Renewable Energy







Notes on Bioenergy, Biofuels




and Energy-from-waste







by




Ludwig Gredmaier









A review of issues with bioenergy and biofuels - v1.1, May 2016



Please report errors to mailto:l.gredmaier@soton.ac.ukl.gredmaier@soton.ac.uk


Contents

Biofuels, bioenergy and biomass

To meet our ever increasing energy demand the world has almost exclusively relied on fossil fuels in recent history. The industrial revolution in the 19th century was firmly grounded on the increasing exploitation of coal as a fuel for steel manufacture, new heavy machinery such as steam engines and railways. Biofuels have only briefly been in fashion in the early days of the internal combustion engine. Rudolf Diesel (http://en.wikipedia.org/wiki/Rudolf_Diesel), the inventor of the self-ignition combustion engine, believed in the use of peanut oil in 1911 to run his machines:
''The Diesel engine can be fed with vegetable oils and would help considerably in the development of agriculture of the countries which will use it. This may appear a futuristic dream but I can predict with great conviction that this use of the Diesel engine may in the future be of great importance.'' (Source: Hall (1981))
Even if the modern motor car of the future may be powered by electric motors, biofuels probably have to power airplanes, simply because of the superior energy density of liquid fuels. In the media and press the term biofuels and bioenergy are often used synonymously, but strictly speaking, the two terms are not identical: The units may provide a way of distinguishing between fuel and energy. A good explanation of biofuels is at http://www.nrel.gov/learning/re_biofuels.html.

Biomass is a term given to biological matter that either grew naturally without human interference, or in a cultured environment (e.g. monocultures), and made use of photosynthesis. This includes grown materials such as

(e.g. Hall and Moss (1983), page 5). Biomass has created interest because it represents a renewable source of energy.

Energy, forms of energy and power

Energy is measured in joules [J], an SI unit, and several forms of energy exist: Power P on the other hand is energy per time and has units of Watts or Joules [J] per time in [s]:
\begin{displaymath}
\textrm{Power } P =\frac{\textrm{energy}}{\textrm{per time}}=\frac{E}{\Delta t}=\textrm{energy flux}
\end{displaymath} (0.1)

with units $ [\frac{J}{s}]=[W]$. Power is energy per time and the term energy flux indicates this time dependence better than then term power. Power stations are typically rated in megawatts [MW] of electrical power (output).

Figure 1: Plants and trees appear green due to the selective absorbtion of certain wavelengths by chlorophyll. Wavelengths that are reflected back from the leaf are seen by the human eye. In autumn chlorophyll gradually stops absorbing solar energy which makes leafs change from green to yellow. A similar process is the ripening of green bananas - green chlorophyll is broken down into sugars. The banana turns yellow.
Image chlorophyll_is_a_pigment

Figure 2: The three main solar energy conversion pathways have different energy conversion efficiencies.
Image solar_energy_conversion_pathways

Physico-chemical characterisation of biological matter

All biological matter contains water. Biomass can be analysed or split into the following compositional fractions:
  1. Carbohydrates
  2. Lipids
  3. Proteins
  4. Fibre or lignocellulose: Biopolymers that give the plant stiffness; fibre consists of the three subgroups
    1. Cellulose: Polymer chains consisting of long strands of $\beta$1-4 glucose units (several thousand glucose units); also called fibrills;
    2. Hemicellulose: Polymers of hexose and pentose (sugars)
    3. Lignin: Complex polymer, consisting of the three types of lignols: p-couramyl (H), guaiacyl (G), and syringyl (S); different plant species have various types of lignin;
    The fibre fraction of biomass is recalcitrant - it does not break down in anaerobic digestion, but can of course be combusted or pyrolysed. In X-ray diffraction analysis, cellulose occurs on both crystalline and amorphous form (e.g. Bos and Donald (1999))
Just like with the digestion of food in the gut, not all theoretical energy in the food (feedstock) can be extracted by the body's metabolism. A certain percentage of the energy content is not digested and remains in the residue (human faecies). With human food the metabolisable energy in the food is somewhat smaller than the energy per mass value [MJ/kg] measured in a bomb calorimeter. Similarly, not all biofuel conversion technologies are equally suited to convert the individual fractions of biomass. For example, woody biomass is not suited for anaerobic digestion or for bioethanol (yet), but is ideal for thermal conversion. Figure 3 illustrates mass and compositional concepts that are relevant in the conversion to biofuel.

Figure 3: Composition of biological matter. Main component is water, which can be dried of. What is left after drying is called TS (total solids).
Image 1000g_of_foodwaste_contains_what

TS (total solids) and ODM (organic dry matter)

A simple but useful measure for biomass material is total solids TS and volatile solids VS. TS is usually, but not always, expressed as a percentage of the original or natural biomass sample that typically includes a substantial amount of water. With unaltered biomass more than half of the volume can be water as shown in Figure 4. For example TS for a biomass material could be expressed as '27 % of total weight'. This means, that after drying a biomass sample with originally 1 gram of mass in an oven at 105$^\circ $C over night, 0.27 grams of solid material is left. The terms organic dry matter and TS are often used interchangeably, but some textbooks do differentiate between the two terms. Water concentration for wood is 8 to 40 % by mass. The emphasis with ODM (organic dry matter) is on 'organic' carbon, excluding inorganic carbon such as calcium carbonate $CaCO_3$.

Figure 4: Illustration of water content and TS (total solids) in biomass. The main component of biological matter, including food waste, is moisture (i.e. water). In 1 kg of foodwaste about 930 g are water. If one kg of food waste is dried at 105$^\circ $C over night, only 70 g remain. This dried fraction is called TS (total solids). If these 70 g are left in a furnace at 550$^\circ $C for several hours, about 5 g of ash are left. What is 'burnt off' is 65 g of VS (volatile solids).
Image foodwaste_fractions

Advantages of biofuels and bioenergy

There were a number of reasons why energy from biomas was and still is considered attractive:
  1. Renewable: Compared to fossil sources biofuels are renewable. They can not be depleted.
  2. Net carbon: When biofuels are combusted no 'new' carbon dioxide is emitted into the atmosphere. All or most of the carbon dioxide is short term cyclic.
  3. Diversification: Not relying on a single, fossil fuel source is a good thing. This increases energy security.
  4. Rural economies: Local, domestic rather than remote economies are strengthend.

1st generation biofuels

Biomass generation primarily relies on renewable solar energy as shown in Figure 1. 1st generation biofuels employ proven technologies available today. 2nd generation biofuels are more advanced, but are less often produced in full scale operating plants, and offer more energy yield per ecological expense. Common 1st generation biofuels together with their feedstock include BTL represents a technology rather than a biofuel and a graphical overview of 1st generation biofuels is shown in Figure 5. In Brazil, the dominant biofuel is ethanol (= ethyl alcohol) from sugar cane. Bioethanol from sugar cane also is a 1st generation biofuel.

What is wrong with 1st generation biofuels

There is growing concern that 1st generation biofuels are not as sustainable as originally thought (Sanderson (2006), Odling-Smee (2007), Manuel (2007)). A variety of biofuel feedstocks and technology pathways exist and not all are beneficial. Biofuels with low sustainability are branded 1st generation biofuels. The most serious issues with (1st generation) biofuels are:
  1. Land use and land use change: Diversion of large portions of land to biofuels.
  2. Potentially increased carbon emissions from land use change: If grass land is ploughed under to grow biofuels, up to 300 tonnes of $CO_2$ per hectare can be released. Forests can release 600 to 1000 tonnes per hectare if converted to energy crops (Source: www.biograce.net).
  3. Potentially increased nitrogen oxide emissions from industrial fertilisers by a factor of 3 to 5 (Crutzen et al. (2007))
  4. Indirect fossil fuel use through artificial fertilisers and soil acidification
  5. Crop residues stabilise soil organic carbon (Reijnders (2008)) and should not be taken off the field.
  6. Water use from irrigation
  7. Soil erosion
  8. Monocultures: Threaten biodiversity of plants and animals.
  9. Herbicide and pesticide use: Leak into the food chain.
So what is the answer to these biofuel Achilles' heels? In response to these drawbacks a more advanced class of biofuels were developed, known as 2nd generation biofuels (https://en.wikipedia.org/wiki/Second-generation_biofuels). As an example, food waste can be converted to biogas and such a feedstock from residue material does not suffer from Clearly, 2nd generation biofuels provide more environmental benefits compared to 1st generation biofuels.

Figure 5: A list of the four most common 1st generation biofuels. The feedstock typically comes from edible biomass which is undesirable.
Image 1st_generation_biofuels

Biofuels and crop residue removal

Harvesting usually leaves behind a crop residue, that is left on the fields and replenishes the soil with nutrients and carbon. Taking away these crop residues for biofuels or for incineration can have negative effects and long term effects are not well known (Lemke et al. (2010), Lal (2005)). Removing crop residue from agricultural fields after the harvest can have the following disadvantages (Lal (2009)): So removing the crop residue to make biofuels may also file under 'unsustainable practice'. This view is not shared by all researchers. It has been claimed that removal of straw for biofuels is acceptable (Gabrielle and Gagnaire (2008)).


Table 1: Despite the fact that they have only recently been re-discovered, biofuels have already been classified into 1st and 2nd generation fuels. Two factors determine which generation the biofuel belongs to - the feedstock and the conversion technology. Ethanol, propanol and butanol are (bio)alcohols.
Type of biofuel 1st generation 2nd generation
biodiesel edible crops, e.g. nonedible biomass
  soybean, sunflower, palm oil waste cooking oil
bioethanol edible crops, e.g. nonedible biomass (cellulosic ethanol)
  sugar cane, maize, wheat wood, straw, wastes
biobutanol edible crops, e.g. nonedible crops
  sugar cane, maize, wheat  
biogas edible crops, e.g. nonedible biomass
  maize, grass, sugar beet manure, slurry, food waste, sewage
biosyngas (coal) municipal waste, waste wood,
(= $CO+H_2$)   dry crop residues


2nd generation biofuels

2nd generation biofuels are produced using more advanced technologies and better feedstock compared to traditional 1st generation biofuels. Table 1 lists a feedstock matrix for current 1st and 2nd generation biofuels. Some of the characteristics and benefits from 2nd generation biofuels are:
  1. Food-fuel debate: Do not eat into our food supply because the feedstock consists of inedible biomass such as biological waste materials (e.g. used cooking oil, animal fats, food waste)
  2. Greenhouse gases: Carbon and other GHG emissions are reduced compared to 1st generation biofuels.
  3. Infrastructure: The currently existing petrol, Diesel or gas networks can be used for distribution of energy to end users.
  4. Advanced conversion to biofuel: Ability to break up lignocellulosic feedstock and convert it to fuel. If cellulose is broken down into fermentable feedstock, this indicates a 2nd generation biofuel - Figure 6.
In a reply to Searchinger et al. (2009), Bent Sorensen Sorensen (2010) points out that 2nd generation biofuels are of two types:
  1. Biofuels derived from cellulosic and lignocellulosic plant biomass grown on marginal land (i.e. land not used for food crops).
  2. Biofuels derived from organic residues or wastes, where the nutrients are taken back to land, implementing nutrient recycling. There is no or little primary energy input into producing these wastes; the energy inputs have already been expended anyway.
Novel methods are researched to exploit the previously recalcitrant lignocellulosic fraction of the plant matter. The challenge is to keep parasitic energy demand low and minimise toxic chemicals used to break down recalcitrant lignocellulose (e.g. Ragauskas et al. (2006)). Examples of 2nd generation biofuels include
  1. Lignocellulosic ethanol: The production of ethanol from lignin and cellulose makes it possible to use nonedible trees or grasses as feedstocks. With advanced conversion methods long chains of cellulose in woody materials are broken down and converted to fuel.
  2. Biodiesel from waste vegetable oil: An example is the Brighton Yellow Lemon bus that runs on waste biodiesel - http://youtu.be/vdJuQ36y3_k.
  3. BTL (biomass-to-liquids): BTL-fuels (BTL = biomass-to-liquid) can be produced from various types of biomass by pyrolysis/gasification and subsequent synthesis of the liquid fuel from the $CO/H_2O$ gas mixture. A subsection of BTL is known as GTL (Gas-to-Liquid), a thermo-chemical (rather than a biological) conversion pathway. A key process in GTL (Gas-to-Liquid) pathway is the Fischer-Tropsch conversion.
  4. Lipids (http://www.chem.qmul.ac.uk/iupac/lipid) from algae are usually branded as a 3rd generation biofuel but the potential of this pathway is highly debatable. Algal biofuels is still in their infancy (e.g. http://www.oilgae.com.)
The Fischer-Tropsch thermo-chemical conversion pathway is not based on a single but a family of chemical reactions. In a first step syngas is generated:
\begin{displaymath}
CH+O_2\longrightarrow \frac{1}{2}H_2+CO
\end{displaymath} (0.2)

A more general version of the above syngas reaction is
\begin{displaymath}
CH_n+O_2\longrightarrow \frac{1}{2}nH_2+CO
\end{displaymath} (0.3)

possibly assisted by a catalyst to instigate the reaction. The actual Fischer-Tropsch reaction can be written as
\begin{displaymath}
H_2+CO\longrightarrow hydrocarbonchain + H_2O
\end{displaymath} (0.4)

with a catalyst such as Fe or Co initiating the reaction. The term 'hydrocarbonchain' consists of $CH_2$ molecules in a chain. The final liquid fuel is obtained in a refining reaction (e.g. http://www.fischer-tropsch.org). Fischer-Tropsch synthesis works with many feedstocks such as coal, natural gas, or biomass. An example of larger scale Fischer-Tropsch BTL (biomass-to-liquid) is Also, part of South Africa's fossil Diesel is produced from coal employing a type of Fischer-Tropsch synthesis, run by the petroleum company Sasol (http://www.sasol.com). This is of course not a biofuel.

Volkswagen have marketed their own brand of 2nd generation BTL (biomass-to-liquid) biofuels under the name sunfuel 0.1. Audi supports the US biotech company http://www.jouleunlimited.com who research into 3rd generation liquid biofuels from algae, grown with waste carbon dioxide. Critics brand these as 'caviar fuels' - they are far too expensive to be produced on a large scale.

Figure 6: The differentiation between 1st and 2nd generation biofuels is often based on the performance in the three key areas (1) food fuel dilemma, (2) biomass-to-fuel conversion efficiency and (3) carbon emissivity per unit of energy delivered.
Image 2nd_generation_biofuels

Figure 7: Organic waste materials can be classified into wet and dry which determines the energy conversion technology.
Image wet_and_dry_waste_biomass

Energy-from-waste/waste-to-energy - a 2nd generation biofuel

Waste can be described as material and goods that have been discarded by society, are now unwanted, and need to be disposed of in some way. Waste can be classified as dry and wet as shown in Figure 7, and it is a clear indicator of a non-cyclic society. In a fully cyclic, recycling society, there should be no or only little waste, as all material is going back into the system for re-use. Societies are currently squandering resources on a large scale by intercepting the cycle, and the term 'waste' is actually inappropriate (Drackner (2005)). In Europe every citizen generates about 3.5 tonnes of waste each year (http://scp.eionet.europa.eu/themes/waste, accessed Feb 2013), and a substantial fraction of municipal tax goes into disposing of this enormous mountain of waste. The incineration of household waste is widely classed as an energy-from-waste technology (http://en.wikipedia.org/wiki/Waste-to-energy). Further energy-from-waste options are: An example of the latter is http://www.edmontonbiofuels.ca and a video about the project is http://youtu.be/4AnMzntAde4. It is clear that energy from waste ticks a lot of boxes, so why are we not doing more of this? The answer is that one box can't be ticked - being able to satisfy the excessive energy demand of Western societies. Figure 8 compares electric output power of energy-from-waste power plants versus conventional fossil and nuclear plants in the UK. Energy-from-waste is comparable to a drop in the ocean and remains a marginal energy supply technology. Further examples of energy from waste are:

Figure 8: Electric output in Megawatts [MW] across various power plants in the UK. Waste-to-energy plants like the incineration plant in Marchwood, Hampshire, UK (http://www.veoliaenvironmentalservices.co.uk/Main/Facilities/Energy-Recovery-Facilities/), provide only a fraction of the electrical power of nuclear plants and large wind farms.
Image renewable_energy_power_plants

Advantages of energy from waste technology

Waste is generated in large quantities in our society. This material needs to be disposed of anyway, and if we can produce energy, then we have a double benefit ('free lunch' effect). Advantages of waste-to-energy technologies are: Potential disadvantages of energy from waste are:

Example of a waste-to-energy process: Net energy balance ratio for food waste to biogas

In many coutries in Europe food waste from domestic households is not collected separately but is 'disposed' off together with the residual waste. This is completely unacceptable and food waste needs to be separated out and either composted or digested to produce biogas, as much of organic waste is water - Figure 4. Advantages of the food-waste-to-biogas process compared to a 1st generation biofuel, are as follows: Figure 9 depicts the food waste to biogas process chain. One way to assess the energy performance of renewables in simple terms, is to establish the net energy balance ratio (NEB ratio). It is important to realise that two types of energy balances can be made, one relative to the calorific value of the generated biomethane and one relative to the electric energy produced by the CHP engines in the anaerobic digestion plant: Attention should therefore be paid to what type of end energy (methane or electricity) is going into the energy balance. The following conclusions can be drawn from conduction an energy balance ratio for an anaerobic digestion plant fed on food waste, that produces biogas:

Figure 9: Illustration of process steps and system boundary in a food waste-to-biogas chain. The energy analysis includes operational energies but not infrastructural energies.
Image foodwaste_to_biogas_process_chain

Household waste in the UK

According to DEFRA statistics http://www.defra.gov.uk/statistics/environment/waste/wrfg23-wrmsannual/ for the period from April 2010 to March 2011 a total of The amount of waste is going down from year to year - Figure 10.

Figure 10: Amount of household waste and recycling in the UK per person up to 2011. Source: 2010-11-ANNUAL-publication_WITHOUTLINKS.xls, available at http://www.defra.gov.uk/statistics/environment/waste/wrfg23-wrmsannual, accessed March 2012.
Image householdwaste_and_recycling_in_England

Figure 11: EU Renewable Energy Directive 2009/28/EC sets target percentages of renewable energy in the overall consumption for the year 2020 for each memeber state. These energy target percentages are plotted here. Source: Renewable Energy Directive 2009/28/EC Annex 1, page 46/page 31; http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:140:0016:0062:en:PDF.
Image RED_Renewable_Energy_Directive2009

EU Renewable Energy Directive 2009/28/EC and Biofuel Directive 2003/30/EC

The Renewable Energy Directive 2009/28/EC, or short RED, is the legislative pillar that carries the energy policy of the EU into the next decade - http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:140:0016:0062:en:PDF. The 'rapporteur' (= appointed person) responsible for dealing with the RED in the European Parliament was Claude Turmes, a Green MEP from Luxemburg - http://www.claudeturmes.lu. Key points in the RED are as follows: According to Article 17 of the RED, carbon emissions are a key sustainability criterion:
''The greenhouse gas emission saving from the use of biofu­els ... shall be at least 35 %.''
Article 17 gradually raises the bar in 2017 and again in 2018 as follows:
''From 1 January 2017, the greenhouse gas emission saving from ... biofuels ... shall be at least 50 %. From 1 January 2018 that green­house gas emission saving shall be at least 60 % for biofuels and bioliquids produced in installations in which production started on or after 1 January 2017.''
To conclude, the RED introduces the following sustainability criteria in Article 17: The RED clarifies the EU Biofuels Directive 2003/30/EC from 8 May 2003, which aims to blend 5.75 % of biofuel into transport fuels in all EU member states by 2010 and 10% by 2020. Biofuel blending mandates vary from country to country as shown in Figure 12.

In order to support and organise the implementatin of the renewable targets, a scheme called 'Concerted Action on the Renewable Energy Sources Directive (CA-RES) started in July 2010 - http://www.ca-res.eu. The RED must be transposed into national law in each member state by 5 December 2010 and on 21 June 2012 the EU Commission pointed its finger at four member states, Cyprus, Ireland, Malta and Slovenia. These four states have not transposed the Directive into national law in time - http://europa.eu/rapid/press-release_IP-12-640_en.htm. Obviously these countries had to tackle other issues that took priority over becoming a more sustainable society.

Figure 12: Biofuel blending mandates in the EU. Summaries of the renewable status per memeber state are at the CA-RES website (Concerted Action on the Renewable Energy (Sources) Directive) at http://www.ca-res.eu/index.php?id=307 (accessed Feb 2013).
Image EU27_biofuels_blending_mandates

Sustainability and performance of biofuels - criteria and biofuel policies

Not long ago biofuels were regarded as the silver bullet for an eternal, future energy source that is able to stamp out our depedency from fossil fuels for once and for all. This phase has certainly waned and a more sobering view has taken over following the food fuel debate among scientists and policy makers. The food fuel dilemma and previously neglected land-use change effects led to the sobering view that biofuels are not as beneficial as originally thought. Land use change and the application of industrial fertilisers in conventional farming can completely wipe out an otherwise beneficial greenhouse gas balance of a biofuel. Fossil energy sources are hard to replace in the manufacture of mineral fertilisers, yet they guarantee high crop yields. Sustainability of biofuels clearly needs to be assessed in some way and this receives increasing attention from scientists and policy makers (e.g. Lovett et al. (2011)). The degree of sustainability of a biofuel is often reduced to looking at the following three environmental impacts:
  1. Food security
  2. GHG emissions over the life cycle or compared to fossil fuel equivalents: Biofuels can be rated according to their global warming emissions, and this indicator is often reduced to the term 'carbon emissions', which distinguishes between three levels of achievement for biofuels as shown in Figure 13:
    1. carbon positive: The biofuel cycle acts as a fossil carbon dioxide emitter.
    2. carbon neutral: The biofuel cycle exhibits an overall carbon balance where emissions equal absorbtion of carbon dioxide over the life cycle of the biofuel.
    3. carbon negative: The biofuel cycle acts as a carbon sink over the life cycle. More carbon is sequestered into plant matter than is released in biofuel combustion - this could for example be achieved with so-called LIHD (low input high diversity) plant species, grown on non-food agricultural land with lower than average fertility (Tilman et al. (2006)).
    $CO_2$ is only one greenhouse gas among several and it is necessary to take other powerful GHGases such as $CH_4$ and $NO_2$ into account. Biofuels can completely defeat their purpose and cause more climate change compared to conventional fossil fuels if forests or grass land are converted to high intensity crop land. In one study, out of 26 biofuels only 21 resulted in a reduction of GHG emissions, the remaining 5 had greater environmental impact than fossil fuel equivalents (Zah et al. (2007)). It is expected that 2nd generation biofuels are less carbon intensive than 1st generation biofuels, which in turn are meant to be less carbon intensive than fossil fuels. Carbon emissivity of biofuels needs to be evaluated thoroughly for each type of fuel and technology pathway. Carbon neutral hydrocarbons have been proposed (Zeman and Keith (2008)), where carbon dioxide is either captured from biomass or from ambient air.
  3. Energy return per energy invested.
However, limiting assessment of a biofuel to the above criteria is inadequate - adoption of a biofuel on a larger scale requires a wider scope of potential impacts including social and labour effects, water consumption, long term soil stability, biodiversity, and numerous forms of pollution into the compartments air, soil and water (e.g. Gasparatos et al. (2013)). Up to about 35 environmental impacts are known (Buchholz et al. (2009)) and a convention needs to be established and adopted that specifies performance targets.

Figure: An important performance indicator for biofuels is GHGas emissions from the complete production chain. The term 'carbon' in 'carbon positive' stands for fossil, anthropogenic carbon dioxide. See also Figure 14.
Image biofuels_carbonnegative_carbonneutral_carbonpositive

Figure 14: Simplistic illustration of the direction of carbon flows with biomass cultivation. Biogenic carbon emissions (section in magenta on the left) represents short-term cyclic carbon and the majority of biofuel studies do not take short term cyclic, biogenic $CO_2$ into their balances.
Image carbon_flows_with_biomass
Both scientists and policy makers recognise that metrics need to be introduced to curb mushrooming of unsustainable biofuels and foster promising pathways more selectively. So-called certification schemes are on the agenda and these have proven successful in various areas other than biofuels:
  1. FSC Forest Stewardship Council https://ic.fsc.org: Provide methods for assessing forest management.
  2. RSPO Round table on Sustainable Palm Oil http://www.rspo.org: Develop principles and criteria for sustainable palm oil production.
  3. GOTS Global Organic Textile Standard http://www.global-standard.org: The website contains a database of producers of organic textiles; certification for the GOTS standard is based on visits to textile factories by accredited, independent bodies;
  4. Soil Association Certification http://www.sacert.org: The largest organic food and farming certification body in the UK.
In terms of biofuels several national and international entities have gone some way towards assessment and certification of biofuel pathways. The UN Food and Agriculture Organization FAO runs the Bioenergy and Food Security Criteria and Indicators (BEFSCI) project at http://www.fao.org/bioenergy/foodsecurity/befsci/compilation/en/ and schemes are:

Biogenic and fossil carbon dioxide

Biofuels have a distinct advantage over fossil fuels - they only emit biogenic $CO_2$ rather than fossil $CO_2$. If fossil $CO_2$ is emitted into the atmosphere the net concentration of $CO_2$ in the atmosphere rises. Scientists consider biofuel combustion as carbon-neutral technology because the $CO_2$ from combustion is cyclic - it has previously been captured by growing plants driven by photosynthesis. The identical amount of $CO_2$ captured during the plant growth phase will later be released in the combustion of the plant matter. Plants rely on water, sun, $CO_2$ and soil nutrients to grow. The net biogenic $CO_2$ balance, however, is only cyclic, if no fossil fertilisers are used in farming.

Environmental impacts of biofuels

To make a biofuel pathway sustainable a number of boxes need to be ticked (e.g. Shilton and Guieysse (2010)):
  1. Carbon emissivity
  2. Long term soil quality
  3. Biodiversity: Monocultures must be avoided.
  4. Nutrient cycling and eutrophication: Nutrient runoff must be minimised.
  5. LUC (land use change) and iLUC (indirect land use change): An unprecedented portion of farm land is now devoted to biofuels throughout the world and two types of land use change can be associated with the recent rise of biofuels:
    1. direct LUC (land use change): Farmland previously used to produce food is now used to produce energy crops.
    2. indirect LUC: Farmland in Europe is increasingly used for energy crops. Cooking oil that was previously made from sunflower and rape seed in Europe, may now be displaced by palm oil. The effect is that more forest is cleared in Malaysia to plant palm oil trees. This is an indirect effect where local farming practices 'at home' are causing remote deforestation, which may make things worse than they were with fossil fuels. On top of loss of biodiversity, this type of land use change may cause a carbon debt which may take as long as 423 years to pay back with the new biofuel that replaced the rain forest (Fargione et al. (2008)).
    Indirect land use change effects are hard to quantify and it is the indirect land use change that the EU wants to incorporate into environmental performance indicators for biofuels. It is not surprising that the policy is highly controversial with farmers and the agricultural lobby.
  6. Water use: Maize (American English: corn) is a C4 plant grown for biofuels in the US for ethanol and in Germany for biogas, requires large amounts of water and nitrogen, which in many cases means artificial irrigation and intensive mineral fertiliser use. Crop rotation is essential to avoid soil nutrient depletion from maize cultivation. Industrial fertilisers (e.g. ammonium nitrate) release nitrogen oxides, an extremely potent GHG with a emission factor of 298 compared to $CO_2$.

Figure 15: A highly sobering experiment - Iain Stewart demonstrates IR radiation absorption by $CO_2$, using a candle and an IR (infrared) camera - http://youtu.be/kGaV3PiobYk. The lit candle provides the source of thermal radiation, simulating solar radiation onto the atmosphere, that is increasingly filled with $CO_2$ and other GHGases.
Image Iain_Stewart_demonstrates_IR_radiation_absorption_by_CO2

Figure 16: Carbon emissions for cars bought in 2010 across Europe. Portugal and Denmark are leading. Surprisingly countries such as Estonia and Latvia are trailing. Data source: http://www.eea.europa.eu.
Image new_vehicle_CO2_emissions_in_grams_per_km_in_Europe_2010

Well-to-tank and well-to-wheel analysis for biofuels

In the EU all countries are required to report carbon emissions for new vehicle sales to the European Commission. Figure 16 shows that there is a large gradient of new vehicle emissions across the 27 EU countries that does not seem to correlate with their economic income. In Denmark and Portugal new cars sold in 2010 exhibit record low emissions of only 127 grams per km whereas many other EU member countries exceed 160 grams of $CO_2$ emitted per km driven. Passenger transport planes emit even more $CO_2$ at 100-250 g/km per passenger, a bus 40-80 g/km, and a train 40-160g/km per passenger (Source: http://www.eea.europa.eu/green-tips/going-on-a-longer-trip-choose-wisely). The $CO_2$ emissions plotted in Figure 16 are vehicle emissions. The system boundary is virtually 'around the vehicle' and excludes the fossil oil well and the oil refinery.

It has been pointed out that well-to-wheel studies adopt a much more holistic view (Geerken et al. (2005)) to assess the whole fuel and vehicle transport chain and therefore take on life cycle analysis concepts. However, there are important differences between well-to-wheel studies and life cycle analysis studies:

  1. Infrastructure: Well-to-wheel studies typically exclude embodied energy of infrastructur and the vehicle.
  2. Environmental impacts: Well-to-wheel studies employ far fewer environmental impacts compared to LCA. In many cases well-to-wheel studies only measure GHG emissions for the various transport fuel process chains.
For example, a 2001 early well-to-wheel study in the US (Wallace et al. (2001)) only considers two environmental impacts - (1) total energy use and (2) the three greenhouse gases $CO_2$, $CH_4$ and $N_2O$. The study claims to conduct a LCA of 27 different fossil and renewable fuel pathways. A CONCAWE well-to-wheel study claims that liquid biofuels such as biodiesel and bioethanol reduce $CO_{2eq}$ greenhouse gas emissions compared to their fossil counterparts by 53 %. If previously ignored $N_2O$ emissions from mineral fertilisers are taken into account in the study, GHG emissions with biodiesel are only a mere 7% better compared to fossil Diesel (Armstrong et al. (2002), page V). CONCAWE is a European oil industry association founded in 1963 and is short for CONservation of Clean Air and Water in Europe (http://www.concawe.eu). It is clear that environmental impacts other than pure GHG emissions also play a role in the environmental performance of a fuel and must be considered as stated in a Swedish study from 2008:
''Thus, the digesting of organic waste produces ... pollutants contributing to eutrophication (ammonia), and these indirect effects might even exceed the direct environmental benefits of replacing fossil vehicle fuels with biogas.'' (Börjesson and Mattiasson (2008), page 10).
Carbon dioxide equivalent emissions for various fossil and biofuels are plotted in Figure 17. Biogas as transport fuel comes out extremely well at less than minus 64 g of $CO_{2eq}$ per MJ of fuel. The biofuel may be much less beneficial if impacts other than GHG are considered. Figure 17 does not take biogenic carbon emissions from the combustion of biogas into account.

Figure 17: Attempt to attach a number to greenhouse gas emissions to various types of transport fuels. Bar graphs depict grams of GHGas emissions as $CO_{2eq}$ per fuel type. Drawn after Börjesson and Mattiasson (2008), Figure 3, page 10.
Image GHG_reductions_for_biofuels_after_Borjesson2012

To assess and rate the environmental performance for transport fuels from biofuels, two types of analyses exist, that differ in their system boundary:

  1. Well-to-tank analysis: identical to well-to-wheel but the system boundary does not include the car, i.e. does not take into account efficiency of the vehicle technology. This can be regarded as a cradle-to-gate analysis.
  2. Well-to-wheel analysis: For a car running on liquid fuel, this would include mining of the oil well, processing/refining crude oil in the oil refinery, transport to the petrol station, conversion of fuel to turn the wheels and propel the car (i.e. conversion from chemical fuel energy into kinetic energy). A well-to-wheel analysis would correspond to a cradle-to-grave approach for both the vehicle fuel and the vehicle technology.
See http://www.mpoweruk.com/fuel_cells.htm for examples of well-to-wheel efficiencies for various propulsion systems. In the literature the fuel well, the fuel refining, and the delivery to the petrol station is sometimes known as the upstream section and the part from the petrol station onwards, i.e. the vehicle, is called downstream section. Well-to-wheel efficiencies can be comparatively low with biofuels, for example between 9 and 11 % (Ahlvik and Brandberg (2001) Figure 9, page 78), but a number of ways of producing well-to-wheel analyses exist (Ma et al. (2011)).

Example: Let's look into a well-to-tank and a well-to-wheel analysis for biomethane. Say we are operating a large anaerobic digestion plant and produce biogas. The digester is fed with various organic materials such as (a) food waste, (b) agricultural waste and (c) garden waste. Let's see what a (1) cradle-to-gate analysis involves and (2) a cradle-to-wheel analysis, for the case where transport fuel is produced from upgraded biogas:

  1. Well-to-tank: The analysis stops after the upgrading of biogas to compressed, pure methane. What we do with the generated biomethane energy is not specified. The analysis stops before the specific end user technology to move 80 kg of passenger from A to B. The system (analysis) boundary in this case is well-to-compressed-gas or cradle-to-gate where the gate is the compressed gas cylinder within the plant after upgrading of biogas. Vehicle technology is not taken into account.
  2. Well-to-wheel: The system (analysis) boundary goes further to the type of vehicle technology used to move an 80 kg passenger from A to B. There are two options how to convert the biogas into kinetic energy: (a) compressing biomethane and feeding an internal combustion engine or in the future a gas fuel cell. (b) Generating electricity from biomethane and charging an electric vehicle. Due to the extended system boundary beyond the factory gate, this scenario represents a well-to-wheel analysis.
Both analyses are useful but have different emphasis. The well-to-tank analysis emphasises the production chain of the biofuel excluding the vehicle technology, whereas the well-to-wheel analysis takes a more holistic view by taking the vehicle technology and its efficiency into the system boundary.

Electric vehicles

There is evidence that well-to-wheel efficiency of electric vehicles is higher than that of vehicles powered by an internal combustion engine. The two transport technologies work with different types of energy sources or wells:
  1. Internal combustion engine powered vehicle: A fuel source, e.g. fossil, crude oil from an oil well, or renewable biomass, is taken to an oil refinery for upgrading to petrol and Diesel.
  2. Electrically powered vehicle: A fuel source, e.g. fossil or possibly renewable biomass is combusted (oxidised) and an electric generator produces electricity. Alternatively wind and solar radiation act as a renewable well to produce electricity with wind turbines or photovoltaic cells.
It is clear that a well-to-wheel analysis of a propulsion technology is complex right from the start. It is important to distinguish between (tank-to-wheel) efficiencies, and carbon intensity. With electric vehicles carbon intensity is measured in units of [ $\frac{{g}\textrm{ of } CO_{2eq}}{km}$] and it is a somewhat nontransparent metric due to the dependency of carbon emissivity on the fuel mix burnt to generate electricity in the first place. The reason for the poorer conversion efficiency of the internal combustion engine is that much of the energy in the combustion engine is converted into low-grade (useless) heat rather than kinetic energy. Of course the electric vehicle motor system also is not 100 percent efficient due to thermal losses in battery charging, but overall, the losses with electric propulsion are lower compared to an internal combustion engine. Figure 19 plots well-to-wheel $CO_{2eq}$ emissions per MJ generated for various biofuels, excluding the vehicle technology. Biogas fuel exhibits very low carbon emissions. Combustion of biogas fuel releases only (short term cyclic) biogenic carbon which does not count towards the (fossil fuel) greenhouse effect.

Figure 18: Example of a well-to-tank analysis. The vehicle technology is not included - it is situated outside the system boundary.
Image well-to-tank_analysis

Figure 19: $CO_{2eq}$ emissions per MJ of fuel energy for various fuels. Biogas or biomethane combustion exhibit low emissivity of 0.2 g of $CO_2eq$, based on the assumption that biogenic carbon emissions are carbon neutral, i.e. do not count towards anthropogenic GHG emissions. Source: Ramesohl et al. (2003).
Image CO2eq_emissions_per_MJ_for_various_fuels-Ramesohl2003

Standard values and constants

Greenhouse gases

The three most common greenhouse gases and their origin in the atmosphere are:
  1. Carbon dioxide $CO_2$: Burning of fossil fuels (gas, oil, coal), or burning of biogenic fuels (wood, straw, biogas) solid waste, trees and wood products generates $CO_2$; also emitted in chemical reactions such as the manufacture of cement and during the production of steel. A certain amount of carbon dioxide is removed from the atmosphere (or 'sequestered') when it is absorbed by plants in the biological carbon cycle.
  2. Methane $CH_4$: Emitted from livestock such as cattle and pigs; mining of coal, oil and natural gas; anaerobic decay of organic matter in landfills;
  3. Nitrous oxide $N_2O$ : Emitted in modern agriculture, as well as in the combustion of fossil fuels in (Diesel) engines and the incineration of solid waste.
  4. Hydrofluorocarbons (HFCs)
  5. Perfuorocarbons (PFCs)
  6. Sulphur hexafluoride (SF6)
Global warming is caused by a number of individual gases, but these are often aggregated into a single carbon dioxide equivalent GHG effect with units of kg of $CO_{2eq}$.

Molecular vibrations of greenhouse gases

A gaseous or liquid molecule vibrates in a complex way when it is excited. This complex vibration, however, can be broken down into a superposition of simpler vibrations, called the normal modes of vibration. The normal modes of vibration consist of simple harmonic motion. Molecular vibrations can be classified into two types:
  1. Stretching (mode) vibration: change of bond length
  2. Deformation (mode) vibration: change of bond angle
Both types of vibrations can be symmetric and antisymmetric. For linear molecules such as $CO_2$, the atoms are in line and then the number of normal modes of vibration for such a linear molecule is given by
\begin{displaymath}
3N-5
\end{displaymath} (0.5)

where N is the number of atoms in the molecule. Since $CO_2$ contains N = 3 atoms (one carbon and two oxygen atoms) this linear molecule is able to vibrate in 3 $\times$ 3 - 5 = 4 modes of vibrations. An animation of the modes of vibration of $CO_2$ is in this video about IR spectroscopy at https://www.youtube.com/watch?v=TMLnUmbLwUI. For nonlinear molecules such as the water molecule $H_2O$ the two hydrogen atoms are not aligned with the oxygen atom - they are not arranged in a straight line. In such a nonlinear molecule the number of normal modes of vibration is
\begin{displaymath}
3N-6
\end{displaymath} (0.6)

The water molecule consists of N = 3 atoms and then the number of modes of vibration for the water molecule is
\begin{displaymath}
3 \times 3 -6 = 9 - 6 = 3
\end{displaymath} (0.7)

The water molecule oscillates in three normal modes of vibration.

Global Warming Potentials GWPs

The global warming potential GWP of a greenhouse gas indicates to what degree (or to what factor) the gas causes global warming compared to the chosen standard $CO_2$. What is the GWP of $CO_2$ ? The answer is 1. $CO_2$ acts as the reference gas and it has been given a GWP of 1. The calculation of the GWP of greenhouse gases is complicated by the fact that the various greenhouse gases don't reside in the atmosphere for good. Instead, greenhouse gases gradually decay in the atmosphere over time. Because the residence time of global warming gases are dynamic, rather than static with time, they are subject to debates among researchers. Decay times or residence times are not identical for the various green house gases - they range from a few years to thousands of years.

The GWP for methane and nitrogen (di)oxide are shown in Table 2. The physics behind the global warming effect is that the gas is absorbing solar, thermal radiation, that would otherwise be reflected back into outer space. Currently adopted values for GWP of methane (Table 2) may be underestimates due to gas-aerosol interactions ( urlhttp://www.giss.nasa.gov/research/news/20091029 and http://methanenet.org/content/methane-has-larger-gwp). The 100-year GWPs for methane may be as large as 33 rather than 21 (Figure 2 in Shindell et al. (2009)).




Example: According to the 4'th assessment report IPCC 2007 Nations (2007), the greenhouse gas dinitrogen monoxide $N_2O$, also known as nitrous oxide or laughing gas (http://en.wikipedia.org/wiki/Nitrous_oxide; do not confuse with nitrogen dioxide $NO_2$) has a GWP of 298. Explain!

Answer: A GWP of 298 means that the gas is 298 times more potent in global warming than $CO_2$. Or in units, one kg of $N_2O$ is as potent as 298 kg of $CO_2$. One kg of $N_2O$ causes as much global warming as 298 kg of $CO_2$.





Table 2: Global warming potentials GWPs of the three most common greenhouse gases relative to $CO_2$ which has been allocated a GWP of 1. The currently accepted values are from the 4'th assessment report IPCC 2007 Nations (2007).
Greenhouse gas $GWP_{100years}$ $GWP_{100years}$
  in IPCC 1996 in IPCC 2007
carbon dioxide $CO_2$ 1 1
methane $CH_4$ 21 25
nitrous oxide $N_2O$ (dinitrogen monoxide) 310 298


No rule exists to make the IPCC 2007 Nations (2007) GWPs mandatory in carbon footprinting. According to http://www.biograce.net the global warming potentials in carbon dioxide equivalents $CO_{2eq}$ for the three major greenhouse gases $CO_2$, $CH_4$ and $N_2O$ are:

The reason to use the above values rather than the IPCC 2007 values is explained at http://www.biograce.net:
''This list of standard values contains the conversion factors that were used for calculating the default values in the Renewable Energy Directive (2009/28/EC) Annex V but for one exception: The Commission calculated its default values using global warming potentials of 25 for $CH_4$ (methane) and 298 for $N_2O$ (nitrous oxide), whereas in Annex V part C the Commission prescribes that global warming potentials of 23 for $CH_4$ and 296 for $N_2O$ should be used for calculations. As to ensure that calculations are in line with the directive's rules, our list of standard values follows the methodology laid down in Annex V part C and uses values of 23 and 296.'' (Source: http://www.biograce.net, viewed April 2016)

Literature and educational videos

Video resources on biofuels:




Video resources on greenhouse gases and the GHG effect:




Climate change sceptics:




Greenhouse gases and global warming:

A simple quiz on biofuels is at https://www.isurvey.soton.ac.uk/7043.


Selected literature about biofuels:




LCAs and sustainability criteria for biofuels

Despite the progress in LCA models, the stability and accuracy of LCA has been debated, and biofuel process chains are often difficult to model.




Waste to energy:




Bioethanol:




Palm oil:




Plant nutrients and nitrogen fertilisers:




Miscellaneous:

Global statistical energy data:

Supporting you for more sustainable living:

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