One tonne of wet biomass is not one tonne of product. Price the water you can remove with usable, coincident heat.
Contents
| Quantity | 50% to 10% moisture, wet basis |
|---|---|
| Incoming wet feed | 1,000 kg |
| Dry solids retained | 500 kg |
| Final product | 500 ÷ 0.90 = 555.6 kg |
| Water evaporated | 1,000 − 555.6 = 444.4 kg |
| Latent heat alone, near 100°C | About 279 kWh; excludes heating feed and all losses. |
| At an illustrative 1 kWh/kg water removed | About 444 kWh of useful dryer heat. |
Buy evaporation capacity, not a waste-heat headline
A generator’s recoverable heat is useful only if it reaches the dryer at a usable temperature and at the same time wet feed is available. The equipment match is between a heat source, a drying process and an operating schedule.
In the table, reducing moisture from 50% to 10% does not mean removing 40% of the original tonne. The denominator changes as water leaves. Keep dry-solids mass constant and solve the final wet mass. That avoids a common sizing error.
Turn megawatts of heat into tonnes of production
At an assumed delivered heat consumption of 1 kWh per kg of water removed, 1 MW of continuous useful heat evaporates 1,000 kg/hour. For this feed, that supports about 2.25 tonnes/hour of incoming wet biomass. Actual dryer demand includes sensible heating, exhaust losses, recirculation and heat-exchanger performance; require a guarantee on your feed and final moisture.
At 6,000 coincident operating hours, that 1 MW delivers 6,000 MWh of heat. If it displaces purchased heat costing an assumed $30/MWh delivered, its gross heat value is $180,000/year. Subtract pumping and fan power, maintenance, downtime and the annualized heat-recovery investment.
Use each heat stream where it fits
Engine exhaust offers higher-temperature heat than jacket water. A low-temperature water loop may suit preheating or an appropriate belt dryer; it cannot be assumed to supply the reaction heat required by torrefaction or pyrolysis. Drying feed upstream can still improve those processes by avoiding the cost of heating and evaporating unnecessary water.
Keep gas cleanliness and heat transfer explicit. Indirect heat exchange can separate exhaust constituents from the product. Pressure drop, corrosion, condensation and fouling can change both engine operation and heat availability.
The project boundary includes the dusty end
Dry material handling, fire and dust controls, dryer turndown and backup heat are part of the plant. A dryer that works only at full engine output may constrain both businesses. Match the heat-duration curve to the feed schedule and provide a safe operating mode when either side trips.