Agricultural and Biosystems Engineers Licensure Examination (ABELE) · Philippines · Professional Regulatory Board of Agricultural and Biosystems Engineering, PRC
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 Agricultural and Biosystems Engineers Licensure Examination spread across the units of Bio-energy Systems: Design & Applications. Difficulty: 19 easy, 28 medium, 5 hard.
| Gasification: updraft and downdraft gasifiers | 7 | |
| Calorific value | 5 | |
| Briquetting and pelleting | 5 | |
| Producer gas and dual-fuel engines | 5 | |
| Biogas plant types: floating drum and fixed dome | 5 | |
| Biogas plant sizing and gas yield | 5 | |
| Biodiesel and transesterification | 5 | |
| Biomass characterisation: proximate and ultimate analysis | 3 | |
| Combustion of biomass | 3 | |
| Bio-ethanol | 3 | |
| Pyrolysis | 2 | |
| Anaerobic digestion stages | 2 | |
| Energy audit and energy ratio in agriculture | 2 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Freshly cut wood contains 40% moisture on a wet basis. Its moisture content on a dry basis is:
C. 66.7%
Take 100 kg of wet wood: 40 kg water and 60 kg dry matter. Dry basis = water ÷ dry matter = 40 ÷ 60 × 100 = 66.7%. The dry-basis figure is always larger than the wet-basis figure for the same wood. 28.6% divides the water by wet mass plus water, 40 ÷ 140.
The difference between the higher (gross) and lower (net) heating value of a fuel is:
B. The latent heat of water vapour in the products, from fuel moisture and hydrogen combustion
HHV assumes all water in the products is condensed to liquid at the reference temperature, releasing its latent heat; LHV assumes it leaves as vapour. The water comes from fuel moisture and from burning hydrogen (9 kg water per kg H). Ash and flue-gas nitrogen losses are part of an appliance's efficiency, not of the HHV–LHV definition.
The main purpose of briquetting loose agricultural residues is to:
D. Raise bulk and energy density for easier storage, transport and uniform burning
Densification raises density by roughly an order of magnitude, so a truck or store holds far more energy and the fuel burns more steadily. It does not change the heating value per kg of dry matter (apart from small moisture changes), and it cannot remove ash; ash that goes in comes out in the briquette.
The dry flue gas from a husk-fired furnace contains 6% O2 by volume. Using the approximation excess air ≈ O2 ÷ (21 − O2), the excess air is about:
B. 40%
Excess air ≈ 6 ÷ (21 − 6) = 0.40, i.e. 40%. Dividing by 21 alone (28.6%) forgets that the excess air itself dilutes the gas. The approximation ignores small CO and fuel-oxygen effects but is the standard field check. 250% inverts the ratio (15 ÷ 6).
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 chamber of a fixed-dome biogas plant into which slurry is pushed as gas accumulates, and from which digested slurry overflows, is called the:
D. Outlet (displacement) chamber
As gas collects under the dome it pushes slurry down in the digester and up into the outlet chamber; the level difference between the two is the gas pressure. When gas is used, slurry flows back. The mixing tank is at the inlet, the guide frame belongs to floating-drum plants, and the water trap removes condensate from the gas pipe.
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).
A distillery ferments 1 t of molasses containing 45% fermentable sugars (expressed as glucose). With 0.511 kg ethanol per kg glucose theoretically and a fermentation efficiency of 90%, the ethanol produced (density 0.789 kg/L) is about:
B. 263 L
Sugar = 1000 × 0.45 = 450 kg. Ethanol = 450 × 0.5114 × 0.9 = 207.1 kg = 207.1 ÷ 0.789 = 263 L. 207 is the mass in kg; applying the yield to the whole molasses mass ignores its water and non-fermentable solids. Distillation losses reduce the product further. 292 L ignores the 90% fermentation efficiency (230.1 ÷ 0.789).
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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