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SECTION 6.0:ALTERNATIVES The availability of the natural gas resource provided by PG&E, as well as the environmental and operational advantages of natural gas technologies,make natural gas the logical choice for the proposed project. 6.6.3 NO. Control Alternatives To minimize NO,,emissions from the LEC,the combustion turbine generators (CTGs) will be equipped with water injection combustors and selective catalytic reduction (SCR) using anhydrous ammonia as the reducing agent. The following combustion turbine NO,,control alternatives were considered: • Steam injection (capable of 25 to 42 parts per million [ppm] NO,,) • Water injection (capable of 25 to 42 ppm NO,) • Dry low NO,,combustors (capable of 15 to 25 ppm NO,,) Water injection or dry low NO,,were selected because these allow for lower acceptable NO,, emissions while being able to achieve an output turndown rate of 30 percent. This turndown is necessary to meet variable load demand. Two post-combustion NO,,control alternatives were considered: • SCR • EMxTM (formerly SCONOXTM) SCR is a proven technology and is used frequently in combined-cycle applications. Ammonia is injected into the exhaust gas upstream of a catalyst. The ammonia reacts with NOx in the presence of the catalyst to form nitrogen and water. EMXTM consists of an oxidation catalyst,which oxidizes carbon monoxide to carbon dioxide and nitric oxide to nitrogen dioxide. The nitrogen dioxide is adsorbed onto the catalyst,and the catalyst is periodically regenerated. The level of emission control effectiveness between the EMx and SCR technologies are approximately the same. However,the EMx technology does not employ the use of ammonia to reduce air emissions. The CEC recently summarized in the EPA's opinion (Colusa Generating Station Final Staff Assessment) "that EMx is no more effective for reducing air quality impacts than selective catalytic reduction(or"SCR",which is what is proposed for CGS), and it also found EMx to be significantly more expensive and arguably less reliable,particularly for larger facilities." Therefore,EMx was not considered for the LEC project. The following reducing agent alternatives were considered for use with the SCR system: • Anhydrous ammonia • Aqueous ammonia • Urea Anhydrous ammonia is used in many combined-cycle facilities for NOx control,but is more hazardous than diluted forms of ammonia;however,because the anhydrous ammonia tank will be shared between the LEC and STIG facility, aqueous ammonia use was not investigated for this site. Urea has not been commercially demonstrated for long-term use with SCR and was eliminated from consideration. 6-16 SAC/371322/082330003(LEC_6.0_ALTERNATIVES.DOC)