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Mr. Frank DeMaris October 18, 2010 Page 2 of 7 • Ameresco proposes to revise the carbon monoxide(CO) emission factor based in part on Bay Area Air Quality Management District's(BAAQMD) "Revisiting BACT for Lean Burn Landfill Gas Fired Internal Combustion Engines"(2009). This document states that improved lean-burn low- NOx combustion design can maintain low NOx emission rates during operation,but under these conditions,the engine CO will increase("creep")over time up to a Not to Exceed(NTE) limit. The upper end of the range of the uncontrolled CO emission factor for an engine that can achieve a 0.6 g/bhp NOx emission rate is 3.6 g/bhp-hr. This NTE level was established by the BAAQMD in an effort to allow flexibility for variable CO levels while achieving low NOx levels. Ameresco's experience at Ox Mountain indicated that a CO Catalyst is required in order for the SCR to sustain its reduction efficiency for NOx emissions. The manufacturer, Miratech, has verified this(the CO Catalyst operates upstream of the SCR in the engine exhaust path). However, with approximately 12,000 hours of operating experience at the Ameresco Ox Mountain facility, it has been shown that the oxidation catalysts were unable to sustain 75%design for CO reduction efficiency,the level posed in the initial permit to construct. Their efficiency continues to be slightly better than 50%at this time(on a sustainable basis), and Ameresco is proposing that this reduction level be applied to the BAAQMD NTE level,resulting in an emission rate of 1.8 g/bhp of CO. Ameresco has been and continues to look for ways to improve the CO Catalyst reduction efficiency at Ox Mountain by looking at changes with the fuel pretreatment system, engine operation and catalyst design and operation. However, any changes toward that end must not negatively impact operation of the SCR. From all this,we are proposing a controlled CO emission level that assumes a minimum 50% reduction efficiency of the catalyst is sustainable and accounts for the natural CO drift that will occur on the uncontrolled emissions up to the NTE level. Also for these reasons,we are proposing this same emission rate Ameresco believes to be sustainable at Ox Mountain. This emission level is more stringent than the BAAQMD CO BACT for LFG engines. Results of the testing at the Ameresco Ox Mountain facility can be found in Appendix B. By including a CO Catalyst, which will operate ahead of the SCR in the engine exhaust path in order to protect the SCR,the CO emissions will be reduced from the original estimate of 145.42 tpy to 104.70 tpy for the two engines. • The volatile organic compound(VOC)emissions will remain the same as the original application. The emissions using 20 parts per million by volume(ppmv) @ 3%oxygen as hexane(outlet) and maximum exhaust flow at 10%oxygen(converted to 3%oxygen)will be used because that is considered the lowest emission level achievable based on discussions with GE Jenbacher,the engine manufacturer. It is based on the maximum exhaust flow rate provided by GE Jenbacher. The 20 ppmv outlet limit is also considered BACT/Lowest Achievable Emission Rates(LAER) in other Districts including Ventura County Air Pollution Control District(VCAPCD), South Coast Air Quality Management District(SCAQMD), BAAQMD, San Diego County Air Pollution Control District(SDCAPCD), in addition to the limit under the landfill New Source Performance Standards(NSPS).