Biomass is organic matter that comes from living organisms, mainly plants and animals. Biomass is a renewable resource, in the sense that it derives - directly or indirectly - from photosynthesis. However, biomass is also a limited resource, meaning that the amounts available in a given year are finite.
Humans rely on biomass for a variety of services: human food, animal feed, biomaterials and bioenergy. In total, it is estimated that the human appropriation of Earth's biomass productivity is already around 30%, and it is expected to increase in future scenarios with high use of biomaterials and bioenergy.
Any kind of biomass can be used for pyrolysis, but common biomass feedstocks include:
Biomass feedstocks have different properties and are available in different amounts. They also have variable supply-chains, which leads to differentiated environmental impacts.
The first thing we may think of are the chemical and physical properties of the biomass. These can be determined by sending a sample of biomass for laboratory analyses. Alternatively, data can be found in existing litterature and databases.
The phyllis database contains information on the composition of biomass and waste (e.g. ultimate analysis, proximate analysis). It is an reknown data source.
For practice, other practical aspects need to be considered to fully characterise a biomass resource:
How wet is the feedstock? It is very dry (<10%), rather wet (around 50%), or very wet (>60%)? High moisture levels affect for instance storage, transportation or pre-treatment requirements.
Is it a problematic or heterogenous feedstock? For instance, the composition of garden waste is known to vary from one place to another, but also in time. Beach cast (algae collected from beach cleaning operations) can be mixed with varying amounts of sand, which is a challenge for some industrial processing techniques.
How much and when is the biomass available? Is the biomass harvested continuously over the year, or only once a year in a given season. Can it be easily stored, for a continuous supply?
Is it primary or waste biomass? Although it is a subjective and changing notion, we commonly hear that biochar can be produced from waste biomass as opposed to primary biomass. Waste biomass is sometimes referred to as secondary, by-product, or side-product biomass. Waste biomass is seen as an undesired product for a given stakeholder. Primary biomass is also referred to as cultivated biomass or dedicated biomass. Primary biomass is the main output of an activity.
The distinction between waste and primary biomass is used to emphasize that using waste biomass has limited environmental impacts, does not affect land use. In fact this claim refers to a subtle notion in environmental systems analysis & life-cycle assessment, a notion called the reference land or biomass use. More about this in the next section.
Synonyms: counterfactual, alternative fate, reference situation, reference fate.
Some theory put simply:
Let’s look at some examples to explain this:
The theory becomes clearer when we take as example the climate impact from burning fossil oil in an engine.
It is clear that burning fossil oil leads to emissions of carbon dioxide. But the notion of a climate impact arises from the assumed reference fate of that fossil oil.
Here, the implicit reference is that, if not extracted and combusted in an engine, fossil oil would remain in the geosphere and not release any carbon to the atmosphere. In the reference situation, the emissions are equal to 0.
This is a rather undebated reference fate when it comes to fossil fuels, but as soon as we look into biomass feedstocks things get more complicated: there are often several possible reference situations.
Peat is a material extracted from water-clogged ecosystems either for energy or horticultural use. In some countries, it is considered a fossil fuel & its use is associated with a high climate change impact.
Peat is formed by the slow accumulation of dead biomass in the upper soil layers of an environment that does not allow its decomposition (water filled, low oxygen, low nutrient, or high acidity environments). Peat in the environment is thereby a stock of carbon.
Harvesting of peat usually involves the drainage of the peatland, and therefore its exposure to oxygen, leading to release of carbon dioxide in the field but also during the use of the peat.
Here, the mainly accepted reference fate for peat is: if peat is not extracted, it remains stable in the environment (like oil from the geosphere, in example 1). Thereby, its extraction and use lead to carbon emissions that can be attributed to this activity.
However, the peat industry has sometimes claimed that peat being already drained, the carbon emissions would happen even if not harvested, and therefore these greenhouse gas emissions should not be accounted for in the climate impact of peat. This is another reference land use.
An objection to this statement is to consider that if peat is not extracted, the peatland can also be rehabilitated.
Cropland [to be written]
Garden waste [to be written]
Beach cast [to be written]
Environmental impacts arise from consumption of products and services, the use of natural resources, and their effects on the environment. Assessing the environmental impacts of biomass production is a dedicated field of research because of its many specifities (e.g. biogenic carbon flows and stocks, direct and indirect land use effects, timing of emissions, crop rotations, stand and lanscape perspectives).
The cradle-to-gate LCA of biomass production and supply should include (but not limited to) the following items:
For crop cultivation or forestry operations (e.g. willow cultivation on cropland):
For waste or secondary biomass (e.g. municipal garden waste):
In this case, it is common to apply a “cut-off” approach. This implies that the no environmental burdens are attributed to the waste biomass for its creation. All upstream burdens are attributed to the main product. However, any operations taking place downstream of the waste creation are considered.
This typically includes:
Remark: The case of multi-product biomass production:
When an activity produces several biomass products, the LCA practionner is facing an issue of attributing the production burdens between the products. Two main approaches can be followed, both being a subjective choice:
If one product is considered as the main product, and the others have little value: all production burdens are attributed to the main product. The co-products are considered burden free (so called cut-off approach for by-products).
If all products are deemed having some value, an allocation of the burdens based on some physical property can be performed.
Both approaches are valid and have been applied to the same feedstocks in different studies.
For instance, in a study by Hammar & al (2015) logging residues are considered a side-product from conventional forestry operation for timber production. Only impacts from harvesting, chipping and transportation are considered (alongside direct land use change). However, in the ecoinvent life cycle database, forestry operations are considered as a multi-functional activity delivering multiple products: timber, bundle energy wood, and woodchips. An allocation of burdens from forestry operation is applied between the three products (dry mass allocation?).
Remark: Biomass production involving use of another biomass
What if to produce wood, we use biofuels made from wood via a fischer-trop process? That’s what we call a loop in LCA. It can be solved but makes calculations sometimes more complicated than if we assume that fossil fuels are used in the supply-chain.
Read more about biomass supply chains in the work of Wolf et al. (2016).
As explained above, there is no unique or objective choice of reference fate.
A reference land use is most often used for dedicated crop cultivation or forestry operations. A reference biomass use is considered for waste or secondary biomass.
The choice of a reference land use allows to quantify direct land use changes, which often contribute largely to the climate change impact of biomass production. Direct land use changes represent emissions arising at the site of cultivation of the biomass related to a change in land cover or land management. This can represent the emissions of carbon dioxide and other greenhouse gases from reforestation but also the loss of carbon in aboveground and belowground stocks when increasing the intensity of residues harvesting.
Examples of reference land uses:
Examples of reference biomass uses:
To explore the topic further, we recommend the following references:
Logging residues: Hammar T, Ortiz C, Stendahl J, et al (2015) Time-Dynamic Effects on the Global Temperature When Harvesting Logging Residues for Bioenergy. BioEnergy Res 8:1912–1924. https://doi.org/10.1007/s12155-015-9649-3
Branches, tops, stumps from long-rotation forestry and willow:
Potential in Sweden: