Towards a circular economy with multiple-product biorefineries
Currently 90 % of organic chemicals are produced from oil. Due to environmental concerns and fluctuating oil prices, biofuels have received much attention in recent times as alternatives to oil. Biomass can also be used for materials and chemicals production. Even if all the heat and electricity humans require could be produced from hydro, wind, solar or nuclear power, we would still need carbon mass such as oil or biomass to produce chemicals and materials we consume on a daily basis; pharmaceuticals, plastics, and detergents.
A biorefinery can produce multiple products from biomass in the same way an oil-based refinery refines crude oil into fuels and chemicals. The same chemical building blocks that are currently derived from oil can be produced from biomass via chemical, biochemical or thermochemical conversion methods.
There are already biorefineries that produce energy, chemicals and materials. Pulp mills, for example, also produce heat, electricity and tall oil, which can be converted into chemicals or fuels. Biogas plants can also produce chemicals. In Finland Envor Biotech’s biogas plant produces ammonium sulphate (NH4)2SO4 for industrial and agricultural use from the biogas plant’s reject water.
Biomass-derived products can also contain value-added properties. A good example is biopolymers in medical applications such as biodegradable implants, sutures and slow release drug delivery devices.
Finland has strong forestry tradition
Wood is an abundant raw material in Finland, where there is extensive experience in forest industry operations such as pulp, paper and saw mills. Wood is a good raw material for biorefineries as it does not compete with food production; it does not need fertilisers, pesticides or artificial irrigation and forests absorb carbon dioxide. The main drawback of wood is that it grows relatively slowly compared to other biomasses like crops. However, forests in Finland are currently growing faster than they are being harvested.
Wood and other lignocellulosic biomasses have a complex chemical structure that consists mainly of cellulose, hemicellulose and lignin. The composition depends on the biomass type. For wood it is around 35–45 % cellulose, 25–35 % hemicellulose and 20–30 % lignin. Lignin makes the structure strong and works like a glue that keeps the structure together.
For pulp mills cellulose is the most desired fraction of wood. In the kraft process, wood chips are treated with sodium hydroxide (NaOH) and sodium sulphide (Na2S), which breaks down the wood structure and separates the fractions.
After that the lignin and hemicellulose degrade in a delignification process and are washed to separate the pulp. In a traditional pulp mill the remaining material, which is called black liquor, is burnt in a recovery boiler to produce heat and electricity and the chemicals are recycled back to the cooking step.
Hemicellulose and lignin have immense potential
Only the cellulose fraction of wood is used in making pulp. Hemicellulose and lignin, which are currently burnt in the kraft boiler, have the potential to be used to produce a wide variety of products that are currently made from fossil resources.
Hemicellulose is a highly branched polymer and it is easily soluble. In the kraft pulping process, it is broken down into its monomers. These monomers could be fermented into organic acids and alcohols, which could then be further processed into various chemicals and biopolymers. There is ongoing research on separating the hemicellulose fraction from wood before kraft pulping.
Lignin is a complex aromatic and amorphous polymer that consists of phenyl propane units. In the kraft pulping process, lignin ends up in the black liquor. It could be extracted via precipitation and used for production of phenol-based glues and resins, carbon fibre and chemicals such as toluene and benzene.
The demand for paper is decreasing because of digitalisation, but consumption of packaging materials for online stores is increasing. Additionally, there are other products that can be produced from pulp such as cellulose derived fabrics, specialty cellulose or construction materials.
High value products essential for biorefineries
The best part of the biomass should be used for high value products, such as pharmaceuticals or fine chemicals and the rest can be used for lower value products and energy production. With multiple products and processes a biorefinery can adjust its production according to market demand and prices. For example, cars are becoming more fuel efficient and, thus, less bioethanol is needed to fuel cars. There are, however, other products, such as acetaldehyde, butadiene, ethylene and propylene that can be produced from bioethanol. Ethylene can e.g. be polymerised into polyethylene (PE), which is used to manufacture plastic bottles.
In Finland a recently announced 1.1 billion euro investment by Metsä Fibre, part of Metsä Group, is a good indication of how viable the concept is. It is the biggest investment in the Finnish forestry industry ever. The main product of the planned bioproduct mill is pulp, but all the side streams will be converted into bioproducts or energy by partner companies that form an industrial ecosystem in the area. The mill will not use fossil fuels and will produce more heat and electricity than its own consumption.
Pulp mills are not the only application area for biorefinery integration. The food industry produces large amounts of side streams, which are often used for animal feed. However, these streams contain carbohydrates, fats and proteins that could be used for higher value products.
Starch technology is another example; the side streams of starch processing could be utilised for acid and alcohol production, which can be processed into chemicals, fuels or biopolymers.
The starch can also be converted into higher value products such as modified starches, biodegradable polyesters, thermoplastics and food additives. Plant residues from the wheat or potatoes the starch is extracted from could also be used for biogas production.
In the chemical industry the waste-to-product ratio usually increases with the price of the product. In the production of fine chemicals and pharmaceuticals only a small fraction of the raw materials is used for the product and the rest is waste. This waste can and should be used more beneficially.
Upgrading an existing plant into a biorefinery that utilises side streams has several advantages. The material efficiency and the profitability of the plant can be improved by producing a new product from a waste stream that currently incurs disposal costs.
Biorefineries can, as such, maximise the value of biomass and minimise the amount of unused waste. It is difficult to come up with good reasons why this should not be done.
Author: Helena Arkkola