# How much fuel can a Bikaner POP furnace save by preheating combustion air from its flue gas?

Canonical: https://senseible.earth/answers/bikaner-pop-furnace-flue-gas-air-preheat

Roughly a tenth of the petcoke. Flue gas leaves Bikaner's plaster of Paris furnaces at 700 to 800 C and could heat combustion air to 250 to 300 C. TERI's rule: every 20 C of air preheat saves 1% of fuel, and it puts the cluster's saving potential at 10 to 15%.

## Why the chimney is where the money goes

- Gypsum in the drum only needs about 150 to 200 C to turn into plaster of Paris; the furnace is run at around 1,000 C.
- The flue gas leaves through a small chimney above 700 C, and nothing recovers it.
- Walls are clay brick with no insulation, the drum has no fins, and firing is judged by eye.
- Petcoke feed is held constant while air is changed by hand, so unburnt coke clogs the burner orifices and shows as black smoke.

## The two rules of thumb

Every 22 C drop in flue gas temperature, or every 20 C rise in combustion air temperature, saves about 1% of fuel. They describe the same heat, so count one, not both. Air heated from about 35 C to 250 C is a rise of 215 C, about 10.7%.

## The rest of TERI's list

- Temperature probes at the crown in at least three places, and on the drum, so firing is not done by eye.
- Coke feed matched to airflow and furnace temperature, so less coke falls unburnt.
- Insulating brick and ceramic fibre in walls, floor and crown.
- Fins on the drum to pass more heat to the gypsum.
- A redesigned furnace with all of these and the recuperator: about 20% less energy, in TERI's estimate.

## Your before figure

Bikaner units burn 50 to 60 kg of petcoke per tonne of gypsum charged. Weigh coke per batch and POP bagged for a month before the change and a month after; that kilogram per tonne is the figure a lender will look for.

## A three shift unit making 6,000 t a year (estimate)

- Petcoke: 330 t a year, 55 kg per tonne of POP
- 10% saved by air preheat: 33 t of coke
- At Rs 10,000 per tonne, the profile's 2016 price: Rs 3.3 lakh a year
- CO2 at the IPCC petcoke default, about 3.17 t per tonne: 33 t x 3.17 = about 105 t a year
- Whole unit: 330 t x 3.17 = 1,046 t + 175,000 kWh x 0.675 = 118 t; about 194 kg CO2 per tonne of POP

Temperatures, rules of thumb, coke use and unit sizes from TERI's Bikaner plaster of Paris cluster profile (2016). The 35 C starting air temperature is an assumption. Petcoke CO2 is our estimate from the IPCC 2006 default (97.5 kg per GJ at 32.5 GJ per tonne); grid factor CEA FY 2025-26.

## What Senseible does here

The green loan check shows what a lender asks before funding a furnace upgrade; Senseible reads your electricity bills into monthly kWh and CO2, so the electrical side of the before and after is on record.

- [Green loan check](https://senseible.earth/green-loan-check)
- [Plaster of Paris CO2 per tonne](https://senseible.earth/answers/plaster-of-paris-calcination-co2-per-tonne)

Sources: [TERI, Cluster profile: Bikaner plaster of Paris industries (2016)](https://sameeeksha.org/pdf/clusterprofile/Bikaner-POP-Industries-Rajasthan.pdf), [IPCC 2006 Guidelines, Volume 2 Energy](https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol2.html), [CEA, CO2 Baseline Database for the Indian Power Sector, version 22.0](https://cea.nic.in/cdm-co2-baseline-view?mappingId=234&lang=en)

Last reviewed 2026-10-31.