Biomass Gasification for Carbon Dioxide Enrichment in Greenhouses

Biomass heating is used more and more by the greenhouse industry to reduce costs and the environmental footprint of production. The objective of this research project was to investigate the possibility of using the carbon dioxide (CO2) from the exhaust gas of a biomass heating system to enrich greenhouses with CO2 and improve crop yield. When compared to direct combustion, biomass gasification technology offers better control, which helps in reducing atmospheric emissions. Gasification is a thermo-chemical reaction, which converts solid biomass into a gaseous fuel, known as syngas. Experiments were performed at McGill University (Montreal, QC, Canada) using a downdraft gasifier to monitor its performance, with sawdust wood pellets as feedstock. Temperature and pressure monitoring provided valuable insights on optimal gasification temperatures, biomass fuel depletion in the reactor, ash grate shaking requirements, micro-explosions detection, char bed packing and pressure drop across the packed bed filter. The gasifier operated with an average equivalence ratio (the actual air to fuel ratio relative to the stoichiometric air to fuel requirement) of 0.17, below the optimal value of 0.25, and achieved a cold gas efficiency of 59%. Syngas combustion emissions produced an average of 8.8 ppm of carbon monoxide (CO), with 60% of the trials below the ASHRAE standards for indoor air quality and 90% below 20 ppm. The sulphur dioxide (SO2) emissions were below ppm levels, while ethylene (C2 H4) emissions were below the critical concentration of 50 ppb for CO2 enrichment. The average nitrogen oxides (NOx) emissions were 23.6 ppm and would need to be reduced to allow commercial operations. From the empirical data, the gasifier operating with sawdust wood pellets, with a consumption of 7.7 kg/hr, could provide a maximum of 22.9 kW of thermal energy and could enrich a maximum of 1540 m2 of greenhouse surface area. Results indicated that biomass, following combustion or gasification, could provide more CO2 for greenhouse enrichment than propane or natural gas per unit of energy. Biomass gasification coupled with syngas combustion could be a promising renewable alternative to propane and natural gas for CO2 enrichment in greenhouses.