A combined heat and power system using Bowman exhaust gas heat exchangers has halved fuel consumption and emissions on a Finnish farm, writes Phil Allman
Grain drying is an essential part of the grain production process in Nordic countries
Credit: EJ Bowman Ltd
Grain drying is one of the biggest energy consumption and cost factors in agriculture. Drying has been developed in many ways and there are several energy sources available for heating. However, the heat energy input is rarely recovered for re-circulation and so this highly valuable resource is lost to the atmosphere.
Now a new system designed by Calefa of Finland, using heat pump technology linked to a Bowman exhaust gas heat exchanger, can reduce the energy cost and fuel consumption of the dryer by as much as 50% with a relatively short investment payback period.
In its first year of operation at one Finnish farm, the system has cut fuel oil consumption by half – saving around 18,000 litres of fuel and halving the farm’s CO2 emissions.
Grain drying is an essential part of the grain production process in Nordic countries such as Finland. After harvesting, the grain has to be dried to reduce its moisture content from around 25% to 12%-14% so that it can be stored and preserved.
Traditionally, the process is carried out in large grain dryers, where hot air is blown through the grain, usually provided via an oil or gas burner. However, the energy costs involved are significant and directly impact on food production costs.
During the traditional drying process, warm air, which has passed through the grain, is exhausted outside to atmosphere, in a ‘total loss’ cycle. Typically this exhaust air still has a temperature of 35oC, plus 90%-100% -> 50%-70% humidity, so a valuable energy source is lost.
It takes up to 24 hours to achieve the required moisture level for a 30-tonne batch of grain, so a farm’s typical drying season can last for up to two months, consuming between 25,000 and 40,000 litres of fuel oil during that time.
The new combined heat and power (CHP) system has been successfully installed by Calefa in Iitti, Southern Finland, at a well-established arable farm. It is the first time this kind of installation has been undertaken in Finland, and possibly the first of its type in the world.
The process works by recovering a significant proportion of the warm exhaust air from the farm’s grain dryer that is traditionally released to atmosphere.
|Recovered energy is heated back to 65oC in the Bowman heat exchanger|
Any contaminants and grain debris are removed from the air prior to it being re-circulated and the ‘cleaned’ air is then transferred through a heat pump and then to a Bowman exhaust gas heat exchanger, where it is heated back up to 65oC.
The CHP system has been set up to recover heat energy from half of the exhaust air that would normally be expelled to atmosphere. The ‘closed loop’ heat recovery system dramatically reduces the amount of energy required and the emissions produced.
|Grain drying from diesel-powered air source heat pump|
The system is designed to operate with outside air temperatures between -50oC to +30oC. which are typical outside air temperatures during the Nordic drying season. It can be retrofitted very easily to existing grain drying systems as well as being supplied as part of new installations.
As an additional bonus, the diesel engine is fitted with a generator with a shaft power of 55 kW – enabling it to provide power to the pumps and other actuators of the heat recovery system.
‘Bowman exhaust gas heat exchangers were chosen for the system because of their outstanding thermal transfer efficiency and reliability. The heat recovery system that Calefa has developed delivers huge savings both in terms of fuel costs and CO2 emissions, for what has historically been an extremely energy intensive process,’ says Tony Carter of E J Bowman.
How it works
1. A closing damper channels warm exhaust air from the drier to an inside duct;
2. Warm air is blown into a cyclone at high speed;
3. The warmed air is cleaned of debris in the cyclone;
4. Heat from the warmed air is transferred to a water-glycol mixture and piped to the heat pump container;
5. The transferable heat pump container houses the diesel engine, compressor and a generator;
6. The diesel powered heat pump increases the temperature of the water/glycol mixture from around 350oC to around 650oC using heat recovered from the diesel engine via the Bowman EGHE;
7. The water/glycol mixture is then transferred to a pre-heating coil and from there to the dryer inlet air stream.