Greener heating for greenhouses

By Annie Webb and Gabrielle Bauer
Published in Canadian Biomass Magazine

Because greenhouse crops can be grown throughout the year, they are becoming increasingly important for the food supply of countries like Canada, which have colder climates and shorter growing seasons. However, greenhouse heating can be one of the highest operating costs for a producer. Heat is typically supplied by non-renewable fossil fuels, such as oil and natural gas. These fuels are also frequently used to enrich the greenhouse with carbon dioxide (CO2) to enhance plant growth. The high cost and environmental impact of fossil fuels has led some greenhouse operators to look for alternative heat sources, such as biomass.

Still, biomass heating systems are far from a panacea. For one thing, the initial capital investment necessary to install these systems can be discouraging for small operations. What’s more, a substantial amount of heat, CO2, noxious gases, and particulates are released when any type of biomass is combusted. Enter the Biomass Furnace Flue Gas Emission Control System (GECS), a process designed to recapture energy and CO2 from the furnace and redirect it to the greenhouse.

The brainchild of researchers and several graduate students at McGill University’s Department of Bioresource Engineering, GECS is “a greener way to use wood pellets for heating greenhouses,” says Dr. Mark Lefsrud, the engineering professor spearheading the project. The system not only recovers heat and purifies noxious gases and particulates from the furnace’s exhaust, it also recycles CO2 back into the greenhouse, which enhances plant growth and yields.

The GECS unit consists of a rigid box air filter coupled with two sets of heating elements, two catalytic converters and two forced air fans. The unit is attached to the chimney of a pellet stove installed in a greenhouse. “Pellet stoves generally produce less harmful emissions than other stove types because the shape of the biomass allows for a better distribution of oxygen, which produces a uniform and complete combustion. This makes the flue gas easier to purify and more suitable for CO2 enrichment”, explains Dr. Lefsrud.

“The air filter removes the particulate matter in the flue gas, while the other elements transform the exhaust gases into less harmful gases,” says Yves Roy, a Master’s student who played a pivotal role in designing the system. He explains that there are 3 steps in the purification process: the first is the mechanical collection of large-scale particulates using a combination of an electrostatic precipitator, and cyclone and a bag filter. Finally, two sets of catalytic converters and heating elements transforms all noxious gases to less hazardous gases.  

Once the GECS prototype was completed, the team tested it on the chimney of a wood pellet biomass furnace. The device passed with flying colours. “We confirmed that it considerably improves the thermal efficiency of the wood pellet heating system since no heat is lost through the flue gas,” says Dr. Lefsrud. The team’s measurements also affirmed the system’s safety: when the exhaust from the furnace chimney was pumped directly into the greenhouse, the air remained well within Health Canada’s air quality guidelines for acceptable levels of indoor gases and contaminants.

The GECS unit is also very cost-effective: “The capital investment required for the GECS is far lower than for alternative heating systems currently on the market,” says Lefsrud. His experiments show that direct combustion exhaust gas recuperation through the purification system reduces greenhouse heating costs by 18.8%. Translated into bottom-line terms, this means both a lower heating bill and a lower carbon footprint, even for small operations. “End-users may even be able to claim a carbon credit,” he adds.

“This project demonstrates that university research can yield marketable products,” says Dr. Lefsrud. BioFuelNet gave legs to the initiative, supporting graduate students to travel to conferences where they showcased the technology and networked with other scientists in the field. One of these students was Roy, who had a chance to present his work at the 2013 International Meeting of Agricultural and Biological Engineers in Kansas City, Missouri. “Even though some of the attendees commented on my strong French accent, they seemed to enjoy my presentation,” he jokes.

Two things need to happen before the GECS goes to market: a patent and a unit suitable for commercial use. The McGill team has already applied for a patent, and intends to enhance the product to make it commercially viable. “We plan to build a control system into the unit to allow growers to adjust CO2 levels,” says Dr. Lefsrud, adding that “the BioFuelNet community is helping us with the commercialization process by connecting us to the right people and information.”

Lefsrud has high hopes for the new technology. “Our piece of equipment has the potential to spur economic development in the agriculture and greenhouse sector and strengthen Canadian food security,” he says. Roy shares Lefsrud’s enthusiasm. “Greenhouses are seeing a steady growth as they offer a way to control the environment,” he says. As weather patterns become increasingly fickle, greenhouses are set to become more popular than ever. “I’m confident our system will make it economically- feasible for greenhouse operations of all sizes to use wood-pellet biomass furnaces.”