Tema 4 CS

In the biochemical process of anaerobic digestion (AD) several microorganisms convert organic matter into biogas. This happens in different stages. Which phase can be the rate-limiting step of AD if the organic substrates such as lipids, cellulose and lignin are not easily biodegradable?. Hydrolysis because complex insoluble polymers must first be broken down into smaller units. Acidogenesis because the pH must be decreased to 5.5-7.0. Acetogenesis because it must maintain low partial pressure of hydrogen to proceed. Methanogenesis because methanogenic bacteria are highly sensitive to pH and temperature.

Which property of Hydrogen makes it dangerous for combustion applications?. It has a narrow flammability range, making ignition rare. It is very dense, meaning that it accumulates near the ground. It requires high ignition energy, reducing accidental ignition risk. It has a very low ignition energy, allowing for it to ignite easily from small sparks.

Why does increasing CO₂ content in biogas reduce combustion performance?. CO₂ increases the reactivity of the fuel mixture. CO₂ increases flame temperature and flame speed. CO₂ acts as a diluent, reducing flame temperature and laminar flame speed. CO₂ increases oxygen availability in the mixture.

Which is the most established technology for Hydrogen production as of today?. PEM electrolysis. Steam Methane Reforming. Thermochemical Watersplitting. Photocatalysis.

In combustion systems using biogas and hydrogen, the composition of the fuel strongly affects flame behaviour and performance. Which of the following statements correctly explains how CO₂ in biogas and hydrogen addition influence combustion characteristics?. Increasing CO₂ in biogas increases flame temperature and improves flame stability, while hydrogen decreases flame speed. Increasing CO₂ in biogas reduces flame temperature and flame speed, while hydrogen increases flame speed and risk of flashback. Increasing CO₂ in biogas has no effect on combustion, while hydrogen only affects emissions but not flame properties. Increasing CO₂ in biogas increases combustion efficiency, while hydrogen reduces ignition probability.

Why is premixed combustion generally preferred over diffusion combustion in modern sustainable fuel applications?. Because premixed combustion always produces more energy than diffusion combustion regardless of fuel type. Because premixed combustion leads to lower soot formation and more efficient combustion due to better fuel air mixing. Because diffusion combustion cannot occur in engines or industrial systems. Because premixed combustion eliminates all types of emissions, including NOx and CO.

Based on the combustion characteristics of hydrogen (H2) compared to methane (CH4), how does the equivalence ratio (ϕ) uniquely influence the laminar flame speed (SL) and NOx emissions, and what is the primary operational challenge when transitioning to hydrogen-rich mixtures?. Hydrogen's SL peaks at a much leaner condition (ϕ≈0.7) than Methane, which allows for lower combustion temperatures and naturally eliminates NOx formation without risking flashback. Unlike Methane, which has a peak SL near ϕ≈1.1, Hydrogen's SL continues to increase significantly in rich conditions, peaking around ϕ≈1.8. This high reactivity across a wide range of ϕ, increases the risk of flashback even in non-stoichiometric mixtures. The equivalence ratio has no impact on NOx emissions for pure hydrogen since there is no carbon present; NOx only increases if the air excess ratio is kept below 1.0. Increasing the hydrogen fraction in a CH4/H2 blend shifts the peak adiabatic temperature to ultra-lean conditions (ϕ < 0.5), which reduces the laminar flame speed to values below those of pure methane.

In biogas combustion, increasing the CO2 content in the fuel mixture will affect the flame characteristics. Which of the following statements is correct?. Increasing the CO2 content increases the flame temperature and laminar flame speed. Increasing the CO2 content decreases the flame temperature and reduces the laminar flame speed. Increasing the CO2 content has no effect on flame temperature but increases the stability of the flame. Increasing the CO2 content increases the combustion rate and widens the burner working region.

Premixed flames are: yellow flames, meaning rich flames and reducing conditions. blue flames, meaning rich flames and oxidizing conditions. yellow flames, meaning lean flames and reducing conditions. blue flames, meaning lean flames and oxidizing conditions.

In the context of the energy transition, the replacement of methane (CH4) with hydrogen (H2) in combustion systems results in a substantial alteration of the operating parameters and flame stability. Which of the following statements correctly describes a difference in behaviour between these two gases based on their properties?. Hydrogen presents a lower risk of accidental ignition than methane, as its minimum ignition energy is significantly higher, making it difficult for combustion to occur from low-energy sources. Hydrogen’s flammability range is narrower than that of methane, which limits combustion to a very narrow range of concentrations when mixed with air. The laminar flame speed of hydrogen is significantly higher than that of methane, which increases the likelihood of flashback occurring in conventional burners. A hydrogen flame is easier to detect visually than a methane flame in daylight, due to the strong incandescent radiation from carbon particles, which gives the flame its characteristic brightness and yellowish colour.

When comparing a diffusion flame (such as a candle or a forest fire) with a premixed flame (such as a blue flame on a kitchen stove), which of the following statements correctly describes their physical characteristics and emission patterns?. Diffusion flames generally exhibit higher temperatures and lower radiation levels than premixed flames. Premixed flames are characterized by a blue color, low radiation, and the absence of soot emission. Diffusion flames appear blue because they possess a higher oxygen concentration during the initial mixing stage. Soot emission is a common characteristic of premixed flames due to the "high" temperature of the chemical reaction.

Regarding combustion and burner design: For a fixed flame speed, mixing hydrogen in a natural gas mixture will require a smaller tube diameter. A mixture of 100% methane produces a reduction in design area. Lower flame speed will increase the risk of blowoff. For a fixed flame speed, mixing hydrogen in a natural gas mixture will require a bigger tube diame.

Hydrogen Fuel Cell vehicles have emerged as an alternative to Battery Electric Vehicles, avoiding the disadvantage of long charging times. Hydrogen, stored under high pressure in onboard tanks, reacts with oxygen from the air inside the Fuel Cell, producing only water as a by-product. The Fuel Cell provides electricity to the electric motor via a combustion reaction of Hydrogen and Oxygen, with a battery used to deliver stable power. The Fuel Cell provides electricity to the electric motor via an electrochemical reaction of Hydrogen and Oxygen, with a battery used to deliver stable power. The hydrogen combustion provides power directly to the wheels, only using the battery to power electronic and auxiliary systems. The Fuel Cell can only be powered by Green Hydrogen, as other hydrogen types would damage the membrane.

Domestic stove burners have the design of a Bunsen burner. The following image represents the stability and operation limits of a Bunsen burner: The fuel originally used by the burner is methane, CH₄. Adding H₂ to the fuel, creating a CH₄ and H₂ mixture, will: Not have any significant effect over the design area. Increase the design area. Decrease the design area. There isn´t enough information on the question statement.

When adapting existing domestic natural gas burners to operate with a blend of methane and hydrogen, the physical behavior of the premixed flame changes. Based on the combustion properties of hydrogen, what is the primary effect of this blending on the laminar flame speed and its associated operational risk?. The laminar flame speed decreases, which significantly increases the risk of the flame blowing off the burner head. The laminar flame speed increases, leading to a higher risk of flashback, where the flame propagates backwards into the burner tube. The laminar flame speed remains constant. The laminar flame speed increases, which makes the flame more stable and eliminates the risk of flashback.

When designing a burner, stability and operation limits need to be considered. Which one of the options is correct?. Yellow tips on the flame mean that there is too much O2. Blowoff of the flame means that the air is entering with too much speed. Flashback of the flame means that the air is entering with too much speed. The risk of the flame turning turbulent decreases directly with a higher laminar flame speed.

O enriquecimento com H₂ pode compensar: Aumento de CO₂. Diminuição da velocidade de chama causada por CO₂.

In premixed combustion systems originally designed to operate with methane (CH₄), the introduction of hydrogen significantly alters flame behaviour due to the different burning characteristics of H₂. Which of the following statements correctly describes the impact of adding hydrogen to a CH₄ based premixed flame?. Adding hydrogen reduces the risk of flashback because its high diffusivity stabilizes the flame. Adding hydrogen increases the risk of flashback because the laminar flame speed more easily approaches the local flow velocity in the premixing region. Adding hydrogen influences only the blowoff limit, not the flashback limit. Adding hydrogen does not significantly affect flame stability.

What are the four processes that describe anaerobic digestion: Hydrolysis, Acidogenesis, Acetogenesis, Methanogenesis. Electrolysis, Acidogenesis, Acetogenesis, Pyrolysis. Hydrolysis, Acidolysis, Glycolysis, Methanogenesis. Hydrolysis, Ammonification, Photolysis, Methanation.

What will be the effect of blending hydrogen into the gas grid on domestic appliances (e.g. boilers and stoves)?. The flame becomes slower, improving stability. The presence of CO₂ increases flame temperature and improves combustion. The flame speed increases, raising the risk of flashback and requiring burner adaptation. The flame becomes yellow with more soot.

Numa chama de difusão CH₄/H₂, o aumento da percentagem de H₂ no combustível leva, visualmente, a: um aumento da luminosidade amarela da chama. uma diminuição da componente amarela da chama.