Nepal Engineering Council Registration Exam — Agricultural Engineering · Nepal · Nepal Engineering Council
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)
How our questions for Nepal Engineering Council Registration Exam spread across the units of Bio-energy Systems: Design & Applications. Difficulty: 8 easy, 19 medium, 5 hard.
| Biogas plant sizing and gas yield | 7 | |
| Anaerobic digestion stages | 5 | |
| Biogas plant types: floating drum and fixed dome | 5 | |
| Briquetting and pelleting | 2 | |
| Gasification: updraft and downdraft gasifiers | 2 | |
| Producer gas and dual-fuel engines | 2 | |
| Biodiesel and transesterification | 2 | |
| Energy audit and energy ratio in agriculture | 2 | |
| Combustion of biomass | 1 | |
| Pyrolysis | 1 | |
| Bio-ethanol | 1 | |
| Biomass characterisation: proximate and ultimate analysis | 1 | |
| Calorific value | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
The correct order of the four stages of anaerobic digestion is:
C. Hydrolysis, acidogenesis, acetogenesis, methanogenesis
Complex polymers (carbohydrates, proteins, fats) are first hydrolysed to sugars, amino acids and fatty acids; acidogenic bacteria ferment these to volatile fatty acids, alcohols, H2 and CO2; acetogens convert the longer acids to acetate, H2 and CO2; methanogenic archaea finally produce methane from acetate or from H2 and CO2.
The optimum pH range for methanogenic digestion is about:
D. 6.8–7.2
Methanogens are the most pH-sensitive group; methane production is best near neutral pH, roughly 6.8–7.2, and falls sharply below about 6.5. A falling pH signals accumulating volatile fatty acids (souring), usually from overloading. pH 4–5 suits yeast fermentation to ethanol, not methanogenesis. pH 8.5–9.5 raises free-ammonia toxicity, and pH 2–3 stops methanogens entirely.
Biogas that is to be compressed and used as a vehicle fuel must first be upgraded, which mainly means:
B. Removing CO2 and impurities
Raw biogas is about 50–70% CH4 with 30–50% CO2, traces of H2S and saturation water. Upgrading by water scrubbing, pressure-swing adsorption, membranes or chemical absorption removes CO2 and impurities to give biomethane with over about 90–97% methane, depending on the specification. H2S must also be removed because it is toxic and corrosive.
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).
For a family biogas plant, the hydraulic retention time chosen at the design stage should be:
A. Longer in cold climates and seasons, because gas production per day falls as temperature drops
Microbial activity slows sharply with temperature, so unheated plants in cooler regions need longer HRT (bigger digesters) for the same degradation. National programmes therefore specify longer HRTs for hill and cold regions than for warm plains. Very short HRTs risk washout of slow-growing methanogens, which is the opposite problem.
A rice mill processes 1000 t of paddy per season. Husk is 20% of paddy mass and has a heating value of 13 MJ/kg. The energy available in the husk is:
C. 2600 GJ
Husk = 1000 × 0.20 = 200 t = 200,000 kg. Energy = 200,000 × 13 MJ = 2,600,000 MJ = 2600 GJ. Using the whole paddy mass gives 13,000 GJ; the other options are unit slips (1 GJ = 1000 MJ). The husk share of paddy commonly lies near 20% but varies by variety and milling.
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