Your Answer : (Not Answered)

Correct Answer : A

Passive solar heating uses large windows facing south.

In the mid-latitudes of the northern hemisphere, the winter sun stays low in the southern sky. Large south-facing glazing admits that sunshine, and thermal mass inside the building stores the heat. Overhangs shade the same windows from the high summer sun.

North-facing windows get almost no direct sun in winter and lose heat. Small south windows admit too little energy, and east or west windows receive only low-angle morning or evening sun and are hard to shade in summer.

Your Answer : (Not Answered)

Correct Answer : A

The provision is baffles on the side walls.

An impeller spinning in an unbaffled vessel makes the whole broth rotate, forming a vortex: mixing and gas dispersion become poor and air is drawn down the shaft. Baffles, vertical metal strips usually four in number and about a tenth of the tank diameter wide, are fixed to the vessel wall to break up this swirl and convert it into top-to-bottom mixing.

A stuffing box seals the agitator shaft where it enters the vessel, a sparger introduces air at the bottom, and an oxygen probe measures dissolved oxygen. None of them prevents a vortex.

Your Answer : (Not Answered)

Correct Answer : D

Conversion is difficult because of the presence of microcrystalline cellulose and lignin.

In straw the cellulose is largely crystalline, its chains packed tightly by hydrogen bonds so that enzymes and water reach them slowly, and the whole is wrapped in lignin, a complex aromatic polymer that blocks cellulase enzymes and is itself hard to degrade. This recalcitrance is why lignocellulosic ethanol needs pretreatment before fermentation.

Amorphous cellulose is the easily digested part. The absence of lignin would make the conversion easier, not harder. Hemicellulose is branched and amorphous and is broken down relatively easily, so its presence is not the main obstacle.

Your Answer : (Not Answered)

Correct Answer : C

The methanogens that do not use hydrogen are the acetotrophic methanogens.

Acetotrophic (acetoclastic) methanogens such as Methanosaeta split acetate directly into methane and carbon dioxide, with no hydrogen needed. This route produces about two-thirds of the methane in a typical digester.

Hydrogenotrophic methanogens reduce CO₂ with hydrogen, and methylotrophic methanogens convert methylated compounds such as methanol and methylamines, some of them using hydrogen to do so. 'Methanotrophic methanogens' is a contradiction: methanotrophs are bacteria that consume methane rather than produce it.

Your Answer : (Not Answered)

Correct Answer : A

Methanogens are inhibited by more oxygen (8 ppm).

Methanogens are strict anaerobes: they lack enzymes such as catalase and superoxide dismutase to cope with oxygen, and key enzymes of methanogenesis are destroyed by it. Dissolved oxygen around 8 ppm, close to saturation in water, therefore stops methane production completely.

A neutral pH of 6.8-7.2 is the optimum range for methanogens, syntrophic bacteria help them by supplying hydrogen and acetate, and the absence of oxygen is exactly the condition they need. None of these inhibits them.

Your Answer : (Not Answered)

Correct Answer : B

The homoacetogen is Acetobacterium woodii.

Homoacetogenic bacteria make acetate as their sole product. Acetobacterium woodii fixes H₂ and CO₂ into acetate through the Wood-Ljungdahl (acetyl-CoA) pathway, and in digesters it competes with methanogens for hydrogen while supplying acetate to the acetoclastic methanogens.

Methanosarcina mazei is a methanogenic archaeon, which makes methane rather than acetate. Pseudomonas carboxydovorans is an aerobic bacterium that oxidises carbon monoxide, and Escherichia coli is a facultative anaerobe that performs mixed-acid fermentation, not homoacetogenesis.

Your Answer : (Not Answered)

Correct Answer : D

The incorrect step is the methane oxidation process.

Anaerobic digestion converts organic matter to biogas in four stages: hydrolysis of complex polymers into sugars, amino acids and fatty acids; acidogenesis into volatile fatty acids; acetogenesis into acetate, hydrogen and carbon dioxide; and methanogenesis, in which archaea produce methane.

Methane oxidation is the opposite of what a digester does: methanotrophic bacteria consume methane, mostly with oxygen, so it would reduce biogas yield. Hydrolysis, acetogenesis and methanogenesis are all genuine stages of the process.

Your Answer : (Not Answered)

Correct Answer : C

While feeding power to the grid, the slip of the induction generator is less than zero.

Slip is \(s = (N_{s}-N_{r})/N_{s}\). An induction machine acts as a generator only when the rotor is driven faster than the synchronous speed set by the grid frequency, so \(N_{r} \gt N_{s}\) and the slip is negative, typically a few per cent.

Zero slip means the rotor turns at synchronous speed and no power is converted. Slip between 0 and 1 is motoring, slip of 1 is a stalled rotor, and slip greater than 1 means the rotor turns against the field, braking (plugging) rather than generating.

Your Answer : (Not Answered)

Correct Answer : A

The nearest combination is X = 6.12 m and Y = 1.28 kW.

Shaft power is the power in the wind through the swept area, times the power coefficient and the drivetrain efficiency:

\[ \begin{aligned} P &= \tfrac{1}{2}\rho A V^{3} C_{p}\eta \\ 2000 &= 0.5(1.225)A(343)(0.36)(0.90) \\ A &= \frac{2000}{68.07} = 29.38\ \text{m}^{2} \\ D &= \sqrt{\frac{4A}{\pi}} = \sqrt{37.43} = 6.12\ \text{m} \end{aligned} \]

With the same rotor, power varies as the cube of wind speed:

\[ \begin{aligned} Y &= 2\left(\frac{6}{7}\right)^{3} \\ &= 2 \times 0.630 = 1.26\ \text{kW} \end{aligned} \]

That is 1.26 kW, and the option set offers 1.28 alongside the correct diameter, so it is the nearest correct combination, as the question asks. X = 3.06 halves the diameter (it is the radius), X = 6.12 with Y = 2.00 ignores the fall in wind speed, and X = 6.75 does not come from the power equation.

Your Answer : (Not Answered)

Correct Answer : D

The lift type wind mill extracts about 20.03 kW.

Power extracted is \(P = C_{p} \cdot \tfrac{1}{2}\rho A V^{3}\). Both machines have the same rotor size, height and wind, so the available wind power \(\tfrac{1}{2}\rho A V^{3}\) is the same and the extracted power scales with the power coefficient alone:

\[ \begin{aligned} P_{lift} &= P_{drag} \frac{C_{p,lift}}{C_{p,drag}} \\ &= 5 \times \frac{0.593}{0.148} \\ &= 20.03\ \text{kW} \end{aligned} \]

The wind speed and height are not needed. 0.593 is the Betz limit, the theoretical maximum, which is why the airfoil (lift) machine can extract about four times what the drag machine can. The smaller options do not follow from the ratio.

Your Answer : (Not Answered)

Correct Answer : B

The process is pyrolysis.

Pyrolysis heats biomass in the absence, or near absence, of oxygen. At around 450-600 °C it breaks the biomass down into a solid char (biochar), a condensable liquid (bio-oil or tar) and non-condensable gases; slow pyrolysis favours char and fast pyrolysis favours bio-oil.

Gasification uses a controlled amount of oxygen or steam at higher temperatures, around 800-1000 °C, to make syngas. Anaerobic digestion is a biological process at 35-55 °C that makes biogas. Steam reforming converts hydrocarbons such as methane into hydrogen.

Your Answer : (Not Answered)

Correct Answer : C

The combustible constituents are CO and \(H_{2}\).

Producer gas from a gasifier is typically about 20% carbon monoxide, 15 to 20% hydrogen, a few percent methane, around 10% carbon dioxide and roughly half nitrogen. Its heating value comes from the combustible gases, chiefly CO and \(H_{2}\).

\(CO_{2}\) is already fully oxidised and, like nitrogen, only dilutes the gas, so every option containing \(CO_{2}\) names a non-combustible component.

Your Answer : (Not Answered)

Correct Answer : A

The constituent is CO, carbon monoxide.

Producer gas from a wood or charcoal gasifier is typically about 20-25% CO, 12-18% H₂, 1-3% CH₄, 8-12% CO₂ and roughly half nitrogen. Nitrogen is the largest constituent but is inert, so among the gases that add heating value, carbon monoxide occupies the largest volume and contributes most of the calorific value.

CO₂ is an incombustible diluent that lowers the calorific value. Hydrogen is combustible but present in a smaller share than CO. Methane has the highest heating value per cubic metre, but there is only a few per cent of it.