Producing valuable products from agricultural residues
Pyrolysis as a sustainable management solution
A new regulation coming in the province of Quebec in 2020 to banish the burying of organic residual materials forces industries to find sustainable solutions in order to manage their wastes and residues. And what if a technology could be used to convert these residues into a high-energy biofuel while reducing greenhouse gas emissions and sequestrating carbon? That is what can be achieved through the pyrolysis process.
A lot of residues are produced from agricultural activities, including crop residues, animal wastes and food wastes. It is estimated that 17.8 millions of dried tons of crop residues (e.g. cereal straw and corn stover) are available per year in Canada, excluding residues that are left on field to maintain soil health and residues collected for animal bedding. That represents an energetic potential of 0,3 Exajoules (EJ).
Similarly, livestock wastes (e.g. poultry and swine manure) could provide 0.065 EJ/year.
Downgraded fruits and vegetables that cannot be sold on the market represent 83 350 tons year-1 in the province of Quebec. Currently, most of these vegetal residues (57%) are disposed in clusters or buried, causing greenhouse gases (GHG) emissions and both underground and surface water pollution.
On-farm pyrolysis of agricultural residues
Pyrolysis, which can be defined as the thermochemical decomposition of biomass at elevated temperature (350 – 700 oC) in the absence of oxygen, could be considered a sustainable management solution for the agricultural residues. The valuable products resulting from pyrolysis are biochar and bio-oil. Non-condensable gas is also produced and is generally used to heat the pyrolysis reaction. Products yields and characteristics depend on feedstock, pyrolysis operational conditions and the type of pyrolysis technology used.
In the perspective of agricultural residues management, mobile pyrolysis reactors could be established in agricultural regions and be shared by many farmers. A mobile pyrolysis unit has the advantage to reduce the transportation of biomass. For example, mobile auger pyrolysis systems that can convert from 20 kg/day to 50 dry tonnes per day are presently manufactured by ABRI-tech. In this reactor, the biomass and the heat carrier (hot sand) are mixed in a heated screw conveyor, which favors the heat transfer.
Biochar as a tool to mitigate climate change
Because of its high porosity and high surface area, biochar has an agronomic value, including soil composition improvement, water retention, nutrient uptake increase and crop yield increase. Even more, biochar is considered as a climate change mitigation tool because it can sequester carbon and reduce soil GHG emissions. Instead of biomass decomposition resulting in emission of large amounts of GHG, pyrolysis sequesters about 50% of the carbon in biochar, which can be stocked in soil over centuries or millennia. Moreover, many studies reported that biochar amendment to soil can help reduce GHG emissions, and particularly N2O emissions. However, more research is needed to identify the exact mechanisms involved following biochar amendment to soil.
Bio-oil to replace fossil fuels on farms
Bio-oil produced from pyrolysis is of great interest due to its energetic potential and chemical composition. Bio-oil produced from pyrolysis could be used, for example, to replace No. 2 oil (heating oil) in furnaces to heat greenhouses or poultry and swine barns.
Bio-oil can also be used as a second-generation transportation fuel after being upgraded. However, a special attention must be taken because bio-oil properties are different from fossil fuels, including a higher density and viscosity, high oxygen content, low pH, high water content of 35 to 50%, and a lower calorific value of 15-19 MJ/kg.
A case study: pyrolysis of pig manure
Due to a regulation in Québec (Canada) restricting phosphorus (P) fertilisation, pig producers are required to manage their manure in excess.
“Solid-liquid separation and valorisation of the solid fraction by pyrolysis appear as a sustainable way of management”, indicated Stéphane Godbout, research scientist at the Research and Development Institute for the Agri-Environment (IRDA) at Quebec City.
In fact, pyrolysis experiments conducted at the IRDA in an auger pyrolyser demonstrated that up to 62% of the solid fraction of pig manure is converted into bio-oil and about 25% is converted into biochar. The produced bio-oil can be stored and substitute fossil fuel in furnace to heat the piggery. Biochar, which concentrates and immobilize P and nitrogen (N) could be easily managed and transported outside of the regions where P excess exists.
A sustainable management solution
There is no doubt that pyrolysis can be considered as a sustainable management solution for the agricultural residues, providing economic, environmental and social benefits. In fact, the establishment and the operation of mobile pyrolysis units can create employment opportunities in rural areas.
With the rise of fossil fuels price, bio-oil could be a cheaper alternative to heat farm facilities. The substitution of fossil fuels with the renewable bio-oil can contribute to reduce GHG emissions.
In addition to carbon sequestration and soil GHG emissions decrease following biochar amendment, biochar is expected to increase soil fertility and it could reduce the need for chemical fertilizers. Knowing that chemical fertilizers production and application to fields emits large quantities of GHG, this is a net gain on the GHG balance. Moreover, biochar production could eventually bring incomes to farmers if there exists a carbon market that recognizes the avoided emissions and carbon sequestration due to application of biochar to agricultural soils.
Why bury feedstock that can provide so many benefits when converted into valuable products through the pyrolysis process?
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