Comparative analysis of agricultural biomass fuels
Calorific value is the number everyone quotes. Ash is the number you live with.
Agricultural residues used as boiler and dryer fuel range from about 3,800 to 4,500 kcal/kg, with cashew shell published as 4,800 kcal/kg. Calorific value decides how much you burn; ash content decides how often you stop. On a day-to-day basis the second number usually matters more.
The published fuel envelope
| Fuel | Calorific value | Form | Practical note |
|---|---|---|---|
| Cashew shell | 4,800 kcal/kg | Loose shell | Highest value in the range; requires CNSL management |
| Groundnut shell | 4,500 kcal/kg | Briquette, 90 × 90 mm | Clean, widely available in the groundnut belt |
| Dried mango peel | 4,200 kcal/kg | Briquette, 90 × 90 mm | Closes the loop on a mango line’s own residue |
| Corn cob | 4,100 kcal/kg | Briquette, 90 × 90 mm | Abundant, seasonal, easy to briquette |
| Cotton stalk | 3,900 kcal/kg | Briquette, 90 × 90 mm | Bulky before densification |
| Rice stalk | 3,800 kcal/kg | Briquette, 90 × 90 mm | Highest ash — expect shorter cleaning intervals |
Cashew shell is published at 4,800 kcal/kg. Two of our own source documents used to disagree — one stated about 4,500 and another about 4,800 — and while that was unresolved we published the range that was true under both rather than pick the comfortable number. The master technical reference is the authority, and 4,800 is now used consistently across the site.
The four properties that actually decide
Calorific value sets how many kilograms you burn per hour, and it is the only property most comparisons mention. Ash content sets how often the chamber has to be cleaned and how much material an operator moves every day, and it varies far more between these fuels than calorific value does — rice-derived fuels are notably high, wood pellets notably low.
Moisture content silently reduces delivered calorific value, because energy spent evaporating water in the fuel is energy not delivered to your product. A fuel rated dry and stored wet is a different fuel. And durability determines how much of your briquette or pellet arrives as fines rather than as fuel, which is a feed-system problem before it is an energy one.
Why the cheapest fuel is often not the cheapest
The comparison that matters is delivered cost per unit of usable heat, plus the operator time the fuel imposes. A residue that is free at the gate but arrives wet, dusty and high in ash can cost more in labour and lost output than a purchased pellet. That is not an argument against residue — for most processors residue wins clearly — but it is an argument for costing the whole picture rather than the invoice.
Seasonality is the other hidden cost. A fuel available for four months of the year is a fuel you must store for eight, or substitute. A multi-fuel burner exists so that substitution is a control-recipe change rather than a capital decision.
Making fuel from what you already have
A briquette line mills, dries and densifies loose residue into 90 × 90 mm briquettes. It is sized alongside the burner, because the residue volume you genuinely have — not the volume in the field — determines how much of your fuel you cover yourself. Most processors end up partly self-supplied and partly buying in, and designing for that from the start is more robust than assuming either extreme.
Frequently asked questions
Which fuel should I specify if I can get several?
The one you can reliably get most of the time, and size against that. The burner will fire the others without mechanical change — 6–8 mm pellets, 90 × 90 mm briquettes or loose residue — but feed geometry and ash handling are specified against a primary fuel, and specifying against the occasional fuel leaves the everyday one poorly served.
How much does fuel moisture really cost me?
More than most people expect, because it hits twice: the water displaces fuel mass, and evaporating it consumes heat that would otherwise reach your product. On fuel bought by the tonne, every 10 % of moisture is 10 % of the mass you are paying to evaporate. It is the single easiest efficiency gain available on most sites, and it costs a roof rather than an engineering change.
Is high ash a reason to reject a fuel?
Not on its own — it is a reason to plan for it. High-ash fuels need shorter cleaning intervals and more operator time, and the ash handling has to be specified for the fuel you will actually burn. Rice-derived residues fire perfectly well anywhere in the 200,000 to 900,000 kcal/h range; they are just not a fuel to specify a plant around and then discover.
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