GATE — Agricultural Engineering (AG) · India · IITs and IISc on rotation (2026: IIT Guwahati)
Original practice questions written to the published syllabus · not past papersTurning crop residues, dung and oils into fuel: briquettes, gasifiers, biogas plants and liquid biofuels, and auditing farm energy.
In the exams: ICAR AIEEA-PG Unit V; ICAR AICE subject 42 Units 1–3 and 8; ICAR AICE subject 39 Unit 9; GATE AG biogas sizing (2 of 8 papers)
GATE — Agricultural Engineering (AG)
Sources of Power section: short numericals rather than theory. Biogas plant sizing (digester volume from dung, dilution and HRT; daily gas; gas holder) has appeared in a minority of recent papers; expect also heating-value, gasifier efficiency and ethanol or biodiesel stoichiometry at one or two marks. Units and bases (wet vs dry, kWh vs MJ) decide marks.
How our questions for GATE spread across the units of Bio-energy Systems: Design & Applications. Difficulty: 7 easy, 25 medium, 10 hard.
| Biogas plant sizing and gas yield | 9 | |
| Gasification: updraft and downdraft gasifiers | 5 | |
| Anaerobic digestion stages | 4 | |
| Bio-ethanol | 4 | |
| Biodiesel and transesterification | 4 | |
| Calorific value | 3 | |
| Combustion of biomass | 3 | |
| Pyrolysis | 3 | |
| Biomass characterisation: proximate and ultimate analysis | 2 | |
| Producer gas and dual-fuel engines | 2 | |
| Briquetting and pelleting | 1 | |
| Biogas plant types: floating drum and fixed dome | 1 | |
| Energy audit and energy ratio in agriculture | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A sample of crop residue has, on an as-received basis, moisture 12%, volatile matter 62% and ash 8%. Its ash content on a dry basis is closest to:
C. 9.09%
Dry-basis value = as-received value ÷ (1 − moisture fraction) = 8 ÷ (1 − 0.12) = 9.09%. Leaving the 8% unchanged ignores the basis; dividing by (1 − moisture − ash) = 0.80 (10.00%) is the dry ash-free conversion, which cannot be applied to ash itself. Multiplying by (1 + M), 8 × 1.12 = 8.96%, is a common but wrong shortcut.
A dry biomass has a higher heating value of 18.5 MJ/kg and contains 6% hydrogen (dry basis). Taking the latent heat of water as 2.442 MJ/kg, its lower heating value (dry basis) is:
D. 17.18 MJ/kg
Water formed = 9 × H = 9 × 0.06 = 0.54 kg per kg fuel. LHV = HHV − 2.442 × 0.54 = 18.5 − 1.32 = 17.18 MJ/kg. Forgetting the factor 9 (1 kg H2 forms 9 kg H2O) gives 18.35 MJ/kg; adding instead of subtracting (19.82 MJ/kg) reverses the definition; 8.51 MJ/kg multiplies the HHV by (1 − 9H) = 0.46 instead of subtracting the latent heat.
A downdraft gasifier consumes 50 kg/h of wood chips and 75 kg/h of air. The stoichiometric air–fuel ratio of the wood is 6 kg/kg. The equivalence ratio is:
B. 0.25
Actual A/F = 75 ÷ 50 = 1.50 kg/kg. ER = actual ÷ stoichiometric = 1.50 ÷ 6 = 0.25, within the usual gasification range of about 0.2–0.4. 1.50 is the actual air–fuel ratio, not the ER; 4.00 is its inverse. 0.67 is the fuel–air ratio (50 ÷ 75).
An air-blown gasifier uses 1.3 Nm³ of air per kg of biomass. The dry producer gas contains 50% N2 by volume, and the fuel's own nitrogen is negligible. Using a nitrogen balance (air is 79% N2 by volume), the dry gas yield is:
B. 2.05 Nm³ per kg biomass
N2 is inert, so N2 in = N2 out: 1.3 × 0.79 = 1.027 Nm³ = 0.5 × gas volume. Gas volume = 1.027 ÷ 0.5 = 2.05 Nm³/kg. Dividing the whole air volume by 0.5 (2.60) forgets that air is only 79% nitrogen. The tie-element method is the standard way to close a gasifier mass balance.
Producer gas contains, by volume, CO 20%, H2 18%, CH4 2%, the rest CO2 and N2. The stoichiometric air requirement (air 21% O2 by volume) is:
A. 1.10 m³ air per m³ gas
O2 needed: CO + ½O2 → CO2 gives 0.2 × 0.5; H2 + ½O2 → H2O gives 0.18 × 0.5; CH4 + 2O2 → CO2 + 2H2O gives 0.02 × 2. Total O2 = 0.10 + 0.09 + 0.04 = 0.23 m³/m³. Air = 0.23 ÷ 0.21 = 1.10 m³/m³. Using 0.232 mixes the mass fraction into a volume calculation. The ratio is close to 1:1, which is why producer-gas mixers admit roughly equal volumes of gas and air.
A family of 5 needs 0.34 m³ of biogas per person per day for cooking. The gas yield is 0.04 m³ per kg of fresh dung, dung is mixed 1:1 with water (slurry 1000 kg/m³), and the HRT is 50 days. The required digester volume is about:
C. 4.25 m³
Gas demand = 5 × 0.34 = 1.70 m³/day. Dung = 1.70 ÷ 0.04 = 42.5 kg/day; slurry = 85 kg/day = 0.085 m³/day. Volume = 0.085 × 50 = 4.25 m³. Multiplying gas demand by HRT (85.0 m³) confuses gas volume with slurry volume. 2.13 m³ forgets the dilution water (42.5 kg dung only, 0.0425 × 50).
A wheat crop has a total energy input of 15000 MJ/ha and an energy output (grain + straw) of 90000 MJ/ha. The energy ratio and the net energy are:
D. 6.0 and 75000 MJ/ha
Energy ratio = 90000 ÷ 15000 = 6.0. Net energy = 90000 − 15000 = 75000 MJ/ha. The inverse (0.17) is input per unit output.
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