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Chapter 3: Capture of CO2 123 pulverized coal, low NOx burner. The system included a heat-transfer test section to simulate fouling conditions. Test conditions included variation in recycle flow and excess O2 levels. Measurements included all gas compositions, ash analysis and tube fouling after a 5-week test run. The work also included a case study on oxy-fuel operation of a 660 MW power boiler with CO2 capture, compression and purification. The main test results were that NOx levels reduced with increase in recycle rate, while SO2 and carbon in ash levels were insensitive to the recycle rate. Fouling in the convective test section was greater with oxy-fuel firing than with air. High-slagging UK coal had worse slagging when using oxy-fuel firing, the higher excess O2 level lowered carbon in ash and CO concentration. pulverized coal fired boiler, hot recycling of the flue gas prior to CO2 purification and compression also reduces the size of all unit operations in the stream leaving the boiler to 1/5 that of similar equipment deployed in conventional air blown combustion systems (Chatel-Pelage et al., 2003). Use of a low temperature gas purification step prior to CO2 compression (see Section 3.4.2.2) will also eliminate the need to deploy conventional selective catalytic reduction for NOx removal and flue gas desulphurization to purify the gas, a practice typically adopted in conventional air-blown combustion processes (see Figure 3.3). The overall reduction in flow volumes, equipment scale and simplification of gas purification steps will thus have the benefit of reducing both capital and operating costs of equipment deployed for combustion, heat transfer and final gas purification in process and power plant applications (Marin et al., 2003). For the combustion of pulverized coal, other pilot-scale tests by Croiset and Thambimuthu (2000) have reported that the flame temperature and heat capacity of gases to match fuel burning in air occurs when the feed gas used in oxy-fuel combustion has a composition of approximately 35% by volume O2 and 65% by volume of dry recycled CO2 (c.f. 21% by volume O2 and the rest nitrogen in air). In practice, the presence of inerts such as ash and inorganic components in the coal, the specific fuel composition and moisture in the recycled gas stream and the coal feed will result in minor adjustments to this feed mixture composition to keep the flame temperature at a value similar to fuel combustion in air. As noted above for pulverized coal, oil, natural gas and biomass combustion, fluidized beds could also be fired with O2 instead of air to supply heat for the steam cycle. The intense solid mixing in a fluidized bed combustion system can provide very good temperature control even in highly exothermic conditions, thereby minimizing the need for flue gas recycling. In principle, a variety of commercial designs for fluidized combustion boilers exist that could be retrofitted for oxygen firing. A circulating fluidized bed combustor with O2 firing was proposed by Shimizu et al. (1999) to generate the heat required for the calcination of CaCO3 (see also Section 3.3.3.4). More recently, plans for pilot testing of an oxy-fired circulating fluidized bed boiler have been published by Nsakala et al. (2003). At conditions that match O2/CO2 recycle combustion to fuel burning in air, coal burning is reported to be complete (Croiset and Thambimuthu, 2000), with operation of the process at excess O2 levels in the flue gas as low as 1-3% by volume O2, producing a flue gas stream of 95-98% by volume dry CO2 (the rest being excess O2, NOx, SOx and argon) when a very high purity O2 stream is used in the combustion process with zero leakage of ambient air into the system. No differences were detected in the fly ash formation behaviour in the combustor or SO2 emissions compared to conventional air firing conditions. For NOx on the other hand, emissions were lower due to zero thermal NOx formation from the absence of nitrogen in the feed gas - with the partial recycling of NOx also reducing the formation and net emissions originating from the fuel bound nitrogen. Other studies have demonstrated that the level of NOx reduction is as high as 75% compared to coal burning in air (Chatel-Pelage et al., 2003). Similar data for natural gas burning in O2/CO2 recycle mixtures report zero thermal NOx emissions in the absence of air leakage into the boiler, with trace amounts produced as thermal NOx when residual nitrogen is present in the natural gas feed (Tan et al., 2002). 3.4.2.2 Assessments of plants converted to oxy-fuel combustion The above and other findings show that with the application of oxy-fuel combustion in modified utility boilers, the nitrogen- free combustion process would benefit from higher heat transfer rates (McDonald and Palkes, 1999), and if also constructed with higher temperature tolerant materials, are able to operate at higher oxygen concentration and lower flue gas recycle flows – both of which will considerably reduce overall volume flows and size of the boiler. to achieve the same temperatures as in air combustion (compatible temperatures with existing materials in the boiler). It should be noted that even when deploying a 2/3 flue gas recycle gas ratio to maintain a 35% by volume O2 feed to a • The CO2-rich flue gas from the boiler is divided into three gas streams: one to be recycled back to the combustor, one to be used as transport and drying gas of the coal feed, and the third as product gas. The first recycle and the product stream are cooled by direct water scrubbing to remove residual particulates, water vapour and soluble acid gases such as SO3 and HCl. Oxygen and entrained coal dust together with the second recycle stream flow to the burners. We now discuss performance data from a recent comprehensive design study for an application of oxy-fuel combustion in a new build pulverized coal fired power boiler using a supercritical steam cycle (see Figure 3.8; Dillon et al., 2005). The overall thermal efficiency on a lower heating value basis is reduced from 44.2% to 35.4%. The net power output is reduced from 677 MWe to 532 MWe. Important features of the system include: • Burner design and gas recycle flow rate have been selected • The air leakage into the boiler is sufficient to give a high enough inerts level to require a low temperature inert gasPDF Image | CARBON DIOXIDE CAPTURE AND STORAGE
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