Diesel Exhaust ORC Retrofit with Backpressure Penalty
A 1.2 MW diesel generator at a textile mill runs 7,000 h/year. Its exhaust is 3.8 kg/s at 430°C; take cp = 1.05 kJ/kg·K. An ORC supplier proposes recovering heat down to 180°C with a thermal efficiency of 11% and auxiliary power of 8 kW. The existing exhaust backpressure is 2.0 kPa; the ORC heat exchanger would raise it to 4.5 kPa. The engine maker states that backpressure above 3.0 kPa increases fuel energy input by 0.4% per kPa above 3.0, and the engine's current fuel-to-electric efficiency is 40%. Electricity costs ₹8.00/kWh. The installed ORC cost is ₹1.10 crore. The fuel contains 0.8% sulphur, and the stack is upstream of the existing chimney. You must decide whether to recommend the project and what conditions to impose.
1. Compute the recovered heat, gross ORC output, and net output after auxiliary power and engine backpressure penalty.
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Exhaust heat recovered = m cp ΔT = 3.8×1.05×(430−180)=997.5 kW. Gross ORC output =0.11×997.5=109.7 kW. Auxiliary=8 kW, so electrical before engine penalty=101.7 kW. Engine fuel input=1.2/0.40=3.0 MW. Backpressure excess=4.5−3.0=1.5 kPa, penalty=0.4%/kPa×1.5=0.6% of fuel input=0.006×3000=18 kW. Net useful output=101.7−18=83.7 kW.
2. Compute annual saving and simple payback, and state whether the project is justified.
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Annual net energy=83.7×7000=585,900 kWh. Saving=585,900×8=₹46.87 lakh/year. Simple payback=1.10 crore/46.87 lakh=2.35 years. Without the backpressure penalty, saving would be 101.7×7000×8=₹56.95 lakh and payback 1.93 years. The project is financially attractive, but the margin is sensitive to the 11% efficiency guarantee and to the actual backpressure penalty. Recommend proceeding only with a performance guarantee and a backpressure limit.
3. What technical conditions should be imposed before recommending the ORC?
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Conditions: (i) heat-exchanger design must keep backpressure ≤3.0 kPa at full load; fit a bypass and alarm; (ii) confirm exhaust outlet temperature stays above the acid dew point for 0.8% S fuel-180°C is likely safe but should be checked against a gas-side acid dew-point calculation; raise to 200°C if needed; (iii) guarantee net output after auxiliaries; (iv) include fouling allowance and cleaning access; (v) verify engine maker's warranty position. If backpressure cannot be held at 3.0 kPa, net output falls by 18 kW and the payback lengthens, so the penalty clause should be in the contract.