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Show Environmental Benefits • NOx reduction: gas zone is operated at low stoichiometry in a reburn mode, using excess stoker oxygen for burnout • Opacity reduction: extend compliance limits during low load, high load, load following, cold startup, episodes of problematic coal; reduced emissions during soot blowing Avoided cost of ESP addition or upgrades for compliance Negotiate relaxed coal specifications • SOx reduction: dilution by proportion of gas use; seasonal gas use Net out of Title V through synthetic minor Negotiate higher sulfur coal contract These benefits have been screened and quantified for several stoker operations which were case studies of cofiring benefits or candidates for field evaluations. Figure 1 shows an example of the relative magnitude of several cofiring credits and debits. To put the various parameters on a comparable basis, the credits and debits are all expressed in terms of operating cost or savings per million Btu fired. Typically, the natural gas fuel differential cost is the dominant debit. For a given site, there are usually several cofiring benefits but one dominant credit which is the decisive offset to the fuel cost. The dominant credit is usually a highly leveraged effect of cofire, commonly either avoided cost of environmental upgrades or recovered derate. Other common themes observed in the cofiring benefits evaluation were: • The zero gas use benefits of startup and standby is usually nearly sufficient to justify the retrofit capital cost, but has little bearing on the decision to cofire • Opacity reduction is a significant cofiring benefit at nearly all sites, either directly as a compliance issue, or indirectly as a cause of derate or a constraint on turndown, load following, or coal quality • Efficiency improvements, environmental credits, and operational flexibility are all highly desirable secondary benefits but usually do not, in themselves, justify cofiring 7 |