Bioenergy: biomass, biogas and biofuels

In short

Bioenergy is energy recovered from biomass - any recently living organic matter such as crop residue, wood, dung, molasses, municipal organic waste or algae - by burning it, converting it to a gas, or converting it to a liquid fuel. Unlike solar and wind it is carbon-based, dispatchable and storable, so it substitutes for coal in boilers, for LPG and CNG in cooking and transport, and for petrol and diesel in vehicles. India has roughly 750 million tonnes of biomass produced annually with about 230 million tonnes surplus, giving an assessed power potential near 28 GW (MNRE estimates; figures are revised periodically), and bioenergy policy is central to the ethanol blending programme, the SATAT compressed biogas scheme and paddy-stubble management in Punjab and Haryana. It is examined in GATE (Energy/ME/CH), university renewable energy papers and the energy sections of UPSC/ESE, and it is tested in interviews at Praj Industries, Thermax, Indian Oil, BPCL, HPCL, EID Parry, Balrampur Chini, Gail, NTPC and sugar-cogeneration companies.

Explained

Biomass: resource, properties and characterisation

• Biomass - organic material of recent biological origin: agro-residues (rice husk, straw, bagasse, cotton stalk, mustard stalk), woody biomass, energy crops (napier grass, sweet sorghum), animal dung, sewage, industrial organic effluent, the organic fraction of municipal solid waste, and algae.
• Carbon-neutral in principle: CO₂ released on combustion was fixed from the air during growth. Not neutral in practice - fertiliser, diesel for harvest and transport, and land-use change add emissions.
• Proximate analysis gives moisture, volatile matter (biomass is high, 70-85% of dry mass, so it ignites and flames readily), fixed carbon and ash. Ultimate analysis gives C, H, O, N, S, ash as percentages - biomass is roughly 45-50% C, 6% H, 40-45% O, with very low S (<0.1%) and low N.
• Compared with coal: biomass has high oxygen, so lower calorific value; very low sulphur, so negligible SO₂; low ash but alkali-rich ash (K, Na, Cl) that causes slagging, fouling and bed agglomeration in boilers. Ash fusion temperature can be below 800 °C for straw.
• Calorific values (approximate, air-dry): rice husk 13-15 MJ/kg, bagasse (50% moisture) 9-10 MJ/kg, wood 16-19 MJ/kg, briquettes 16-18 MJ/kg, biogas 20-25 MJ/m³, producer gas 4-6 MJ/m³, ethanol 26.8 MJ/kg, biodiesel 37-40 MJ/kg, diesel 42-45 MJ/kg.
• HHV vs LHV: higher (gross) calorific value includes the latent heat of the water vapour formed; lower (net) value excludes it. Biomass is high in H and moisture, so the gap is large - always state which is used.
• Bulk density and energy density are the real problem: loose straw is ~50-100 kg/m³, so transport beyond roughly 50-100 km is uneconomic. Hence densification: briquetting (60-90 mm, screw-press or ram-piston) and pelletisation (6-10 mm, used for co-firing in thermal plants and for industrial boilers).
• Dulong-type correlation estimates HHV from ultimate analysis; it is approximate for biomass because of the high oxygen content, and bomb-calorimeter testing is preferred.

Thermochemical conversion

1. Direct combustion - complete oxidation with excess air in a grate-fired, fluidised-bed (BFB/CFB) or suspension-fired boiler, raising steam for a Rankine cycle. Typical biomass power plant efficiency is only 20-28% because of small size and modest steam parameters.
2. Co-firing - biomass pellets fired with coal in existing thermal units. India's policy (MoP/CEA directions) mandates biomass pellet co-firing in coal-based plants - commonly cited at 5%, with a higher target announced for later years; the mandated percentage and dates have been revised more than once, so quote the current MoP revised policy rather than a fixed number.
3. Gasification - partial oxidation at 700-1,200 °C with a sub-stoichiometric air (or O₂/steam) supply to give producer gas (CO 18-22%, H₂ 15-19%, CH₄ 1-5%, CO₂, N₂; LHV 4-6 MJ/m³ with air). Zones in a downdraft gasifier from the top: drying → pyrolysis → combustion (oxidation) → reduction.
   • Key reactions: C + O₂ → CO₂ (exothermic, supplies heat); Boudouard C + CO₂ ⇌ 2CO (endothermic); water-gas C + H₂O ⇌ CO + H₂; water-gas shift CO + H₂O ⇌ CO₂ + H₂; methanation C + 2H₂ ⇌ CH₄.
   • Types: updraft (counter-current - high tar, high thermal efficiency, suitable for direct heat); downdraft (co-current - gas passes through the hot oxidation zone and tar cracks, so lowest tar; standard for engine applications up to ~1 MW); cross-draft (fast response, charcoal); fluidised bed (bubbling/circulating - uniform temperature, scalable, fuel-flexible, moderate tar); entrained flow (large scale, syngas for chemicals).
   • Equivalence ratio (actual air / stoichiometric air) of about 0.2-0.3 is the gasification window; below it, pyrolysis dominates, above it, combustion. Tar is the main practical failure mode - it fouls cyclones, filters and engine valves; cleaning uses cyclone + scrubber + fabric/sand filter, or catalytic/thermal cracking.
   • Use: thermal (kilns, dryers), 100% producer-gas spark-ignition engines (derate ~50% of diesel rating), or dual-fuel diesel engines (60-80% diesel replacement).
4. Pyrolysis - thermal decomposition at 400-600 °C without oxygen, giving char, condensable bio-oil and gas. Slow pyrolysis maximises charcoal; fast pyrolysis (very short vapour residence time, rapid quench) maximises bio-oil at 60-75%. Bio-oil is acidic (pH 2-3), oxygen-rich, high viscosity and unstable - it needs hydrodeoxygenation before use as a transport fuel.
5. Torrefaction - mild pyrolysis at 200-300 °C; produces a dry, brittle, hydrophobic, grindable "bio-coal" with ~20-23 MJ/kg, suited to co-firing.
6. Liquefaction / HTL - high pressure, 250-370 °C in water; handles wet feedstock without drying.

Biochemical conversion: biogas and anaerobic digestion

• Anaerobic digestion (AD) - microbial breakdown of organic matter in the absence of oxygen in four stages: hydrolysis (polymers → sugars, amino acids, fatty acids) → acidogenesis (→ volatile fatty acids) → acetogenesis (→ acetate, H₂, CO₂) → methanogenesis (→ CH₄ + CO₂ by archaea). Hydrolysis is rate-limiting for lignocellulosic feed; methanogens are the most sensitive step.
• Biogas composition: CH₄ 50-70%, CO₂ 30-45%, plus H₂S (traces to 1%), N₂, water vapour, NH₃. CV about 20-25 MJ/m³ at 55-60% CH₄.
• Operating conditions: pH 6.8-7.2; temperature mesophilic 30-40 °C (robust, common in India) or thermophilic 50-57 °C (faster, higher gas rate, less stable); C/N ratio 20-30:1 (dung ~25, poultry litter and night soil too low, straw too high - so co-digestion); total solids 7-9% for wet digestion, >20% for dry; HRT 30-50 days in India (shorter where warm), 15-25 days thermophilic; gentle stirring; exclusion of air, antibiotics, detergents and heavy metals.
• Inhibition: free ammonia (from high-N feed), H₂S, VFA accumulation with falling pH, sudden organic overload (high OLR), and temperature swings - all show up as a drop in gas and a rise in CO₂.
• Plant types (Indian): KVIC floating-drum (constant gas pressure, visible gas store, mild-steel drum corrodes), Deenbandhu fixed-dome (brick dome, no moving part, cheapest, variable pressure, leaks if masonry cracks), Janata fixed-dome, flexible balloon/bag digesters, and prefabricated HDPE/FRP units now widely sold. Appliances are specified under IS 8749 for biogas stoves and related IS standards for fixed-dome plants and pipe fittings - quote the clause only from the current edition.
• Slurry/digestate is a valuable fertiliser: nitrogen is converted to the more available ammoniacal form, pathogens and weed seeds are reduced. This is why the national programme is framed around GOBARdhan and fermented organic manure.
• Upgrading to CBG (compressed biogas): remove CO₂ (water scrubbing, PSA, amine, or membrane), remove H₂S (iron sponge, activated carbon, biological), dry, then compress to 200-250 bar. CBG is >90% CH₄ and interchangeable with CNG; IS 16087 is the Indian specification for biogas/CBG as automotive fuel (confirm the current amendment). SATAT (Oil & Gas marketing companies, since 2018) assures offtake of CBG; CBG blending in CNG/PNG is being implemented in phases under the CBG-CGD blending obligation.
• Landfill gas is AD occurring in a landfill - roughly 50% CH₄; must be collected and flared or used, since methane has a far higher global warming potential than CO₂ (about 28-30 times over 100 years, IPCC AR5/AR6 values differ slightly).

Liquid biofuels: ethanol, biodiesel and advanced fuels

• Bioethanol (1G) - from sugar/starch: sugarcane juice, B-heavy and C-heavy molasses, sugar syrup, damaged food grain and surplus maize/rice. Steps: milling → (for starch) liquefaction and saccharification with α-amylase and glucoamylase → fermentation by *Saccharomyces cerevisiae* → distillation → dehydration by molecular sieve to anhydrous (99.5%+) fuel ethanol. Azeotrope with water at ~95.6% by mass is why plain distillation cannot reach fuel grade.
• Theoretical yield: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂, i.e. 0.511 kg ethanol per kg glucose (51.1%); plants achieve 85-92% of this.
• 2G ethanol - from lignocellulose (rice straw, bagasse, cotton stalk, bamboo): pretreatment (steam explosion, dilute acid, alkali) → enzymatic hydrolysis of cellulose and hemicellulose → fermentation of C6 and C5 sugars → distillation. Lignin is burnt for process energy. India's 2G refineries include Panipat (IOCL), Bargarh (BPCL) and Numaligarh; costs remain higher than 1G.
• Properties and engine effects: ethanol has ~2/3 the energy per litre of petrol (21.1 vs ~32 MJ/L), high octane (RON ~108), high latent heat (cooling, cold-start issues), is hygroscopic and corrodes some metals and elastomers. E20 petrol (20% ethanol) rollout is complete across the country, with E20-compliant and flex-fuel vehicles mandated/being introduced; fuel economy falls a few percent on E20 in older material-compatible-only vehicles.
• Biodiesel - fatty acid methyl esters (FAME) made by transesterification: triglyceride + 3 methanol ⇌ 3 FAME + glycerol, with KOH/NaOH (alkali) catalyst at 55-65 °C, methanol-to-oil molar ratio ~6:1, 1 hour. High free fatty acid feed (used cooking oil, non-edible oils) needs acid esterification first (H₂SO₄) to avoid soap formation. Indian feedstock is non-edible and waste: jatropha, pongamia (karanja), mahua, rice bran acid oil and used cooking oil (RUCO scheme, FSSAI) - edible oil is not diverted.
  • Key specs: IS 15607 for B100 biodiesel, IS 1460 for diesel blends; cetane number is high (48-60), sulphur near zero, lubricity good, but cloud/pour point is poor, oxidation stability is low and it is a solvent that can clean deposits into filters. B7 is permitted up to 7% in diesel; B20 is notified for specified applications. Straight vegetable oil is not biodiesel - its viscosity is roughly 10 times that of diesel.
• Bio-CNG/CBG, bio-hydrogen, bio-methanol, DME, HVO and bio-ATF (SAF) are the advanced routes; methanol blending (M15) and SAF blending targets (indicative 1% from 2027, 2% from 2028 for international flights) are being implemented - treat the exact targets as policy that is still being notified.
• Generations: 1G - food crops (food-versus-fuel debate); 2G - agricultural and forest residue; 3G - algae (high oil yield per hectare, no arable land, but costly harvesting/dewatering); 4G - engineered organisms and carbon-capture-coupled routes.

Indian policy, institutions and economics

• National Policy on Biofuels, 2018 (amended 2022) - advanced the 20% ethanol blending target to ESY 2025-26 (achieved; E20 is now the mainstream petrol grade), permits more feedstock, and indicative 5% biodiesel blending by 2030.
• MNRE programmes: National Bioenergy Programme (2021-26) with sub-schemes - Waste to Energy, Biomass (briquette/pellet and non-bagasse cogeneration) and Biogas (family-size and medium-size plants); GOBARdhan and SBM-G for rural biogas; SATAT for CBG; Waste to Energy and biomass co-firing on the power side.
• Electricity sale is through PPAs at SERC-determined tariffs under the Electricity Act, 2003; renewable purchase obligations and the Renewable Generation Obligation apply, and Energy Savings Certificates/carbon credit trading under the Energy Conservation (Amendment) Act, 2022 and the CCTS now cover biomass-based savings.
• Environmental compliance: CPCB/SPCB consent under the Water Act 1974 and Air Act 1981, EIA notification where applicable, and the Solid Waste Management Rules, 2026 (which replaced the 2016 Rules from 1 April 2026) for processing the organic fraction of MSW; boilers are certified under the Boilers Act, 2025 with the Indian Boiler Regulations, 2026. Occupational safety at plants falls under the OSH&WC Code, 2020 and the other Labour Codes in force from 21 November 2025.
• GST: biogas/CBG and most renewable devices and parts sit in the lower slabs after the 22 September 2025 rationalisation to 5%/18%/40% - verify the current HSN-wise rate before quoting it in a costing.
• Advantages: dispatchable and storable renewable; uses waste; rural employment and farmer income; reduces stubble burning and import of crude and LPG; digestate replaces chemical fertiliser; existing boiler and engine infrastructure can be reused.
• Limitations: low energy density and seasonal, scattered supply with volatile prices; large land and water footprint for energy crops; food-versus-fuel and feed/fodder competition; slagging and fouling from alkali ash; tar in gasifiers; high cost of 2G and CBG upgrading; air pollution (PM, CO, NOx) if burnt badly; methane leakage can negate the climate benefit.

Key points

• Biomass - recently living organic matter used as fuel; carbon-neutral only on a full life-cycle basis.
• Volatile matter - 70-85% of dry biomass, so it ignites easily and burns with a long flame; needs secondary air above the grate.
• Low sulphur, alkali-rich ash - negligible SO₂ but serious slagging, fouling and bed agglomeration.
• Densification - briquetting (60-90 mm) and pelletisation (6-10 mm) raise bulk density and make transport and co-firing viable.
• Gasification - partial oxidation at equivalence ratio 0.2-0.3 giving producer gas of 4-6 MJ/m³ with air.
• Downdraft gasifier - lowest tar because gas passes through the hot zone; the standard choice for engines.
• Tar - the main cause of gasifier engine failures; removed by cracking, cyclones, scrubbers and filters.
• Pyrolysis - oxygen-free thermal decomposition; fast pyrolysis gives 60-75% bio-oil, slow gives charcoal.
• Torrefaction - 200-300 °C upgrade to hydrophobic, grindable bio-coal of ~20-23 MJ/kg.
• Anaerobic digestion - hydrolysis → acidogenesis → acetogenesis → methanogenesis; hydrolysis is usually rate-limiting.
• Biogas - 50-70% CH₄, 20-25 MJ/m³; best at pH 6.8-7.2, 30-40 °C mesophilic, C/N 20-30, HRT 30-50 days.
• Fixed-dome vs floating-drum - fixed dome has no moving part and variable pressure; floating drum gives constant pressure but the drum corrodes.
• CBG - biogas upgraded to >90% CH₄ and compressed to 200-250 bar; marketed under SATAT, specified by IS 16087.
• Ethanol - 0.511 kg per kg glucose theoretical; anhydrous fuel grade needs molecular sieve because of the water azeotrope.
• E20 - 20% ethanol in petrol, now the mainstream grade; ~2/3 the volumetric energy of petrol but RON ~108.
• Transesterification - oil + methanol with alkali catalyst → FAME + glycerol; high-FFA oils need acid pre-esterification.
• Biodiesel specs - IS 15607 for B100, IS 1460 for blends; high cetane, poor cold flow and oxidation stability.
• Generations - 1G food crops, 2G residues, 3G algae, 4G engineered; India's policy pushes 2G and waste-based routes.

Formulas and examples

Example 1 - Rice-husk power plant sizing. A plant burns 1 tonne/hour of rice husk of CV 14 MJ/kg at an overall efficiency of 24%. Find the electrical output and annual generation at 75% plant load factor.

• Fuel heat input = 1,000 kg/h × 14 MJ/kg = 14,000 MJ/h = 14,000/3,600 = 3.889 MW thermal.
• Electrical output = 0.24 × 3.889 = 0.933 MW ≈ 933 kW.
• Heat rate = 3,600/0.24 = 15,000 kJ/kWh (against ~9,500-10,500 for a large coal unit - the penalty of small size).
• Annual generation = 933 kW × 8,760 h × 0.75 = 6.13 million kWh. At ₹7/kWh the revenue is about ₹4.29 crore/year; the husk bill at ₹2,500/tonne and 6,570 tonnes consumed is about ₹1.64 crore/year.

Example 2 - Family biogas plant for a dairy household. Five cattle give 50 kg dung/day. Dung is diluted 1:1 with water; biogas yield is 0.036 m³ per kg of fresh dung; HRT = 40 days; biogas CV = 22 MJ/m³; a stove needs 0.25 m³/h.

• Daily gas = 50 × 0.036 = 1.8 m³/day → energy 1.8 × 22 = 39.6 MJ/day.
• Slurry = 50 kg dung + 50 kg water ≈ 100 litres = 0.1 m³/day → digester volume = 0.1 × 40 = 4 m³ (so a 4 m³ Deenbandhu or KVIC plant; gas holder is usually sized for 40-60% of a day's gas, i.e. 0.7-1.1 m³).
• Cooking hours = 1.8/0.25 = 7.2 hours/day - enough for a family of five.
• LPG displaced: LPG CV ≈ 46 MJ/kg, and allowing a stove efficiency of 55% for LPG and 55% for biogas, useful energy 39.6 × 0.55 = 21.8 MJ/day; equivalent LPG = 21.8/(46 × 0.55) = 0.86 kg/day ≈ 26 kg/month, i.e. nearly 1.8 domestic cylinders of 14.2 kg a month.

Example 3 - COD-based methane from a distillery effluent. A distillery discharges 500 m³/day of spentwash at COD 100,000 mg/L; 80% COD is removed in a UASB reactor; 0.35 m³ CH₄/kg COD removed; biogas is 65% CH₄.

• COD load = 500 m³ × 100 kg/m³ = 50,000 kg/day. COD removed = 0.8 × 50,000 = 40,000 kg/day.
• CH₄ = 40,000 × 0.35 = 14,000 m³/day → biogas = 14,000/0.65 = 21,538 m³/day.
• Energy, taking CH₄ LHV 35.8 MJ/m³ = 14,000 × 35.8 = 501,200 MJ/day = 5.80 MW thermal.
• At 35% engine efficiency, electrical output = 0.35 × 5.80 = 2.03 MW, about 48,700 kWh/day.

Example 4 - Ethanol from C-heavy molasses. A distillery processes 300 tonnes/day of C-heavy molasses containing 45% total fermentable sugars; fermentation and distillation efficiency is 88% of theoretical; ethanol density 0.789 kg/L.

• Sugar = 300 × 0.45 = 135 tonnes/day = 135,000 kg.
• Theoretical ethanol = 135,000 × 0.511 = 68,985 kg → actual = 0.88 × 68,985 = 60,707 kg/day.
• Volume = 60,707/0.789 = 76,942 L/day ≈ 77 KLPD. Yield = 76,942/300,000 kg ≈ 256 L per tonne of molasses (the field figure for C-heavy is about 220-260 L/t, so this is consistent).

Example 5 - E20 energy and mileage effect. Petrol CV ≈ 32 MJ/L, ethanol ≈ 21.1 MJ/L.

• E20 CV = 0.8 × 32 + 0.2 × 21.1 = 25.6 + 4.22 = 29.82 MJ/L, a drop of (32 − 29.82)/32 = 6.8%.
• So, at the same engine efficiency, a car giving 18 km/L on E0 would give 18 × 0.932 ≈ 16.8 km/L on E20. Engines calibrated for E20 recover part of this through higher compression ratio and spark advance using ethanol's octane.

Where it is used

• Sugar-industry bagasse cogeneration - Maharashtra, UP and Karnataka mills export surplus power to the grid at SERC tariffs; high-pressure boilers (67-110 bar) with extraction-cum-condensing turbines. Roles: boiler and turbine operation, cogeneration design, energy audit.
• Distilleries and ethanol plants - Praj Industries, Balrampur Chini, EID Parry, Shree Renuka; dual-feed 1G plants, 2G refineries at Panipat, Bargarh and Numaligarh; roles in fermentation process engineering, distillation, molecular sieve operation and ETP/biomethanation.
• CBG plants under SATAT - IOCL/BPCL/HPCL/GAIL offtake; feedstock press mud, napier grass, cattle dung and MSW organics; roles in digester design, upgrading (PSA/membrane), compression and gas-grid injection.
• Rural and institutional biogas - KVIC and Deenbandhu plants, GOBARdhan and SBM-G projects, gaushala-scale digesters; roles in rural energy, implementation and subsidy administration for MNRE/state nodal agencies.
• Biomass pellet supply and co-firing - pellet manufacturing for NTPC and state thermal plants (notably for paddy straw around Delhi NCR); roles in fuel supply chain, pellet plant operations and boiler fuel testing.
• Gasifier-based thermal and captive power - rice mills, dyeing units, ceramic and foundry furnaces replacing furnace oil/LPG; biomass boiler and gasifier OEMs (Thermax, Ankur Scientific, Chanderpur).
• Waste-to-energy and municipal projects - biomethanation of wet waste under the Solid Waste Management Rules, 2026 and landfill gas recovery; roles with municipal corporations and WtE developers.

Interview questions

• Q: Why is biomass called carbon-neutral, and when is that claim wrong? A: The CO₂ released on burning was absorbed during growth, so the biogenic carbon cycles. It is wrong if the biomass is harvested faster than it regrows, or if land-use change, fertiliser and transport emissions are large.
• Q: What is the difference between combustion, gasification and pyrolysis? A: All are thermochemical: combustion uses excess air to get heat, gasification uses sub-stoichiometric air (ER 0.2-0.3) to get a combustible gas, pyrolysis uses no oxygen to get char, bio-oil and gas.
• Q: Why is a downdraft gasifier preferred for engine applications? A: The pyrolysis gases pass down through the hot oxidation zone, where tar is cracked, so the gas has the lowest tar content and does not foul the engine.
• Q: Which biomass suits anaerobic digestion and which suits gasification? A: Wet, easily biodegradable, low-lignin feed (dung, food waste, press mud, effluent) goes to digestion; dry, low-moisture, sized lignocellulosic feed (husk, wood chips, briquettes) goes to gasification or combustion.
• Q: Why does a biogas plant stop giving gas in winter, and what is done about it? A: Mesophilic methanogens slow sharply below about 20 °C; remedies are insulation, a greenhouse cover, warm feed water, slurry recirculation, or heating the digester with part of the gas.
• Q: What is the significance of the C/N ratio? A: 20-30:1 balances carbon as the energy source against nitrogen for cell growth; too low releases free ammonia which inhibits methanogens, too high starves the bacteria and gas falls.
• Q: Why must H₂S be removed before using biogas in an engine or grid? A: It forms sulphuric acid with moisture, corroding engine parts, pipelines and compressors, and it gives SO₂ on combustion; iron sponge, activated carbon or biological scrubbing is used.
• Q: Why can't plain distillation produce fuel-grade ethanol? A: Ethanol and water form an azeotrope at about 95.6% ethanol by mass, so dehydration by molecular sieve (or azeotropic/extractive distillation) is needed for 99.5%+ anhydrous ethanol.
• Q: Why does mileage fall on E20 but power need not? A: E20 carries about 7% less energy per litre, so more fuel is used per km; but ethanol's high octane and charge cooling allow higher compression and advanced timing, which recovers power in an engine designed for it.
• Q: Why is straight vegetable oil not used in place of biodiesel? A: Its viscosity is about ten times that of diesel, so atomisation is poor and it causes injector coking and oil dilution; transesterification cuts the viscosity and the molecular weight.
• Q: What goes wrong when the feed oil has a high free fatty acid content? A: The alkali catalyst reacts with the FFAs to form soap, which emulsifies the mixture, consumes catalyst and ruins glycerol separation - so acid esterification is done first.
• Q: Biomass is low in sulphur, so why do biomass boilers have ash problems? A: The ash is rich in potassium, sodium, chlorine and silica with a low fusion temperature, causing slagging on tubes, fouling of superheaters and agglomeration of fluidised beds.
• Q: Why is the economic transport radius for loose biomass so small? A: Bulk density of loose residue is 50-100 kg/m³, so a truck carries very little energy; beyond roughly 50-100 km freight exceeds fuel value unless the biomass is briquetted or pelletised.

Common mistakes

• Mixing HHV and LHV, or quoting an as-received calorific value against a dry-basis one, and then reporting an efficiency above 100%.
• Treating producer gas (4-6 MJ/m³) as comparable to natural gas (~36 MJ/m³) and forgetting that engines derate about 50% on 100% producer gas.
• Using the whole mass of wet feed instead of volatile solids in a biogas yield calculation, which overstates gas by several times.
• Sizing a digester from the dung weight alone and ignoring the dilution water - digester volume is daily slurry volume × HRT.
• Confusing HRT with SRT, or confusing the digester volume with the gas-holder volume (the holder stores only part of a day's gas).
• Assuming the theoretical 0.511 kg ethanol per kg sugar is achieved; real plants reach only 85-92% of it.
• Calling straight vegetable oil "biodiesel", or omitting the acid pre-treatment step for high-FFA used cooking oil.
• Quoting the old 2016 solid waste rules, pre-2022 blending targets or withdrawn IS numbers; and assuming biomass power is as cheap per kWh as solar - fuel cost is a continuing expense that solar does not have.

Related topics

Renewable energy systems: solar, wind and storage · Hydrogen and the energy transition · Small hydro, geothermal and ocean energy · Steam power plants and boilers · IC engines and alternative fuels · Environmental pollution control and waste management