The forest biomass has been transformed through the process of pyrolysis, and biochar (solid) and bio-oil (liquid) have been obtained, with different uses depending on their properties.
The optimum temperature for producing bio-oils, higher quality useful compounds and biochar from forest biomass was found to be 450 °C. The optimum temperature for the production of biostimulants used in agriculture from the solid fraction, however, is 300 ºC.
The products obtained from the bio-oil at 450 °C from Aleppo pinewere antioxidants (4 g per kg biomass), sugars (11 g per kg biomass), acids (35 g per kg biomass) and phenolic resins. The end product with the highest added value is the sugars, both in terms of the quantity obtained and their high market price.
A comprehensive multi-objective extraction and separation process based on biorefinery techniques of value-added compounds from bio-oil has been successfully developed, so that with a single process all the chemicals described can be obtained, with a purity equal to or higher than expected.
The biochar generated in this process has been shown to have no inhibitory effect on microbial activity in the soil and can therefore be used as a soil structuring agent or for water retention. This biochar can also be used as biofuel with a low calorific value range of 23-30 MJ/kg.
The bio-stimulants obtained from torrefied forest biomass at 300 °C are of high quality and could be improved further. Their properties are similar to biostimulants obtained from plant compost.
Efficiency of the different types of forest biomass tested
Similar amounts of useful compounds were obtained from the different types of forest biomass processed during the project. It can therefore be concluded that there are no major differences between hardwood and softwood tree species, or between these and shrub species.
This implies that, firstly, it is viable to use all the forest species tested in this type of biorefinery and, secondly, that they could be mixed, thereby exploiting biomass with few alternative uses such as i) brush and small trees with bark and leaves from undergrowth or sapling thinning operations, ii) unstripped cork oak (wood and cork) from selective cutting operations, or iii) mixed wood of different species and types from post-disturbance measures that generate small volumes of wood.
Among the tree species studied, pine (especially Aleppo pine) were the best for producing antioxidant compounds (such as guaiacol, catechol, phenol and vanillin). Chestnut and cork oak gave the best results for the production of humic extracts used to stimulate soil microbial activity and plant growth. Shrub species such as erica were very good for producing acids such as acetic acid.
Table 1.1. Yields at 400 ºC (g product/kg biomass) and potential use of the products obtained from processing biomass from the demonstration stands in the LIFE BIOREFFORMED microbiorefinery.
Yields and impact of forest biomass extraction
The use of biomass in biorefineries can make the partial extraction of biomass from adaptation/restoration measures viable. This biomass is often left chopped up in the forest.
The obstacles to using biomass from adaptation/restoration measures are the low yields/ha (the average yield in the demonstration stands was 13 t/ha)and the high cost of extracting it, which is currently done manually. The data sheets complementing this document give details of the values obtained from the different demonstration stands.
When extracting biomass for biorefinery use in mature forests it is vital that the crowns are left in place, as the crown (branches and leaves) contains 60% of the nitrogen and potassium in the aerial part of the tree and up to 50% of the phosphorus, which is very difficult to recover naturally. Biomass from regenerated Aleppo pine or brush is extracted in many places, with the removal of the whole tree. However, harvesting is concentrated in accessible areas (strips along the sides of roads), which at stand scale represents 10% of the total biomass. It also contributes to preventing forest fires.
These operations logically reduce the above-ground carbon stock due to the cutting and removal of part of the biomass. However, the reduction in CO2 stocks can only be counted as a net CO2 emission if the material is used for biofuel in the short term. A CO2 sink effect is likely, due to increased growth following the measures, but this remains imperceptible over the short periods involved.
Forestry management that also exploits the historical socio-ecological heritage (tangible and intangible) of the forest can help strengthen the Cultural Ecosystem Services that forestry estates provide. Measures involving the preservation of oral lore and traditional knowledge about forests and how to manage them help foster the transmission of knowledge, while serving as a strategy to promote resilience.
Biorefinery yields and impact
For the biorefinery to be profitable, inputs of 1-2 t of woodchips per hour are needed, approximately 6,000 to 10,000 t of biomass per year. The optimal scenario would be to make the biorefinery profitable with small plants that use less than 1 tonne per hour (microbiorefineries). This would reduce the cost and impact of transport, while there is also the possibility of the plant being mobile.
Within the operating costs of the biorefinery, the highest cost is the purchase of biomass followed by the cost of electricity. The high consumption of electricity makes it difficult to locate them in areas with difficult access to electricity and it is necessary to explore alternatives to generate energy such as the use of biomass, biochar or solar energy.
Biomass extraction and transport operations must be optimised to improve the profitability of the whole process while also reducing its environmental impact. This involves finding strategic locations for the biorefineries and optimising pre-treatment operations (storage and second chipping) and the loading of the product to be processed.
. The biorefinery, with a pyrolysis temperature of 400° C, has turned out to be a viable and sustainable alternative: the production of bio-oil and acids is 3.7 times less impactful than the combined production of the same products through industrial processes. The overall impact in terms of CO2 eq emissions is 0.5 kg CO2 eq/kg biomass input for most products, with the largest proportion associated with chipping, followed by forestry work and transport.
The profitability of the biorefinery and its overall impact could be further improved if the sale of the products generated were to be complemented by the potential sale of CO2, credits linked to the production of biochar as a carbon sink.
