Question 1

Pulverized coal transport safety: Preheated air for conveying pulverized coal to boiler burners is typically limited to about 300°C mainly to avoid the __________.

Accepted Answer

Introduction: Pulverized coal is conveyed pneumatically with hot air from mills to burners. While preheating improves drying and ignition, excessive air temperature can create a fire/explosion hazard by igniting coal dust within the transport lines. Given Data / Assumptions: Coal dust–air mixtures can be explosible within specific concentration and temperature ranges. Typical plant practice limits primary air to around 300°C. Lines include bends, valves, and potential ignition sources (hot spots). Concept / Approach: Coal dust has a minimum ignition temperature; raising the carrier air too high increases the chance of autoignition or ignition from smoldering deposits. The catastrophic consequence is a deflagration in ducts or mills. Therefore, a conservative temperature limit mitigates explosion risk. Step-by-Step Solution: Identify hazard: dust–air mixture with ignition source + high T.Note industry limit ~300°C as a safe upper bound.Conclude primary reason is explosion risk minimization. Verification / Alternative check: NFPA guidance and utility boiler practices emphasize temperature control of primary air and mill outlet to prevent fires/explosions. Why Other Options Are Wrong: Chamber size, clinker, incomplete combustion, erosion: Influenced by many factors, but not the main reason for the strict 300°C cap. Common Pitfalls: Overlooking smoldering coal in mills as an ignition source when air is too hot. Final Answer: risk of explosion

Question 2

Coke sizing for ironmaking: Blast-furnace grade metallurgical coke is usually sized within which of the following ranges (in millimetres)?

Accepted Answer

Introduction: Proper coke size is vital for permeability, burden support, and stable gas flow in a blast furnace. Oversized or undersized coke degrades bed hydraulics and can cause pressure drop or excessive fines generation. Given Data / Assumptions: Standard blast-furnace operations handling sinter/pellet burden. Need for strong, relatively uniform coke lumps. Concept / Approach: Typical metallurgical coke is screened to a mid-size fraction that balances surface area for reaction (e.g., with CO2) and structural strength. Industry practice commonly uses about 25–80 mm as the accepted size range for BF coke charged at the top. Step-by-Step Solution: Recall standard screen cuts: fines removed; extreme sizes avoided.Identify accepted plant specification ≈ 25–80 mm.Select the matching option. Verification / Alternative check: Ironmaking texts and plant specs specify coke of roughly 25–80 mm for large furnaces to ensure good permeability and CSR/CRI performance. Why Other Options Are Wrong: 0–15, 5–10, 15–25 mm: Too small; cause gas channelling and pressure drop. >100 mm: Too large; poor packing and reactivity. Common Pitfalls: Assuming that smaller always reacts faster; mechanical strength and bed permeability constrain the size upward. Final Answer: 25–80

Question 3

Properties of bituminous coal: Which statements about bituminous coal are generally true?

Accepted Answer

Introduction: Coal rank strongly influences ignition, flame appearance, and coking behavior. Bituminous coals are mid-rank coals used widely for power and metallurgical coke production. This question checks basic recognition of their combustion and coking traits. Given Data / Assumptions: Anthracite is a higher-rank coal with low volatiles and higher ignition temperature. Many bituminous coals are coking (though not all); coking depends on rank and petrography. Volatile matter in bituminous coal yields luminous, smoky flames. Concept / Approach: Bituminous coal contains more volatile matter than anthracite, so it ignites more readily and burns with a smoky yellow flame due to soot precursors. Many bituminous coals possess the plasticity needed for coking, making them suitable for metallurgical coke when blended appropriately. Step-by-Step Solution: (a) is false: anthracite, not bituminous, is harder to ignite.(b) is generally true: numerous bituminous coals are coking.(c) is true: higher volatiles produce luminous, smoky flames.Hence, (d) “both (b) and (c)” is correct. Verification / Alternative check: Proximate analysis (volatile matter) and coking tests (Gieseler fluidity, Roga index) corroborate these characteristics. Why Other Options Are Wrong: (a) reverses the ignition ease; (e) conflicts with true statements (b) and (c). Common Pitfalls: Assuming all bituminous coals are coking; some are non-coking and require blending. Final Answer: both (b) and (c)

Question 4

Blending practice in coking: Why is prime coking coal blended with medium or non-coking coal prior to carbonization in coke ovens?

Accepted Answer

Introduction: Coal blending before carbonization is a critical operational step in coke making. Prime coking coals develop strong plastic layers and can swell markedly during heating, potentially exerting damaging pressures on oven walls if not moderated by suitable blend components. Given Data / Assumptions: Coke ovens are refractory-lined chambers sensitive to mechanical stress. Prime coking coals have high fluidity and swelling indices. Medium or non-coking coals reduce excessive plasticity and adjust coke quality. Concept / Approach: Excessive swelling can cause high wall pressures and structural damage. Blending balances plastic layer development, controls expansion, and tailors coke CSR/CRI, porosity, and strength after reaction. The objective is oven safety and target coke properties, not to increase ash or because prime coking coal makes “unreactive” coke by itself. Step-by-Step Solution: Identify risk: prime coking coal → high swelling → wall pressure.Mitigation: blend with less coking coals to moderate plasticity.Outcome: safe operation and desired coke strength/reactivity. Verification / Alternative check: Operational guidelines use Swelling Index, Gieseler fluidity, and Audibert–Arnu dilatation to design blends that keep wall pressure within limits. Why Other Options Are Wrong: (b) is incorrect; prime coking coal does not inherently make “unreactive” coke. (c) includes (b) and is therefore incorrect; (e) is undesirable—higher ash harms coke quality. Common Pitfalls: Equating “more coking ability” with “better” without considering mechanical pressures and oven integrity. Final Answer: to prevent excessive swelling and high oven wall pressure that could damage the coke oven

Question 5

Constituents of coal relevant to coke making: Which constituent of coal is the most critical for forming metallurgical coke during carbonization?

Accepted Answer

Introduction: Metallurgical coke formation relies on a plastic stage during heating when coal particles soften, fuse, and resolidify into a porous, strong mass. The coal constituent that governs plastic behavior and coking ability is crucial for coke quality. Given Data / Assumptions: Carbonization is conducted in the absence of air. Plasticity and caking arise from thermally decomposing volatile matter. Final coke is predominantly carbon, but how we get there depends on coal composition. Concept / Approach: Volatile matter content and composition drive the plastic stage: tar precursors soften the coal, allowing particles to coalesce and form a coherent mass before resolidifying as semicoke/coke. While total carbon determines ultimate yield and calorific value, it is the presence and nature of volatiles that enable caking and coke structure formation. Moisture and ash are generally detrimental; sulfur affects quality but not the fundamental ability to form coke. Step-by-Step Solution: Identify which constituent controls plasticity: volatile matter.Connect volatile-driven softening to caking and coke formation.Select “Volatiles” as the most critical for producing coke. Verification / Alternative check: Standard tests (Gieseler fluidity, Roga index, Free Swelling Index) correlate coking ability with volatile-driven plastic behavior. Why Other Options Are Wrong: Moisture/ash: reduce efficiency and strength; not drivers of coking. Carbon: necessary, but does not create plasticity by itself. Sulfur: a quality concern, not the coking mechanism. Common Pitfalls: Assuming “more carbon” alone guarantees better coke; without adequate plasticity from volatiles, coke will be weak and friable. Final Answer: Volatiles

Question 6

Stack design and dispersion: A large thermal power plant typically discharges flue gas through its chimney at about what velocity (metres per second)?

Accepted Answer

Introduction: Flue-gas exit velocity from stacks influences plume rise, dispersion, and the potential for stack-tip downwash. Power plant stacks are designed to balance acceptable pressure drop with sufficient momentum for effective dispersion. Given Data / Assumptions: Utility-scale thermal power plant with tall chimney. Normal operating draft and induced-draft fans. Velocity values need only be order-of-magnitude correct. Concept / Approach: Typical stack exit velocities range around 10–20 m/s. Much lower values cause poor dispersion and downwash; extremely high velocities would demand excessive fan power and are rarely practical. Thus, a representative benchmark value is approximately 10 m/s for big plants. Step-by-Step Solution: Consider energy cost and dispersion needs → moderate velocities preferred.Industry practice → design around ~10–20 m/s.Choose 10 m/s as the nearest standard value among the options. Verification / Alternative check: Environmental permitting guidelines often cite stack exit velocity targets near this range to limit downwash and meet dispersion modeling assumptions. Why Other Options Are Wrong: 0.5 or 2 m/s: Too low; risk of downwash and re-entrainment. 50 m/s: Unusually high and inefficient for large stacks. 500 m/s: Physically unrealistic for stack flows. Common Pitfalls: Confusing flue-gas duct velocities (which can be higher) with final stack exit velocities constrained by environmental and energy considerations. Final Answer: 10

Question 7

Combustion fuels and biomass: Approximately what is the carbon content by weight (mass %) in air-dried wood commonly used as a solid biofuel?

Accepted Answer

Introduction / Context: Understanding the elemental composition of common fuels helps in estimating heating value, carbon emissions, and combustion stoichiometry. Air-dried wood is a widely used biomass fuel in boilers and gasifiers. Knowing its typical carbon percentage allows quick back-of-the-envelope calculations for energy and CO2 output. Given Data / Assumptions: Air-dried wood (not oven-dry), with moisture still present. Typical proximate/ultimate analyses are assumed for mixed hardwood/softwood. We seek a representative mass-percent of carbon, not an exact species-specific value. Concept / Approach: Biomass contains carbon, hydrogen, oxygen, small amounts of nitrogen, ash, and moisture. For air-dried wood, ultimate analyses commonly report carbon near one-half of the dry mass. Moisture lowers the apparent fraction slightly versus oven-dry, but the round-number estimate widely used for calculations remains about 50% carbon by mass. Step-by-Step Solution: Start from typical ultimate analysis (air-dried): C ~ 48–52%, H ~ 5–7%, O ~ 40–45%, minor N/S/ash.Round to the standard engineering estimate used in preliminary designs.Thus select 50% as the closest representative value. Verification / Alternative check: Handbooks and biomass databases consistently list wood near 50% carbon by mass on an air-dried basis, confirming the rule-of-thumb. Why Other Options Are Wrong: 10% and 25% underestimate carbon severely for lignocellulosic solids. 80% carbon corresponds to high-rank coal or char, not fresh air-dried wood. Common Pitfalls: Confusing oven-dry and air-dry bases; moisture content changes values. Using bark or char data for fresh wood fuel; those differ markedly. Final Answer: 50%

Question 8

Fuel gas composition: Coke-oven gas produced during coal carbonisation consists mainly of which constituents?

Accepted Answer

Introduction / Context: Coke-oven gas (COG) is a valuable by-product of high-temperature coal carbonisation in coke plants. Its composition determines calorific value, safety considerations, and treatment steps (e.g., tar, ammonia, benzol recovery, and gas cleaning for use as fuel or feedstock). Given Data / Assumptions: High-temperature by-product coke ovens (about 900–1100°C). Typical plant-average gas after initial tar/condensate removal. We seek the principal constituents by volume. Concept / Approach: COG is characteristically rich in hydrogen and methane with smaller fractions of carbon monoxide, carbon dioxide, nitrogen, and light hydrocarbons. Thus the pair that best represents the dominant components is hydrogen (H2) and methane (CH4). Step-by-Step Solution: Identify major species in COG → H2 (often 50–60% v/v) and CH4 (~25–30% v/v).Minor components include CO, CO2, N2, and C2–C6 hydrocarbons.Therefore, the principal pair is H2 and CH4. Verification / Alternative check: Standard coke-plant references list H2 and CH4 as the two largest fractions, aligning with the selected option. Why Other Options Are Wrong: CO and CO2 are present but not dominant; H2 and CO together omit methane, a key constituent; CH4 and CO omit the dominant hydrogen fraction. Common Pitfalls: Confusing COG with blast-furnace or basic oxygen furnace gases, which are CO-rich and H2-poor. Using site-specific numbers as universal; ranges vary but qualitative dominance persists. Final Answer: H2 and CH4

Question 9

Combustion fundamentals: During the combustion of gaseous fuels, what is the primary effect of insufficient (deficient) air on the flame appearance and length?

Accepted Answer

Introduction / Context: Air-to-fuel ratio strongly influences flame shape, stability, temperature, and emissions. Operators and designers monitor flame characteristics to ensure complete combustion, reduce CO and unburnt hydrocarbons, and protect equipment. Given Data / Assumptions: Gaseous fuel fired into a burner with adjustable air supply. Observation focuses on qualitative flame behavior under air deficiency. Steady firing conditions assumed. Concept / Approach: With insufficient air, oxygen becomes locally limiting. Fuel takes longer to find oxygen, combustion completes farther from the nozzle, and soot precursors form, making the flame more luminous and elongated. The apparent (measured) temperature may drop due to incomplete combustion and heat losses, even though local rich zones can be hot. Step-by-Step Solution: Reduce air below stoichiometric → oxygen deficit near burner exit.Mixing-controlled burning region extends → flame length increases.Increased soot formation leads to yellow, luminous flame.Overall efficiency and completeness of combustion deteriorate. Verification / Alternative check: Empirical burner tuning shows lean flames are short and blue, whereas rich, air-deficient flames are long and luminous, matching the rule. Why Other Options Are Wrong: Shortening occurs with excess air and high momentum mixing. Claims of no effect contradict basic mixing/kinetics. Higher adiabatic temperature is incorrect; air deficiency usually lowers effective flame temperature due to incomplete combustion and radiation losses. Common Pitfalls: Confusing visible luminosity with higher efficiency; luminous does not mean hotter overall. Interpreting transient blow-off or flashback phenomena as steady effects of air deficiency. Final Answer: It lengthens the flame (luminous, sooty).

Question 10

Pulverised coal firing practice: Why does the ratio of primary air to secondary air generally increase with an increase in the rank of coal?

Accepted Answer

Introduction / Context: In pulverised-coal combustion, primary air transports and begins drying/ignition of coal particles, while secondary air provides additional oxygen for complete burnout. Coal rank affects volatiles, ignition characteristics, and air staging strategy. Given Data / Assumptions: Comparison among low-rank (high-volatile) and high-rank (low-volatile, high fixed-carbon) coals. Conventional burner with distinct primary and secondary air streams. Goal is stable ignition and complete burnout. Concept / Approach: As coal rank increases, volatile matter decreases and fixed-carbon proportion increases. More primary air (relative to secondary) helps transport, dry, and ensure ignition/burnout of the higher fixed-carbon fraction within the burner zone, hence the primary/secondary air ratio tends to rise with rank. Step-by-Step Solution: High rank → higher fixed carbon / volatile ratio.Lower volatiles reduce early flame stability; more primary air promotes ignition and transport.Therefore, the primary-to-secondary air ratio increases. Verification / Alternative check: Combustion handbooks show burner air-staging tuned to fuel volatiles; low-volatile coals need stronger primary stream and longer residence for char burnout. Why Other Options Are Wrong: (a) Reverses the trend; higher rank generally has lower volatiles. (c) While oxygen content may vary, it is not the primary control for the air split. (d) Calorific value trends are not the direct reason for air staging changes. Common Pitfalls: Assuming one air split suits all coals; burner tuning must match fuel rank. Confusing transport (primary) air function with overall excess air requirements. Final Answer: Because the ratio of fixed carbon to volatile matter increases.

Fuels and Combustion Questions