Fuel Moisture and Calorific Value

Two penalties, not one

Wet fuel costs twice. The first penalty is dilution: water displaces fuel in the hopper, so a tonne of wet residue contains less combustible matter than a tonne of dry. The second is the evaporation load — the burner has to boil that water off before the fuel will burn properly, and that heat is taken out of the same fire that is meant to be drying your product.

The second penalty is the one people miss, and it compounds. A wet charge lowers combustion temperature, which lowers combustion quality, which raises unburned carbon and tar. On cashew shell, where CNSL is already looking for somewhere cool to condense, that is how a clean-running chamber starts fouling.

What the numbers look like across the range

The published envelope on this platform runs from about 3,200 to 5,100 kcal/kg depending on residue and moisture. Those two variables are not independent — the same fuel sits at different points in the range depending on how it was stored.

Fuel	Calorific value	Moisture as received
Cashew shell	4,800 kcal/kg	4–8 % w.b.
Groundnut shell	4,500 kcal/kg	Under 8 % briquetted
Dried mango peel	4,200 kcal/kg	Briquetted from a dried feed
Corn cob	4,100 kcal/kg	Field-dependent, briquette to stabilise
Cotton stalk	3,900 kcal/kg	Field-dependent
Rice stalk	3,800 kcal/kg	Field-dependent, the wettest of the set

The pattern is that the top of the range is a shelled residue arriving from a process, and the bottom is a field residue arriving from a field. Process residues are drier because they have already been through a factory.

Storage is part of the specification

Covered, ventilated, off the ground. That is the whole requirement and it is routinely the first thing dropped from a budget. Rain leaches CNSL out of cashew shell, which is the fraction carrying about 11,380 kcal/kg — so a wet shell heap is not just diluted, it has had its best part washed out.

Pellets and briquettes are worse again. A 90 × 90 mm briquette that gets wet crumbles, and crumbled briquettes bridge in a hopper and stall the feed. The failure shows up as an intermittent burner rather than as a fuel problem, which is why it is often diagnosed late.

How to compare two fuel offers

Not on price per tonne, and not on calorific value. Compare delivered cost per unit of usable heat: price per tonne, divided by the calorific value, corrected for the moisture it actually arrives at, and with ash-handling time added. A cheap wet residue with 12 % ash frequently loses to an expensive dry one.

The correction is arithmetic anyone can do and almost nobody does. It is also the only comparison that survives contact with a second season, when the cheap supplier’s quality has drifted and the expensive one’s has not.

Frequently asked questions

How much does moisture reduce delivered heat?

Both by dilution and by the heat spent boiling the water off. A fuel published at 4,800 kcal/kg dry delivers meaningfully less at 20 % moisture than at the 4–8 % cashew shell normally arrives at, and the second effect also lowers combustion temperature, which costs quality as well as quantity.

Should fuel be dried before burning?

Storing it dry is nearly always cheaper than drying it. Covered, ventilated storage off the ground costs a roof; active fuel drying costs the same heat you were trying to save. The exception is a field residue arriving straight off the land, where a short air-dry down towards the under 8 % a briquette needs pays for itself.

Does briquetting change the calorific value?

Not per kilo of dry matter — compression changes density, not chemistry. What it changes is everything around the number: bulk density rises from around 150–200 kg/m³ loose to about 1.2 g/cm³ briquetted, so a hopper holds a shift instead of an hour, and the feed becomes consistent.

Fuel moisture and what it does to calorific value