Biomass Energy: Resources, Composition, Characterisation, Conversion and Biofuels

This chapter belongs to the Engineering Sciences (XE) paper and sits in Section XE8.1 (Energy Resources and Conversion Technologies) of XE8 Energy Science, one of the nine optional sections, any two of which a candidate chooses in the exam. It answers the heading “Biomass: Biomass resources, biomass composition, characterisation, conversion methods (pyrolysis, gasification, steam reforming), biofuels”. It teaches the biomass resource classes, lignocellulosic composition, proximate and ultimate analysis with the basis conversions and heating values, thermochemical conversion (combustion, pyrolysis, gasification, steam reforming and the shift reaction) and, in outline, anaerobic digestion, then bioethanol, biodiesel and biogas. The numericals are basis conversion of a proximate analysis, lower heating value from a higher heating value, ethanol yield, biogas energy and the air needed at a given equivalence ratio; every constant is stated in the question.

1. Biomass resources

Biomass is organic matter of recent biological origin that can be used as fuel or feedstock. Its carbon was taken from the atmosphere as carbon dioxide by photosynthesis, so when it is grown and used sustainably the carbon released on use is balanced by the carbon taken up by regrowth: biomass is renewable and, ideally, close to carbon-neutral, apart from the fossil energy used to grow, harvest, transport and process it.

Biomass resource classes
Resource classExamplesMain concerns
Woody biomassforestry residue, fuelwood, sawdustmoderate moisture, low ash, sustainable harvest
Agricultural residuesstraws, husks, bagasse, stalkshigh ash, seasonal supply, collection and storage
Energy cropssugar and starch crops, oil seeds, fast-growing grasses and treesland and water use, competition with food
Wastesanimal manure, sewage, the organic part of municipal solid wastewet and variable, contaminants, but a disposal benefit
Aquatic biomassalgae, water weedshigh water content, lipid or sugar rich, harvesting cost

Raw biomass has a low energy density: high moisture, low bulk density and a dispersed supply make collection and transport costly, so it is used near its source or is densified (briquettes, pellets) or dried or torrefied to raise its energy per unit volume and ease handling.

2. Biomass composition

Plant dry matter is mostly lignocellulose, three polymers in a composite cell wall. Cellulose is a linear polymer of glucose, partly crystalline and the largest fraction (of order 40 to 50 % in wood and straw). Hemicellulose is a shorter, branched, amorphous polymer of five- and six-carbon sugars (of order 20 to 30 %), the easiest to break down. Lignin is a cross-linked aromatic polymer (of order 15 to 30 %) that binds and protects the cellulose, has the highest carbon and energy content of the three, and resists biological conversion. Smaller parts are extractives (oils, resins, sugars) and mineral ash; these ranges vary with species and are indicative only.

The composition decides the route. A high sugar and starch content suits fermentation; high oil content suits transesterification; dry lignocellulose suits combustion, pyrolysis and gasification; wet manure and sewage suit anaerobic digestion because drying them would cost more energy than it yields. Lignin is the barrier in turning lignocellulose into ethanol, so a pretreatment step must open the structure before enzymes can reach the cellulose.

3. Characterisation: proximate and ultimate analysis, heating value

Proximate analysis gives mass percentages of moisture, volatile matter (VM), fixed carbon (FC) and ash: moisture is the loss on drying, VM the further loss on heating out of contact with air, ash the residue on burning, and FC is found by difference (100 − moisture − VM − ash). Ultimate analysis gives the elemental mass percentages of C, H, N, S and O (O usually by difference), separately from ash. Biomass is rich in volatiles and has little fixed carbon, which is why it ignites readily and why much of its energy is released from burning gases.

Reporting basis matters. As-received (ar) includes moisture; dry basis divides by (1 − M); dry ash-free (daf) basis also removes ash. Worked example. As received: moisture 12 %, VM 60 %, FC 18 %, ash 10 %. Dry-basis VM = 60/0.88 = 68.2 %; dry-basis FC = 18/0.88 = 20.5 %; dry-basis ash = 11.4 %. On a daf basis, FC = 18/(60 + 18) = 23.1 %.

The higher heating value (HHV) counts the water in the products as liquid; the lower heating value (LHV) counts it as vapour, so LHV = HHV − (latent heat) × (kg of water per kg of fuel). The water comes from the fuel moisture and from the burning of hydrogen (each kg of H gives 9 kg of water). Worked example. As received: HHV 15.0 MJ/kg, moisture 20 %, hydrogen 5.0 % (both of the as-received mass); latent heat 2.44 MJ per kg of water. Water per kg of fuel = 0.20 + 9 × 0.05 = 0.65 kg, so LHV = 15.0 − 2.44 × 0.65 = 13.41 MJ/kg.

Ash matters beyond its percentage. It dilutes the fuel, must be removed, and its alkali content (especially in straws and husks, which are high in ash) lowers the ash melting temperature, so deposits (slagging and fouling) form on boiler surfaces and in gasifier beds at lower temperatures than for coal-like ash. Ash fusion behaviour is therefore a design input beside the heating value.

⚠️ Do not mix bases or the two heating values
Percentages on different bases cannot be added or compared; dry-basis ash is always higher than as-received ash for a moist fuel. HHV is larger than LHV, never the reverse, and moisture lowers the usable energy twice: it dilutes the fuel and its evaporation takes latent heat.

4. Conversion methods

Conversion routes compared
RouteConditionsMain products
Combustionexcess air, 800 to 1000 °C flame zonehot flue gas, ash; heat for steam or direct use
Pyrolysisno oxygen; about 400 to 600 °Cchar, condensable bio-oil, non-condensable gas
Gasificationpartial oxidation with air, oxygen or steam; about 800 to 1000 °Cproducer gas: CO, H₂, CH₄, CO₂, N₂ with air
Anaerobic digestionno oxygen; mesophilic about 35 °C or thermophilic about 55 °Cbiogas (CH₄ and CO₂) and digestate

Pyrolysis heats biomass without oxygen. Slow pyrolysis (low heating rate, long residence) favours char; fast pyrolysis (very high heating rate, about 500 °C and vapour residence of the order of a second or two before quenching) maximises the liquid bio-oil. Gasification burns only part of the fuel: the equivalence ratio ER = actual air/stoichiometric air is well below 1 (typically 0.2 to 0.4); a higher ER gives a hotter gasifier and more CO₂ and N₂, so the heating value of the gas falls.

The main gasification reactions, per mole of carbon, are: C + H₂O → CO + H₂ (steam gasification, endothermic, +131 kJ/mol); C + CO₂ → 2CO (Boudouard, endothermic, +172 kJ/mol); CO + H₂O ⇌ CO₂ + H₂ (water-gas shift, mildly exothermic, −41 kJ/mol), which is used to raise the H₂ share. Steam reforming of methane, CH₄ + H₂O → CO + 3H₂ (endothermic, +206 kJ/mol), makes hydrogen-rich gas from biogas or from the methane in producer gas, and so does reforming of pyrolysis vapours.

Anaerobic digestion, in outline, has four stages: hydrolysis of polymers to soluble sugars, amino acids and fatty acids; acidogenesis to volatile fatty acids; acetogenesis to acetate and hydrogen; and methanogenesis by archaea to methane and CO₂. It runs near neutral pH (about 6.8 to 7.5), needs a balanced carbon-to-nitrogen ratio (of order 20 to 30), and is slowed by oxygen and by inhibitors. Worked example (air for gasification). For biomass CH₁.₄O₀.₆ (molar mass 23.02 g/mol) the stoichiometric oxygen is 1 + 1.4/4 − 0.6/2 = 1.05 mol per mol. With air as 21 mol % O₂ and molar mass 29 g/mol, stoichiometric air is 1.05/0.21 = 5 mol = 145 g per 23.02 g fuel, 6.30 kg/kg. At ER = 0.3 the air supplied is 0.3 × 6.30 = 1.89 kg per kg of fuel.

5. Biofuels: bioethanol, biodiesel and biogas

Bioethanol is made by yeast fermentation of sugars without oxygen: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. With molar masses 180.16 and 46.07 g/mol the theoretical yield is 2 × 46.07/180.16 = 0.511 kg ethanol per kg glucose (and 0.511 kg CO₂). Starch must first be hydrolysed to glucose, gaining a mass of water: 180.16/162.14 = 1.111 kg glucose per kg starch. Sugar crops ferment directly, starch crops need saccharification, and lignocellulose needs pretreatment and cellulase hydrolysis first. The beer is distilled and then dehydrated to fuel grade. Worked example. 1000 kg of dry starch at 90 % of the theoretical yield gives 1000 × 1.111 × 0.511 × 0.90 = 511 kg of ethanol.

Biodiesel is made from vegetable oils, animal fats or used oils by transesterification: one triglyceride plus three methanol gives three fatty acid methyl esters (FAME) and one glycerol, with an alkali (NaOH or KOH) catalyst. The ester is separated from the denser glycerol and washed. Free fatty acids in the feed must be low for an alkali catalyst, or they form soap. By mass, 885 kg of triolein and 96 kg of methanol give about 890 kg of methyl oleate and 92 kg of glycerol, so the ester yield is a little above the oil mass. Biodiesel is used neat or blended with petroleum diesel.

Biogas is mainly methane (typically 50 to 70 % by volume) and CO₂, with traces of H₂S that must be removed. Its yield depends on the volatile solids (VS) destroyed. Worked example. 100 kg VS per day is fed, 60 % is destroyed, and the yield is 0.8 m³ biogas per kg of VS destroyed with 60 % methane by volume: methane = 100 × 0.6 × 0.8 × 0.6 = 28.8 m³ per day. At a stated 36 MJ/m³ and a boiler efficiency of 0.55, the useful heat is 28.8 × 36 × 0.55 = 570 MJ per day. Upgrading removes CO₂ and H₂S to give a gas near pipeline quality.

Key takeaways

  • Biomass is renewable, near carbon-neutral when sustainably grown, but low in energy density: moisture, bulk density and dispersed supply drive the choice of route and the need for densification.
  • Lignocellulose is cellulose (crystalline glucose polymer), hemicellulose (amorphous sugars) and lignin (aromatic, resists conversion), plus extractives and ash. Lignin is the barrier to ethanol and calls for pretreatment.
  • Proximate: moisture, VM, FC (by difference), ash; ultimate: C, H, N, S, O. Convert bases with 1/(1 − M); LHV = HHV − latent heat × (moisture + 9H) per kg fuel.
  • Combustion uses excess air; pyrolysis has no oxygen (fast gives bio-oil, slow gives char); gasification is partial oxidation at ER of about 0.2 to 0.4; steam gasification, Boudouard and steam reforming are endothermic, water-gas shift is mildly exothermic.
  • Anaerobic digestion: hydrolysis, acidogenesis, acetogenesis, methanogenesis. Ethanol: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (0.511 kg/kg theoretical). Biodiesel: triglyceride + 3 methanol → 3 FAME + glycerol.

Practice questions (18)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. Which component of lignocellulosic biomass is a cross-linked aromatic polymer that protects the cellulose and resists biological conversion?

    1. Cellulose
    2. Lignin
    3. Hemicellulose
    4. Starch
    Show answer

    Answer: B — Lignin

    Lignin is the aromatic, cross-linked polymer of the cell wall. Hemicellulose is the amorphous sugar polymer that is easiest to break down, and cellulose is the glucose polymer that lignin shields.

  2. In the proximate analysis of a biomass sample, which quantity is normally obtained by difference?

    1. Moisture content
    2. Volatile matter
    3. Ash content
    4. Fixed carbon
    Show answer

    Answer: D — Fixed carbon

    Moisture, volatile matter and ash are measured by successive mass losses or the residue, and fixed carbon = 100 − moisture − VM − ash. The ash is weighed as the residue left on burning, not found by difference.

  3. A fuel is reported to have the same dry composition, but one sample has been left in the rain and now contains more moisture. Which statement is correct for the wet sample, compared with the dry one?

    1. Its as-received ash percentage is lower and its lower heating value is lower
    2. Its as-received ash percentage is higher and its lower heating value is higher
    3. Its as-received ash percentage is unchanged, because ash is inert
    4. Its dry-basis ash percentage is lower, because the water dilutes it
    Show answer

    Answer: A — Its as-received ash percentage is lower and its lower heating value is lower

    Moisture adds mass without ash, so ash per kg as received falls, while the energy per kg falls and extra heat is needed to evaporate the water, lowering the LHV. The dry-basis value is by definition independent of moisture.

  4. Which conditions favour the maximum yield of liquid bio-oil from biomass pyrolysis?

    1. Low heating rate, long vapour residence time and about 400 °C, so that the vapours crack and condense slowly into a heavy liquid
    2. Excess air and about 900 °C
    3. Very high heating rate, about 500 °C and a short vapour residence time followed by rapid cooling
    4. Steam at high pressure with a catalyst and without any external heating of the feed
    Show answer

    Answer: C — Very high heating rate, about 500 °C and a short vapour residence time followed by rapid cooling

    Fast pyrolysis at about 500 °C with a vapour residence of a second or two limits secondary cracking of the vapours, so the liquid yield is greatest. Slow pyrolysis with long residence favours char, and excess air is combustion, not pyrolysis.

  5. How does gasification differ from combustion in the air supply?

    1. Gasification uses more air than stoichiometric, so that all the carbon is burnt to CO₂ and the gas carries no fuel value
    2. Gasification uses no oxidant at all and relies on external heating of the feed alone
    3. Gasification is run at equal air and fuel masses
    4. Gasification uses less air than stoichiometric (equivalence ratio below 1), giving a fuel gas rich in CO and H₂
    Show answer

    Answer: D — Gasification uses less air than stoichiometric (equivalence ratio below 1), giving a fuel gas rich in CO and H₂

    Partial oxidation at ER below 1 gives CO and H₂ (and CH₄) as a combustible gas, while combustion uses excess air to reach CO₂ and H₂O. Using no oxidant at all describes pyrolysis.

  6. The water-gas shift reaction CO + H₂O ⇌ CO₂ + H₂ is applied downstream of a gasifier mainly to

    1. raise the hydrogen share of the gas, at the cost of converting some CO to CO₂
    2. remove all the nitrogen from the gas, so that the remaining gas is undiluted by air
    3. raise the heating value of the gas by making more CO from the steam and the CO₂
    4. convert the char left in the gasifier into condensable bio-oil
    Show answer

    Answer: A — raise the hydrogen share of the gas, at the cost of converting some CO to CO₂

    Shift converts CO and steam to H₂ and CO₂ and is mildly exothermic (−41 kJ/mol). It makes more H₂, not more CO, and has no effect on nitrogen or on char.

  7. Which is the correct order of the stages of anaerobic digestion?

    1. Methanogenesis, hydrolysis, acetogenesis, acidogenesis
    2. Acidogenesis, hydrolysis, methanogenesis, acetogenesis
    3. Hydrolysis, acidogenesis, acetogenesis, methanogenesis
    4. Hydrolysis, methanogenesis, acidogenesis, acetogenesis
    Show answer

    Answer: C — Hydrolysis, acidogenesis, acetogenesis, methanogenesis

    Polymers are first hydrolysed to soluble monomers, then acidified to volatile fatty acids, then converted to acetate and hydrogen, and last converted to methane and CO₂ by methanogens. Methanogenesis cannot come first, because it needs the products of the other stages.

  8. In the production of biodiesel by transesterification of a triglyceride with methanol, the by-product is

    1. Ethanol, three moles for each mole of triglyceride
    2. Glycerol, one mole for each mole of triglyceride
    3. Carbon dioxide, three moles for each mole of triglyceride
    4. Water only, with no second liquid phase
    Show answer

    Answer: B — Glycerol, one mole for each mole of triglyceride

    One triglyceride and three methanol give three methyl esters and one glycerol, which settles as a denser second phase. No CO₂ is released and no ethanol is formed in this route.

  9. Which statement about producing ethanol from straw (lignocellulose), as opposed to from sugarcane juice, is correct?

    1. A pretreatment and enzymatic hydrolysis must first release the sugars locked in cellulose and hemicellulose, because lignin shields them
    2. The straw can be fermented directly by yeast, since its sugars are free and need no prior treatment
    3. The straw needs transesterification with methanol first, as is done with vegetable oils
    4. The straw is first converted to biogas, which yeast then ferments to give the ethanol
    Show answer

    Answer: A — A pretreatment and enzymatic hydrolysis must first release the sugars locked in cellulose and hemicellulose, because lignin shields them

    The sugars in straw are bound in polymers shielded by lignin, so they must be released before yeast can use them. Sugarcane juice carries free sucrose that ferments directly. Transesterification belongs to oils, not to carbohydrates.

  10. In a biomass gasifier the equivalence ratio is raised from 0.25 to 0.40 with the same feed. Which pair of changes is expected?

    1. The gasifier gets cooler and the gas heating value rises, since less air is burnt
    2. The gasifier becomes a pyrolyser with no oxygen, giving mainly char and bio-oil
    3. Both the temperature and the heating value are unchanged, since the feed is the same and only the air differs
    4. The gasifier gets hotter and the gas heating value falls, with more CO₂ and N₂ in the gas
    Show answer

    Answer: D — The gasifier gets hotter and the gas heating value falls, with more CO₂ and N₂ in the gas

    More air burns more of the fuel, which releases heat and raises the temperature, but the products are CO₂ and H₂O and the nitrogen of the air dilutes the gas, so its heating value falls. Cooling with a higher heating value would follow from reducing the air.

  11. Which of the following statements about anaerobic digestion are correct?

    1. It takes place in the absence of oxygen
    2. The biogas is mainly methane and carbon dioxide
    3. The digestate left over can keep nutrients and be used as a fertiliser
    4. The methane-forming organisms work best at a strongly acidic pH of about 3
    Show answer

    Answer: A — It takes place in the absence of oxygen; B — The biogas is mainly methane and carbon dioxide; C — The digestate left over can keep nutrients and be used as a fertiliser

    The process is anaerobic, produces CH₄ and CO₂, and leaves a nutrient-bearing digestate. Methanogens prefer a near-neutral pH of about 6.8 to 7.5; strong acidity inhibits them.

  12. Which of the following statements about the characterisation of biomass fuels are correct?

    1. Proximate analysis reports moisture, volatile matter, fixed carbon and ash
    2. Fixed carbon is found by subtracting moisture, volatile matter and ash from 100 %
    3. The lower heating value of a fuel is always greater than its higher heating value
    4. For a moist fuel the dry-basis ash percentage is greater than the as-received value
    Show answer

    Answer: A — Proximate analysis reports moisture, volatile matter, fixed carbon and ash; B — Fixed carbon is found by subtracting moisture, volatile matter and ash from 100 %; D — For a moist fuel the dry-basis ash percentage is greater than the as-received value

    The statements on the proximate analysis items and on the by-difference rule are standard, and dividing by (1 − M) raises the ash percentage. HHV exceeds LHV by the latent heat of the water formed or present.

  13. Which of the following statements about biofuels are correct?

    1. Biodiesel made with methanol is a mixture of fatty acid methyl esters
    2. Fermentation of glucose to ethanol releases carbon dioxide
    3. Biogas is produced by aerobic digestion in the presence of excess air
    4. Ethanol fermentation by yeast needs a large supply of oxygen
    Show answer

    Answer: A — Biodiesel made with methanol is a mixture of fatty acid methyl esters; B — Fermentation of glucose to ethanol releases carbon dioxide

    FAME is the product of methanol transesterification, and C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ releases CO₂. Biogas comes from anaerobic digestion and ethanol fermentation is anaerobic.

  14. A biomass sample as received has moisture 12 %, volatile matter 60 %, fixed carbon 18 % and ash 10 % (all by mass). The volatile matter on a dry basis, in percent and to one decimal place, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 68.2

    Dry basis divides by (1 − 0.12) = 0.88: 60/0.88 = 68.18, i.e. 68.2 %. Verified in Python.

  15. A fuel as received has HHV = 15.0 MJ/kg, moisture 20 % and hydrogen 5.0 % (both by mass of the as-received fuel). Each kg of hydrogen burns to 9 kg of water, and the latent heat of water is 2.44 MJ/kg. The lower heating value in MJ/kg, to two decimal places, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 13.41

    Water per kg of fuel = 0.20 + 9 × 0.05 = 0.65 kg; LHV = 15.0 − 2.44 × 0.65 = 15.0 − 1.586 = 13.414, i.e. 13.41 MJ/kg. Leaving out the hydrogen water would give 14.51.

  16. 1000 kg of dry starch is hydrolysed to glucose (180.16 kg of glucose from 162.14 kg of starch) and fermented via C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ at 90 % of the theoretical yield. Take molar masses of glucose 180.16 and ethanol 46.07 g/mol. The mass of ethanol in kg, rounded to the nearest whole number, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 511

    Glucose = 1000 × 180.16/162.14 = 1111.2 kg; theoretical ethanol = 1111.2 × 2 × 46.07/180.16 = 568.3 kg; at 90 % = 511.4, i.e. 511 kg. Verified in Python (511.44).

  17. A digester is fed 100 kg of volatile solids per day, of which 60 % is destroyed. The yield is 0.8 m³ of biogas per kg of volatile solids destroyed, and the biogas is 60 % methane by volume. Taking the heating value of methane as 36 MJ/m³ and a boiler efficiency of 0.55, the useful heat in MJ per day, rounded to the nearest whole number, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 570

    VS destroyed = 60 kg; biogas = 60 × 0.8 = 48 m³; methane = 48 × 0.6 = 28.8 m³; heat = 28.8 × 36 = 1036.8 MJ; useful = 1036.8 × 0.55 = 570.2, i.e. 570 MJ/day.

  18. Biomass of empirical formula CH₁.₄O₀.₆ has a molar mass of 23.02 g/mol. Air is taken as 21 mol % oxygen with a molar mass of 29 g/mol. For complete combustion to CO₂ and H₂O the stoichiometric oxygen is 1.05 mol per mol of biomass. A gasifier runs at an equivalence ratio of 0.3. The mass of air supplied per kg of biomass, in kg and to two decimal places, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 1.89

    Stoichiometric air = 1.05/0.21 = 5.00 mol per mol fuel = 5.00 × 29 = 145 g per 23.02 g = 6.299 kg/kg. At ER = 0.3: 0.3 × 6.299 = 1.890, i.e. 1.89 kg/kg. Verified in Python.