The benefits of greenhouse crop production are widely documented and recognized as valuable technique to produce food throughout the year by room temperature control.
The greenhouse allows the producer to control a variety of other parameters that influence plant growth.
The greenhouse operator can control humidity, temperature, air circulation, water and nutrient cycles, lighting, intensity and duration, and the concentration of carbon dioxide in the ambient air (Mortensen 1982).
According to Statistics Canada (2012), the total greenhouse area in Canada is 2316 hectares, which is used predominantly to produce vegetables (1255 hectares) and flowers (859 hectares).
- Greenhouses in northern climates have an important heat requirement that is mainly supplied by non-renewable fuels such as heating fuel and natural gas.
- The objective of this project was the development of an improved biomass furnace capable of recovering the heat and CO2 available in the flue gases and using them in the greenhouse.
- A combustion gas purification system was designed, built and installed in the chimney of a wood pellet furnace (SBI Caddy Alterna). The purification system consists of a rigid box air filter (MERV 14 classification, pores of 0.3 μm) followed by two sets of heating elements and a catalytic converter.
- The air filter eliminates particles present in the flue gas while heating elements and catalysts transform harmful gases into less harmful gases.
- The gas analysis was sampled at different locations in the system using a TESTO 335 combustion gas analyzer.
- The purification system reduces CO concentrations from 1100 cm3 m−3 to less than 1 NOx from 70 to 5.5 cm3 m−3 SO2 of 19 cm3 m−3 at less than 1 cm3 m−3 and the particles trapped up to 0.3 μm with an efficiency greater than 95%.
- These results are satisfactory since they guarantee human and plant safety after dilution in the greenhouse’s ambient air.
- The recovery of combustion gas has several obvious benefits, since it increases the capacity of heat use per unit of biomass and greatly improves the recovery of CO2 from biomass heating systems for the benefit of greenhouse-grown plants.
- The biomass furnace shows a high potential for the enrichment of greenhouse carbon dioxide.
- The recovery of flue gases significantly increases the thermal efficiency of an oven.
- The catalytic converter can reduce CO and NOx below the exposure limit of humans and plants.
- Particle control is essential to maintain the efficiency of catalytic conversion.
- The CO2 recovery of biomass heating systems reduces the farmer’s dependence on fossil fuel.
- The enrichment of carbon dioxide (CO2) in the greenhouse from biomass residues was investigated using exhaust gases from the combustion of synthesis gas produced by gasification.
- Almost complete combustion of synthesis gas is essential to achieve CO2 levels that increase plant yields and maintain a safe environment for workers.
- Wood pellets were supplied to a downdraft gasifier to produce synthesis gas fed to a steel swirl burner.
- The preliminary results were encouraging and represented a first step towards a successful development of this technology.
- The burner required an equivalence ratio (the actual air-to-fuel ratio in relation to stoichiometric air-to-fuel requirements) of 2.6 for almost complete combustion.
- The concentrations of sulfur dioxide (SO2) and ethylene (C2H4) emissions were lower than the critical or insignificant concentrations.
- In 60% of the trials, carbon monoxide (CO) emissions were below ASHRAE standards for indoor air quality.
- However, the average nitrogen oxide (NOx) emission was 23.6 ppm, and would need to be reduced below 0.05 ppm to meet ASHRAE standards.
- The proposed improvements to the synthesis gas burner design to reduce NOx emissions and increase efficiency are: integration of a low swirl design, mesh catalysts, a higher quality refractory material and a more efficient heat exchange.
- Theoretically, combustion or gasification of biomass could provide more CO2 for greenhouse enrichment than propane or natural gas per unit of energy.
Energy requirement and greenhouse gas emission
The current study has shown that a purification system composed of a rigid box air filter (MERV 14 rating,0.3 µm pores), heating elements, catalytic converters and two fans that force combustion gas.
The system is a suitable system to filter the exhaust gases of the wood pellet furnace for greenhouse CO2.enrichment.
The analysis shows that the purification system installed in the chimney of the oven configured in an input power of 35.17 kW was able to reduce the CO concentration from 1100 to less than 1 ppm, NOx of70 to 5.5 ppm, SO2 from 19 to less than 1 ppm and particles removed up to 0.3 µm with a efficiency greater than 95%.
These results are satisfactory since they meet the air quality criteria for greenhouse environment after dilution in the greenhouse environment air.
It was found that the maximum theoretical rate of CO2 production is 10.89 kg of CO2 hr-1.
This production rate was able to maintain the concentration of CO2 inside a tunnel double layer polyethylene greenhouse at 670 ppm for summer, 910 ppm during fall / spring and 5132 ppm during winter.
Consequently, a sensor will be configured necessary to ensure that the CO2 concentration does not rise above the maximum desirable CO2 concentration in the 900 ppm greenhouse.
In addition, the recovery of combustion gases through the purification system was able to reduce greenhouse heating costs by 18.8%, which represents a saving of USD 14.7 per week.
Thus, the ability of biomass-based CO2 enrichment system will continue to increase in the years to come.