Low quality waste heat recovery for a gas gathering compressor package

Thermodynamic analysis has been performed on the exhaust gas of Waukesha engine model 7042GL (1,100 kW lean burn) and 7042GSI (1,100 kW stoichiometric burn) to determine if adequate recoverable heat exists for use in a Rankine power cycle. The first law and dew-point analysis illustrated that the exhaust gas could be cooled to 120°C, yielding maximum recoverable heat amounts of 728.4 kW and 576.0 kW for the 7042GSI and 7042GL, respectively. The pinch-point analysis showed that many organic fluids could reduce the exhaust gas temperature below the dew-point temperature, resulting in heat addition efficiencies greater than 100%.; A simulation program was written to reference the thermodynamic library Stanmix to test a total of 14 working fluids for net work output and thermodynamic efficiencies. Three different expansion scenarios were simulated in the Rankine cycle: saturated expansion, superheated expansion and supercritical expansion. It was found that the superheated and supercritical expansion produced the highest net work output and cycle efficiencies.; Of the 14 fluids tested, n-pentane operating on a superheated Rankine cycle proved to be the working fluid with the highest net work and thermal efficiency of 155kW and 22.6%, and 123kW and 22.6% for the 7042GSI and 7042GL, respectively. In addition, n-pentane proved to have ideal thermal and safety characteristics.; It was found that by placing the waste heat recovery unit on the 7042GSI, 12.3% more natural gas could be transported without increasing the current power and fuel consumption of the compressor package. Similarly, the 7042GL could transport 10% more natural gas. At {dollar}7.00/mcf of natural gas, the operational savings of this was calculated to be {dollar}103,000/year and {dollar}76,000/year for the 7042GSI and 7042GL, respectively. The capital savings of not having to purchase larger engines to achieve the same work was determined to be {dollar}190,000 and {dollar}150,000, respectively. The capital costs for both systems was determined to be {dollar}309,000 with operating costs at roughly {dollar}9,000/year and overhaul expenses at {dollar}20,000 every three years. The simple pay back period calculated at a cost of natural gas of {dollar}7.00/mcf was 2 years for the 7042GSI and 3.4 years for the 7042GL. Over the 20 year lifetime of the project, the net cash flow was determined to be approximately {dollar}1.8 million for the 7042GSI and {dollar}1.25 million for the 7042GL.; The thermodynamic and cost analysis on the potential use of a waste heat recovery unit on a gas gathering compressor package has yielded very attractive results both financially and environmentally. It has been shown that the waste heat recovery unit will turn a profit over the lifetime of the project, reduce the annual fuel consumption by over 15 million cubic feet, and reduce greenhouse gas emissions by 5800 tons each year.