Do the Bureau of Energy Efficiency Jamnagar reports for 200, 300 and 350 kg gas pit furnaces show different savings per tonne of brass?

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No. The three Bureau of Energy Efficiency (BEE) Jamnagar reports all burn 103.74 normal cubic metres (Nm3) of gas in place of 192.16 kilograms of coal per tonne of brass. Payback falls from 3.61 to 2.38 to 2.02 years only because they assume 400, 600 and 700 tonnes melted a year. Baseline: 192.16 kilograms (kg) of coal per tonne of brass. It comes from one audit melt of 255 kg on 49 kg of coal at 6,500 kilocalories (kcal) a kg, a furnace efficiency of 9.71%. Proposed furnace: 103.74 Nm3 of gas per tonne, at 17% efficiency, with waste heat recovery and automatic temperature control. Gas is taken at 8,600 kcal a kg and 0.8 kg a cubic metre. Prices: coal at rupees (Rs) 18 a kg and gas at Rs 24 a Nm3, which the reports print as a saving of Rs 969.01 a tonne. A 5 horsepower (HP) blower adds 7,460 kilowatt hours (kWh) a year, Rs 37,300 at Rs 5 a kWh, in every report, whatever the furnace size. 2,000 operating hours a year, seven days of downtime and 13 weeks to install. 200 kg an hour: 400 tonnes a year, 76.86 tonnes of coal replaced by 41,497 Nm3. Saving Rs 3.50 lakh, cost Rs 12.65 lakh (Rs 11.50 lakh furnace plus Rs 1.15 lakh erection), payback 3.61 years, net present value (NPV) Rs 1.35 lakh, internal rate of return (IRR) 12.73%. 300 kg an hour: 600 tonnes, 115.29 tonnes of coal, 62,245 Nm3. Saving Rs 5.44 lakh, cost Rs 12.94 lakh, payback 2.38 years, NPV Rs 6.82 lakh, IRR 24.82%. 350 kg an hour: 700 tonnes, 134.51 tonnes of coal, 72,620 Nm3. Saving Rs 6.41 lakh, cost Rs 12.94 lakh, payback 2.02 years, NPV Rs 10.53 lakh, IRR 32.25%. In each report the tonnes equal the rated kg an hour times 2,000 hours. The audit melt behind the 192.16 figure was 255 kg. So the 300 and 350 kg results assume the furnace melts a full rated charge every hour of the year (my reading of the printed sums). The 300 and 350 kg reports carry the same Rs 12.94 lakh price (Rs 11.76 lakh plus Rs 1.18 lakh erection), so the larger furnace looks better partly because its extra output is free. All three print the gas at about 2.02 kg of CO2 a Nm3: 84 tonnes for 41,497, 126 for 62,245, 147 for 72,620. The coal is where they split. The 200 kg report prints 153 tonnes of CO2 for 76.86 tonnes of coal, about 2.0 a tonne. The 300 and 350 kg reports print 300 for 115.29 and 349 for 134.51, about 2.6 a tonne. Same coal, same cluster, so both cannot hold. The net cuts printed are 69, 174 and 201 tonnes a year. At the 1.76 kg of CO2 a kg we use for Indian coal, and counting the blower at 0.675 kg a kWh, the cuts are about 46, 72 and 85 tonnes, or 116, 120 and 121 kg per tonne of brass. The 300 kg report's cluster line also prints 348 for 200 units. At 174 tonnes each that is 34,800; the 350 kg report's 40,200 is 200 times 201, so the 300 kg line looks like a slip. Weigh the coal and the brass for a month on your existing pit. Coal per tonne is the one number all three reports borrow from a single melt. Size the furnace to the tonnes you really melt, not the rated hour. A furnace that melts 300 tonnes a year does not earn the 600 tonne saving. Ask the gas supplier for the heat content and billing unit on the invoice. The reports use 8,600 kcal a kg, and piped gas may differ.

Do the Bureau of Energy Efficiency Jamnagar reports for 200, 300 and 350 kg gas pit furnaces show different savings per tonne of brass?

What is the same in all three reports

What changes: tonnes melted, and so payback

The reports disagree on carbon dioxide (CO2)

Before you pick a size

The 300 kg furnace at 600 and at 300 tonnes

What Senseible does here