Biogas & Biomass CHP no longer useful

In brief: new cheap power generation from wind means that the electricity generated by CHP is now less useful than it used to be, and may be unnecessary.

Preamble on natural gas and grids

It is not widely understood that natural gas CHP (Combined Heat and Power*) has largely reached the end of its useful life as a contributor to decarbonisation in the UK. This was fairly conclusively shown by DECC in their 2014 study “Bespoke natural gas CHP analysis” which was then reinforced by the 2015 ERPUK study which also showed that from the early 2030s, gas CHP has a carbonising, not a decarbonising effect: “Managing Flexibility Whilst Decarbonising the GB Electricity System” . [This is because while CHP initially substitutes for CCGTs, models show that from the early 2030s it increasingly substitutes for lower carbon generation.]

Before I continue, I must point out that whether CHP is decarbonising or not depends on the characteristics of the electricity grid it feeds. Thus the modelling results referred to above apply only to the UK and are not necessarily applicable to other grids. As a rule of thumb, if a country has a much cleaner grid than the UK (e.g. France or Norway), then more CHP is likely to be actively harmful. The EU rules on qualifying CHP (even fossil-fueled CHP) under the Renewable Energy Directive are thus a vast over-simplification as they are uniform across the entire EU.

Bioenergy CHP

Bioenergy CHP has long been considered immune from these arguments because the combustion emissions from biogenic carbon have been accounted as zero. Whether this is “true” or not depends on what accounting framework you use, but even if one accepts the zero emission assumption, bioenergy CHP is no longer a safe investment indefinitely.

The alternative to a CHP plant is a biomass-fuelled boiler for heat, coupled with a grid connection to provide low-carbon electricity. A pure boiler is much more efficient at turning fuel into heat than a CHP plant: the electricity generator in a CHP plant does not work “for free” because in a CHP plant a greater proportion of the heat from burning fuel is not turned into useful heat.

Flexible generation – or not

There is another characteristic of CHP plants which makes the electricity less useful: the time of day that they generate. Most CHP runs continuously (except for maintenance) because either the fuel is produced continuously (e.g. biogas from anaerobic digestion, sewage gas or from landfill) or because the heat is continuously required by an industrial process or for a hospital or for district heating (in Winter). Thus most CHP is producing inflexible baseload electricity.

[UPDATE: 16 March 2018. This post contains the implication of a misleading simplification. See link to updated discussion at the end of this post.]

In a grid with a high penetration of irregular renewables there is some benefit of having baseload generation, but there is much greater value in having flexible generation that can meet peak demands when the wind does not blow. CHP is rather inflexible because (a) the large thermal mass of the heating part of the system means that it cannot ramp up and down quickly, and (b) the heat production is required to be constant because that is the reason for having a CHP unit: the heat.

There is an interesting exception with micro-CHP where a domestic-sized system replaces a central heating boiler. These systems would run in heat-following mode, which for most UK housing means that the electricity generated will be largely synchronised with the evening peak electricity demand, because that’s when people have their central heating on. However no one expects micro-CHP to be powered by biomass as it adds very significantly to the complexity of a pellet boiler. Most expect that micro-CHP is likely to be fuel-cell based and to burn fossil natural gas (including the small proportion of injected biomethane).

In a country operating under the constraints of a carbon budget (which means all signatories of the Paris agreement) biomass should be burned in the most efficient process possible if needed for heat, and if used for electricity generation it should be used only in a peaking plant to fill the gaps when the wind does not blow. Currently many CHP plants benefit from a double subsidy incentive: because they are CHP and because they are biomass. New policies are required that would encourage one or the other, but penalise those projects attempting to do both.

Postscript

Fossil gas fueled CHP is still “encouraged” in the UK because the pricing structure for electricity access means that it is cost effective for a company which needs both heat and power to generate its own, even if the overall system impact is higher CO2 emissions. Presumably this perverse incentive will be fixed some time during the 2020s, possibly by applying a carbon tax on all natural gas use.

[Follow-on post: coming soon]

EU Directive Reference

See Annex II on page 9 of “Directive 2004/8/EC of the European Parliament and of the Council of 11 February 2004” on the promotion of cogeneration based on a useful heat demand in the internal energy market and amending Directive 92/42/EEC (or from 5 June 2014 onwards, “European Union Directive 2012/27/EU on energy efficiency”) which sets a common number across the whole EU.

“The definition of high-efficiency cogeneration is the same as under the CHP Directive. High efficiency cogeneration must achieve at least 10% primary energy savings compared to separate heat and electricity production” page 13 of http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52013SC0449&from=EN irrespective of time of generation: baseload or peaking. This metric ignores the degree of decarbonisation of the electricity grid: it is entirely about efficiency, not decarbonisation.

It would be interesting to see an analysis comparing primary energy saving (e.g. biomass or gas fuel calorific value for generating electricity) compared with wind electricity, where the electricity itself is primary. But also see  that the “efficiency” definition itself may be warped for some fuels: Bogus “Biomass Conversion Coefficient”.

*CHP is also known as “cogeneration” in Europe and the USA.

What is a Bioenergy Strategy ?

• What is a bioenergy strategy useful for?
• What does the process of devising and agreeing a bioenergy strategy achieve?

How a bioenergy strategy is used

A completed bioenergy strategy is a tool used by people constructing new policies. A bioenergy strategy rules out some bioenergy technologies and declares different priorities for different feedstocks. This is extremely useful as there is a much reduced range of possibilities to be examined for each new policy and many prior arguments can be treated as settled. This speeds up policy making immensely.

If the bioenergy strategy proposals are clear and are widely agreed, then the public, local authorities and commercial organisations can make outline plans in advance of detailed legislation. This speeds up adoption of the policies when they arrive and may even make some legislation entirely unnecessary.

Bioenergy issues always cross government departmental boundaries, so a strategy which allocates responsibilities clearly between departments is vital to prevent sterile and interminable disagreements between officials. These disagreements are hidden from public view and can never be resolved quickly, however unimportant they really are.

Creating and agreeing a bioenergy strategy

The UK has three bioenergy strategy documents:

• 2011 – the Committee for Climate Change Bioenergy Review
• 2012 – the cross-departmental government Bioenergy Strategy
• 2018 – the Committee for Climate Change Biomass in a low carbon economy

and the first two are now severely out of date. Each of these was published alongside technical papers, models and annexes which described the issues in substantially more depth and breadth. Since 2012 there has been a quite extraordinary amount of work globally and in the UK, and special mention must be made of the ETI bioenergy report repository which is now public.

These are two different types of strategy: the CCC reports are open reviews which propose specific policy options but are somewhat in the style of an academic consolidated review paper. The government strategy is a “closing down” rather than an “opening up” document, because that is what is necessary for an authoritative statement about choices and departmental responsibilities. Government strategies will generally also include ad hoc rules because of interactions with non-energy and non-climate policies. A common example (since 2012) is forbidding the use of human food crops for energy (whilst still allowing the use of livestock feed crops).

The later 2018 CCC report does have a few “closing down” elements, e.g. section 3.7 on sub-optimal uses of biomass: “These end-uses should not be supported by Government”.

Counterfactuals

Any bioenergy strategy has to cover much more than bioenergy. This becomes obvious as soon as one looks at the key decision options to be decided but this means that any group attempting to produce such a strategy can run into the difficulty of being accused of extending its work beyond its remit. So any body preparing to produce a bioenergy strategy would be well advised to prepare for this in advance.

A strategy recommendation that some bio-resource is to be used for energy can only be made if all the alternative non-energy uses for that same feedstock (and their climate implications) are understood. Thus the methane emissions from the farming practice of using open ponds for farmyard slurries become relevant, because those emissions would be reduced if the slurry was instead carefully used for anaerobic digestion and biogas production.

A strategy recommendation must also consider what alternative fuel is being displaced by the bioenergy. Thus the case for using biomethane to fuel trucks can be very different if the fuel being displaced is (fossil) diesel rather than compressed (fossil) natural gas. Since 2016 we have a government policy of removing all coal-fired electricity generation so one can no longer propose that biomass burned after 2025 replaces coal. [Sometimes the displacement is explicit: in the recent contract for difference auction, offshore wind was competition for the same budget as biomass-CHP. But one suspects that the cost-benefit analysis for the proposed biomass-CHP plants did not envisage wind as the “fuel” being displaced, even though that was the case.]

In those examples of slurries and trucks there is clear encroachment on the area normally considered to be entirely within the purvey of the ministries of agriculture (slurries) or transport (trucks). This is especially contentious when the climate benefit of using that resource for bioenergy can be significantly undermined by non-energy changes in the practice of the alternative use: thus covering slurry ponds is a good thing, but would reduce the optimal subsidy to be paid for biogas.

Discussion and agreement

The activities involved in creating a review-type bioenergy strategy require a wide-ranging survey of technical and academic knowledge and can be carried out largely in public. This public discussion tests and verifies the assumptions and models thus improving their quality and helping to understand the limits of their applicability. Whereas creating a government strategy requires negotiation between representatives of distinct policy authorities: private negotiations which require access to previously-agreed data and assumptions, but where it is more important that those assumptions and models are agreed than that they are correct. [New measurements and technologies will revise the data and assumptions, thus undermining the negotiated agreements, which is an incentive to keep those agreed models and data private. But the UK government actually published as much as it could in 2012 in a technical annex, but the proprietary AUB model itself was published only in report form.]

Pitfalls

The 2012 Bioenergy Strategy defined four basic principles which are almost unarguably useful (at least the first three are, the fourth is just an admission that the world changes and any strategy has a finite life):

1. Policies that support bioenergy should deliver genuine carbon reductions that help meet UK carbon emissions objectives to 2050 and beyond…
2. Support for bioenergy should make a cost effective contribution to UK carbon emission objectives in the context of overall energy goals…
3. Support for bioenergy should aim to maximise the overall benefits and minimise costs (quantifiable and non-quantifiable) across the economy…
4. At regular intervals and when policies promote significant additional demand for bioenergy in the UK, beyond that envisaged by current use, policy makers should assess and respond to the impacts of this increased deployment…

But then almost all the specific recommendations in the strategy were based on the results of one proprietary model: the Appropriate Uses of Bioenergy (AUB) model by Redpoint. The model was commissioned (I believe) by government but copyright remained with Redpoint who declined to publish it (or to publish the specific tables of cost and performance data on which it depended). Thus no one in the public domain could tell when any new published report invalidated the assumptions behind an important strategy recommendation or not. Thus the strategy recommendations were almost instantly out of date as they could not be supported or challenged.

It is particularly frustrating that the strategy did not make the logical deductions from principles 2 and 3, deductions which would have been useful – if they had been made explicit – when new data and new applications arose. This would have helped future-proof it to some extent and to improve its coverage to those areas on which it was silent:

The combination of cost-effectiveness with a “whole economy” scope means that for any use of bioenergy, it is the cost of the alternative way of supplying that demand in a similarly low carbon way which needs to be considered.

• Thus when using biomethane to power trucks, the comparative lowest cost would be using biodiesel instead.
• But when using biomethane injected into gas mains to heat poorly-insulated houses, the comparative lowest cost would be insulation plus heat-pumps (or possibly mass conversion to hydrogen).

Thus the subsidy level for each use of biomethane should be scaled according to the alternative (and not the fuel displaced, which is irrelevant according to the principles). Since biodiesel is fairly cheap, the strategic decision should be to put all biomethane into the gas grid, and not to put it into trucks. But if biodiesel is ruled to be a bad thing because of land-use change effects, then all biomethane should go into trucks as there is no sensible alternative for heavy goods vehicles.

But that example illustrates a systemic failure of the 2012 bioenergy strategy principles: although it projected economy changes to 2050, it did not suggest that the appropriate uses of bioenergy should vary with the date. As we know from recent modelling for Carbon Budget 5, it is actually cheaper for the economy averaged over time if conversion of heavy goods transport is left until much later, and that all bioenergy (including biomethane) is used with carbon-capture and storage (CCS) to buy time. We now know that CCS changes everything in that bioenergy strategies with- and without-CCS are radically different (as illustrated by the ETI BVCM work).

[Addendum: The AUB model is very impressive: it is a model of the whole UK energy system including transport (but not agriculture or forestry) and uses the AIMMS underlying software (as does ESME from ETI). Creating it was no doubt a very significant piece of work and it is a great shame that the dataset on which it was based has not been made public. See here for a list of models of this type.]

[Addendum: This post updated 4 December 2021 to include a reference to the later 2018 CCC report “Biomass in a low carbon economy” and supporting research.]

A footnote on regulation

The emphasis on the utility of a bioenergy strategy as a tool to help “regulation construction” may seem bizarre to many people. But for anyone who has had to develop a specific policy to support a policy goal it will seem entirely natural.

Suppose that there is a policy goal to help the “ice-cream poor” and a minister has made a public statement that we must eliminate “ice-cream poverty”. Those charged with constructing policy options have to look at the following options:

• Should this be an obligation on ice-cream producers? An obligation to give away a proportion of their production (perhaps controlled by some kind of certificate trading like the renewables obligation)?
• Or should this be an incentive to voluntary action by subsidy, either an incentive to produce (similar to a feed-in tariff), or an incentive to consume (similar to a boiler replacement subsidy), or an incentive to both produce and consume (similar to the renewable heat incentive)?
• whatever is chosen, it should not increase the price of ice-cream generally as that will create more “ice-cream poor”: which is usually an argument that the cost should come from general taxation and not just by moving funds within the ice-cream industry.
• also any policy must not be easily “gamed” by anyone who can obey the letter but not the intent, it will not be liable to any fraud, and it will not accidentally support another industry which is intended to be phased out by another policy;
• in an ideal world, the policy will reward the virtuous and penalise the unworthy (though the reality is that while political speeches always claim this, the reality is that effective policies mostly work by bribing the unworthy, and any payments to people who “would have done it anyway” are termed “deadweight”),
• the policy will induce society or technical changes which will lead to its own obsolescence and will not create a new industry entirely dependent on subsidy which will then lobby loudly and determinedly for its own continued existence.

Thus the earmarking of all bioliquids to be primarily for transport in a bioenergy strategy has the effect that there will be no policy to subsidise adding a small percentage of bio-oils to heating oil: a market which is very dispersed across the UK, is hard to police, which has a history of fraud, and where fossil oil-fired heating is intended to be replaced by heat-pumps, solar-thermal and biomass boilers supported by the renewable heat incentive. [Unfortunately this is very hard luck for any companies valiantly trying to produce non-transport liquid biofuels, but at least the situation is (eventually) clear and unambiguous. Any bioenergy strategy will not be perfect. There will always be useful and valuable special cases which are pointlessly and unfairly eliminated.]

The Conversational Home

Speculations about “the intelligent home” are more than half a century old. The idea is that a house is instrumented with sensors and enabled with actuators and control systems to allow it to respond to circumstances and to residents’ wishes.

The assumption has been that the sensors and individual control comes first, and that the “expensive” intelligence and control system comes later. But this year we can this that this long-held assumption is probably backwards: the intelligence and interaction can come first, and need only later be followed by sensors and control.

Alexa is a first exemplar of a “conversational intelligent entity” in the home, though Siri, Cortana and “OK Google” are apparently intelligent entities in your pocket. But the location-specific nature of Alexa, which is (in the initial version) by default tied to home WiFi, is actually a great advantage and affords qualities that the others don’t match: “Alexa, what is the weather outside?” reveals how people feel about the interaction.

As revealed by conversations, we see that many Alexa users accept that their “house” has a personality, however rudimentary. This is the first step. (Small children behave as if Alexa is another inhabitant of the home, not a personification of the home itself.)

It is simple to make the device aware that “it” is located in a specific house and most WiFi databases will do it automatically (for flats and WiFi-congested areas a it more would be needed). That is all that is required for globally available, location- and address-specific data to be available to give context to all the conversations, such as “How cold is it outside?”, without requiring any sensors whatsoever. The device also only needs WiFi to detect and recognize all the mobile phones in close proximity, and thus nearly all the people in the house and all the regular visitors. With a bit of configuration, it would also have online access to electricity, internet and gas bills, and thus acquire a good sense of how the house is used. An Amazon device would know when deliveries are expected, and a Google device would know who has a flight to catch.

I think we will be surprised at how little extra hardware we need to install before our house becomes a “person” in the way we talk to it.

22 – Holistic viewpoints versus item-by-item analysis

Many people prefer to take a holistic approach to bioenergy and sustainability discussions. I take the view that a single narrative is useful for communicating a conclusion, but is actively harmful when one is trying to understand the issues. (There is no rule that the universe has to make sense to the story-oriented default human mindset: sometimes there really are a number of disconnected items which must be understood individually.)

So if anyone insists on using a single holistic viewpoint, be aware that they are acting as a campaigner or lobbyist. You should instead insist on understanding each matter individually. Don’t let yourself get swept along with a “compelling story”. It might be compelling, but that doesn’t mean that it is true, complete or fair.

Stories are a good way of communicating issues in a memorable way, but they actively obscure getting at the truth of complex and complicated matters.

21 – Modifying forest management to increase carbon retention

If we subsidise burning biomass using public money (to reduce carbon emissions) then we should also subsidise other uses of biomass which save carbon otherwise we will be distorting markets and not achieving the best value for money overall.
So to reduce carbon at least cost we should widen our range of counterfactuals to include everything that we might do with an area of forested land. Any slight tweak to forestry management which increases soil carbon (or any long-lived carbon pool) should be investigated and costed. Top of the list is probably preventing residues being burned in open fires (which emits some methane, not just carbon dioxide). Preserving carbon in standing trees is also likely to be profitable when we use a common price for carbon across all potential actions in forestry: this results from practices such as selective felling (instead of clear-cutting) and extending rotation periods slightly. The number of potential management actions of this sort has not yet been investigated across all the worlds’ forests and plantations.
This is the sort of thing implied by “stronger coherence and joint strategies in the EASAC report published on May 11th: http://www.easac.eu/fileadmin/PDF_s/reports_statements/Forests/EASAC_Forests_web_complete.pdf

20 – The degradation of forestry residues with time: a useful viewpoint

The discussion around carbon debt has always been about re-growth of trees, but this is narrow-minded. So long as there is continuous leaf-

cover and no bare earth, there will be photosynthesis and there will be carbon draw-down from the atmosphere. The issue is that much of this green biomass degrades: it rots or is eaten, and gives up its CO2 to the atmosphere quite quickly.

So we can get additional insights using an alternative framing of the issue: focusing on foliage decay rather than tree growth. This has the useful unifying effect of consolidating what appeared to be several different issues into one: the length of time that CO2 captured by photosynthesis spends locked down away from the atmosphere:

1. annual foliage decay and digestion by forest wildlife (mostly invertebrates)
2. fine residues (months)
3. coarse residues (years)
4. pulp-wood used for paper products (up to 6 recycles)
5. sawdust in particle-board furniture & fittings (10 years?)
6. wood in fine furniture (30 years?)
7. structural timber in buildings (40-100 years?)
8. direct sequestration, either entirely or partially as biochar (average 150 years?)

One other result from this viewpoint is that we should look at choosing a scope for the calculations based around the photosynthetically-productive land area rather than the trees growing on it.

Addition: recent papers

Forest residues for bioenergy: climate impacts and cumulative carbon budgets, presented at ‘Understanding the Climate Effects of Bioenergy Systems’ workshop, 16 May 2017, Chalmers University, Gothenburg, Sweden.
http://task38.ieabioenergy.com/wp-content/uploads/2017/05/3.-Johansson.pdf

All workshop presentations: http://task38.ieabioenergy.com/gothenburg-sweden-2017/

Climate impact assessments of forest bioenergy affected by decomposition modelling – comparison of the Q and Yasso models, Report to the IEA-Bioenergy Task 43 & Task 38 (2017) http://www.ieabioenergytask43.org/wp-content/uploads/2017/04/EXCO-2017-05.pdf

19 – Is particle board or oriented-strand board an alternative to wood pellets ?

At its simplest, the material used to make wood pellets destined for European boilers is pulp-wood: the same stuff that is used to make paper. This is the term for trees that are not big enough or straight enough to be used for saw-logs for construction.

So are these pulp-wood trees now just wastes or residues? If we assume that “they have to be cut down anyway” (a big assumption, when thinking several decades into the future), is there really no alternative to burning them ? Yes there is, and in future there will be more.

Particle-board, more commonly known as MDF (medium-density board) in the UK, is very widely used in making furniture and fittings in buildings. Oriented-strand board (OSB) has been taking over from plywood for 20 years in the construction industry. Particle-board uses sawdust and OSB uses thin strips of wood from small trees. Both substitute for more massive wood lumber, and so reduce the carbon footprint of wooden buildings as more of the forest is used: so fewer trees need to be cut down to meet the demand for buildings.

But does this then cause longer forest rotation periods and thus higher levels of standing carbon than would otherwise be the case? Or does it incentivise cutting trees earlier as more of the smaller trees can be used productively ? We don’t know, but in principle the economic model SRTS should be able to tell us, if only we knew what the future demands for OSB and MDF will be.

One thing should be clear though: the amount of pulp-wood “that will be cut down anyway” will not all be available to make wood pellets in the same volume in the future as it is now, because these competing uses of the same feedstock are increasing. So we should see an increase in the proportion of pellets made from plantation wood cut for pulp-wood only. That at least makes the carbon accounting easier as the pellets then form the only product and there are no confusions with definitions of residues.

There is one final use of pulp-wood which may also come into  play in the 2040s: geo-sequestration of biomass to remove the carbon from the biosphere. When carbon prices rise to the levels we fear they might (from the whole energy system modelling), this will become a commercially competitive “use”.

18 – Does saw-log timber displace fuel-intensive construction materials ?

In some carbon accounting models it is assumed that the timber is used in construction and off-sets the carbon footprint of concrete and steel. This is available as an option in some BEaC model scenarios.

But what matters is construction in the locality where the saw-logs come onto the market, not the construction in the country using wood pellets from forest residues. In the USA domestic construction is almost entirely based around wood, unlike in the UK. There is no question that timber will be used in construction, and making more timber available won’t mean that more is used in construction. Presumably there is some price sensitivity in the demand for floor joists (concrete) and roof beams (steel), but not for most of the timber. Nevertheless it would be good to know what that price sensitivity is as that marginal behaviour would be a fossil-fuel displacement, even though it would only be a small part of the total timber harvest.

There is a more interesting case when considering increasing the amount of engineered timber used in future in Europe. Here there is quite definitely displacement of concrete and steel, and current UNFCCC inventory rules mean that the carbon benefit accrues to the country in Europe (but only if it makes its on steel and cement). This benefit is irrespective of whether the wood is considered carbon neutral or not, it is simply because there is less fossil fuel being used to make the steel and cement. (Importing steel and cement instead of making it within the country also works to reduce the UNFCCC carbon footprint, but this usually has an economic disincentive.)

 

 

17 – Which woody materials are wastes and which are forestry co-products?

There has been immense confusion caused by arguments made without being clear about definitions of wastes. Some have taken it as axiomatic that “whole trees” cannot be wastes, whereas others take existing forest practices as inviolate.

While there are many ecological arguments against clear-cutting practices, a strict life-cycle analysis (LCA) carbon accounting has to stick to clear boundaries. Discussing whether those boundaries are appropriate and correct is what we are talking about here.

The EU Renewable Energy Directive (RED) lists in great detail a number of different residues, co-products and by-products, and there is also an over-arching definition of wastes as “anything which would be discarded”. But when you get down to actually use these in drafting regulations you discover that they are almost hopeless: they are ambiguous, they over-specify some things and omit others entirely. The RED also says that how transport carbon costs are counted depends on whether the material is a waste or a residue. [Clearly the RED is the result of negotiation between actors who do not all fully understand the subject matter. Unfortunately, what we have seen in 2016 of draft proposals for RED 2.0 are no better.]

Very simply, if a material is a co-product or by-product of saw-timber planks, then it shares all the upstream carbon costs of producing that lumber. Whether this is shared on the  basis of mass, or monetary value, or calorific value (which is odd if some products are not fuels) depends. Which you choose should depend on which is most useful. Since these footprints are going to be used to compare the products against the footprints of alternative products, choosing a metric which makes that comparison easiest is a good idea. But of course the different co-products will be competing against radically different other products: hog fuel against natural gas, and lumber against rolled-steel joints, so which metric is most useful is not at all clear.

If the material is a waste, such as bark or “empty fruit bunches” from oil palm, then it has no monetary value. It might have a negative value in that there may be a disposal cost if it is not used.

If the material has several uses, such as sawdust form a sawmill, then it is generally called a residue but sometimes it is arguably a co-product. There is competition for sawdust from the particle-board industry and the wood-pellet industry, but if transport to those plants is not available then it may be burned on-site as fuel.

So the same material may be a waste, a residue or a co-product for LCA purposes depending on where it is produced, unless there is some arbitrary regulation defining it as one or the other (such as RED).

So is a “whole tree” a valid residue of a clear-cutting felling operation to harvest timber? Only if clear-felling is unavoidable, and a discussion of that is outside the scope of this set of articles. If we assume that clear-felling is unavoidable because of the terrain and the commercial cost of doing otherwise (and of course if it is legal), then logically yes: everything cut down which isn’t a saw-log is waste, or a residue or co-product of the saw-timber for LCA purposes.

The challenge is not to fight over definitions of residues, but to construct structures for doing LCAs that capture the issues we really care about for the climate. For example, suppose that selective logging of only large-diameter trees is feasible in a particular forest, but that this increases the cost of collection significantly. How do we find out from a “social good perspective” whether this (or clear-felling) should be allowed or not? How do we construct carbon prices to capture the value of leaving (some) bent trees growing as well as the fossil-fuel offset by using the slash as fuel, in addition to the social utility of the timber? That is the real issue we need to do more work on.  (But we do also need much better definitions of residues!)

Footnote on “cascading use”

Don’t think that simply adhering to “the waste hierarchy” gets you out of this mess. When you look carefully at the principle of cascading woody biomass, you discover that the same issues arise when deciding how high in the hierarchy a particular use comes, according to economic and carbon accounting. The big issue is still that all these classifications transfer the power of resource allocation from open markets to policy interest groups.

16 – Should we not bother with the energy in woody wastes? Just bury it ?

Following on from the issue that the fossil fuel counterfactual is reducing in effectiveness, what happens to waste woody material when all the alternatives emit less carbon than burning the biomass ?

The simplistic solution is to bolt-on a carbon capture plant onto the bioenergy system to reduce the carbon emissions below those of competing systems for producing heat or electricity. But what is really going on here is that the value of the carbon sequestered is now greater than the value of the energy embodied in the biomass. So in many situations it may be more cost effective to miss out the energy generation step entirely and go straight to the sequestration. For example, mixing waste wood chip with sand, bagging it and storing it in a deep, cold anoxic lake or sea. That would

• remove the carbon from the biosphere for a few centuries
• be fairly easily retrievable if a use could be found for the biomass in a future bioeconomy
• entirely avoid the cost of building and operating a heating system or electricity generation plant
• allow the operation to happen far from any demand for energy
• allow the operator to get paid for sequestering the carbon (under some future UNFCCC implementation protocol or national carbon storage subsidy).
• but there would be a continuous monitoring cost to prevent disturbance and to prevent any GHG emissions from the store.

The practical and legal obstacles to creating such policies would be severe, but the benefit could be a globally significant carbon removal.

Half-way from efficient biomass-fired electricity generation to the pure 100% sequestration with no generation would be a cheap, inefficient plant with carbon capture. Such a plant has been modelled by a team at Imperial College in 2017 and it is indeed environmentally optimal in some conditions.

There is one further issue which is relevant. Generating electricity or heat from a fairly pure stream of natural gas is easy and efficient, whereas new technology needs to be developed for every slightly different variety of woody biomass. Also biomass produces tars which create costly difficulties in the furnace or gasifier. This can all be avoided if the biomass is just sent straight to storage and the fossil fuel is burned. Sometimes the best use of biomass is not to use it for bioenergy, but just use it for sequestration. As an example of the pointless technological contortions that people go through trying to turn biomass into fuel instead of just burying it, see this presentation about liquid biofuels.