especially CO 2 sequestration. Current conventional coal-fired boiler plants burn coal using 15-20% excess air producing a flue gas, which is only approximately 15% CO 2. Several different technologies for separation of the CO 2 from fluegas have been proposed, including amine-based absorption and membrane gas absorption. However, these techniques require substantial energy for regeneration, typically from low-pressure steam. A promising approach to capturing carbon dioxide is utilizing high efficiency ultra-supercritical oxyfuel power plants which reduce CO 2 by 20-30% via high cycle efficiency and produce combustion products which are only CO 2 and water vapor with small amounts of impurities. The water vapor is condensed, yielding a nearly-pure CO 2 stream that can be easily purified and compressed for sequestration. The CO 2 effluent as a liquid or supercritical fluid is piped from the plant to the sequestration site. To protect ultra-supercritical oxy-fired boilers from potential corrosion problems, a test program is being conducted that subjects candidate boiler materials to the furnace exit gas conditions and the furnace microclimate conditions predicted for new and retrofitted boilers; these tests will identify which materials are best suited for oxy-fired boilers. The program utilizes CFD modeling to identify representative gas conditions, coats potential material specimens with synthesized ash, exposes the coated material coupons to predicted gas conditions in electricallyheated tube furnaces, and analyzes the exposed material specimens to identify/confirm their suitability for use in oxy-fired boilers. Conceptual oxyfuel designs were developed for a 750 MWe ultra-supercritical PC power plant burning either low-sulfur PRB or high-sulfur bituminous coal. Designs were made for low-o 2 (26%, vol.) conditions, suitable for retrofit and capable of running either on air or O 2 and high- O 2 (41%, vol.) conditions new plant. The high O 2 -fired boiler has approximately 25% less tube weight than the low-o 2 boiler, although it requires higher grade materials. Based on plant simulation studies, the overall plant efficiency is predicted to be reduced by approximately 9% points for the low-o 2 design and by 8.5% points for the high O 2 -design. Based on CFD furnace simulations, compared to air-firing, maximum wall CO is predicted to be nearly the same for low-o 2 firing without OFA, double for low-o 2 firing with OFA, and nearly double for high-o 2 firing without OFA. Compared to air-firing, maximum wall H 2 S is predicted to be about 70% for low-o 2 firing without OFA, 1.5 times for low-o 2 firing with OFA, and nearly the same for high-o 2 firing without OFA. ACKNOWLEDGEMENT This work was prepared with the support of the U.S. Department of Energy, under Award No. DE-FG26-01NT41175 and the Ohio Coal Development Office/Ohio Department of Development (OCDO/ODOD) under Grant Agreement Number CDO/D However, any

11 opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE and/or the OCDO/ODOD. LEGAL NOTICE/DISCLAIMER This report was prepared by Foster Wheeler North America Corp. pursuant to a Grant partially funded by the U.S. Department of Energy (DOE) under Instrument Number DE-FG26-01NT41175 and the Ohio Coal Development Office/Ohio Department of Development (OCDO/ODOD) under Grant Agreement Number CDO/D NO WARRANTY OR REPRESENTATION, EXPRESS OR IMPLIED, IS MADE WITH RESPECT TO THE ACCURACY, COMPLETENESS, AND/OR USEFULNESS OF INFORMATION CONTAINED IN THIS REPORT. FURTHER, NO WARRANTY OR REPRESENTATION, EXPRESS OR IMPLIED, IS MADE THAT THE USE OF ANY INFORMATION, APPARATUS, METHOD, OR PROCESS DISCLOSED IN THIS REPORT WILL NOT INFRINGE UPON PRIVATELY OWNED RIGHTS. FINALLY, NO LIABILITY IS ASSUMED WITH RESPECT TO THE USE OF, OR FOR DAMAGES RESULTING FROM THE USE OF, ANY INFORMATION, APPARATUS, METHOD OR PROCESS DISCLOSED IN THIS REPORT. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Department of Energy and/or the State of Ohio; nor do the views and opinions of authors expressed herein necessarily state or reflect those of said governmental entities.