the feasibility and costs of biochar deployment in the uk · biochar is a relatively new concept...

Biochar is a relatively new concept that has been promoted primarily as a form of carbon storage but also for its poten-tial benefits for bioenergy production (e.g., syngas, bio-oil and heat), soils and crop productivity [1–3]. Other poten-tial benefits include reduction of nitrate leaching [4–10], adsorption of contaminants, such as arsenic and copper from soils [11,101] and reduction of trace-gas emissions from soils (nitrous oxide [N

2O] and methane) [4,5,7,12]. Biochar

might also be useful to the waste-processing industry in allowing the recovery of waste as a potentially useful by-product [10]. However, many of these potential bio-char functions remain highly uncertain. The most cer-tain function is the carbon storage and reasonably good estimates can be given of the recalcitrant carbon from different biochar types and production methods [13–15]. Biochar is produced by thermochemical conversion in the absence of oxygen (slow pyrolysis) and a simple schematic of the process of production, supply and use is illustrated in Figure 1.

In this paper, we examine the potential opportunities for deployment of biochar in the UK through:

� Creating scenarios of biomass resource that might be used for producing biochar, which, combined with life cycle assessment data, can be used for

putting forth preliminary estimates of net carbon equivalent abatement from biochar production and use;

� Estimating the costs of producing biochar in the UK context. Combining the feedstock-specific data on carbon abatement and production cost, a provisional marginal abatement cost curve can be derived.

Scenarios for available feedstock, biochar supply & technology scaleThe resource pyramid approach [16] was used to dis-tinguish between ‘theoretical available resources’ (the total amount currently accessible), ‘realistic available resources’ (which applies a first level of pragmatic judgement to limit the supply) and ‘viable available resources’ (which applies a second level of pragmatic judgement to further limit supply, taking particular account of likely or possible other demands in the mar-ket place). Three different biomass supply scenarios of viable available resources were then developed to reflect the high level of uncertainty over availability given uncertain demand and supply factors. These supply scenarios include lower-, higher- and high-supply of feedstocks available for pyrolysis (Table 1).

Carbon Management (2011) 2(3), 335–356

The feasibility and costs of biochar deployment in the UK

Simon Shackley†1, Jim Hammond1, John Gaunt2 & Rodrigo Ibarrola1

Biochar allows long-term (multi-centennial) soil carbon storage, with potential benefits for agricultural sustainability (e.g., productivity, reduced environmental impacts and water retention). Little is know about the costs of producing biochar and this study attempts to provide a ‘break-even selling point’ for biochar, accounting for costs from feedstock to soil application and revenues from electricity generation and gate fees. Depending on the assumptions used, biochar in the UK context may cost between GB£-148 t-1 and 389 t-1 (US$-222 to 584) produced, delivered and spread on fields, which is a provisional carbon abatement value of (GB£-144 tCO2

–1 to 208 tCO2–1). A negative cost indicates a profit-making activity. The most profitable

source of biochar is from wastes, but such materials will face complex regulatory issues and testing.

Review

1UK Biochar Research Centre, University of Edinburgh, King’s Buildings, Edinburgh EH9 3JN, UK2Carbon Consulting Ltd., 95 Brown Road, Ithaca, NY 14850, USA†Author for correspondence: Tel.: +44 131 650 7862; E-mail: [email protected]

future science group 335ISSN 1758-300410.4155/CMT.11.22 © 2011 Future Science Ltd

We distinguish between two general classes of mate-rial; virgin and non-virgin biomass [17]. Biomass derived from whole (or parts of) plants and trees or from the

processing of biomass, where this does not involve chemical or bio-logical transformation, amendment or treatment is virgin biomass. Any biomass that does not fall under the definition of virgin biomass resource is non-virgin biomass. The distinc-tion is particularly important for the risk assessment and regulation of biochar.

A wide range of (although not all potential) virgin and non-vir-gin biomass feedstocks have been included in the scenarios (Table 1); for example, wood pellets, as a relatively high value fuel, are not included, only one imported feed-stock has been included (wood chip from Canada), and not all types of organic municipal and industrial waste are included. Chicken litter waste has not been included because there are already several large facili-ties for fluidized bed combustion of this material with sale of the result-ing ash to farmers. It is unlikely that supplies of chicken litter will increase to the extent that new treat-ment facilities would be required

[18]. Municipal solid waste (MSW) treated through mechanical and biological treatment (MBT) was also excluded since such material is highly heterogeneous and typically contains a wide-range of contaminants. (There might be suitable non-soil applications of bio-char from such materials and regulatory differences between legislatures.)

The estimated theoretical available resource values in Table 1 are based upon other published or Government assessments, with the realistic available and viable avail-able resource levels being estimated through group discussions. The assessment of sustainable bioenergy feedstocks in the UK by the Supergen Biomass and Bioenergy consortium [18] was an important resource. A spreadsheet listing all the sources and assumptions can be made available. In general, the lower viable resource scenario for virgin biomass is 25% of the realistic avail-able scenario while the higher and high viable resource scenarios are 75% and 100% of the realistic available scenario, respectively. These percentages are judge-ments, but no robust methodology for arriving at such values is currently available. We have moderated these percentages for the high viable available value in the case of forestry, owing to the difficulty of applying biochar to forestry systems (without which the removal of 100% of harvestable biomass might result in a long-term loss of nutrients).

There is no use of non-virgin resources in the lower viable available scenarios, owing to concerns over its contamination and potential pollution effects, while percentage utilizations are 50 and 75%, respectively, for the higher and high scenarios, the scaling-down

TransportUK£0–19

Transport£8–44

Multiple biomass sources

Virgin feed: £54–247Non-virgin feed: £0

Avoided gated-fees (wastes): £89–124

Application of biochar to soil £5

Storage £7–15

Pyrolysis unit

Capital: £45–101

Operation (gas, labor and maintenance): £9–119

Electricity sales: £37 and subsidy £74

Carbon Management © Future Science Group (2011)

Figure 1. Pyrolysis-biochar system: from source to sink. Numbers indicate cost ranges (in GB£ per ton of biochar [£t-1]) per process stage.

Key terms

Biochar: The porous carbonaceous solid produced by thermochemical conversion of organic materials in an oxygen-depleted atmosphere that has physiochemical properties suitable for the safe and long-term storage of carbon in the environment and, potentially, soil improvement.

Pyrolysis: The thermochemical decomposition of organic material at elevated temperatures (usually > 400°C) in the absence of oxygen.

Biomass: The biodegradable fraction of products, waste and residues from agriculture (including vegetal and animal substances), forestry and related industries, as well as the biodegradable fraction of industrial and municipal waste.

Carbon abatement: The net effect of changes in GHG fluxes that result from some action, process or intervention; this paper considers only the recalcitrant carbon in the biochar.

Virgin biomass: Biomass derived from whole plants and trees, or the parts therefore, or otherwise from the processing of biomass, where this does not involve chemical or biological transformation, amendment or treatment.

Carbon Management (2011) 2(3) future science group336

Review Shackley, Hammond, Gaunt et al.

Table 1: UK biomass resource availability scenarios used for generating three supply scenarios.

Feedstock Theoretical available biomass

resource for bioenergy (t yr-1

o.d.)

Realistic available biomass

resource for pyrolysis-

biochar systems (t yr-1 o.d.)

Viable available resource assumed available for pyrolysis

Lower resource(t yr-1 o.d.; %)

Higher resource(t yr-1 o.d.; %)

High resource(t yr-1 o.d.; %)

Virgin Biomass Resources

Wheat straw 6,300,000 4,725,000 25 1,181,250 75 3,543,750 100 4,725,000Small scale 413,438 1,240,313 1,653,750Large scale 767,812 230,343 3,071,250Barley straw 2,400,000 1,800,000 25 450,000 75 1,350,000 100 1,800,000Small scale 157,500 472,500 630,000Large scale 292,500 877,500 1,170,000OSR straw + other cereals 3,150,000 2,362,500 25 590,625 75 1,771,875 100 2,362,500Small scale 206,719 620,156 826,875Large scale 38,390 115,171 1,535,625Forestry residues 880,000 800,000 25 200,000 75 600,000 75 600,000Arboricultural arisings 341,000 341,000 25 85,250 75 255,750 100 341,000Sawmill co-product 1,606,000 86,000 25 21,500 75 64,500 100 86,000Wood pellets 4,107,505 4,107,505 0 0 0 0 0 0Miscanthus, switchgrass and short rotation coppice – cultivated on agricultural land

2,012,500 2,012,500 25 503,125 75 1,509,375 100 2,012,500

Reed canary grass 50,000 50,000 25 12,500 75 37,500 100 50,000Short rotation forestry – contaminated or public land

210,000 210,000 25 52,500 50 105,500 75 157,500

Total virgin biomass resource 21,057,005 16,494,505 3,096,750 9,237,750 12,134,500

Non-virgin biomass resources

Construction and demolition wood waste

5,040,000 2,520,000 0 0 50 1,260,000 75 1,890,000

Commercial and industrial wood waste

4,481,000 2,240,500 0 0 50 1,120,250 75 1,680,375

Municipal solid waste – wood 1,065,000 798,750 0 0 50 399,375 75 599,063MBT waste 6,250,000 6,250,000 0 0 50 3,125,000 75 4,687,500Green and food waste 3,600,000 3,600,000 0 0 50 1,800,000 75 2,700,000Sewage sludge 1,509,000 1,509,000 0 0 50 754,500 100 1,509,000Commercial and industrial animal and vegetable waste

540,000 405,000 0 0 50 202,500 75 303,750

Poultry litter 4,276,000 2,138,000 0 0 50 1,069,000 75 1,603,500Total non-virgin biomass resource 26,761,000

19,461,250 0 9,730,625 14,973,188

Total minus MBT and poultry waste

16,235,000 11,073,250 0 5,536,625 8,682,188

Total virgin and non-virgin biomass resource (including MBT and poultry waste)

47,818,005 35,955,755 3,096,750 18,968,375 27,107,688

Total virgin and non-virgin biomass resource (excluding MBT and poultry waste)

37,292,005 27,567,755 3,096,750 14,774,375 20,816,688

MBT: Mechanical and biological treatment; o.d.: Oven dry; OSR: Oil seed rape.

The feasibility & costs of biochar deployment in the UK Review

future science group www.future-science.com 337

compared with virgin resources being due to the greater difficulty in using non-virgin materials. (An exception is sewage sludge, a feedstock that appears to be bet-ter suited for use in slow pyrolysis, with commercial examples in Japan [19]; hence, 100% of the resource is assumed to be available in the high scenario.) The main categories of waste considered are: wood waste (con-struction and demolition, commercial, from MSW), greenwaste (segregated and collected urban arisings), animal and vegetable waste (commercial and industrial) and sewage sludge.

Our estimates of the realistic and viable available straw resource (just over 2 million over dry ton per annum [odtpa]) to nearly 9 million odtpa in the high scenario) are greater than those given by [18] (1 mil-lion odtpa). Thornley et al. assume that less than 50% of the straw could be removed from field in order to maintain the soil organic carbon (SOC) level to an acceptable sustainable level [18]. However, where that straw is being used for producing biochar, and where all or some of that biochar is being returned to the same fields where the arable crop was cultivated, this con-straint can be relaxed since carbon is being returned to the soil, albeit in a largely recalcitrant form. The amount of carbon returned to soil from keeping 50% of the straw on the field is approximately 0.25 tons per tonne (tt-1) dry straw; returning biochar from all the straw to the same field returns c. 0.21 tt-1 dry straw, assuming the carbon content of the dry char by weight is 60% [201]. Nutrients such as P, K and metals would be better conserved where straw biochar is returned to soil than where 50% of straw is removed, although straw would probably contain more available nitrogen that straw char. More research is required on the relative benefits and costs of each strategy.

The scenarios suggest that there are between 3 and 12 million tons of virgin biomass resource and from 0 to 9 million tons of non-virgin biomass resource (exclud-ing poultry waste and MBT) available in the UK for producing biochar. An important proviso is that the sce-narios are based upon existing or likely future biomass, meaning already planned for or likely to be available in the next decade. Consequently, potential changes in biomass availability over the longer time-scale have not been characterized in the scenarios. Various reviews of the role of bioenergy and biofuels in the UK con-text have been undertaken (e.g., [20,21]). In 2005, the Biomass Task Force estimated that approximately

1 million hectares of land could be made available for non-food uses in general, yielding 8 million tons of energy crops. In retrospect, these numbers appear rather large, implying as they do a large-scale

shift from use of existing arable land (of which there are approximately 6 million ha in the UK in total). Since the Biomass Task Force report was published, food security and terrestrial carbon debt issues have risen up the policy agenda and the reluctance of British farmers to shift towards the cultivation of energy crops has become apparent [22]. Hence, estimates of energy crops have been scaled-down and a far more modest estimate of energy crop potential has been used here (of just over 2 million odtpa) [18]. Estimates of the avail-ability of waste wood by the Biomass Task Force are more consistent with the estimates in our scenarios: in the case of wastes, there is not the same concern with impacts on alternative land-use and hence food security, although there are still of course alternative uses for the waste (non-virgin) biomass.

Large-scale expansion of forestry has not been accounted for, although it could, potentially, add sub-stantially to virgin and non-virgin biomass resources. The National Assessment of UK Forestry and Climate Change (the ‘Read Report’) published in 2009, pro-posed that woodland planting be increased from 8360 ha per year to 23,200 ha per year to 2050 [23]. The report suggests a large increase in the use of woody biomass for permanent uses (e.g. as building materials) as well as for bioenergy. An assessment of the impact of increased forestry in Scotland for the availability of biomass for biochar production has been undertaken [24]. This sug-gests that large-scale expansion of forestry in Scotland (by 15,000 ha per year, consistent with the ambitions of the Read Report) would increase the available feedstock for biochar production by 75–150%.

� Technology scales The standard approach in evaluating technology costs is to develop empirical relationships between compo-nent costs and power output (e.g., [25] for bioenergy plant). This is difficult in the case of pyrolysis-biochar systems (PBS) however, since there is a lack of peer-reviewed data available on the realistic costs of slow pyrolysis (contra fast pyrolysis) at different scales. Therefore, it has been necessary to assemble the best available data on the possible costs at different scales from the published and grey literature and through discussions with experts. However, a high degree of uncertainty inevitably surrounds the estimates. It will be difficult to reduce these uncertainties in the absence of data from demonstration or commercial facilities.

In the absence of scale correlations, it was decided to represent three potential sizes of pyrolysis units (PUs): small-scale (~2000 odtpa), medium-scale (~16,000 odtpa) and large-scale (~185,000 odtpa). The small-scale unit could take in feedstocks from a number of farms, small woodmills and from city green

Key term

Pyrolysis-biochar systems (PBS): A combination of a specified pyrolysis technology, transport, distribution and storage infrastructure and application of biochar.



waste sources and so on. The medium-scale unit is an industrial facility that produces electric power (and potentially heat) for a single large consumer or for sup-ply to the national electricity grid (with heat being sup-plied to a district heating system). The large-scale unit is a large bioenergy power plant (but still very small compared with a 1GW coal power plant). Another obvious scale that could be included is a farm-scale unit (processing approximately 200 t y-1). Assuming a farm of c. 200 ha growing arable crops, a farm-scale unit could process approximately 50% of the straw produced in a given year. Inclusion of this scale in future work is important.

Technical assumptions with respect to the energy per-formance of the three PU scales are shown in Tables 2 & 3, although some of the information on the medium-scale unit is commercial-in-confidence. Feedstocks are dis-tributed between the different-sized pyroylsis units, either to the two larger-scale units (large and medium), or to the smaller-scaled unit. The proportions are shown in Table 3 and are judgements based upon the num-ber of units in each scenario and the feedstock type; for example, where large-scale collection, distribution and production occurs (e.g., large-scale forestry and municipal and industrial waste feedstocks), these are used exclusively in medium-/large-scale units, while arboricultural arisings (which are typically much more dispersed) are used exclusively in small-scale units. Straw and energy crops (short rotation coppice [SRC], reed canary grass [RCG]) can be used both at the small and large-scales, with two-thirds to four-fifths being

utilized at the larger-scales. The proportion of feedstock going to the medium-/large-scale units is determined by the biomass flow requirements of such units. However, it is also influenced by the very large number of smaller units that would be required to take a larger proportion of the feedstocks.

For the medium-scale PU (16 k odtpa), data from one of the few demonstration slow pyrolysis units were made available. The identity of the company and the precise details have had to remain confidential. The plant-specific data used is the value of electricity gen-erated, the cost of natural gas to initiate the process, the labor costs and other operational costs. For the small-scale unit (2 k odtpa) we have simply divided the medium-scale unit electricity generation by eight, while for the large-scale unit (185 k odtpa), we have multiplied the medium-unit electricity generation by 11.55, on the crude assumption that the conversion efficiencies will be equivalent for the three differently-sized units. As a check, we also calculated the electric-ity generated from the large-scale unit by using the Biomass Environmental Assessment Tool (BEAT

2)

[26,202]. BEAT2 is a publicly-available tool provided by

the UK government and developed by UK energy con-sultants (North Energy Ltd. and AEAT Ltd.), which can be used in bioenergy life cycle process-model-ing [202]. Using best estimates from the literature [27], BEAT

2 was modified to include slow pyrolysis.

The approach we have adopted is, in some ways, simpler than that of McCarl et al. [28] and of Roberts et al. [29] who distinguish between pre-treatment

Table 2. Energy performance features of the three pyrolysis plant scales, number of pyrolysis units and biochar production.

System parameter Small-scale PU Medium-scale PU Large-scale PU

Feedstock consumption (odt per year) 2000 16,000 184,800Process energy required (% of total energy generated)†

10 10 10

Energy loss (% of total energy generated) 15–20 15–20 15–20 Total electricity efficiency (% energy content of feedstock as delivered electricity) (modelled)

As for medium-scale Commercial-in-confidence

As for medium-scale

Number of pyrolysis units for lower resource scenario‡

875 50 13

Number of pyrolysis units for higher resource scenario‡

3000 117 39

Number of pyrolysis units for high resource scenario‡

4250 251 43

Biochar production (odt per year) § 727 5396 38,202Biochar production yield (%) 36% 36% 36%†This refers to the energy used to dry the feedstock and to drive the pyrolysis process and other operations as a proportion of the total energy in the bio-oil and syngas. The value can vary considerably depending upon moisture and system configuration [58]. ‡Calculation of numbers of pyrolysis units assumes load factors of 0.4 (small), 0.6 (medium) and 0.8 (large). §Biochar production is calculated over a 10-year period with 2 years start-up conditions; hence reduced output compared to years 3–10: 61% of final efficiency is achieved in year 1, 67% in year 2 and 100% in year 3. odt: Oven dry tons; PU: Pyrolysis units.



of biomass (including reception, drying, size-reduction, storage and feeding) and the pyrolysis process per se. In our ana lysis, some of the pre-treatment operations are covered by the feedstock costs (drying and size-reduction), while others are covered by the plant capital and operational costs (e.g., reception, storage, feeding). Our ana lysis is also simpler than Roberts [29] in terms of the modeling of the pyrolysis process, although the medium-scale unit data is based on a real-plant. The extrapolation of data from the medium to the other scales remains a source of considerable uncertainty.

It is evident that the three technology scales are point selections from an unknown range of options. Crucially, the values are not validated against actual experience with the exception of the medium-scale unit and are not feedstock-specific (given the lack of any appropriate data).

Review of previous cost–benefit analysis studies Turning to the question of costs, the economic assess-ment of biochar should consider the total costs and benefits of developing, implementing and managing

Table 3. Assumptions and sources of data regarding costs and revenues†.

Scale Small-scale PU Medium-scale PU Large-scale PU

System parameter

Electricity generation ‡ Scaled-down from medium-scale plant

Provided by company based on real-plant experience

Scaled-up from medium-scale plant.

Natural gas (to initiate pyrolysis) Scaled-down from medium-scale plant

Provided by company based on real-plant experience

As for medium-scale in the absence of a better way of estimating.

Labor Assumes 1 person FTE using standard wage rates

4 staff (manager, 2 workers, 1 admin)


Other operating costs Personal communication, Jason Cook, based upon farm-based pyrolyser

Provided by company (based on real-plant experience)


Handling costs (capital) Assumes use of 55-66 hp tractor Included in capital costs Included in capital costs

Distribution of feedstocks to small-scale and to medium/large-scale pyrolysis units (%)

Wheat, barley, oil-seed rape straw§ 35 65Forestry residues ¶ 0 100Arboricultural arisings# 100 0Sawmill co-products† † 0 100Miscanthus ‡ ‡ 0 100Imported Canadian woodchips§ § 0 100Short rotation coppice ¶ ¶ 20 80Switchgrass## 100 0Reed canary grass† † † 30% lower supply

23% higher supply25% high supply

70% lower supply77% higher supply

75% high supply Short rotation forestry, waste wood, green and garden waste, sewage sludge, animal, vegetable and food wastes‡‡‡

0 100

† The operational costs and revenues take account of the fact that the first 2 years of operation are not at full efficiency. All feedstocks are modelled equivalently with respect to these costs and revenues. Not enough is known at present to distinguish between feedstocks with respect to their costs and properties. ‡ The electricity generation of the large-scale unit was validated against data derived from the BEAT2 model assuming gas yield of 31.9% with a CV of 11MJ/kg. All process energy assumed to be derived from bio-oil. Values agreed to within a few percentage. § Straw could be used at all scales.¶ Forestry residues from commercial operations would be collected more efficiently for use in larger units.# Arboricultural arisings are typically collected in reasonably small volumes hence could be utilised in small units.† † More efficient use of sawmill waste is in larger units ‡ ‡ It is assumed the crop would be sold to larger users§ § Only efficient if large volumes imported.¶ ¶ Could be grown and used at range of scales.##Unlikely to be a major energy crop in UK.† † † May have more potential as energy crop than switchgrass in UK and could be grown and used at range of scales. ‡‡‡Production, collection, sorting, separation and use more efficient at larger scales. BEAT2: Biomass Environmental Assessment Tool (2); CV: Calorific value; FTE: Full time equivalent employee; PU: Pyrolysis units.



PBS. A simple framework is presented in Table 4. Information on many of these costs and benefits is cur-rently not available as dominant technological designs and management systems for biochar production and application have not yet been developed. A related tech-nology to pyrolysis is gasification but there are not, to our knowledge, many commercially-viable biomass gasification plants in industrialized countries. Biomass gasification plants in Europe have tended to be unsuc-cessful in commercial terms, despite a considerable investment in their development [30]. However, we have used data from gasification plants to inform the calcu-lation of capital and operating expenditure (CAPEX and OPEX) for slow pyrolysis plants. Furthermore, the financial benefits to agriculture of biochar application to soil are insufficiently understood, both in the UK context and globally. This leads to our reluctance to undertake a full cost–benefit analysis (CBA). A more modest approach is adopted here, whereby an attempt is made to calculate the production costs of biochar.

Some existing studies have attempted to undertake a full life cycle assessment and/or CBA for biochar, but have needed to assume the benefits of biochar in terms of elevated crop yield and other agronomic impacts; for example, Gaunt and Cowie [31] explored three scenarios (low, medium and high) to represent soil responses to a biochar addition at 5 t ha-1. This included not only a yield response, but also avoided CO

2-equivalent emis-

sions per hectare arising from the following changes: N

2O emissions from soil, field operations, fertilizer sav-

ings and increased levels of SOC. While three scenarios are used, a zero value for agronomic impacts is only used in one-sixth of cases, while increased emissions from biochar addition are not assumed in any case. There is an absence of definitive published evidence (and in some cases no peer-reviewed publications) to support many of these values either way.

For example, evidence regarding the impacts of bio-char on N

2O emissions are rather mixed, although sup-

pression does appear to be the more frequent effect. N

2O suppression may be a pH-related effect, since wood

ash can also suppress N2O emissions [32]. There are,

as yet, no convincing studies on the impacts of bio-char on the costs of field operations such as ploughing and irrigation. The evidence relating to the impact of biochar upon yield with and without fertilizer addi-tions is currently ambiguous [33–35]. The impact of bio-char upon SOC from non-biochar sources is currently uncertain, with some studies of boreal soils suggest-ing a reduction in SOC from biochar additions [36], although other evidence, (e.g., studies of terra preta soils) suggest an increase [37–40]. The accumulation of (non-biochar) SOC arising from biochar addition is an important feedback in accounting for the net carbon equivalent abatement of pyrolysis-biochar in some life cycle assessments [41,42]. There is reasonably good evi-dence that biochar increases pH by 0.5–1 unit in most cases for application rates of 30 t ha-1 [10]. There are no published studies on the effects of biochar on product quality, while effects on disease suppression and induced resistance to disease are anecdotal.

McCarl et al. assume a 5% increase in crop yield from a 5 t ha-1 biochar addition, as well as a reduction in lime and nutrient requirements [28]. Collison et al. assume that an unspecified level of biochar applica-tion would result in a 5% increase in feed wheat and potato yields, a 3% uplift in quality, a 10% reduction in fertilizer use and a 5% reduction in cultivation costs in the East of England region [43]. The overall effect was a reduction in the total variable costs in the case of biochar addition and a significant increase in per hectare profitability (by GB£143 ha-1 for feed wheat and GB£545 ha-1 for potatoes). Roberts et al. [29] assume a 7.2% increase in fertilizer use efficiency in

Table 4. Summary of costs and benefits associated with pyrolysis-biochar systems (assuming that the biochar does not contain contaminants).

Total costs: cost of producing, delivering and applying biochar Total benefits: value of biochar

Biochar production

Transportation and storage

Application Energy production

Agricultural gains

Carbon storage

Diffuse pollution abatement

Feedstock Equipment Equipment Electricity value Yield gain C abatement Reduced nitrate run-off

Transport Labor Labor Heat value Quality Utilities New covered

storage facilitiesMonitoring, verification and reporting

Reduced fertiliser

Maintenance & operation

Soil workability

Labor Water retentionCapital costsGate fee



the US corn-belt context from biochar application, although the supporting reference refers to a study of an Amazonian soil.

However, the empirical evidence is less clear. A meta-ana lysis of the impacts of biochar on crop yields [44] has shown an average from all studies (which have used rep-lication) of a +10% response to biochar; however, the studies are heavily skewed towards (sub-) tropical con-ditions on degraded soils. Field trial results from more temperate regions are few and far between but tend to show smaller (or no) yield increases compared with sub-tropical field trials (e.g., [33], UK Biochar Research Centre (UKBRC) [unpublished data] and East Malling Research). Yield responses in pot trials or in controlled glasshouses are not readily extrapolated to real-world field conditions [10,34,35]. Biochar additions at rates of 10, 25, 50 and 100 t ha-1 led to statistically significant increases in crop yields compared with a control with no addition, although other studies using 40 and 65 t ha-1

did not show any statistically significant yield increase. Verheijen et al. speculate that the reasons for the wide-ranging response are variability in the biochar, crop and soil types [44]. They also note that the mean yield response for each application rate are positive and that no single biochar application rate had a statistically sig-nificant negative effect on crop productivity. Negative impacts upon plant growth have been reported from bio-char addition and it has been speculated that sorption of nitrogen by the char is one potential mechanism in an N constrained context [45,46].

McCarl et al. undertook a full CBA for biochar in the context of the US mid-west [28]. This is for a 70,000 t (feedstock) y-1 plant, costing US$24 million (GB£14.5 million), assuming a 20-year lifetime and a discount rate of 12%. McCarl et al. conclude that slow and fast pyrolysis of maize residue for biochar production and energy by-products is not profitable. The net present value for fast pyrolysis is -US$45 t-1

feedstock and -$70 t-1 feedstock for slow pyrolysis. It is understood that the data for the slow pyrolysis opera-tion in the McCarl et al. study were inferred, not derived from an oper-ating pilot plant. Given that 1 t feed-stock converted to biochar results in approximately 1 t CO

2 equivalent

(CO2e) abatement [26], and remov-

ing the US$4 t CO2e abatement

value assumed by McCarl et al., the carbon abatement cost is therefore $49 to US$74 t-1 CO

2. This is simi-

lar to, or lower than, the abatement costs commonly reported for many renewable energy technologies and

for CO2 capture and storage, in industrialized countries,

although cheaper carbon abatement options of course exist [41, 47,48].

There is a risk in putting too much credence on quan-titative net present values in a situation of such high uncertainty. It is, arguably, preferable not to attempt to quantify variables where there is no empirical validation. A total of 40–60% of the agronomic value of biochar in the above studies arises from unconfirmed impacts other than crop yield (e.g., from changes in quality and reduced fertilizer application) and the factors included in different studies are not consistent (e.g., the impact on crop quality is included in Collison et al. [43], but not in McCarl et al. [28]). A further complication with undertaking a CBA from a societal perspective, is that transfer payments from one firm in the economy to another should be excluded, such as revenue streams that are, in effect, a cost (including as a lost revenue) to another firm. This would apply to revenue from tip-ping fees, avoided landfill tax and payment of renewable obligation certificates (ROCs) or feed-in tariffs.

A more limited approach than full CBA is to esti-mate the cost of producing, transporting and applying biochar to the field and then working backwards to what the break-even selling point (BESP) would be per ton of biochar. The BESP is a production cost for a ton of biochar; hence, the accumulative agronomic and soil benefits of biochar plus any carbon-storage ben-efits that could be claimed (all expressed in monetary terms) need to exceed BESP for a biochar strategy to be financially viable (although with no return on invest-ment at the margin). It is also valid in calculating the BESP to include the effect of incentive schemes, avoided gate-fees and landfill tax as it is the production cost from the perspective of a prospective firm. This strategy does assume that these financial (dis)incentives and costs will continue into the near future, which is of course not guaranteed. A schematic of the system that is modeled is shown in Figure 1.

A legitimate criticism of our approach is that we are as guilty of using unconfirmed numbers and assumptions as the above authors whose quantitative approach we have questioned. Our defense is that a BESP cannot cur-rently be calculated without resorting to assumption and estimates. At least by not including additional uncertain-ties regarding agronomic and soil impacts, we are avoid-ing piling one uncertainty on top of another. Bracketing uncertainties in this way also enables a better apprecia-tion to be gained of the sensitivity of results to the uncer-tainties of particular interest in this paper – namely the costs of feedstock provision, transportation, processing and deployment on farm. This bracketing thereby helps in targeting future research on what aspects need to be better understood to estimate costs more accurately.

Key terms

Renewables obligation certificate: A green certificate issued to an accredited generator for eligible renewable electricity generated within the UK and supplied to UK customers by a licensed electricity supplier. One ROC is issued for each megawatt hour (MWh) of eligible renewable output generated.

Feed-in tariff (FIT): a policy mechanism to encourage deployment of renewable energy sources that provide for: guaranteed grid access, long-term contracts for the electricity supplied, and a purchase price that reflects generation costs and tends towards grid parity (i.e. enabling cost equivalence with average grid power).



Costs of pyrolysis-biochar process stages � Feedstock costs

A recent report from the National Non-Food Crops Centre [49] has provided estimates of bioenergy feed-stock and transportation costs. In our study, transporta-tion from the feedstock production site to the pyrolysis unit is assumed to occur in a single stage for small-scale operations, or in two stages (with an intermediate stor-age facility on field or side of road or in an existing waste handling facility) for large-scale operations. The data we have used on feedstock and transportation costs is largely derived from the study by Mortimer N et al [49] and represents assumptions, practice and understanding for the UK situation (Box 1).

Included in the feedstock costs are the effects of biochar production on nutrient balance in soils where straws are employed as a feedstock. The key issue here is to compare the production of biochar from straw with the alternative options, which are either to cut up straw in situ and incorporate into field or to bale up and remove for use or sale. There are different costs and benefits associated with all three options, as detailed in Box 2.

Waste feedstocks costs were calculated from the estimated gate fee charged to companies wishing to dispose of waste. Since the pyrolysis unit operator typi-cally receives a gate fee, the waste feedstock costs are, in many instances, a source of revenue. The gate fee

Box 2. Arable straw: use for producing biochar, field integration or sale.

It is assumed that: a) 100% of straw from a field would be removed (but that the stubble remaining after harvest will remain on field); b) most (90%) of the phosphorus and potassium would be retained during pyrolysis while 45% of nitrogen is retained; and c) that the biochar produced from the straw of a given field is returned to that same field. The nutrient content in straws is assumed to be: wheat: 5kg N, 1.3kg P

2O

5, 9.3kg K

2O per ton; barley: 6kg N, 1.5kg P

2O

5, 12.3kg K

2O per ton; oilseed rape: 7kg N, 2.2kg P

2O

5, 11.5kg K

2O per

ton, with a value of £1.13/kgN, £1.45/kg P2O

5 and £1.00/kgK

2O [71]. Assuming that the straw, if not removed for pyrolysis, would have

been incorporated into the soil, the value of the loss of nutrients through pyrolysis is £4.23 per ton wheat straw, £5.18 per ton barley straw and £5.82 per ton oilseed rape straw (using straw yields from [72] and [73]). The net benefit of straw incorporation was found to be £16 (wheat), £20 (barley) and £21 (oil seed rape [OSR]) per ton. Where straw is removed from the field for sale, the costs of baling and removal need to be accounted for and reduce the net benefits to £11 (wheat) and £17 (barley) (using prices in [72] and [204]).The net benefit of biochar production using an in situ mobile pyrolysis unit and incorporation into the same field (hence no need for baling) (and only including the costs of straw management, not pyrolysis costs, biochar application costs or other benefits of the biochar) was £11 (wheat) or £15 per ton (barley, OSR). However, where the straw is baled and removed for pyrolysis off-field, the net benefits are -£7 (wheat) to -£10 (barley, OSR) per ton. The market price for straw ought to reflect the value of the nutrients contained within, although the numbers above imply that the value of nutrients is not fully appreciated in the market. In order to take account of this, we add the difference between the net benefit of direct straw incorporation and production of biochar in situ to the costs of feedstock management (£4, 5 and 6 a ton for wheat, barley and OSR, respectively). Other estimates in the literature propose higher levels of nutrient loss from biochar production. For example, the paper by Roberts KG et al [29] suggest that there is no loss of P and K but that no N is conserved. The quantity of N conserved during pyrolysis is highly variable, depending on feedstock and production conditions [74] and it is unclear whether (or how much) the N in biochar is available to the plant or to microorganisms. Compared with our assumptions, assuming no available N would reduce the nutrient value of straw biochars by approximately 10% and would have little effect on the additional cost of nutrient management.

Box 1. Detailed assumptions on transport and storage costs.

The detailed assumptions for each feedstock are presented in Mortimer N et al. [49]; assumptions about transportation are also presented. Data on the gate fees paid by waste producers who dispose of their wastes has been used [68,69]. Values for short rotation coppice and short rotation forestry. in [49] assume bulk drying with electric fans; all other feedstocks are dried naturally. Values for Miscanthus chips short rotation forestry credible within a pyrolysis-biochar systems. The values for sawmill residues have been inferred from [49,48] from the value for a range of wood waste sources chipped and dried naturally. The cost of existing farm storage is calculated at £0.35 per ton per week (cost of grain storage in [70], assuming storage for an average 20-week period. For medium- and large-scale pyrolysis, it is assumed that biochar deployment will be contracted-out and storage will occur in specially constructed units at a cost of £130 m2 floor space for construction of dedicated facilities (200 x 100 x 5m, load factor 0.5, with a capital recovery factor of 0.149 over 10 y). The annualized cost is £15 per ton of biochar. We assume that the load factor can be increased to 0.8, reducing the costs of storage to £10 t-1. We also considered a lower-cost storage option, namely storing biochar at the margin of fields in appropriate flexible containment. This would eliminate the need for lengthy indoor storage periods. A nominal value of £1 t-1 was selected for this approach, although the extent to which this approach might be achievable and practicable is as yet unknown. The results reported in the paper assume that off-farm storage is practised for biochar production from medium- and large-scale units, whilst on-farm storage is the norm for production at the small-scale. For the non-virgin waste feedstocks, it is assumed that some transportation is required in the baseline case (e.g. from disposal point to landfill site, windrow or anaerobic digestor). An estimate of this transportation distance from the transport from pyroylsis unit to the farm has been deducted, which is why the non-virgin waste feedstocks incur no additional transport requirement in this phase.



charged by pyrolysis unit operations was assumed to be 10% lower than the current landfill tax or gate fee costs of the alternative waste management options, which were assumed to be landfills for wood waste, windrows for composting for green waste, and anaerobic digestion for sewage sludge and food waste. These alternative management gate fee costs are rated at the moment at GB£50 per ton of wood waste, £22 per ton of garden and green waste, and £35 and £45 per ton of food waste and sewage sludge, respectively. Wood waste gate fee revenues at pyrolysis treatment plants could be lower than the values that we present in this ana lysis if wood recycling is considered as the alternative management option, which at the moment has a gate fee of approxi-mately £18 per ton of wood. The attraction of recycling will increase as landfill taxes continue to rise, reaching approximately £80 per ton in 2015.

� Pyrolysis plant costs There is a wide variation in the specific capital costs for bioenergy systems, with smaller plants costing substantially more per unit of installed capacity than large plants, but advanced technologies not necessarily costing significantly more per unit of installed capacity than conventional technologies [50]. There are three reasons for this. First, biomass feedstocks are bulkier and require larger storage areas and handling facilities than fossil fuels. These costs commonly constitute 20% of the overall capital costs, this proportion remaining constant regardless of the technology. Second, bio-energy systems require significant engineering design input and this does not scale with capacity. Third,

many of the component parts (especially steam plant) are optimized for larger-scale utilization and so procur-ing small-sized components costs proportionally more than in the case of a larger-sized component.

Existing specific capital-cost estimates of bioen-ergy plant can be used to provide an estimate of what a slow pyrolysis unit might cost. Two such systems are described in Table 5; a 2 MWe wood gasifier and a 25 MWe wood combustion facility [Patricia Thornley;

Pers. Comm] There is a large difference in the specific capital costs of the two cases, due more to the differ-ence in scale than to the differences in technology. As with bioenergy systems in general, there are strong economic drivers towards implementation of larger-scale technological systems. Some analyses of costs and recent policy incentives (e.g., 2009 banded Renewable Obligation Certificate values in the UK) suggest that a better return on investment will derive more from larger, more-centralized units, than from smaller-scale units [50] (though this could be subject to future redesign of government incentive structures).

The capital costs selected in the study include all design, equipment, construction, civils and commis-sioning costs: they are GB£575k (US$900k) for the small-scale unit (2k odpta), £5,330k (US$8000k), for the medium-scale unit (16k odtpa) and £27,500k ($41,250k) for the large-scale unit (184.8k odtpa). The capital costs for the three-sized units are obtained from Table 5 assuming that the medium- and small-size PU is comparable with the 2MW facility and the large-size PU is comparable with the 25MW facility. We have assumed that the project lifetime is over 20 years and

Table 5. Specific capital costs of bioenergy plant that is analogous to pyrolysis plant†.

Plant analogy Wood gasification (power only) Wood combustion (power only)

Installed capacity (MWe) 2 25 Specific investment costs (in £ kWe-1) 2400 1100Feedstock rate at 30% moisture (kg s-1) 0.64 7.19

Cost (£m kg s-1 installed capacity) 7.5 3.824Major design differences for pyrolysis Pyrolysis unit in place of downdraft gasifier Pyrolysis unit in place of

combustion chamber Addition of conveyors, augers, separation equipment and storage silos for biochar (replacing existing ash discharge unit)

Addition of conveyors, augers, separation equipment and storage silos for biochar (replacing existing ash discharge unit)

Smaller rated engine for reduced gas throughput

Possible: smaller rated engine replacing boiler and steam turbine

Remarks on costs Material requirements similar; design costs higher

Material requirements similar; higher control and design costs

Direction of cost changes for pyrolysis-biochar unit There might be a small reduction in costs compared with gasification unit, although unlikely to be more than -20%

There might be a small increase in costs compared with combustion due to additional handling, design and control costs. Unlikely to exceed +20%

†Specific capital costs of 250 kW gasifier with engine using woodchips assumed GB£2300 kWh-1.



that the discount rate is 8%. These are favorable bor-rowing conditions made under the assumption that projects would be attractive to governments who might support such lending. The yearly capital recovery factor under these assumptions is 0.102.

The capital costs per ton feedstock for the small- and medium-scale units are lower than other estimates in the literature for a fast pyrolysis facility (Table 6) (by three-times for the small-scale and by 1.5-times for the medium-scale unit). However, fast pyrolysis is a more complex process than slow pyrolysis, owing to the need to extract large amounts of bioliquid of a reasonably high quality, hence its associated capital costs are expected to be greater and a direct com-parison is difficult. For the large-scale unit, there is good agreement with the capital costs per ton feed-stock of an existing 255k odtpa slow pyrolysis plant

in Japan (Hinode-cho, Tokyo) (Table 6). Expressed in terms of electricity generation, the large-scale unit costs £1100 kWh-1, which compares well with the slow pyrolysis unit in McCarl et al. [28], with a value of £1185 kWh-1 ($1896 kWh-1) for a 12.5 MWe pyrolysis facility.

� Operational costs The derivation of the operational and labor costs and electricity revenues for each PU scale are presented in Table 3 while the actual values are shown for small-, medium- and large-scale PUs in Tables 7–10. It can be seen that the operational costs vary mark-edly between the three technology scales. The operational costs for the medium-scale unit are from the

Table 6. Comparison of capital and operating costs for a range of pyrolysis plants (2007 USD).

Studies Yearly feed (odt)

Total capital costs (US$m)†

Yearly capital costs (20 yr., 8% interest) (US$m)

Capital cost per odt feedstock (US$)

Total operating costs per odt feedstock (US$)

Ref.

Hinode-cho, Tokyo 255,500 55.5 5.66 22.2 [19]

McCarl et al. 70,080 14.2 1.45 20.7 31.6 [28]

Bridgwater et al. (2002) fast pyrolysis

17.05 1.739 15.4‡ 25.0 [75]

Bridgwater (2009) small- scale fast pyrolysis

2000 2.7 0.28 140 26§ [51]

Bridgwater (2009) medium- scale fast pyrolysis

16,000 11.0 1.12 70 13.2§ [51]

Bridgwater (2009) large- scale fast pyrolysis

160,000 52 5.3 33.1 6.2§ [51]

Coaltec 12.53 10.2‡ [29]

UKBRC large-scale 184,800 41.25 4.21 22.8 5.0 UKBRC medium-scale 16,000 8.0 0.816 51 60.5 UKBRC small-scale 2000 0.9 0.092 46 54.5 Notes: US$/£ exchange rate of 1.5. †The capital costs include both the pre-treatment and pyrolysis equipment except for the case of Bridgwater [51] and Coaltec [29]. ‡Not including pre-treatment. §The operational costs are calculated in Bridgwater (2009) as 12% of the yearly capital charge. Since the authors used a capital recovery factor of 16%, we have calculated the operational costs using that value rather than our value 10.2 odt: Oven dry ton. UKBRC: UK Biochar Research Centre.

Table 7. Net costs of producing biochar at small-, medium- and large-scales, GB£/t biochar applied to field.

Straw SRC Arboricultural arisings

SRC and FRs Miscanthus Sawmill residues

SRF Canadian FRs Waste wood

Green waste and

sewage sludge

C&I veg and animal

waste

Small scale 234 289 142Medium scale

298 323 366 277 344 389 17 51 44

Large scale 135 166 216 107 188 230 -148 -114 -120C&I: Commercial and industrial; FR: Forestry residue; SRC: Short rotation coppice; SRF: Short rotation forestry.

Key term

Capital recovery factor: the ratio of a constant annuity to the present value of receiving that annuity for a given length of time.



demonstration pilot-plant. The small-scale unit capi-tal costs are largely estimated separately, although the medium-scale costs have been scaled-down to obtain the cost of natural gas to power the pyrolysis process.

Bridgwater estimates the operational costs of a fast pyrolysis plant as being 12% of the yearly capital charge (the latter being calculated as 16% of the overall capital cost) [51]. The values obtained by this method broadly agree with the values obtained by commer-cial consultants for large (30 MW) and medium-scale (2 MW) electric biomass power plants (between 10 and 12% of capital charges, assuming yearly 16% capital charge)[52].

Table 6 shows that our estimates of operational costs at small- and medium-scale are much higher than if we had used Bridgwater’s methodology. Using the figure of 12% of our estimated capital costs (and assuming 16% per year capital charge) we would obtain the following operational cost figures: small-scale US$8.3 t-1 feedstock (compare our value of US$55 t-1); and medium-scale US$9.6 t-1 feedstock (compared with US$60 t-1). The smaller difference between our estimated operational costs for small- and medium-scale units, and those of equivalent-sized fast pyrolysis units estimated by Bridgwater (ours being 2× and 4× those of Bridgwater for small- and medium-scale, respectively) is explained by the higher capital cost of the fast relative to the slow pyrolysis plant. We chose to maintain the high opera-tional costs in our default ana lysis because they were derived from a ‘real-world’ demonstration slow pyrolysis unit. However, it is clearly possible that these real-world operational costs are not truly representative of com-mon practice as it evolves and as learning-processes take effect [48]. Furthermore, different companies express their plant costs in very different ways, so these values could include a significant ‘over head’.

It was decided to use a lower value for operational costs per unit feedstock in the case of the large-scale unit, given that such a large unit is very unlikely to be built unless significant learning to lower costs has already taken place. In the absence of any objective method for calculating operational costs, the values for the medium-scale unit were carried over to the large-scale unit, giving a value of US$5 t-1 feedstock. (The same labor force should be able to operate and

Table 8. Costs of biochar produced from a small-scale pyrolysis unit, GB£/t biochar applied to field.

Straw Short rotation coppice

Arboricultural arisings

Sales of electricity -37 -37 -37Renewable obligation certificates

-74 -74 -74

Avoided gate fee 0 0 0Capital cost 87 87 87Feedstock 137 184 54Transport 10 18 0Storage 7 7 7Natural gas 11 11 11Labor 41 41 41Plant costs 49 49 49Application to field 5 5 5Net cost 234 289 142

Table 9. Costs of biochar produced from medium-scale pyrolysis, £/t biochar applied to field.

Straw SRC+FRs Miscanthus Sawmill Residues

SRF Canadian FRs

Waste Wood

Green waste & sewage

sludge

C&I veg and animal

waste

Sales of electricity -37 -37 -37 -37 -37 -37 -37 -37 -37Renewable obligation certificatess

-74 -74 -74 -74 -74 -74 -74 -74 -74

Avoided gate fee 0 0 0 0 0 0 -124 -89 -96Capital cost 101 101 101 101 101 101 101 101 101Feedstock 147 166 176 157 176 247 0 0 0Transport 22 29 62 -8 39 13 12 12 12Storage 15 15 15 15 15 15 15 15 15Natural gas 11 11 11 11 11 11 11 11 11Labor 48 48 48 48 48 48 48 48 48Plant costs 60 60 60 60 60 60 60 60 60Application to field 5 5 5 5 5 5 5 5 5Net cost 298 323 366 277 344 389 17 51 44C&I: Commercial and industrial; FR: Forestry residue; SRC: Short rotation coppice; SRF: Short rotation forestry.



manage a large-scale PU as is required to operate a medium-scale PU.) This compared well with the oper-ational costs of Bridgwater’s large-scale fast pyroly-sis plant (US$6 t-1) and with the value calculated for our large-scale plant using Bridgwater’s methodology (US$4.3 t-1). Conversely, this approach may unfairly bias the ana lysis in favor of the large-, compared with the medium-scale PUs.

� Storage & transport from pyrolysis unit to farm & application to soil Once the biomass has been pyrolysed, it needs to be taken from the pyrolysis unit to a storage unit either on the farm, or in a dedicated storage facility. If biochar is to be widely deployed, large-scale storage facilities are necessary. It could be argued that biochar can be piled-up on a field margin or other space in the open at low-cost and utilized as and when needed. However, because biochar is frequently a dusty material when dry, prone to absorb large quantities of water (e.g., a ton of biochar can hold a ton of water without increasing in

volume), and could even pose a fire risk if stored inap-propriately, it is questionable whether such a storage approach would be practicable or acceptable, especially if biochar were to become widely adopted. Therefore, some sort of containment is likely to be necessary.

Virgin biomass resources will be available intermit-tently throughout the year; for example, at harvest times through summer–autumn, although some energy crops (e.g., Miscanthus), are harvested in the spring. Some non-virgin biomass resources will be more consistently available throughout the year, although the availability of others such as green waste will be skewed towards the growing season. Opportunities for biochar application to soils will also be skewed towards certain times of the year; for example, spring and autumn, when crops are not growing and fields are suitable for coping with tractors and implements. The availability of biochar in adequate quantities at the appropriate time would there-fore probably require large storage capabilities. Such storage facilities might be already available on farm, or could be constructed on-farm for relatively small

Table 10. Costs of biochar produced from large-scale pyrolysis, UK£/t biochar applied to field.

Straw SRC+FRs Miscanthus Sawmill residues

SRF Canadian FRs

Waste Wood

Green waste & sewage sludge

C&I veg and animal waste

Sales of electricity -37 -37 -37 -37 -37 -37 -37 -37 -37ROCs -74 -74 -74 -74 -74 -74 -74 -74 -74Avoided gate fee 0 0 0 0 0 0 -124 -90 -96Capital cost 45 45 45 45 45 45 45 45 45Feedstock 147 179 195 156 190 258 0 0 0Transport 29 29 62 -8 39 13 12 12 12Storage 10 10 10 10 10 10 15 15 15Natural gas 1 1 1 1 1 1 1 1 1Labor 4 4 4 4 4 4 4 4 4Plant costs 5 5 5 5 5 5 5 5 5Application to field 5 5 5 5 5 5 5 5 5Net cost 135 166 216 107 188 230 -148 -114 -120C&I: Commercial and industrial; FR: Forestry residue; ROC: renewable obligation certificates; SRC: Short rotation coppice; SRF: Short rotation forestry.

Box 3. Cost of applying biochar to soils.

For the small-scale pyrolysis unit case where 727 tons of biochar are being applied per year (Table 2), assume a fertiliser spreader conveys 6 tons of biochar per journey and assume that it takes 1 h for loading, wetting, transport to field, application and return journey, or 121 hours. Assuming 40 hours a week, this is 3 weeks’ work. Assuming the spreader costs £78,000, and a capital recovery factor of 0.149 (8% interest rate paid over 10 years), this is a yearly cost of £11,600. The proportion of this attributed to biochar application is: 3/52 X 11,600 = £669 per year. Labor costs are assumed to be £278 per week [70] and it is assumed that biochar application is a two person job. The cost is therefore 3 x 2 x 278 = £1668. (We follow the convention in life cycle assessment work in allocating time use of equipment, hence it is assumed that the farm is extracting an economic value from the spreader and tractor for the other 49 weeks of the year.)The tractor used to pull the spreader is assumed to be a 2-wheel drive, 43–49 kW, costing £5110 to operate for 500 hours operation [70]. The tractor cost is therefore: 121/500 x 5110 = £1237. The total application costs are therefore £3574 for 727 tons, or c. £5 a ton. This value of £5 t-1 is assumed to hold for all cases, although it is likely that a contractor could reduce application costs by using purpose-built machinery and undertaking application on a much larger scale. In the absence of specific information about such equipment and practices, however, we have applied a common application to all scenarios.



quantities of biochar. Further details on storage costs are provided in Box 1 while application to soil costs are explained in Box 3.

� Electricity generation It is assumed that electricity is generated from the syngas produced during pyrolysis at c. 35% efficiency of conver-sion [53,54]. The medium-scale PU demonstration unit was used to estimate the quantity of electricity generated and we simply scaled this up and down (i.e., divided this by eight [small-scale PU] or multiplied by 11.55 [large-scale PU]). The bioliquids are not used for electricity generation; such liquids are used to generate heat for feedstock drying. While it may be feasible in the future to extract more useable energy from the bio-liquids, at the current time there is technical uncertainty and no clear dominant design.

It is also assumed that the conversion process would qualify as a form of advanced pyrolysis assuming a syngas calorific value of over 4 MJ per m3, which appears feasible given that values in the literature are approximately 11 MJ per kg from intermediate pyrolysis [55] and typically 13–16 MJ per kg for syngas [56], and hence would attract double ROCs for each MWh-1 generation under the UK Government’s current banding scheme for renewable energy incentives [57]. The value of a MWh-1 is assumed to be £50 (a reasonable market price in the 2009–10 period) [203], hence the ROC value is £100 and the over-all revenue from electricity generation is £150 MWh-1.

� The total cost of biochar delivered to the field The total cost per ton biochar delivered to field from small-, medium- and large-scale PUs is shown in Table 7 and Figure 2 for individual feedstocks. It is worthy of note that we use the simplifying assumption that the costs of feedstocks are the same regardless of the supply scenario (low, higher and high). Clearly, in reality, feedstock costs would vary depending upon (highly uncertain and unpre-dictable) supply and demand factors. The cost of produc-tion is calculated by subtracting revenue from the total costs over a 10-year period and then dividing that number by the quantity of biochar produced over that period. In other words, any loss is distributed across the quantity of biochar produced, giving a break-even selling point value. Therefore, the numbers in Table 7 represent the minimum price that would have to be charged per ton of biochar generated in order for the operation to be at least break-even. With the exception of the upfront capital costs, we decided not to use a discount factor given consider-able uncertainty over the value of the future operational costs and plant revenues. Where a profit is made by the operation, we have indicated a negative production cost. It may be preferable, in future, to ascribe a cost to biochar based upon the relative value of all the by-products and a reasonable rate of return on investment.

Depending on the assumptions used, biochar may ‘cost’ between -£148 t-1 and £389 t-1 (-$222–584) deliv-ered and spread on fields. There are potentially attractive opportunities for producing biochar in terms of cheaper

Small scaleMedium scaleLarge scale

GB£/t biochar applied to field

Straw

SRC and FRs

Miscanthus

Sawmill residues

SRF

Canadian FRs

Waste wood

Green waste andsewage sludge

Commercial and industralvegetable and animal waste

SRC


5004003002001000-100-200

Figure 2. Net cost of biochars produced with small-, medium- and large-scale pyrolysis.FR: Forestry residue; SRC: Short rotation coppice; SRF: Short rotation forestry.



virgin feedstocks, such as arboricultural arisings and some waste woods. Under scenarios where wastes can be used as non-virgin feedstocks, biochar can be pro-duced as a product at a profit. By using these, instead

of more expensive wood or straw feedstocks, it is pos-sible to reduce the costs per ton of biochar produced in a medium-scale unit by 60–90% (Table 9). Owing to much lower operating costs assumed for the large-scale


Sales of electricty

Avoided gate feeCapital costFeedstockTransportStorageNatural gasLaborPlant costsApplication to field

ROC

-300 -200 -100 0 100 200 300 400 500 600

Straw

SRC and FRs

Miscanthus

Sawmill residues

SRF

Canadian FRs

Waste wood

Green waste and sewage sludge

Commercial and industral vegetable and animal waste

Figure 4. Breakdown of costs of biochar production at medium scale.FR: Forestry residue; ROC: Renewable obligation certificates; SRC: Short rotation coppice; SRF: Short rotation forestry.


Short rotation coppice

Sales of electricty


ROC

-300 -200 -100 0 100 200 300 400 500 600


Straw

Figure 3. Breakdown of costs of biochar production at small scale. ROC: Renewable obligation certificates.



unit, biochar production costs are quite scale-depen-dent with a big premium on producing at larger-scale. It needs to be re-iterated that our assumed operational costs at larger-scale remain speculative and optimistic. Clearly, if operational costs at the smaller-scales could be similarly reduced, then the biochar BESP could be similarly reduced, although the opportunities for such cost reduction at smaller-scales is less obvious.

Some lower-cost straw options were explored, since there can be a wide temporal and spatial variation in the price of straw feedstock. For example, whilst wheat and barley straw prices have been £30–60 t-1 during much of 2008 and 2009, in 2007 the price of wheat straw was more typically in the range £20–30 t-1, and £30–50 t-1 for barley straw [204]. At a price of £10 t-1 the costs of produc-ing biochar from straw comes down significantly – espe-cially at the large-scale production unit where the cost reduction is 75% (based on the data in Table 10).

Analysis of the costs & sensitivity ana lysis The breakdown of the costs is shown in Tables 8–10 and Figures 3–5 for small-, medium- and large-scale units, respectively. The greatest costs are those for feedstock, borrowing capital and operation. Small-scale biochar pro-duction benefits from lower transport cost, large-scale pro-duction from much lower capital and operational costs. Avoided gate fees provide an important revenue stream when the non-virgin feedstocks are utilized (although

gate-fees for some waste woods are likely to come down as competition from other uses and from recycling grows and could become a feedstock charge). Transport costs for non-virgin feedstocks are also low because PBS intro-duces few additional transport requirements beyond those already accounted for in waste management.

The ability to raise additional revenue through avoided gate fees and the low transport costs all help to explain why the use of non-virgin biomass waste resources pro-vides a much more favorable economic outlook for a PBS. Conversely, pyrolysis of such materials will probably pose greater risks and more difficulty in addressing regulatory questions and issues. Use of such materials might also encounter skepticism and resistance from some farm-ers and land-owners. Another attractive option is to use virgin feedstocks that are also relatively low cost, such as arboricultural arisings and sawmill residues. At prices of less than GB£20 t-1, straw also looks to be an attractive option, especially if storage costs can be minimized.

The production cost of biochar is highly sensitive to capital and operational costs (CAPEX and OPEX). A sensitivity ana lysis showed that the relationship between biochar production cost and medium to high CAPEX and OPEX is linear. Biochar production costs are not so sensitive to CAPEX and OPEX changes at lower levels. A halving of the value of OPEX / CAPEX assumed in our ana lysis produces only a c. 15–20% reduction in biochar production costs.

-300 -200 -100 0 100 200 300 400 500 600


Sales of electricty


ROC

Straw

SRC and FRs

Miscanthus

Sawmill residues

SRF

Canadian FRs

Waste wood

Green waste and sewage sludge

Commercial and industral vegetable and animal waste

Figure 5. Breakdown of costs of biochar production at large scale.FR: Forestry residue; ROC: Renewable obligation certificates; SRC: Short rotation coppice; SRF: Short rotation forestry.



Since the main economic value of biochar in the UK is likely to be carbon storage, it is useful to con-vert the production costs into a marginal carbon abate-ment cost curve (MACC). This is done assuming the following carbon contents of different types of fresh biochar: wood-based virgin feedstocks: 75% [58]; straw virgin feedstocks: 60% [59,201]; wood waste: 72% [60]; greenwaste and sewage sludge: 44% [61] and com-mercial and industrial animal and vegetable waste: 55% [201]. It is also assumed that 68% of the carbon in the fresh biochar remains stabilized in the long-term (after 100 years, a timescale relevant for climate change policy) [26]. This is a simplification since the long-term stability of recalcitrant carbon will vary depending upon the feedstock, the pyrolytic pro-duction conditions and the receiving soil. However, at present, there is no reliable method for calculat-ing the long-term stability, hence a common value is used for each feedstock here. The proportion of long-term stable carbon content within biochar (by mass) therefore varies between 0.3 (greenwaste) to 0.5 (woody feedstocks).

For each feedstock, the cost of abatement is then plotted against the quantity of carbon abatement, which is feasible under a given resource scenario. Figure 6 is an example of one such MACC for bio-char in the UK, in this instance for the higher feedstock scenario utiliz-ing small-, medium- and large-scale PU technology. Included in the net carbon equivalent abatement value is the recalcitrant carbon in the biochar, plus the offset carbon emissions arising from bioelectricity generation (compared with the UK electricity grid average), as detailed in [41]. The range of biochar production costs from different feedstocks versus CO

2 abatement amounts is shown

in Figure 7.We calculate the net carbon equivalent abatement

excluding the potential indirect impacts of biochar in soil for reasons already provided. The MACC shows that there are some attractive economic opportunities for approximately 6 million tons of CO

2e abatement

φ 57

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-144

-86 -86

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144 155156

163

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Figure 6. Biochar marginal abatement cost (GB£tCO2e-1) for higher feedstock supply scenario. Values do not include indirect effects of biochars in soils on net CO2 equivalent abatement.L: Large scale; M: Medium scale; S: Small scale.

Key terms

Marginal carbon abatement cost: an assessment of the level of emissions reduction that (a range of) measure(s) could deliver at a given point in time, against a projected baseline level of emissions. They show how much CO2 each measure could save (the level of abatement potential) and the associated cost per ton of CO2.



from biochar beyond which the abatement cost rises quite steeply and biochar becomes a relatively expensive abatement option if more than 8 million tons of CO

2e

abatement is attempted. The carbon abatement cost decreases if the stability of

the carbon in the biochar is higher, and/or if the carbon content of the biochar is higher. For woody feedstocks, the stability was increased to 95% and the carbon content

to 85% (giving a long-term stable carbon fraction of 0.8), and this reduced the abatement cost by 37% (relative to the values in Figure 6). For non-woody feedstocks, stability appears to be lower hence was kept at 68% and carbon content increased by 5%. These changes reduced the car-bon abatement cost by 8% (straws) to 15% (some wastes). If the indirect impacts of biochar are included as repre-sented in [41, Ibarrola R, Shackley S, Hammond J; sumitted data],

Figure 7. Proctution cost curve for biochar from different feedstocks (cost in GB£ per ton versus quantity of CO2 abated). L: Large scale; M: Medium scale; S: Small scale.

400

350

300

250

200

150

100

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216 234 277

289 298344

389

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Table 11. Preliminary and provisional estimate of annual biochar production and carbon abatement in the UK using three scenarios for virgin and non-virgin biomass feedstock and the resulting land-use implications.

Feedstock availability

Abatement per annum (Mt CO2 eq.)

Biochar produced per annum (thousands of tons)

Applied land area (thousands of hectares)

CO2 C Virgin biomass resources

Non-virgin biomass

resources

Total available for use on land

Scenario 30/1: 30 t ha-1 first

year, then 1 t ha-1

(20-y horizon)

Scenario 10/1: 10 t ha-1 first year,

then 1 t ha-1 (20-y horizon)

Lower resource 3.590 0.98 1019 0 1019 416 703Higher resource 15.915 4.34 2547 1934 3514 1434 2423 High resource 21.867 5.96 3267 2902 4718 1926 3254



then the net carbon equivalent abatement of biochar increases substantially (for many feedstocks it is doubled compared with the baseline used here). This, in turn, can halve the carbon abatement cost for virgin feedstocks; however, too much credence should not be given to these lower abatement costs given scientific uncertainty regard-ing the indirect effects of biochar. For this reason, we present Figure 6 as no more than a provisional marginal abatement cost curve for a single scenario from a wide range of potential scenarios. Defining the credible range in the underlying data (e.g., on biochar carbon stabil-ity and carbon content and on the indirect impacts of biochar) emerges as a key research requirement.

Table 11 provides data on the total carbon abatement in the UK from PBS deployment for each scenario (including indirect effects and offset fossil fuel emissions as specified in [26], (from 1 million tons C [equivalent] per year in the low supply scenario to 6 million tons C per year in the very high scenario, this representing 0.3 – 3% of 1990 UK GHG emissions or 1.5 – 10% of the emission reductions required by 2020). Table 11 also indicates the area of land that might be implicated in biochar deployment under each scenario. The key mes-sage is that available or suitable land is much less likely to be a constraint than availability of suitable feedstocks.

Implications & conclusions At present, estimating biochar production costs is fraught with uncertainties. It is nonetheless important that attempts to provide such estimates are undertaken since costs are a crucial indicator in directing future investment in RD&D. What is clear is that greater cer-tainty on biochar production costs will not be forthcom-ing without much better data being available on the costs of constructing and operating slow pyrolysis facilities, as well as on their operational performance. The overriding conclusion from this ana lysis is that there is a very wide variation in biochar production costs. Where gate fee revenues are available, as in the case of waste streams, then producing biochar can be profitable. Where virgin feedstocks are utilized, production costs can be very high (hundreds of pounds per ton), translating into a high per ton CO

2 abatement costs (>£100 tCO

2–1).

Given that the current carbon price on the EU Emissions Trading Scheme is 15 tCO

2–1, it can be seen

that only waste feedstocks are feasible contenders for car-bon abatement purposes at present. Conversely, many other renewable energy options such as offshore wind, wave and tidal power, are also similarly expensive and first generation CO

2 capture and storage is anticipated to

cost approximately 80 tCO2

–1 [48,62]. Since many risk and regulatory issues will arise from producing biochar from waste streams and applying to agricultural land, it is vital that a concerted effort to understand better the potential

hazards and how they may be ameliorated through feed-stock quality control and process engineering is progressed with urgency. Furthermore, the MACC ana lysis does not account for the potential agronomic value of biochar. One implication of the ana lysis is that efforts to maximize the agronomic value of biochar are of great importance. The ideal combination would, potentially, be a waste stream-derived biochar incorporated into soils used for cultivating a high-value agricultural crop where the benefits of the biochar are well demonstrated and reproducible.

All new technologies face the challenge of high costs in the early stage of their development, and pyrolysis is no different. The route to market for new technologies has frequently been through identification and exploita-tion of a favorable ‘niche’, in which a new technology is (at least somewhat) protected from the harsh forces of the unbridled market [63]. Governments frequently have a role to play along with the private sector in identify-ing such niches and helping to nurture innovation, a good example being the Dutch Government’s Energy Transitions Directorate [64]. Examples of possible niche development of biochar are use of arboricultural, green waste and wood waste arising from urban centers for biochar production. More detailed techno-economic evaluation of such options is an important next-step.

The model developed here needs to be further devel-oped such that a fair comparison with other uses of the same biomass can be undertaken. For instance, if the price of electricity goes up, the operator would be incentivized to produce less char and more electricity. The model we have developed does not allow us to examine this par-ticular question since it does not include a comparison of PBS with, say, biomass combustion and gasification.

Future perspective At the present time, biochar does not have any recognized value for carbon storage, soils, agriculture or anything else. It is likely, in many cases, to be illegal to spread it upon land [65] and at present we do not known how local communities might respond to the prospects of biochar projects, although previous advanced biomass technol-ogy (gasification) projects have been abandoned, in part due to local opposition [66]. Some environmental groups have already expressed opposition [67], although the larger environmental NGOs have not yet published a position. There is, currently, no mechanism for ascribing a financial value to the recalcitrant carbon within the biochar and nor is there an obvious route by which a value could be given. The best prospect is likely to be with the estab-lishment of a methodology for biochar that meets the requirements of the Verified Carbon Market. Another possibility is for inclusion of biochar within environmen-tal stewardship schemes under the Common Agricultural Policy of the EU.



The agronomic value of biochar is very poorly under-stood and the ability to predict its impact is very low. Before farmers are likely to take-up the use of biochar, it is probably necessary for the positive (and any negative) effects of biochar addition to be properly understood and more reproducible and predictable. Competition for biomass resources is, meanwhile, intensifying as incen-tive schemes are developed for power, heat and chemical feedstocks from biomass in Western Europe, the USA and elsewhere. The competition is driving up the scarcity and prices of feedstocks and there is a very real possibil-ity that many large-scale sources of biomass will be tied into reasonably long-term contracts with large energy utilities in the next 5–10 years. PBS face a problem in this competition, in that they incur a large energy pen-alty, the deliverable electrical energy being under 50% of that from a comparable combustion unit. Current policy incentives are far more focused upon electricity produc-tion than upon carbon abatement, so the greater carbon abatement efficiency of PBS does not necessarily win out.

All these factors make commercialization of large-scale biochar production and deployment seems unlikely in the short-term future, at least in the UK. What might seem more likely, however, is the use of residues and wastes from other biotechnological con-version processes as feedstock for pyrolysis, primarily

driven by the economics of waste disposal. This might be from second generation fermentation, anaerobic and aerobic digestion, hydrolysis, hydrothermal con-version, biosolids production and Fischer-Tropsch conversion among other things. Biochar may become an integral part of the new wave of bio-refinery tech-nologies that are currently in development. This may be the way that biochar establishes a niche for itself, which might then ‘grow-out’ to encompass other more main-stream applications.

Acknowledgements The feedstock scenario evaluation was jointly undertaken by the current authors with inputs from Patricia Thornley and Saran Sohi. Biochar storage and application costs were calculated with Jason Cook’s input. We would like to thank: Patricia Thornley and Ondřej Mašek for information on the costs of existing bioenergy and pyrolysis plant in the UK and Japan; Saran Sohi for sharing his evaluation of the literature on biochar impacts; Sohel Ahmed for help with graphics; and Peter Brownsort and Rebecca Rowe for their comments on the manuscript. We thank the following for funding the research: UK Engineering and Physical Sciences Research Council (EPSRC) Science & Innovation Award; and the European Regional Development Fund IVb pro-gramme (North Sea Region): ‘Biochar: climate saving soils’ project. Finally, we thank the Editor and the five reviewers, whose comments helped improve the quality of the manuscript.

Executive summary

Scenarios for available feedstock, biochar supply & technology scale � Three scenarios for the UK context were developed to estimate the potential biomass resource available for the production of biochar: a

lower, higher and high scenario are presented. A distinction between virgin (no chemical or biological amendment) and non-virgin (all other) bio-feedstocks is introduced; this is important with respect to regulatory and risk assessment issues for biochar.

� The scenarios suggest that there are between 3 and 12 million tons of virgin biomass resource and between 0 to 9 million tons of non-virgin biomass resource (excluding poultry waste and mechanically-biologically treated municipal solid waste) available in the UK for producing biochar.

Review of previous cost–benefit analysis studies � A high degree of uncertainty surrounds the indirect impacts of biochar in soils (effects on productivity, water retention, pollution

reduction, etc.), which precludes precise valuation of costs and benefits. � A more modest approach is to attempt to calculate the biochar production cost, taking account of the full value-chain from feedstock

cultivation to biochar application to soils including capital and operational costs, transport, storage and feedstock preparation costs; including revenues from electricity generation and waste management. This does not bypass uncertainty, but limits it to some extent.

Costs of pyrolysis-biochar process stages � Three indicative sizes of pyrolysis technology were modelled: small (<2000 t feedstock yr-1), medium (<16,000 t yr-1) and large

(<185,000 t yr-1). The costs were provided for a medium-sized demonstration plant, and estimated for the small- and large-scale unit by comparison with the demonstration unit as well as existing plants. Economies of scale is an important factor in reducing capital and operational costs of production in larger units.

� The costs of producing biochar in the UK context range from between £-148 per ton to £389 per ton delivered and spread on fields – a provisional carbon abatement cost of £-144 tCO2

-1 to £208 tCO2-1 for the higher resource scenario. A marginal carbon abatement cost can be

estimated by plotting biochar production levels from the three production units against costs, although the latter are static with respect to feedstock supply.

� The greatest expense incurred in pyrolysis-biochar systems are the capital costs, feedstock costs and operational costs, while the largest sources of revenue are from electricity generation and from received gate fee for wastes.

� Biochar from imported wood chips, Miscanthus and short rotation forestry are among the most expensive types, while straw-based biochar is close behind; wood waste and greenwaste-derived biochar are much cheaper (with a carbon abatement cost from (£-144 tCO2

-1 to £19 tCO2-1).

� The attractiveness of wastes as a feedstock requires concerted effort on the risk assessment and appropriate regulation of the resultant biochar; it also assumes continued gate fees and landfill tax at current levels.



Financial & competing interests disclosureJohn Gaunt has a financial involvement in a com-pany which is attempting to develop the commercial market for biochar. The authors have no other rel-evant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materi-als discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.

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