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Tuning into radio-frequency heating

An emerging technology called radio-frequency (RF) heating has the potential to be a rapid, non-toxic, efficient and safe method to disinfest various food and agricultural products. The technology is starting to have some commercial applications for treating products like nuts and spices. Now, Saskatchewan researchers are developing RF heating to control insects in stored grains on the Prairies.

Dr. Oon-Doo Baik, a professor of chemical and biological engineering at the University of Saskatchewan, is leading this research. “Radio-frequency is a kind of electromagnetic wave, like a microwave. RF heating works similarly to microwave heating, but RF heating has a greater penetration depth,” he explains.

“Radio-frequency heating is very effective, energy-efficient and fast heating, and provides uniform heating. It can also be used for selective heating or targeted heating, which allows killing of insects in grain without damaging the grain quality.” Insects have very different electrical properties than grains, so at certain RF frequencies the insects heat up much faster than the grain. As a result, the insects can be killed while the grain remains at a moderate temperature.

“There are clear advantages to using RF technology compared to other conventional methods to control insect pests in grain,” says Baik. “Conventional methods include using chemicals or non-chemical methods such as [conventional] heating, which is not efficient and takes a large amount of energy. Chemicals can be toxic, and they are specific to certain insect species and certain stages in the insect’s life cycle.

“In contrast, radio-frequency heating is non-toxic, it efficiently kills all stages of the insect’s life cycle – the egg, larva, pupa and adult – and it works for all different kinds of insect pests. It’s cheaper in the long run and it’s safe.”

Baik and his research team, Dr. Bijay Shrestha and Daeung Yu, recently completed a study to use RF heating to control rusty grain beetles in bulk spring wheat samples at a lab scale. Rusty grain beetle is one of the most common stored-grain insects. The researchers tested the technology with wheat at three moisture contents (12, 15 and 18 per cent) and at temperatures between 15 and 75 C. They examined the effects of RF heating on the wheat and the insect.

“By heating the bulk wheat sample to about 60 C for three minutes with our 1.5 kW lab-scale RF unit, we were able to get 100 per cent mortality of the rusty grain beetle [at all life stages],” says Baik. “Also, we observed no significant quality degradation in terms of wheat germination and wheat flour properties.”

Following up on these promising results, Baik is now leading a new project to develop a prototype RF heating system as the next step towards the practical use of RF technology in Prairie agriculture. Funding for this project is from the Saskatchewan Ministry of Agriculture’s Agriculture Development Fund and the Western Grains Research Foundation.

Developing a prototype
Baik and his research team will be developing the prototype RF technology for use with a variety of grain storage and handling systems commonly found on the Prairies. “This technology is very flexible and can be used for all different types of applications – grain elevators, grain storage bins, any type of grain storage system – because the main body of the RF heating system can be separate from the applicator,” he explains.

Coaxial cables can be used to connect the RF generator to the RF applicator, with the generator up to about 30 feet away. “The applicator is basically composed of two electrodes. We can install the two electrodes in an inlet or outlet of a grain storage system, so the grain can be treated with RF heating as it flows past the applicator.”

The electrodes can range in size from about 20 cm to 1 m long and can have various shapes like a plate or a tube, depending on the characteristics of the specific grain handling system and the needs of the people who want to disinfest the grain.

In an exciting advance, the researchers will be using a sophisticated 3D computer simulation system to design and make the applicators. “Using a computer simulation, we will test the RF system with different shapes of applicators, different configurations and sizes of electrodes, and different grain handling systems, like augers, conveyor belts and bin-to-bin and bin-to-chute transportation systems. The simulation calculates the electromagnetic field generation and then the volumetric heat generation, heat transfer, mass transfer, momentum transfer (fluid flow), et cetera,” says Baik.

“So multiple physical effects can be calculated and simulated, and then that can be used for in-house fabrication and implementation of the technology. So this is a powerful tool for theoretical understanding and for scientifically solid virtual protoyping. Based on this, we will fabricate applicators and then test them with a real system as well.”

The researchers will be testing the prototype technology on rusty grain beetles in spring wheat and on red flour beetles, another common stored-grain pest, in canola. The RF treatments will be done for all life stages of the two insects, and for a range of moisture contents and temperatures for the wheat and canola.

Before and after each RF treatment, the researchers will measure germination of the wheat and canola, as well as the wheat’s flour properties and the canola’s oil properties. And they will determine the insect mortality rates.

This new project involves somewhat different RF technology (50 ohm technology) than the researchers used in their earlier project (self-oscillator technology). As well, they will be using a more accurate way to measure the selective heating rate of the insects. With this greater accuracy, Baik suspects they may be able to achieve 100 per cent insect mortality with even less energy input and shorter process time than they used in their initial study due to more rapid selective heating at higher RF power.

Towards practical use
According to Baik, there are a couple of issues that could be hurdles in the adoption of RF heating to disinfest stored grain.

“The first one could be an advantage, or disadvantage, depending on your point of view. Based on our preliminary research, we need to heat the grain moderately up to about 60 C for about three minutes or less to kill the insects, so there might be some drying of the grain when you use RF heating. However, many farmers want to dry their grain for storage, so it could be an advantage,” he notes.

“The second issue is the initial capital cost of the RF technology. At present a moderate scale RF generator costs about $20,000 to $50,000. However the capital cost per kW of power output is dropping significantly. I am expecting the fall in cost will be like the case of the microwave; initially microwave technology was expensive, but now we buy it very cheaply. Also, the capital cost will be paid back over time because [RF operating costs] are cheaper than the conventional methods.”

To assess the relative costs, the researchers calculated the operating costs for RF heating compared to chemical treatment for the example of wheat stored in a medium-sized bin with a diameter of about 27 feet, a height around 19 feet, and a capacity of about 8,750 bushels.

“The cost to treat that volume of wheat with RF heating was approximately $25, based on the provincial energy cost of 11.7 cents per kilowatt hour. To treat that same volume of wheat with a pesticide or fumigant, the chemical cost plus the operating costs, based on our preliminary research, ranged from about $250 to something like $2,500. So the chemical treatment cost is much higher than the RF heating,” says Baik. “Of course, there is the initial capital cost to purchase the RF heater, but that cost is falling. And RF heating is safer than using chemicals, it can also dry the grain, if that’s required, and it’s fast and efficient. So I think it’s promising.”

 


December 15, 2014
By Carolyn King

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