Biomass is a modern name for the ancient technology of burning plant or animal material for energy production (electricity or heat), or in various industrial processes as raw substance for a range of products.[1] It can be purposely grown energy crops (e.g. miscanthus, switchgrass), wood or forest residues, waste from food crops (wheat straw, bagasse), horticulture (yard waste), food processing (corn cobs), animal farming (manure, rich in nitrogen and phosphorus), or human waste from sewage plants.[2]

The argument that biomass combustion is carbon neutral is contested.[3][4] The traditional use of wood in cook stoves and open fires produces pollutants, which can lead to severe health and environmental consequences. The EU and UN consider biomass a renewable energy source.

Biomass feedstocksEdit

Biomass plant in Scotland.
Wood waste outside biomass power plant.
Bagasse is the remaining waste after sugar canes have been crushed to extract their juice.
Miscanthus x giganteus energy crop, Germany.

Historically, humans have harnessed biomass-derived energy since the time when people began burning wood fuel.[5] Even in 2019, biomass is the only source of fuel for domestic use in many developing countries. All biomass is biologically-produced matter based in carbon, hydrogen and oxygen. The estimated biomass production in the world is approximately 100 billion metric tons of carbon per year, about half in the ocean and half on land.[6]

Wood and residues from wood, for instance spruce, birch, eucalyptus, willow, oil palm, remains the largest biomass energy source today.[5] It is used directly as a fuel or processed into pellet fuel or other forms of fuels. Biomass also includes plant or animal matter that can be converted into fuel, fibers or industrial chemicals. There are numerous types of plants, including corn, switchgrass, miscanthus, hemp, sorghum, sugarcane, and bamboo.[7] The main waste energy feedstocks are wood waste, agricultural waste, municipal solid waste, manufacturing waste, and landfill gas. Sewage sludge is another source of biomass. There is ongoing research involving algae or algae-derived biomass.[8] Other biomass feedstocks are enzymes or bacteria from various sources, grown in cell cultures or hydroponics.[9][10]

Based on the source of biomass, biofuels are classified broadly into three major categories:[11]

First-generation biofuels are derived from food sources, such as sugarcane and corn starch. Sugars present in this biomass are fermented to produce bioethanol, an alcohol fuel which serve as an additive to gasoline, or in a fuel cell to produce electricity.[12]

Second-generation biofuels utilize non-food-based biomass sources such as perennial energy crops (low input crops), and agricultural/municipal waste. Proponents argue that there is huge potential for second generation biofuels, but the resources are currently under-utilized.[13] Third-generation biofuels refer to those derived from microalgae.

Biomass conversionEdit

Cofiring with biomass has increased in coal power plants, because it makes it possible to release less CO2 without the cost associated with building new infrastructure.[citation needed] Co-firing is not without issues however, often an upgrade of the biomass is most beneficial.[citation needed] Upgrading to higher grade fuels can be achieved by different methods, broadly classified as thermal, chemical, or biochemical.[citation needed]

Thermal conversionsEdit

Straw bales

Thermal conversion processes use heat as the dominant mechanism to upgrade biomass into a better and more practical fuel. The basic alternatives are torrefaction, pyrolysis, and gasification, these are separated principally by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature).[14]

There are other less common, more experimental or proprietary thermal processes that may offer benefits, such as hydrothermal upgrading.[15] Some have been developed for use on high moisture content biomass, including aqueous slurries, and allow them to be converted into more convenient forms.

Chemical conversionEdit

A range of chemical processes may be used to convert biomass into other forms, such as to produce a fuel that is more practical to store, transport and use, or to exploit some property of the process itself. Many of these processes are based in large part on similar coal-based processes, such as the Fischer-Tropsch synthesis.[16] Biomass can be converted into multiple commodity chemicals.[17]

Biochemical conversionEdit

As biomass is a natural material, many highly efficient biochemical processes have developed in nature to break down the molecules of which biomass is composed, and many of these biochemical conversion processes can be harnessed. In most cases, microorganisms are used to perform the conversion process: anaerobic digestion, fermentation, and composting.[18]

Glycoside hydrolases are the enzymes involved in the degradation of the major fraction of biomass, such as polysaccharides present in starch and lignocellulose. Thermostable variants are gaining increasing roles as catalysts in biorefining applications, since recalcitrant biomass often needs thermal treatment for more efficient degradation.[19]

Electrochemical conversionsEdit

Biomass can be directly converted to electrical energy via electrochemical (electrocatalytic) oxidation of the material. This can be performed directly in a direct carbon fuel cell,[20] direct liquid fuel cells such as direct ethanol fuel cell, a direct methanol fuel cell, a direct formic acid fuel cell, a L-ascorbic Acid Fuel Cell (vitamin C fuel cell),[21] and a microbial fuel cell.[22] The fuel can also be consumed indirectly via a fuel cell system containing a reformer which converts the biomass into a mixture of CO and H2 before it is consumed in the fuel cell.[23]

Carbon neutralityEdit

Above-ground biomass in forest per hectare[24]

The argument that biomass combustion is carbon neutral is contested.[3][4] Agricultural production releases CO2 and on combustion, a plant's absorbed CO2 is released into the atmosphere.[25] The “carbon payback period” – the time it takes for regrowth of the forest to reabsorb the emissions from biomass – is a key measure, since increasing the amount of carbon in the atmosphere, even for a few decades, might be counterproductive.[26]

CO2 emissionsEdit

Biomass burning is carbon based and therefore produces air pollution in the form of carbon dioxide, carbon monoxide, volatile organic compounds, particulates and other pollutants.[27][28][29] In 2009 a Swedish study of the giant brown haze that periodically covers large areas in South Asia determined that two thirds of it had been principally produced by residential cooking and agricultural burning, and one third by fossil-fuel burning.[30] The use of wood biomass as an industrial fuel has been shown to produce fewer particulates and other pollutants than the burning seen in wildfires or open field fires.[31]

When combusted, oven dry wood emits slightly less CO2 per unit of energy produced, compared to oven dry coal.[32] However, since raw biomass can have a relatively high moisture content compared to coal, the amount of CO2 emitted per unit of energy can be higher, due to the fact that some of the inherent energy must be spent solely on evaporating the moisture. Some critical research groups (e.g. Chatham House, EASAC) therefore argue that "[...] woody biomass for energy will release higher levels of emissions than coal and considerably higher levels than gas".[26][33] Other research groups (e.g. IEA Bioenergy) counter with the argument that this higher amount of CO2 is irrelevant since it will be absorbed back by new plant growth (here assuming sustainable forestry practices). They criticize the bioenergy sceptics for blurring the distinction between fossil and biogenic carbon, and state that "it is incorrect to determine the climate change effect of using biomass for energy by comparing GHG emissions at the point of combustion."[34]

CO2 absorptionEdit

It is the total amount of CO2 emissions and absorption together that determines if the GHG life cycle cost of a biofuel project is positive, neutral or negative. If emissions during agriculture, processing, transport and combustion are higher than what is absorbed, both above and below ground during crop growth, the GHG life cycle cost is positive. Likewise, if total absorption over time is higher than total emissions, the life cycle cost is negative.

Many first generation biomass projects have a positive GHG life cycle cost, especially if emissions caused by direct or indirect land use change are included in the GHG cost calculation. Some have even higher total GHG emissions than some fossil based alternatives.[a][b][c] Transport fuels might be worse than solid fuels in this regard.[d]

During plant growth, ranging from a few months to a decades, CO2 is re-absorbed by new plants.[25] Depending on the size of the newly grown biomass, the previously combusted CO2 is partially or fully absorbed.[citation needed] If non-sustainable forestry techniques are employed, previously combusted CO2 is only partially re-absorbed.[35][36][37][38]

In addition to absorbing CO2 and storing it as carbon in its above-ground tissue, biomass crops also sequester carbon below ground, in roots and soil.[citation needed] Typically, perennial crops sequester more carbon than annual crops because the root buildup is allowed to continue undisturbed over many years. Also, perennial crops avoid the yearly tillage procedures (plowing, digging) associated with growing annual crops. Tilling induces soil aeration, which accelerates the soil carbon decomposition rate, by stimulating soil microbe populations, releasing carbon into the atmosphere.[e]

Soil organic carbon has been observed to be greater below switchgrass crops than under cultivated cropland, especially at depths below 30 cm (12 in).[39] McCalmont et al. compared a number of individual European reports on Miscanthus x giganteus carbon sequestration, and found accumulation rates ranging from 0.42 to 3.8 tonnes per hectare per year,[f] with a mean accumulation rate of 1.84 tonne (0.74 tonnes per acre per year),[g] or 25% of total harvested carbon per year.[h] A large meta-study of 138 individual studies, done by Harris et al., revealed that second generation perennial grasses (miscanthus and switchgrass) planted on arable land on average store five times more carbon in the ground than short rotation coppice or short rotation forestry plantations (poplar and willow).[i] Compared to fossil fuels, the greenhouse gas (GHG) savings are large—even without considering the GHG effect of carbon sequestration, miscanthus fuel has a GHG cost of 0.4–1.6 grams CO2-equivalents per megajoule, compared to 33 grams for coal, 22 for liquefied natural gas, 16 for North Sea gas, and 4 for wood chips imported to Britain from the USA.[j]

Carbon negative (miscanthus) and carbon positive (poplar) production pathways.

Since emissions originating from combustion can be absorbed by next season's plant growth, a carbon negative life cycle is possible if the below-ground carbon accumulation more than compensates for the non-combustion-related emissions (mainly emissions originating from agriculture, processing and transport). For instance, Whitaker et al. argue that a miscanthus crop with a yield of 10 tonnes per hectare per year sequesters so much carbon that the crop more than compensates for both farm operations emissions and transport emissions. The chart on the right displays two CO2 negative miscanthus production pathways, and two CO2 positive poplar production pathways, represented in gram CO2-equivalents per megajoule. The bars are sequential and move up and down as atmospheric CO2 is estimated to increase and decrease. The grey/blue bars represent agriculture, processing and transport related emissions, the green bars represents soil carbon change, and the yellow diamonds represent total final emissions.[k]

Relationship between above-ground yield (diagonal lines), soil organic carbon (X axis), and soil's potential for successful/unsuccessful carbon sequestration (Y axis). Basically, the higher the yield, the more land is usable as a GHG mitigation tool (including relatively carbon-rich land).

Successful sequestration is dependent on planting sites, as the best soils for sequestration are those that are currently low in carbon. The varied results displayed in the graph highlights this fact.[l] For the UK, successful sequestration is expected for arable land over most of England and Wales, with unsuccessful sequestration expected in parts of Scotland, due to already carbon rich soils (existing woodland) plus lower yields. Soils already rich in carbon includes peatland and mature forest. Milner et al. further argue that the most successful carbon sequestration in the UK takes place below improved grassland.[m] However, Harris et al. notes that since the carbon content of grasslands vary considerably, so does the success rate of land use changes from grasslands to perennial.[n] The bottom graphic displays the estimated yield necessary to achieve CO2 negativity for different levels of existing soil carbon saturation. The higher the yield, the more likely CO2 negativity becomes.

Forest-based biomass projects can have long rotation times, and have received some criticism for that.[40][41] Forest-based biomass projects have also received criticism for ineffective GHG mitigation from a number of environmental organizations, including Greenpeace.[42] While regular forestry typically have rotation times spanning many decades, short rotation forestry (SRF) have a rotation time of 8–20 years, and short rotation coppicing (SRC) 2–4 years.[43] Perennial grasses like miscanthus or napier grass have a rotation time of 4–12 months.

Economics of biomassEdit

The economic viability of biomass is dependent on government mandates and subsidies, due to high costs of infrastructure and ingredients for ongoing operations relative to other sources of energy.[44]

Biomass offers a ready disposal mechanism by burning municipal, agricultural, and industrial organic waste products. As part of the Food vs. fuel debate, several economists from Iowa State University found in 2008 "there is no evidence to disprove that the primary objective of biofuel policy is to support farm income."[45]

Power production compared to other renewablesEdit

To calculate land use requirements for different kinds of power production, it is essential to know the relevant area-specific power densities. Smil estimates that the average area-specific power densities for biofuels, wind, hydro and solar power production are 0.30 W/m2, 1 W/m2, 3 W/m2 and 5 W/m2, respectively (power in the form of heat for biofuels, and electricity for wind, hydro and solar).[46] The average human power consumption on ice-free land is 0.125 W/m2 (heat and electricity combined),[47] although rising to 20 W/m2 in urban and industrial areas.[48] The reason for the low area-specific power density for biofuels is a combination of low yields and only partial utilization of the plant when making liquid fuels (for instance, ethanol is typically made from sugarcane's sugar content or corn's starch content, while biodiesel is often made from rapeseed and soybean's oil content).

Smil estimates the following densities for biofuels:


Jet fuel


Combusting solid biomass is more energy efficient than combusting liquids, as the whole plant is utilized. For instance, corn plantations producing solid biomass for combustion generate more than double the amount of power per square metre compared to corn plantations producing for ethanol, when the yield is the same: 10 t/ha generates 0.60 W/m2 and 0.26 W/m2 respectively.[55]

Oven dry biomass in general, including wood, miscanthus[56] and napier[57] grass, have a calorific content of roughly 18 GJ/t.[58] When calculating power production per square metre, every t/ha of dry biomass yield increases a plantation's power production by 0.06 W/m2.[o] Consequently, Smil estimates the following:

In Brazil, the average yield for eucalyptus is 21 t/ha (1.26 W/m2), but in Africa, India and Southeast Asia, typical eucalyptus yields are below 10 t/ha (0.6 W/m2).[60]

FAO (Food and Agriculture Organization of the United Nations) estimate that forest plantation yields range from 1 to 25 m3 per hectare per year globally, equivalent to 0.02–0.7 W/m2 (0.4–12.2 t/ha):[p]

  • Pine (Russia) 0.02–0.1 W/m2 (0.4–2 t/ha or 1–5 m3)[p]
  • Eucalyptus (Argentina, Brazil, Chile and Uruguay) 0.5–0.7 W/m2 (7.8–12.2 t/ha or 25 m3)[p]
  • Poplar (France, Italy) 0.2–0.5 W/m2 (2.7–8.4 t/ha or 25 m3)[p]

Smil estimate that natural temperate mixed forests yield on average 1.5–2 dry tonnes per hectare (2–2,5 m3, equivalent to 0.1 W/m2), ranging from 0.9 m3 in Greece to 6 m3 in France).[61]

As mentioned above, Smil estimates that the world average for wind, hydro and solar power production is 1 W/m2, 3 W/m2 and 5 W/m2 respectively. In order to match these power densities, plantation yields must reach 17 t/ha, 50 t/ha and 83 t/ha for wind, hydro and solar respectively. This seems achievable for the tropical plantations mentioned above (yield 20–25 t/ha) and for elephant grasses, e.g. miscanthus (10–40 t/ha), and napier (15–80 t/ha), but unlikely for forest and many other types of biomass crops. To match the world average for biofuels (0.3 W/m2), plantations need to produce 5 tonnes of dry mass per hectare per year.

Yields need to be adjusted to compensate for the amount of moisture in the biomass (evaporating moisture in order to reach the ignition point is usually wasted energy). The moisture of biomass straw or bales varies with the surrounding air humidity and eventual pre-drying measures, while pellets have a standardized (ISO-defined) moisture content of below 10% (wood pellets)[q] and below 15% (other pellets).[r] Likewise, for wind, hydro and solar, power line transmission losses amounts to roughly 8% globally and should be accounted for.[s] If biomass is to be utilized for electricity production rather than heat production, note that yields has to be roughly tripled in order to compete with wind, hydro and solar, as the current heat to electricity conversion efficiency is only 30–40%.[62] When simply comparing area-specific power density without regard for cost, this low heat to electricity conversion efficiency effectively pushes at least solar parks out of reach of even the highest yielding biomass plantations, power density wise.[t]

See alsoEdit


  1. ^ "The environmental costs and benefits of bioenergy have been the subject of significant debate, particularly for first‐generation biofuels produced from food (e.g. grain and oil seed). Studies have reported life‐cycle GHG savings ranging from an 86% reduction to a 93% increase in GHG emissions compared with fossil fuels (Searchinger et al., 2008; Davis et al., 2009; Liska et al., 2009; Whitaker et al., 2010). In addition, concerns have been raised that N2O emissions from biofuel feedstock cultivation could have been underestimated (Crutzen et al., 2008; Smith & Searchinger, 2012) and that expansion of feedstock cultivation on agricultural land might displace food production onto land with high carbon stocks or high conservation value (i.e. iLUC) creating a carbon debt which could take decades to repay (Fargione et al., 2008). Other studies have shown that direct nitrogen‐related emissions from annual crop feedstocks can be mitigated through optimized management practices (Davis et al., 2013) or that payback times are less significant than proposed (Mello et al., 2014). However, there are still significant concerns over the impacts of iLUC, despite policy developments aimed at reducing the risk of iLUC occurring (Ahlgren & Di Lucia, 2014; Del Grosso et al., 2014)." Whitaker et al. 2018, p. 151.
  2. ^ "The impact of growing bioenergy and biofuel feedstock crops has been of particular concern, with some suggesting the greenhouse gas (GHG) balance of food crops used for ethanol and biodiesel may be no better or worse than fossil fuels (Fargione et al., 2008; Searchinger et al., 2008). This is controversial, as the allocation of GHG emissions to the management and the use of coproducts can have a large effect on the total carbon footprint of resulting bioenergy products (Whitaker et al., 2010; Davis et al., 2013). The potential consequences of land use change (LUC) to bioenergy on GHG balance through food crop displacement or 'indirect' land use change (iLUC) are also an important consideration (Searchinger et al., 2008)." Milner et al. 2016, pp. 317–318.
  3. ^ "While the initial premise regarding bioenergy was that carbon recently captured from the atmosphere into plants would deliver an immediate reduction in GHG emission from fossil fuel use, the reality proved less straightforward. Studies suggested that GHG emission from energy crop production and land-use change might outweigh any CO2 mitigation (Searchinger et al., 2008; Lange, 2011). Nitrous oxide (N2O) production, with its powerful global warming potential (GWP), could be a significant factor in offsetting CO2 gains (Crutzen et al., 2008) as well as possible acidification and eutrophication of the surrounding environment (Kim & Dale, 2005). However, not all biomass feedstocks are equal, and most studies critical of bioenergy production are concerned with biofuels produced from annual food crops at high fertilizer cost, sometimes using land cleared from natural ecosystems or in direct competition with food production (Naik et al., 2010). Dedicated perennial energy crops, produced on existing, lower grade, agricultural land, offer a sustainable alternative with significant savings in greenhouse gas emissions and soil carbon sequestration when produced with appropriate management (Crutzen et al., 2008; Hastings et al., 2008, 2012; Cherubini et al., 2009; Dondini et al., 2009a; Don et al., 2012; Zatta et al., 2014; Richter et al., 2015)." McCalmont et al. 2017, p. 490.
  4. ^ "Significant reductions in GHG emissions have been demonstrated in many LCA studies across a range of bioenergy technologies and scales (Thornley et al., 2009, 2015). The most significant reductions have been noted for heat and power cases. However, some other studies (particularly on transport fuels) have indicated the opposite, that is that bioenergy systems can increase GHG emissions (Smith & Searchinger, 2012) or fail to achieve increasingly stringent GHG savings thresholds. A number of factors drive this variability in calculated savings, but we know that where significant reductions are not achieved or wide variability is reported there is often associated data uncertainty or variations in the LCA methodology applied (Rowe et al., 2011). For example, data uncertainty in soil carbon stock change following LUC has been shown to significantly influence the GHG intensity of biofuel production pathways (Fig. 3), whilst the shorter term radiative forcing impact of black carbon particles from the combustion of biomass and biofuels also represents significant data uncertainty (Bond et al., 2013)." Whitaker et al. 2018, pp. 156–157.
  5. ^ "Any soil disturbance, such as ploughing and cultivation, is likely to result in short-term respiration losses of soil organic carbon, decomposed by stimulated soil microbe populations (Cheng, 2009; Kuzyakov, 2010). Annual disturbance under arable cropping repeats this year after year resulting in reduced SOC levels. Perennial agricultural systems, such as grassland, have time to replace their infrequent disturbance losses which can result in higher steady-state soil carbon contents (Gelfand et al., 2011; Zenone et al., 2013)." McCalmont et al. 2017, p. 493.
  6. ^ "[...] it seems likely that arable land converted to Miscanthus will sequester soil carbon; of the 14 comparisons, 11 showed overall increases in SOC over their total sample depths with suggested accumulation rates ranging from 0.42 to 3.8 Mg C ha−1 yr−1. Only three arable comparisons showed lower SOC stocks under Miscanthus, and these suggested insignificant losses between 0.1 and 0.26 Mg ha−1 yr−1." McCalmont et al. 2017, p. 493.
  7. ^ "The correlation between plantation age and SOC can be seen in Fig. 6, [...] the trendline suggests a net accumulation rate of 1.84 Mg C ha−1 yr−1 with similar levels to grassland at equilibrium." McCalmont et al. 2017, p. 496.
  8. ^ Given the EU average peak yield of 22 tonnes dry matter per hectare per year (approximately 15 tonnes during spring harvest). See Anderson et al. 2014, p. 79). 15 tonnes also explicitly quoted as the mean spring yield in Germany, see Felten & Emmerling 2012, p. 662. 48% carbon content; see Kahle et al. 2001, table 3, page 176.
  9. ^ "A systematic review and meta-analysis were used to assess the current state of knowledge and quantify the effects of land use change (LUC) to second generation (2G), non-food bioenergy crops on soil organic carbon (SOC) and greenhouse gas (GHG) emissions of relevance to temperate zone agriculture. Following analysis from 138 original studies, transitions from arable to short rotation coppice (SRC, poplar or willow) or perennial grasses (mostly Miscanthus or switchgrass) resulted in increased SOC (+5.0 ± 7.8% and +25.7 ± 6.7% respectively)." Harris, Spake & Taylor 2015, p. 27.
  10. ^ "Our work shows that crop establishment, yield and harvesting method affect the C. cost of Miscanthus solid fuel which for baled harvesting is 0.4 g CO2 eq. C MJ−1 for rhizome establishment and 0.74 g CO2 eq. C MJ−1 for seed plug establishment. If the harvested biomass is chipped and pelletized, then the emissions rise to 1.2 and 1.6 g CO2 eq. C MJ−1, respectively. The energy requirements for harvesting and chipping from this study that were used to estimate the GHG emissions are in line with the findings of Meehan et al. (2013). These estimates of GHG emissions for Miscanthus fuel confirm the findings of other Life Cycle Assessment (LCA) studies (e.g., Styles and Jones, 2008) and spatial estimates of GHG savings using Miscanthus fuel (Hastings et al., 2009). They also confirm that Miscanthus has a comparatively small GHG footprint due to its perennial nature, nutrient recycling efficiency and need for less chemical input and soil tillage over its 20-year life-cycle than annual crops (Heaton et al., 2004, 2008; Clifton-Brown et al., 2008; Gelfand et al., 2013; McCalmont et al., 2015a; Milner et al., 2015). In this analysis, we did not consider the GHG flux of soil which was shown to sequester on average in the United Kingdom 0.5 g of C per MJ of Miscanthus derived fuel by McCalmont et al. (2015a). Changes in SOC resulting from the cultivation of Miscanthus depend on the previous land use and associated initial SOC. If high carbon soils such as peatland, permanent grassland, and mature forest are avoided and only arable and rotational grassland with mineral soil is used for Miscanthus then the mean increase in SOC for the first 20-year crop rotation in the United Kingdom is ∼ 1–1.4 Mg C ha−1 y−1 (Milner et al., 2015). In spite of ignoring this additional benefit, these GHG cost estimates compare very favorably with coal (33 g CO2 eq. C MJ−1), North Sea Gas (16), liquefied natural gas (22), and wood chips imported from the United States (4). In addition, although Miscanthus production C. cost is only < 1/16 of the GHG cost of natural gas as a fuel (16–22 g CO2 eq. C MJ-1), it is mostly due to the carbon embedded in the machinery, chemicals and fossil fuel used in its production. As the economy moves away from dependence on these fossil fuels for temperature regulation (heat for glasshouse temperature control or chilling for rhizome storage) or transport, then these GHG costs begin to fall away from bioenergy production. It should be noted, the estimates in this paper do not consider either the potential to sequester C. in the soil nor any impact or ILUC (Hastings et al., 2009)." Hastings et al. 2017, pp. 12–13.
  11. ^ See Whitaker et al. 2018, pp. 156, Appendix S1
  12. ^ "Whilst these values represent the extremes, they demonstrate that site selection for bioenergy crop cultivation can make the difference between large GHG [greenhouse gas] savings or losses, shifting life‐cycle GHG emissions above or below mandated thresholds. Reducing uncertainties in ∆C [carbon increase or decrease] following LUC [land use change] is therefore more important than refining N2O [nitrous oxide] emission estimates (Berhongaray et al., 2017). Knowledge on initial soil carbon stocks could improve GHG savings achieved through targeted deployment of perennial bioenergy crops on low carbon soils (see section 2). [...] The assumption that annual cropland provides greater potential for soil carbon sequestration than grassland appears to be over‐simplistic, but there is an opportunity to improve predictions of soil carbon sequestration potential using information on the initial soil carbon stock as a stronger predictor of ∆C [change in carbon amount] than prior land use." Whitaker et al. 2018, pp. 156, 160.
  13. ^ "Fig. 3 confirmed either no change or a gain of SOC [soil organic carbon] (positive) through planting Miscanthus on arable land across England and Wales and only a loss of SOC (negative) in parts of Scotland. The total annual SOC change across GB in the transition from arable to Miscanthus if all nonconstrained land was planted with would be 3.3 Tg C yr−1 [3.3 million tonnes carbon per year]. The mean changes for SOC for the different land uses were all positive when histosols were excluded, with improved grasslands yielding the highest Mg C ha−1 yr−1 [tonnes carbon per hectare per year] at 1.49, followed by arable lands at 1.28 and forest at 1. Separating this SOC change by original land use (Fig. 4) reveals that there are large regions of improved grasslands which, if planted with bioenergy crops, are predicted to result in an increase in SOC. A similar result was found when considering the transition from arable land; however for central eastern England, there was a predicted neutral effect on SOC. Scotland, however, is predicted to have a decrease for all land uses, particularly for woodland due mainly to higher SOC and lower Miscanthus yields and hence less input." Milner et al. 2016, p. 123.
  14. ^ "In summary, we have quantified the impacts of LUC [land use change] to bioenergy cropping on SOC and GHG balance. This has identified LUC from arable, in general to lead to increased SOC, with LUC from forests to be associated with reduced SOC and enhanced GHG emissions. Grasslands are highly variable and uncertain in their response to LUC to bioenergy and given their widespread occurrence across the temperate landscape, they remain a cause for concern and one of the main areas where future research efforts should be focussed." Harris, Spake & Taylor 2015, p. 37 (see also p. 33 regarding SOC variations). The authors note however that "[t]he average time since transition across all studies was 5.5 years (Xmax 16, Xmin 1) for SOC" and that "[...] the majority of studies considered SOC at the 0–30 cm profile only [...]." Harris, Spake & Taylor 2015, pp. 29–30. Low carbon accumulation rates for young plantations are to be expected, because of accelerated carbon decay at the time of planting (due to soil aeration), and relatively low mean carbon input to the soil during the establishment phase (2-3 years). Also, since dedicated energy crops like miscanthus produce significantly more biomass per year than regular grasslands, and roughly 25% of the carbon content of that biomass is successfully added to the soil carbon stock every year (see Net annual carbon accumulation), it seems reasonable to expect that over time, soil organic carbon will increase also on converted grasslands. The authors quote a carbon building phase of 30-50 years for perennials on converted grasslands, see Harris, Spake & Taylor 2015, p. 31.
  15. ^ Cf. Smil's estimate of 0.60 W/m2 for the 10 t/ha yield above. The calculation is: Yield (t/ha) multiplied with energy content (GJ/t) divided by seconds in a year (31 556 926) multiplied with the number of square metres in one hectare (10 000).
  16. ^ a b c d For yield estimates see FAO's "The global outlook for future wood supply from forest plantations", section 2.7.2 – 2.7.3. Scot's pine, native to Europe and northern Asia, weighs 390 kg/m3 oven dry (moisture content 0%). The oven dry weight of eucalyptus species commonly grown in plantations in South America is 487 kg/m3 (average of Lyptus, Rose Gum and Deglupta). The average weight of poplar species commonly grown in plantations in Europe is 335 kg/m3 (average of White Poplar and Black Poplar.
  17. ^ "The raw material for wood pellets is woody biomass in accordance with Table 1 of ISO 17225‑1. Pellets are usually manufactured in a die, with total moisture content usually less than 10 % of their mass on wet basis." ISO (International Organization for Standardization) 2014a.
  18. ^ "The raw material for non-woody pellets can be herbaceous biomass, fruit biomass, aquatic biomass or biomass blends and mixtures. These blends and mixtures can also include woody biomass. They are usually manufactured in a die with total moisture content usually less than 15 % of their mass." ISO (International Organization for Standardization) 2014b.
  19. ^ Transmission loss data from the World Bank, sourced from IEA. The World Bank 2010.
  20. ^ Additionally, Smil estimates that newly installed photovoltaic solar parks reaches 7–11 W/m2 in sunny regions of the world. Smil 2015, p. 191.


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