ASRB NET — Farm Machinery & Power / Land & Water Management Engineering / Agricultural Structures & Process Engineering · India · Agricultural Scientists Recruitment Board (ICAR)
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)
ASRB NET — Farm Machinery & Power
Theory-heavy MCQs across the whole subject: comparison of gasifier types, biogas plant designs and their parts, digestion microbiology, briquetting machines and binding, bio-oil and charcoal, biofuel properties, and energy audit terminology.
How our questions for ASRB NET spread across the units of Bio-energy Systems: Design & Applications. Difficulty: 25 easy, 32 medium, 1 hard.
| Biomass characterisation: proximate and ultimate analysis | 7 | |
| Anaerobic digestion stages | 6 | |
| Briquetting and pelleting | 5 | |
| Gasification: updraft and downdraft gasifiers | 5 | |
| Biogas plant types: floating drum and fixed dome | 5 | |
| Bio-ethanol | 5 | |
| Biodiesel and transesterification | 5 | |
| Calorific value | 4 | |
| Combustion of biomass | 4 | |
| Pyrolysis | 4 | |
| Producer gas and dual-fuel engines | 4 | |
| Energy audit and energy ratio in agriculture | 4 |
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 value obtained directly from an oxygen bomb calorimeter is:
A. Gross heating value at constant volume
In the bomb, the products cool back to near room temperature and the water formed condenses, so the heat measured includes the latent heat: it is the gross heating value, determined at constant volume. The net value is calculated from it using the hydrogen and moisture content. Conversion to a dry or daf basis is a separate arithmetic step.
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.
A compression-ignition engine cannot run on producer gas alone without modification because:
A. Producer gas will not self-ignite on compression, so a pilot diesel spray is needed
The gas–air mixture is inducted and compressed but does not auto-ignite reliably; a small injection of diesel ignites and the flame spreads through the gas–air charge. Removing diesel entirely needs conversion to spark ignition (and usually a lower compression ratio). CO and CH4 are knock-resistant, so knock is not the obstacle.
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.
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.
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).
Ordinary distillation of a fermented ethanol–water mash cannot yield anhydrous ethanol because:
C. Ethanol and water form an azeotrope at about 95.6% ethanol by mass
At the azeotropic composition the vapour and liquid have the same composition, so further distillation cannot concentrate the ethanol. Fuel ethanol for blending must be nearly water-free, so the rectified spirit is dehydrated, most commonly with molecular sieves (or by azeotropic distillation with an entrainer). Ethanol boils at about 78 °C, below water.
Alkali-catalysed transesterification is usually run near 60 °C mainly because:
B. Just below methanol's boiling point (about 65 °C), giving fast reaction without pressurising
Reaction rate rises with temperature, but methanol boils at about 64.7 °C at atmospheric pressure; running just below that keeps it liquid without a pressure vessel or heavy reflux. Supercritical processes run far hotter but at high pressure. The catalyst works at room temperature too, just more slowly.
The two main combustible components of producer gas from air-blown gasification of wood are:
B. Carbon monoxide and hydrogen
CO and H2 together usually make up about 35–40% of the gas and supply most of its energy; methane adds a few percent. Nitrogen (about half the gas) and CO2 are inert diluents. Methane is the main fuel in biogas, not producer gas.
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