Conceptual Design of Oxygen-Based PC Boiler (2005)

Coal is presently the world's primary fuel for generating electrical power and, being more abundant and less expensive than oil or natural gas, is expected to continue its dominance into the future. Coal, however, is more carbon intensive than natural gas and oil and consequently coal-fired power plants are large point source emitters of carbon dioxide (CO{sub 2}). Since CO{sub 2} is a greenhouse gas, which may have an adverse impact on the world's climate/weather patterns, studies have been conducted to determine the feasibility and economic impact of capturing power plant CO{sub 2} emissions for pipeline transport to a sequestration/storage site. The stack gas that exhausts from a modern coal-fired power plant typically contains about 15% CO{sub 2} on a dry volume basis. Although there are numerous processes available for removing CO{sub 2} from gas streams, gas scrubbing with amine solvent is best suited for this application because of the large gas volumes and low CO{sub 2} concentrations involved. Unfortunately the energy required to regenerate the solvent for continued use as a capturing agent is large and imposes a severe energy penalty on the plant. In addition this ''back end'' or post combustion cleanup requires the addition of large vessels, which, in retrofit applications, are difficult to accommodate. As an alternative to post combustion scrubbing, Foster Wheeler (FW) has proposed that the combustion process be accomplished with oxygen rather than air. With all air nitrogen eliminated, a CO{sub 2}-water vapor rich flue gas will be generated. After condensation of the water vapor, a portion of the flue gas will be recirculated back to the boiler to control the combustion temperature and the balance of the CO{sub 2} will be processed for pipeline transport. This proposed oxygen-carbon dioxide (O{sub 2}/CO{sub 2}) combustion process eliminates the need for CO{sub 2} removal/separation and reduces the cost of supplying a CO{sub 2} rich stream for sequestration. FW has developed a conceptual design of an O{sub 2} fired boiler to determine overall plant performance and economics. Five subtasks were conducted: (1) a literature review, (2) a system design and analysis, (3) a low NOx burner design and analysis, (4) a furnace and heat recovery area design analysis, and (5) an economic analysis. The objective of the literature search is to locate any data/information relevant to the Oxygen-Based PC Boiler conceptual design. The objective of the system design and analysis task is to optimize the PC boiler plant by maximizing system efficiency within practical considerations. Simulations of the oxygen-fired plant with CO{sub 2} sequestration were conducted using Aspen Plus and were compared to a reference air-fired 460 MW plant. Flue gas recycle is used in the O{sub 2}-fired PC to control the flame temperature. Parametric runs were made to determine the effect of flame temperature on system efficiency and required waterwall material and thickness. The degree of improvement on system efficiency of various modifications including hot gas recycle, purge gas recycle, flue gas feedwater recuperation, and recycle purge gas expansion were investigated. The selected O{sub 2}-fired design case has a system efficiency of 30.6% compared to the air-fired system efficiency of 36.7%. The design O{sub 2}-fired case requires T91 waterwall material and has a waterwall surface area of only 65% of the air-fired reference case. The objective of the low NOx burner design and analysis task is to optimize the burner design to ensure stable ignition, to provide safe operation, and to minimize pollutant formation. The burners were designed and analyzed using the Fluent CFD computer program. Four burner designs were developed: (1) with no OFG and 65% flue gas recycle, (2) with 20% OFG and 65% flue gas recycle, (3) with no OFG and 56% flue gas recycle and (4) with 20% OFG and 56% flue gas recycle. A 3-D Fluent simulation was made of a single wall-fired burner and horizontal portion of the furnace from the wall to the center. Without primary gas swirl, coal burnout was relatively small, due to the low oxygen content of the primary gas. Consequently, the burners were modified to include primary gas swirl to bring the coal particles in contact with the secondary gas. An optimal primary gas swirl was chosen to achieve sufficient burnout. The objective of the furnace and heat recovery area design and analysis task is to optimize the location and design of the furnace, burners, over-fire gas ports, and internal radiant surfaces. The furnace is designed with opposed wall-firing burners and over-fire air ports. Water is circulated in the furnace by natural circulation to the waterwalls and divisional wall panels. Compared to the air-fired furnace, the oxygen-fired furnace requires only 65% of the surface area and 45% of the volume. Two oxygen-fired designs were simulated: (1) without over-fire air and (2) with 20% over-fire air.