How much extra air should a gas kiln burn with, and what does each percent of leftover oxygen in the flue gas cost in fuel, according to the Morbi manual?
The Morbi manual says a natural gas kiln should run with 5 to 7% extra air, against 30 to 40% for coal and 35 to 40% for lignite, and that each 1% cut in leftover oxygen in the flue gas saves 1% of fuel. It also found Morbi units did not measure it.
What excess air means, in plain words
A burner needs air to burn its fuel. The exact amount the fuel needs is the theoretical air. Burners are run with some extra air on top, so every drop of fuel burns fully. That extra is called excess air. Too little leaves unburnt fuel. Too much just heats air that goes straight up the chimney, which wastes fuel.
You cannot see excess air, but you can read it: leftover oxygen in the flue gas, the gas in the chimney, tells you how much extra air went in. A hand-held flue gas analyser reads it in seconds.
How do the two readings fit? The air ratio, which is the air used divided by the air needed, equals 21 divided by (21 minus the oxygen percentage). So 5 to 7% extra air is an air ratio of 1.05 to 1.07, or roughly 1% to 1.4% oxygen in the flue gas. A reading of 6% oxygen means about 40% extra air, and 3% oxygen means about 17%.
The manual's table of fuels
- Coal: 10.8 kilograms of air for each kilogram of fuel; recommended excess air 30% to 40%.
- Lignite: 8.4; 35% to 40%.
- Wood: 5.8; 20% to 25%.
- Furnace oil: 13.9; 12% to 15%.
- Natural gas: 17.34; 5% to 7%.
- The manual gives no reason for the spread. My reading is that solid fuel mixes with air less well than gas, so it needs a bigger margin.
Three rules of thumb it gives for kilns
- A 1% reduction of leftover oxygen in the flue gas cuts fuel use by 1%.
- Every 20 degrees Celsius (°C) rise in the temperature of the combustion air raises the kiln's thermal efficiency by 1%.
- A 22 °C fall in the flue gas temperature cuts fuel use by 1%.
- The manual calls these tips and gives no Morbi measurement for them. Treat them as a first estimate.
What it found in Morbi
The manual says that in the cluster there is no practice of measuring excess air, either by carbon dioxide or by oxygen in the flue gas, and that it is important to start. Its own auditors measured carbon dioxide in the flue gas with a chemical test (Orsat apparatus) and cross-checked it with an oxygen analyser. It also saw preheated combustion air at most 250 °C in Morbi kilns, when current burners accept up to 600 °C.
What to do
- Read leftover oxygen in the flue gas at the same point each week and write it down with the kiln zone.
- Compare with the burner maker's setting. If the reading is well above it, ask the supplier to retune.
- Re-read after any change to the blowers, since the manual says the variable speed drives on kiln blowers should be set to a standard frequency and checked regularly.
- Track gas for each square metre packed before and after, so output does not hide the change.
What 3 points of oxygen could be worth, example numbers
- A kiln burning 3,500 standard cubic metres (scm) a day (the manual's typical medium wall tile unit, all heat uses together)
- Oxygen reading falls from 6% to 3%, a 3 point cut
- The manual's rule: 3 x 1% = 3% of fuel, or 3,500 x 0.03 = 105 scm a day
- Over 330 days (an example): 34,650 scm a year
- Carbon dioxide at 2.05 kg a scm: about 71 tonnes a year
A first estimate from a rule of thumb, applied to a number that includes the dryer and the spray dryer as well as the kiln. Measure your own kiln before and after.
What Senseible does here
Senseible reads fuel invoices and electricity bills into monthly records, so a unit's own gas, oil and power lines are on file to set beside the report's figures. It does not measure a kiln or replace an audit.
- Furnace air ratio and oxygen target
- Boiler excess air and fuel saving
- Roller kiln waste heat recovery
- Morbi reports ranked by payback
- Green loan readiness check
Sources: BEE, Manual on Energy Conservation Measures in Ceramic Industry, Morbi cluster (SAMEEEKSHA), IPCC, 2006 Guidelines for National Greenhouse Gas Inventories, Volume 2 Energy
Last reviewed 2026-11-07. Table 11 and the kiln tips are from the April 2010 Bureau of Energy Efficiency (BEE) Morbi manual, section 3.3.1. The manual calls the rules of thumb tips and gives no Morbi measurement for them. Check your burner maker's figures.