How to Choose a Heat Generator for a Grain Dryer

Choosing between direct heating and a heat exchanger is not only about efficiency and cost, but also about the cleanliness of the drying medium for the intended use of the grain.

Direct heating (mixing air with combustion products) delivers 95–98% efficiency and a lower cost—an option for technical grain and biofuel. A system with a heat exchanger supplies the dryer with only clean heated air (80–90% efficiency) and is used for food and seed grain, as well as oilseeds for edible oils. Below are structured selection criteria: the batch’s intended use, requirements for drying-agent cleanliness, and budget and maintenance allowances.

What the Heat Generator Does in a Grain Dryer System

A heat generator is the grain dryer’s source of thermal energy: it heats air to the set temperature and supplies it to the drying chamber. The heated air passes through the grain layer, removes moisture, and is exhausted from the system. Without a heat generator, a grain dryer is just a shell with no drying process.

Heat generators operate on gas, diesel fuel, pellets, husks, wood chips, and other types of biomass. This unit is what produces the heat for the drying process.

Class 1 — direct heating. Fuel is burned in the furnace; combustion products mix with atmospheric air and are fed as a mixture into the drying chamber; the grain comes into contact with this mixture. The design is simpler and cheaper than heat-exchanger solutions, with 95–98% efficiency.

Class 2 — indirect heating via a heat exchanger. Fuel is burned in an isolated chamber; combustion products pass through a heat exchanger and are exhausted through the flue pipe; atmospheric air is heated through the walls without contact with flue gases and is supplied to the dryer as a clean agent. The system is more complex and expensive, has two circuits (flue and air), and a typical efficiency of 80–90%.

Direct heating: operating principle and applications

In a direct-flow heat generator, fuel is burned in the furnace, and the combustion products are mixed with atmospheric air and fed as this mixture into the drying chamber; the grain comes into direct contact with the drying medium. The absence of an intermediate heat exchanger provides 95–98% efficiency, a lower cost, and a simple design.

Possible fuel types: natural gas, diesel, pellets, husk, wood chips, straw. Typical scenarios include drying technical grain (feed), feedstock for biofuel, and industrial oilseeds (not for edible oils); for food and seed grain, heat generators with a heat exchanger are usually used.

Direct heating: operating principle and applications

Risks of direct heating for food and seed grain

When any organic fuel is burned (gas, diesel, biomass), polycyclic aromatic hydrocarbons (PAHs) are formed, including benzo[a]pyrene. In direct-flow operation, the combustion products mix with air and are supplied as the drying medium, i.e., they come into direct contact with the grain. This creates a risk of PAH transfer and associated products of incomplete combustion into batches of food and seed grain.

According to IARC data, benzo[a]pyrene is classified as a Group 1 carcinogen, and PAH contamination of cereals is associated with gas- and flame-drying processes. This transfer mechanism confirms that grain contact with flue gases is a critical risk factor for food chains.

The FAO/WHO (Codex Alimentarius) position: direct contact of oilseeds and grain with combustion products during drying is a source of PAHs and should be excluded; contact of foodstuffs with flue gases should be minimized, and combustion should be brought to completion. The European Commission’s Scientific Committee on Food (SCF) also notes PAH contamination of oilseeds and vegetable oils during direct-flow drying with raw material contact with combustion products.

AHDB points to the risk of hydrocarbon contamination during direct-flow drying on petroleum fuel; incomplete combustion forms PAHs, and recirculation of exhaust gases exacerbates the problem. A soybean study (Brazil, 2021) found PAHs in all 22 samples after direct-flow drying on firewood, with exceedances for PAH4/PAH8 and benzo[a]pyrene detected in all batches. Taken together, this confirms the technological risk of grain contamination under direct heating.

Why filters do not solve the problem of gaseous PAHs

Mechanical filtration is designed to capture solid particles by size and mass, not to remove the gas phase. Gaseous PAHs are volatile organic compounds that pass through such barriers. According to EPA data (TO-13A method), their volatility makes it impossible to effectively collect them using filtering materials alone; a sorption cartridge is required.

Cyclones are effective for particles ≈5 µm and larger; multi-tube designs reach 80–85% for particles from ~3 µm. Even high-efficiency multi-tube cyclones work on particles, not on the gas phase. The molecular sizes of PAHs are incomparably smaller than mechanical filter cells, so they are not retained.

Industry patents on direct-fired grain dryers explicitly note that it is impossible to remove harmful components from hot air using physical methods alone. For gaseous PAHs, only the sorption method on activated carbon is effective; for benzo(a)pyrene, efficiency of up to 99.7% is demonstrated. In industrial direct-fired systems, such units are not used due to high cost and the need for regular sorbent replacement.

Hence the conclusion: installing cyclones and mechanical filters in direct-fired heat generators reduces only the dust load and soot, but does not eliminate gaseous PAHs in the drying agent. To prevent their contact with the grain, a fundamentally different approach is required—separating the combustion and drying circuits with a heat exchanger. This mode ensures that clean heated air is supplied to the dryer and aligns with FAO/WHO practices, which require excluding contact between food raw materials and combustion products.

Heat generator with a heat exchanger: how it works and why it’s needed

Indirect heating is implemented via an insulated combustion chamber: combustion products pass through the heat exchanger and are removed via a separate flue line. Atmospheric air is heated through the walls of the recuperator (without contact with flue gases) and is supplied by a fan into the drying chamber; the assembly includes a combustion chamber, a heat exchanger (fire-tube or plate), a drying-air fan, and an induced-draft fan. To increase efficiency, 3-pass heat exchangers are used.

Only clean heated air enters the dryer outlet; the system efficiency is 80–90%, which is lower than the direct-flow option but ensures the cleanliness of the drying agent. The scheme is used for food-grade grain, seed material, and oilseeds for food processing. This approach aligns with the FAO/WHO position on the need to exclude contact between food raw materials and combustion products.

Heat generator with a heat exchanger: how it works and why it’s needed

Comparison of Two Classes of Heat Generators

Parameter	Direct Heating	With Heat Exchanger
Efficiency	Efficiency 95–98%; no heat-exchanger losses.	Efficiency 80–90%; some heat is lost in the heat exchanger.
Equipment cost	Lower due to a simple design and no heat exchanger.	Higher due to the heat exchanger and a dual-circuit design.
Drying agent purity	A mixture of air and combustion products enters the dryer.	Only clean heated air is supplied to the dryer.
Presence of PAHs in the drying agent	PAHs are present because combustion products come into contact with the agent.	PAHs are absent; contact with flue gases is excluded.
Compliance with requirements for food grain (FAO/WHO)	Does not comply: direct contact with flue gases is unacceptable.	Complies: clean air without combustion products.
Food and seed grain	Not used for food and seed grain.	Used; provides a clean drying agent for food and seed material.
Technical grain and biofuel	Used for feed and technical raw materials.	Also applicable; chosen when higher cleanliness requirements apply.
Maintenance complexity	Lower: fewer units and circuits.	Higher: two circuits and a heat exchanger require scheduled maintenance.

A summary of operational trade-offs across key parameters.

Sources and regulatory documents

List of primary sources used: international organizations, scientific publications, industry and technical documents. The names are provided for fact-checking.

NCBI / IARC — Benzo[a]pyrene. Chemical Agents and Related Occupations
PMC / NCBI — Polycyclic aromatic hydrocarbons in soybean grains, 2021
FAO Codex Alimentarius — Code of Practice for the Reduction of Contamination of Food with PAH, CXP 068-2009
European Commission SCF — PAH. Occurrence in foods, dietary exposure and health effects
AHDB — In-store grain drying: high-temperature and near-ambient air approaches
US EPA — Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air, TO-13A
USPTO — Patent 9696089. Grain drying apparatus using exhaust heat
Chemical Engineering Transactions, Vol. 89, 2021 — The Effective Polycyclic Aromatic Hydrocarbons Removal

Selecting a heat generator for your facility

Request a calculation for your grain dryer: we’ll take into account the crop, drying volume, and operating mode (agent temperature, air flow rate, shift schedule). As part of an Aetern consultation, we’ll select the required capacity, recommend the fuel type and connection scheme, and provide payback guidance. The outcome is a technical and economic feasibility study with implementation options.