Understanding moisture and ash specifications

For solid biomass fuels, moisture and ash do most of the work in a specification. They are also the two parameters most often quoted without stating the basis, which makes comparison between offers meaningless.

Moisture: you are buying water

Moisture directly reduces the energy delivered per tonne, because water absorbs heat during combustion instead of releasing it. It also increases freight cost per useful unit of energy — you pay to ship water across an ocean. A three-point difference in moisture is not a technicality; it is a price difference.

Always confirm the basis. As-received, air-dried and dry basis figures are not interchangeable, and an offer quoted on a favourable basis can look better than a genuinely superior one quoted honestly.

Ash: what is left, and what it does

Ash is the incombustible residue. High ash reduces calorific value and, more importantly, creates operational cost — disposal, more frequent cleaning, and in some cases slagging and fouling on boiler surfaces. Ash content limits are usually set by what the buyer's equipment tolerates rather than by an abstract quality preference.

• Moisture — energy delivered per tonne, freight efficiency, storage and mould risk
• Ash — residue handling, cleaning frequency, slagging and fouling risk
• Foreign matter — often the real cause of an out-of-spec ash result
• Calorific value — the outcome the other parameters drive, best confirmed by testing

Where a shipment fails on ash, foreign matter picked up during handling and storage is a frequent cause. It is usually cheaper to fix at the yard than to argue about at the destination.

Agree tolerance and consequence together

A specification without tolerance bands and an agreed consequence is incomplete. Decide in advance what happens if a lot lands slightly outside the range: a price adjustment on a defined scale, or rejection. Agreeing the mechanism in the contract turns a dispute into an arithmetic exercise.