What did the Bureau of Energy Efficiency test on three wood-fired hot air generators in Ahmedabad, and do the 60,000 kilocalorie report figures tie?

Gas replaces 259.2 tonnes of wood a year in the Bureau of Energy Efficiency (BEE) Ahmedabad report for a 60,000 kilocalorie (kcal) an hour hot air generator. It costs rupees (Rs) 3.19 lakh, saves Rs 1.36 lakh a year and pays back in 2.35 years. Three old units tested at 37, 34 and 23% efficiency.

What the machine does, in plain words

After filtering, chemical powder is still wet. A hot air generator heats air, and a fan blows it over trays until the powder dries. Think of a hairdryer for chemicals. Here the old ones burn wood and the proposed one burns gas.

Efficiency is how much of the fuel's heat ends up in the hot air. Think of carrying water in a leaky bucket: it is the share that reaches the tank. The report is a detailed project report (DPR), a costed plan for one upgrade, for chemical units in Ahmedabad, Gujarat.

The three wood-fired tests

• Unit 1: 1,794.59 kilograms of air an hour warmed from 28.5 to 112 degrees Celsius, an 83.5 degree rise, at 0.24 heat per kilogram per degree = 35,964 kcal out. Wood 36 kilograms an hour x 2,700 kcal = 97,200 in. 37%.
• Unit 2: 1,713.96 kilograms of air, 27.3 to 122 degrees, 94.7 rise = 38,955 out. Wood 42 x 2,700 = 113,400 in. 34.35%.
• Unit 3: 812.04 kilograms of air, 28.5 to 98 degrees, 69.5 rise = 13,544 out. Wood 22 x 2,700 = 59,400 in. 22.8%.
• Plain average (37 + 34.35 + 22.8) / 3 = 31.4%. The text says 'around 32%'. The average of the three direct tests is 31.4%, and Unit 1's indirect test prints 32.31%. The report does not say which it used. It proposes a gas unit at 70%.
• Wood is valued at 2,700 kcal a kilogram here. The Vapi reports use 3,800 for wood. On 3,800, unit 1 would read 35,964 / (36 x 3,800) = 26.3% instead of 37%. The reports do not explain the difference.

The saving and the money

• Wood: 72 kilograms an hour x 12 hours x 300 days = 259,200 kilograms, 259.2 tonnes. Gas: 8.3 normal cubic metres (Nm3) an hour x 3,600 hours = 29,880 Nm3 of liquefied natural gas (LNG).
• Wood at Rs 3 a kilogram = Rs 7.78 lakh. Gas at Rs 19 an Nm3 = Rs 5.68 lakh. Fuel saving Rs 2.098 lakh. A lakh is 100,000.
• Extra power 14,760 kilowatt hours (kWh) x Rs 5 = Rs 0.738 lakh. Net Rs 2.098 minus 0.738 = Rs 1.36 lakh.
• Cost Rs 3.19 lakh = 2.90 equipment + 0.26 fitting + 0.03 contingency. Payback 3.19 / 1.36 = 2.35 years. Own money Rs 0.80 lakh, loan Rs 2.39 lakh.

Printed figures that do not line up

• Power: Annexure 3 prints the connected load as 5.5 kilowatts. Table 2.2 gives 5.5 horsepower, which is 4.1 kilowatts, and 4.1 x 3,600 hours = 14,760, the printed kWh. So the 5.5 under kilowatts looks like horsepower (our reading).
• Gas: Table 2.2 says 9 cubic metres an hour; the saving uses 8.3 Nm3. Nm3 is gas measured at a standard temperature and pressure, so the same gas takes up a different volume at a different temperature. Check which one your supplier bills.
• Wood per kilogram of product: Table 1.2 says 3.0 kilograms and 1.2 kWh. The unit above it burns 350 tonnes for 110 tonnes of product and uses 130,000 kWh: 350 / 110 = 3.18 and 130,000 / 110,000 = 1.18 (our division).
• Drying cost: the text puts it at Rs 3 a kilogram of product, about 30% of energy cost. If Rs 3 is 30% of energy cost, energy would be about Rs 10 a kilogram of product. Wood alone at 3.0 kilograms x Rs 3 is Rs 9 and power adds more, so the Rs 3 and the 30% cannot both describe the same cost (our reading).
• Sensitivity: a 5% fall in fuel savings drops the internal rate of return (IRR, the yearly return the project earns on the money put in) from 25.46% to 12.53%, the net present value (the total gain over the project's life, counted at the value of money now) from Rs 1.76 lakh to Rs 0.27 lakh and the cover ratio (how many times the year's cash covers the loan payments) from 1.71 to 1.20. The power bill takes 0.738 / 2.098 = 35% of the fuel saving, so the margin is thin (our arithmetic).

Carbon dioxide (CO2)

The report counts 259 tonnes of wood as 243 tonnes of CO2, subtracts 58 tonnes for the gas and prints a 185 tonne cut. 58 tonnes over 29,880 Nm3 is 1.94 kilograms an Nm3 (our division, no factor printed), and the power is not counted.

Our reading, following the Greenhouse Gas (GHG) Protocol Corporate Standard: CO2 from burning wood is called biogenic, because the tree took that CO2 from the air as it grew, so it goes on a separate line outside Scope 1 (the emissions from fuel you burn on your own site). Gas is fossil and goes inside. Scope 2 is the emissions behind the electricity you buy. Your reported fossil CO2 would rise, not fall. With the report's own 58 tonnes plus the power at 0.675 kilograms a kWh, that is about 58 + 10 = 68 tonnes more a year.

Fossil CO2 change for one generator (our arithmetic)

• Gas, the report's figure: 58 tonnes (Scope 1)
• Extra power: 14,760 kWh x 0.675 = 9.96 tonnes (Scope 2)
• Wood CO2 moves to the separate biogenic line
• Change in reported fossil CO2: 58 + 9.96 = about 68 tonnes more a year

Gas figure is the report's. Grid factor is the Central Electricity Authority's for 2025-26. The report prints a 185 tonne cut by counting wood as fossil.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so gas and electricity bills sit in one record and the change a switch brings shows up month by month. Hot air temperature and wood burnt are read on site, or through sensors set up with the Senseible team on request.

• Green loan readiness check
• Wood to gas hot air generator: CO2 effect
• Efficient tray dryer wood saving
• Ahmedabad chemical cluster footprint
• Ahmedabad gas hot air generators: 60,000 against 120,000

Sources: BEE, Detailed project report: energy efficient gas fired hot air generator 60,000 kCal/hr, Ahmedabad chemical cluster (SAMEEEKSHA), GHG Protocol Corporate Standard (biogenic CO2 reported separately), CEA CO2 Baseline Database, version 22.0

Last reviewed 2026-11-07. Figures are from the report named above, written in 2010; costs, fuel prices and paybacks are at that year's prices. Check today's prices and a current maker's quote before you rely on the payback. The Scope 1 reading is ours, and the grid factor for the year you report should come from the Central Electricity Authority's current table.