In this article
A biomass project can be buying fuel for a dryer while a nearby generator rejects useful heat. Bringing those systems together may improve the project, but the first calculation is a moisture balance and a temperature match. “Waste heat available” is not enough information to size a dryer or promise a reactor’s throughput.
Drying and thermal conversion are different duties
Drying removes water. Torrefaction and pyrolysis change the material itself. DOE describes pyrolysis as thermal decomposition without oxygen. In one NREL-origin experimental study, researchers torrefied several biomass feedstocks at 200, 250 and 300°C and found feedstock-dependent behavior. Those conditions illustrate why a low-temperature water loop cannot simply be substituted for a thermal-conversion reactor’s heat source. [1] [2]
Our preferred screening sequence is to investigate lower-temperature drying or preheating first. Then ask the process supplier whether higher-grade recovered heat can support a specific reactor duty. The reactor temperature, residence time, oxygen exclusion, product quality and heat-transfer approach remain supplier design questions.
Begin with a transparent moisture balance
Consider an illustrative 1,000 kg/h incoming stream at 50% moisture on a wet basis. It contains 500 kg/h of dry solids and 500 kg/h of water. At a target of 20% wet-basis moisture, total outgoing mass is 500 ÷ 0.80 = 625 kg/h. That stream contains 125 kg/h of water, so the dryer must remove 375 kg/h of water.
Using an assumed latent heat of 2.257 MJ/kg gives about 235 kW solely for phase change. This is a thermodynamic reference calculation, not a dryer specification. Heating the feed, heating or moving air, exhaust losses, operating temperature and recovery efficiency increase or otherwise change the practical duty. A vendor must provide a full heat and mass balance for the actual biomass and drying process.
Build a heat inventory before buying equipment
| Stream to investigate | Potential use to evaluate | Evidence to request |
|---|---|---|
| Engine cooling circuits | Indirect drying-air preheat or another low-temperature sink | Available temperatures, flow and return-temperature limits |
| Exhaust recovery | Hot water, steam or a higher-temperature transfer circuit | Recoverable duty, acceptable backpressure and corrosion limits |
| Process offgas or recovered process heat | Integration with the thermal process | Composition, variability and an engineered combustion/emissions scope |
These are review categories, not statements that every engine can supply every duty. Ask for available heat at the expected operating load and seasonal conditions. Include all exchanger approach temperatures and piping losses between the generator and process. A nearby heat source can be less useful than it appears if production schedules do not coincide.
Keep the generator and process independently operable
Our procurement checklist calls for a bypass or alternative heat-rejection route when the dryer stops, and a startup or backup heat plan when the generator is unavailable. Obtain an engineered response for dust, fire, offgas and oxygen-control hazards. Direct contact between engine exhaust and product should never be assumed acceptable from a general equipment listing.
The business case should value delivered useful heat, not total rejected heat. Deduct added parasitic electricity, maintenance, integration capital and downtime exposure. Compare the result with the dryer fuel actually displaced and test a low-utilization case. Existing engines and heat exchangers may offer a starting point, but compatibility must be established before purchase.
Source review: 24 September 2026. Worked example by EnergyEquipment.com, using stated assumptions. Concept screening only; a qualified process designer must establish a safe equipment and operating scope.