Biomass Energy Questions and Answers

Q: The constituent of producer gas which occupies the highest percentage by volume and helps in increasing its overall calorific value is

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.

Q: Constituents of the producer gas contributing to its heating value are

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.

Q: Which one of the following biomass treatment processes occurs in an oxygen limited condition approximately between 450 ℃ and 600 ℃, and produces biochar and bio-oil as primary products?

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.

Q: What is incorrect about the mechanism of organic matter conversion to biogas in anaerobic digestion system ?

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.

Q: Which of the homoacetogenic bacteria utilize H2 and CO2 during anaerobic digestion of feedstock ?

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.

Q: Activity of methanogens in biomethanation digester is totally inhibited due to :

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.

Q: Which methanogen does not utilize hydrogen to reduce the organic compound or CO2 to methane during anaerobic digestion process ?

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.

Q: Why microbial conversion of lignocellulosic straw feedstock to bioethanol is difficult ?

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.

Q: What provision a typical bioreactor possess to overcome the vortex formation during fermentation ?

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.

Q: What is the range of methane (CH4) percentage in the biogas ?

Methane in biogas ranges about 40-70 per cent.Biogas from the anaerobic digestion of dung, sewage sludge or food waste is mostly methane, typically 50 to 70 per cent and lower with poor feedstock, with carbon dioxide making up most of the rest (around 30 to 45 per cent) and traces of hydrogen sulphide, nitrogen and water vapour. The methane content is what makes the gas burn.30-60 per cent sets the range too low, and 1-5 and 20-30 per cent describe gas that would barely burn. The 40-70 range covers the values reported for working digesters.

The constituent of producer gas which occupies the highest percentage by volume and helps in increasing its overall calorific value is

A.CO
B.\(\text{CO}_{2}\)
C.\(\text{H}_{2}\)
D.\(\text{CH}_{4}\)

Exam Year(s)

GATE AG - 2012,

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.

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Constituents of the producer gas contributing to its heating value are

A.\(CO\) and \(CO_{2}\)
B.\(CH_{4}\) and \(CO_{2}\)
C.\(CO\) and \(H_{2}\)
D.\(CO_{2}\) and \(H_{2}\)

Exam Year(s)

GATE AG - 2018,

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.

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Which one of the following biomass treatment processes occurs in an oxygen limited condition approximately between 450 ℃ and 600 ℃, and produces biochar and bio-oil as primary products?

A.Gasification
B.Pyrolysis
C.Anaerobic Digestion
D.Steam Reforming

Exam Year(s)

GATE XE - 2026,

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.

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What is incorrect about the mechanism of organic matter conversion to biogas in anaerobic digestion system ?

A.Hydrolysis process
B.Methanogenesis process
C.Acetogenesis process
D.Methane oxidation process

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

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.

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Which of the homoacetogenic bacteria utilize H₂ and CO₂ during anaerobic digestion of feedstock ?

A.Methanosarcina mazei
B.Acetobacterium woodii
C.Pseudomonas carboxydovorans
D.Escherichia coli

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

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.

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Activity of methanogens in biomethanation digester is totally inhibited due to :

A.More oxygen (8 ppm)
B.Neutral pH (6.8-7.2) of feedstock slurry
C.Presence of syntrophic microbes
D.Absence of oxygen

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

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.

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Which methanogen does not utilize hydrogen to reduce the organic compound or CO₂ to methane during anaerobic digestion process ?

A.Methylotrophic methanogens
B.Methanotrophic methanogens
C.Acetotrophic methanogens
D.Hydrogenotrophic methanogens

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

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.

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Why microbial conversion of lignocellulosic straw feedstock to bioethanol is difficult ?

A.Presence of more insoluble amorphous cellulose
B.Absence of insoluble branched lignin fraction
C.Presence of more insoluble branched hemicellulose
D.Presence of microcrystalline cellulose and lignin

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

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.

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What provision a typical bioreactor possess to overcome the vortex formation during fermentation ?

A.Baffles on the side walls
B.Stuffing boxes
C.Sparger
D.Oxygen probe

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

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.

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10.

What is the range of methane (CH₄) percentage in the biogas ?

A.30-60
B.40-70
C.1-5
D.20-30

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

Methane in biogas ranges about 40-70 per cent.

Biogas from the anaerobic digestion of dung, sewage sludge or food waste is mostly methane, typically 50 to 70 per cent and lower with poor feedstock, with carbon dioxide making up most of the rest (around 30 to 45 per cent) and traces of hydrogen sulphide, nitrogen and water vapour. The methane content is what makes the gas burn.

30-60 per cent sets the range too low, and 1-5 and 20-30 per cent describe gas that would barely burn. The 40-70 range covers the values reported for working digesters.

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