Q: Minimising stack (flue) heat losses: Which operational adjustment most directly reduces stack heat losses in a fired heater or furnace?

Introduction / Context: Stack heat loss is the sensible heat carried away by hot, oxygen-rich flue gases. It is a major efficiency penalty in boilers, heaters, and furnaces. Operators can trim this loss through combustion control and heat recovery strategies. Given Data / Assumptions: Normal air firing without recuperators/regenerators. Objective: reduce sensible heat leaving with the stack gas. Safe operation (avoid CO formation, soot) is maintained. Concept / Approach: Excess air above stoichiometric ensures complete combustion but also increases nitrogen and unused oxygen mass flow through the stack, raising both flue gas volume and temperature—hence more sensible heat loss. Tight control to the minimum necessary excess air reduces stack losses while preserving complete combustion. Step-by-Step Solution: Recognize stack loss ∝ flue gas flow * mean temperature rise.Excess air increases flue gas flow (more N2 and O2) → higher losses.Trim excess air with O2/CO analyzers to optimal setpoints.Therefore, controlling excess air most directly reduces stack loss. Verification / Alternative check: Efficiency curves show minimum stack loss near optimal excess O2 (e.g., ~2–3% O2 dry for natural gas, higher for solid fuels), validating the control strategy. Why Other Options Are Wrong: Oxygen enrichment often increases flame temperature and may raise stack temperature unless coupled with heat recovery. Low C.V. fuels do not inherently reduce stack losses and can require more flue volume. Proper draft is important for safety/operation but does not primarily target stack sensible heat. Common Pitfalls: Over-trimming excess air leading to CO/smoke; balance is essential. Ignoring air preheat and economizers which also affect flue temperature. Final Answer: controlling the excess air

Q: Adiabatic flame temperature dependence: The adiabatic flame temperature of a fuel–air system depends on the initial temperature of which streams?

Introduction / Context: Adiabatic flame temperature (AFT) is the theoretical maximum temperature achieved by complete combustion without heat loss to surroundings and without dissociation (or including dissociation in a refined model). AFT sets upper bounds for thermal efficiencies and materials selection. Given Data / Assumptions: Closed adiabatic control volume; no heat loss or shaft work. Complete combustion to equilibrium products. Specified initial temperatures of reactants (fuel and oxidizer). Concept / Approach: Energy conservation gives: sensible enthalpy of products at AFT equals chemical heat release plus sensible enthalpies of reactants at their inlet temperatures. Preheating either fuel or air increases the reactant sensible enthalpy term, raising the AFT; preheating both amplifies the effect. Thus AFT depends on the initial temperature of both streams. Step-by-Step Solution: Write balance: Σ h_products(T_ad) = Q_release + Σ h_reactants(T_in).Increase T_in,fuel → larger Σ h_reactants → higher T_ad.Increase T_in,air → same effect; both together give the largest increase.Therefore, AFT depends on both fuel and air initial temperatures. Verification / Alternative check: Calculated flame temperatures for gas turbines and furnaces rise with air preheat (regenerators/recuperators) and with fuel preheat, consistent with practice. Why Other Options Are Wrong: Options limiting dependence to one stream ignore symmetry of the energy balance. Claiming 'neither' contradicts conservation of energy. Common Pitfalls: Neglecting dissociation at very high temperatures, which moderates the increase. Confusing adiabatic temperature with actual flame temperature under heat losses. Final Answer: both (a) & (b)

Q: Flash point testing methods: The Pensky–Martens closed-cup apparatus is used for oils whose flash points are in which range (°F)?

Introduction / Context: Flash point is the lowest temperature at which vapors above a liquid ignite momentarily on exposure to a flame. Different standardized apparatus are specified for different volatility ranges to ensure safety and reproducibility. Given Data / Assumptions: Pensky–Martens is a closed-cup method. Used for medium-to-high flash point petroleum products (e.g., fuel oils, lubricants). Temperature range expressed in °F per the options. Concept / Approach: Closed-cup methods generally yield lower (more conservative) flash points than open-cup, and specific methods cover specific ranges. Pensky–Martens is intended for products with flash points typically above about 120 °F (≈ 49 °C). For very low flash points, other apparatus (e.g., Abel, Tag) are used. Step-by-Step Solution: Identify method applicability: medium/high flash point oils.Map to range: above 120 °F.Select the correct option: '>120' °F. Verification / Alternative check: Standards (ASTM D93) specify Pensky–Martens for a wide range that includes and emphasizes higher-flash products; low-flash gasoline-range materials use other tests (e.g., ASTM D56 Tag). Why Other Options Are Wrong: Ranges below 120 °F correspond to low-flash fuels requiring alternate apparatus; 90–110 °F is too low for the typical Pensky–Martens selection. Common Pitfalls: Confusing open-cup vs. closed-cup results; closed-cup values are typically lower for the same liquid. Applying the wrong standard method outside its validated range. Final Answer: >120

Question 1

Coal briquetting binders: Which of the following is NOT commonly used as a binder in coal briquetting operations?

Accepted Answer

Introduction / Context: Coal briquetting agglomerates fine coal into usable blocks. Binders provide cohesion during pressing and strength after curing. Knowing typical binders aids in selecting recipes that balance strength, cost, and combustion behavior. Given Data / Assumptions: Focus on common industrial binders for coal fines. Options include organic tars/pitches and alternative additives. Single best answer expected. Concept / Approach: Classic binders include coal tar, pitch (bituminous binders), and molasses (often with lime as a hardener but not as the primary binder). Lime is primarily a flux/desulfurizer or hardening additive rather than the main binding medium; it lacks the viscoelastic properties of tars/pitches that provide strong green and cured strength. Step-by-Step Solution: Identify principal binders: tar and pitch are standard.Molasses can serve as a carbohydrate-based binder (often combined with lime or cement).Lime (CaO) itself is not typically the binder; it is used to modify chemistry/strength but not as the main cohesive agent.Therefore, the item not used as a binder, in the primary sense, is lime. Verification / Alternative check: Industrial manuals list tar and pitch as dominant binders; molasses–lime systems classify molasses as the actual binder with lime acting as a hardener/activator. Why Other Options Are Wrong: Tar and pitch are standard petroleum-derived binders; molasses is a recognized alternative binder. Common Pitfalls: Confusing hardeners/fluxes (lime, cement) with true binders. Assuming any additive that improves strength qualifies as the binder. Final Answer: Lime (CaO)

Question 2

Gross vs. net calorific value (GCV vs. NCV): For which of the following gaseous fuels are the gross and net calorific values practically the same?

Accepted Answer

Introduction / Context: Gross calorific value (GCV) includes the latent heat of water vapor formed during combustion, assuming complete condensation of products. Net calorific value (NCV) excludes this latent heat. Fuels with significant hydrogen content produce more water and hence a larger gap between GCV and NCV. Given Data / Assumptions: Composition trends: blast furnace gas (BFG) is mainly CO, CO2, N2 with very little H2. Coke oven gas (COG) contains a high fraction of H2 and light hydrocarbons. Basic oxygen furnace (L.D.) gas is rich in CO with low H2. Concept / Approach: The GCV–NCV difference is dominated by the water formed from hydrogen combustion. If hydrogen content is negligible, little water forms and the two values converge. BFG, with minimal hydrogen, therefore shows almost identical GCV and NCV, whereas COG shows a large difference because of high H2 and hydrocarbons. Step-by-Step Solution: Assess hydrogen content: BFG ≪ LDG < COG (approximate trend).Infer latent heat penalty: smallest for BFG → GCV ≈ NCV.Select the fuel where the two values are practically the same: blast furnace gas. Verification / Alternative check: Typical analyses: BFG ~ 20–28% CO, 2–5% H2; COG ~ 45–60% H2, hydrocarbons present; LD gas mainly CO with low H2. The GCV–NCV gap mirrors hydrogen level. Why Other Options Are Wrong: COG has high hydrogen, so NCV is appreciably lower than GCV. LD converter gas also shows some difference but again greater than BFG. Common Pitfalls: Confusing CO-derived energy (no water produced) with H2-derived energy (water produced). Assuming GCV and NCV differ equally for all gaseous fuels. Final Answer: blast furnace gas

Question 3

Minimising stack (flue) heat losses: Which operational adjustment most directly reduces stack heat losses in a fired heater or furnace?

Accepted Answer

Introduction / Context: Stack heat loss is the sensible heat carried away by hot, oxygen-rich flue gases. It is a major efficiency penalty in boilers, heaters, and furnaces. Operators can trim this loss through combustion control and heat recovery strategies. Given Data / Assumptions: Normal air firing without recuperators/regenerators. Objective: reduce sensible heat leaving with the stack gas. Safe operation (avoid CO formation, soot) is maintained. Concept / Approach: Excess air above stoichiometric ensures complete combustion but also increases nitrogen and unused oxygen mass flow through the stack, raising both flue gas volume and temperature—hence more sensible heat loss. Tight control to the minimum necessary excess air reduces stack losses while preserving complete combustion. Step-by-Step Solution: Recognize stack loss ∝ flue gas flow * mean temperature rise.Excess air increases flue gas flow (more N2 and O2) → higher losses.Trim excess air with O2/CO analyzers to optimal setpoints.Therefore, controlling excess air most directly reduces stack loss. Verification / Alternative check: Efficiency curves show minimum stack loss near optimal excess O2 (e.g., ~2–3% O2 dry for natural gas, higher for solid fuels), validating the control strategy. Why Other Options Are Wrong: Oxygen enrichment often increases flame temperature and may raise stack temperature unless coupled with heat recovery. Low C.V. fuels do not inherently reduce stack losses and can require more flue volume. Proper draft is important for safety/operation but does not primarily target stack sensible heat. Common Pitfalls: Over-trimming excess air leading to CO/smoke; balance is essential. Ignoring air preheat and economizers which also affect flue temperature. Final Answer: controlling the excess air

Question 4

Adiabatic flame temperature dependence: The adiabatic flame temperature of a fuel–air system depends on the initial temperature of which streams?

Accepted Answer

Introduction / Context: Adiabatic flame temperature (AFT) is the theoretical maximum temperature achieved by complete combustion without heat loss to surroundings and without dissociation (or including dissociation in a refined model). AFT sets upper bounds for thermal efficiencies and materials selection. Given Data / Assumptions: Closed adiabatic control volume; no heat loss or shaft work. Complete combustion to equilibrium products. Specified initial temperatures of reactants (fuel and oxidizer). Concept / Approach: Energy conservation gives: sensible enthalpy of products at AFT equals chemical heat release plus sensible enthalpies of reactants at their inlet temperatures. Preheating either fuel or air increases the reactant sensible enthalpy term, raising the AFT; preheating both amplifies the effect. Thus AFT depends on the initial temperature of both streams. Step-by-Step Solution: Write balance: Σ h_products(T_ad) = Q_release + Σ h_reactants(T_in).Increase T_in,fuel → larger Σ h_reactants → higher T_ad.Increase T_in,air → same effect; both together give the largest increase.Therefore, AFT depends on both fuel and air initial temperatures. Verification / Alternative check: Calculated flame temperatures for gas turbines and furnaces rise with air preheat (regenerators/recuperators) and with fuel preheat, consistent with practice. Why Other Options Are Wrong: Options limiting dependence to one stream ignore symmetry of the energy balance. Claiming 'neither' contradicts conservation of energy. Common Pitfalls: Neglecting dissociation at very high temperatures, which moderates the increase. Confusing adiabatic temperature with actual flame temperature under heat losses. Final Answer: both (a) & (b)

Question 5

Flash point testing methods: The Pensky–Martens closed-cup apparatus is used for oils whose flash points are in which range (°F)?

Accepted Answer

Introduction / Context: Flash point is the lowest temperature at which vapors above a liquid ignite momentarily on exposure to a flame. Different standardized apparatus are specified for different volatility ranges to ensure safety and reproducibility. Given Data / Assumptions: Pensky–Martens is a closed-cup method. Used for medium-to-high flash point petroleum products (e.g., fuel oils, lubricants). Temperature range expressed in °F per the options. Concept / Approach: Closed-cup methods generally yield lower (more conservative) flash points than open-cup, and specific methods cover specific ranges. Pensky–Martens is intended for products with flash points typically above about 120 °F (≈ 49 °C). For very low flash points, other apparatus (e.g., Abel, Tag) are used. Step-by-Step Solution: Identify method applicability: medium/high flash point oils.Map to range: above 120 °F.Select the correct option: '>120' °F. Verification / Alternative check: Standards (ASTM D93) specify Pensky–Martens for a wide range that includes and emphasizes higher-flash products; low-flash gasoline-range materials use other tests (e.g., ASTM D56 Tag). Why Other Options Are Wrong: Ranges below 120 °F correspond to low-flash fuels requiring alternate apparatus; 90–110 °F is too low for the typical Pensky–Martens selection. Common Pitfalls: Confusing open-cup vs. closed-cup results; closed-cup values are typically lower for the same liquid. Applying the wrong standard method outside its validated range. Final Answer: >120

Question 6

High- vs. low-temperature carbonisation of coal: Compared to low-temperature carbonisation, which statement is NOT correct for high-temperature carbonisation?

Accepted Answer

Introduction / Context: Coal carbonisation thermally decomposes coal to produce coke, coke-oven gas (COG), tar, and byproducts like ammonia. Operating temperature profoundly affects product distribution and properties, which is critical for metallurgical coke quality and byproduct recovery. Given Data / Assumptions: High-temperature carbonisation: around 900–1100 °C. Low-temperature carbonisation: around 500–700 °C. Focus is on relative changes in ammonia yield, gas composition, tar character, and COG calorific value. Concept / Approach: At higher temperatures, stronger cracking and dehydrogenation occur. This elevates the hydrogen content in COG (more H2, less heavy hydrocarbons), increases aromaticity of the remaining tar (due to severe cracking and polycondensation), and typically reduces the gas calorific value because H2 has a lower volumetric heating value than heavier hydrocarbons and CO. Ammonia is thermally unstable at such temperatures, so its net yield declines. Step-by-Step Analysis: Ammonia: less at high T → statement (a) is correct.Tar aromaticity: high T → tar becomes more aromatic, not less → statement (b) is incorrect.COG composition: high T → more H2 → statement (c) is correct.COG calorific value: with cracked, lighter gas and more H2, CV tends to be lower than low-T process → statement (d) is correct. Verification / Alternative check: Process texts document that low-temperature carbonisation yields more tar (less aromatic, more oxygenated compounds) and richer gas in higher hydrocarbons, whereas high-temperature regimes favor aromatics in tar and H2-rich COG. Why Other Options Are Wrong: (a), (c), and (d) reflect well-established trends for high vs. low temperature carbonisation. Common Pitfalls: Assuming more severe heating always increases calorific value; composition matters. Confusing tar yield (quantity) with tar character (aromaticity). Final Answer: aromatic content of tar is low

Question 7

Coal carbonisation — In low-temperature carbonisation of coal, what is the typical yield of coke oven gas, expressed approximately in normal cubic metres (Nm³) per ton of dry coal?

Accepted Answer

Introduction: Coke oven gas yield is a key performance indicator in coal carbonisation. Low-temperature carbonisation (about 500–700 °C) aims to produce smokeless domestic coke and by-products such as tar and gas. Understanding the approximate gas yield helps with capacity planning for gas cleaning, storage, and use as a fuel. Given Data / Assumptions: Process: low-temperature carbonisation of dry coal. Quantity sought: approximate coke oven gas yield in Nm³ per ton of dry coal. Normal conditions indicate volume at standard temperature and pressure. Concept / Approach: At low temperatures, coal devolatilises gently, favoring tar production and yielding a moderate quantity of gas. In contrast, high-temperature carbonisation (about 900–1200 °C) maximises metallurgical coke strength and tends to yield substantially more gas but less tar. Typical reference values place low-temperature gas yield near the lower hundreds of Nm³/ton. Step-by-Step Solution: Identify operating regime: low-temperature (not metallurgical/high-temperature) carbonisation.Recall typical ranges: gas yield for low-temperature carbonisation ≈ 120–200 Nm³/ton.Choose the nearest listed figure within the plausible range: 160 Nm³/ton. Verification / Alternative check: Process handbooks typically report that high-temperature carbonisation produces higher gas volumes (several hundred Nm³/ton), confirming that figures like 500 or 750 Nm³/ton are characteristic of higher-temperature operations rather than low-temperature practice. Why Other Options Are Wrong: 60: Too low for typical low-temperature practice. 500 and 750: More representative of high-temperature or different processes. 300: Still on the high side for low-temperature carbonisation. Common Pitfalls: Confusing low-temperature with high-temperature yields; ignoring that “normal” cubic metres refer to standard conditions (important for fair comparison). Final Answer: 160

Question 8

Gaseous fuels — Among the following industrial and by-product gases, which has the lowest calorific value (heating value)?

Accepted Answer

Introduction: Designing burners, waste-heat recovery, and fuel-mix strategies requires a clear understanding of the calorific values of industrial gases. Blast furnace gas, converter gas, refinery gas, gobar gas, and coke oven gas differ widely in composition and heating value. Given Data / Assumptions: Comparison is qualitative (lowest heating value). Typical industrial compositions are assumed. Lower heating value trends are sufficient to rank choices. Concept / Approach: Heating value depends on combustible components (CO, H₂, CH₄, higher hydrocarbons). Gas streams heavily diluted with CO₂ and N₂ have low calorific value. Blast furnace gas is notably dilute: high in CO₂ and N₂ with modest CO, giving a very low calorific value compared to biogas or refinery by-product gases that contain substantial CH₄ and higher hydrocarbons. Step-by-Step Solution: Identify gas compositions: BF gas is CO-lean and CO₂/N₂-rich → low CV.Compare to refinery gas/coke oven gas: rich in hydrocarbons → high CV.Gobar gas (CH₄ + CO₂) has moderate CV, still far above BF gas.Converter gas has low-to-moderate CV but typically exceeds BF gas. Verification / Alternative check: Typical LHV ranges: blast furnace gas ~ 700–900 kcal/m³ (very low), converter gas ~ few thousand kcal/m³, gobar gas ~ 4500–6000 kcal/m³, refinery gas and coke oven gas ~ 9000–12000+ kcal/m³. Why Other Options Are Wrong: Gobar gas: contains significant methane → much higher than BF gas. Refinery gas: rich in hydrocarbons → high CV. Converter gas: higher than BF gas though still low. Coke oven gas: very high due to H₂, CH₄, and light hydrocarbons. Common Pitfalls: Assuming all by-product gases are similar; overlooking dilution by inert gases. Final Answer: Blast furnace gas

Question 9

Coal carbonisation — During the carbonisation of coal, the temperature range over which the coal softens and forms a plastic layer is approximately:

Accepted Answer

Introduction: Caking and plastic behavior of coal during heating govern coke quality and by-product recovery. The plastic layer stage is when coal softens, fuses, and then resolidifies, affecting the development of coke structure. Given Data / Assumptions: Thermal treatment of coal under carbonising conditions. Focus on the softening/plasticity temperature interval. Concept / Approach: As coal is heated, moisture removal precedes softening. Around mid-range temperatures, coal becomes plastic, enabling coalescence of particles; at higher temperatures, it resolidifies to form semicoke and then coke. The plastic range is characteristically in the few hundreds of degrees Celsius, not at the very low or very high ends of the carbonisation temperature spectrum. Step-by-Step Solution: Eliminate very low temperatures (100–150 °C) associated with drying.Eliminate very high temperatures (> 550 °C) where resolidification/coking occurs.Select the commonly cited plastic layer window: 350–450 °C. Verification / Alternative check: Empirical tests (e.g., Gieseler fluidity, Audibert–Arnu dilatometry) confirm the onset and peak fluidity in this range for many coking coals. Why Other Options Are Wrong: 100–150 °C: drying/initial devolatilisation, not plasticity. 550–650 °C and 700–850 °C: largely post-plastic, resolidifying and coke-strength development regimes. 250–300 °C: below typical softening onset for coking coals. Common Pitfalls: Confusing the plastic range with ignition or with final coking temperatures used in metallurgical coke ovens. Final Answer: 350 to 450 °C

Question 10

Fuel testing — Under what condition will the gross calorific value (GCV) and net calorific value (NCV) of a fuel be numerically the same?

Accepted Answer

Introduction: GCV (higher heating value) includes the latent heat released when the water formed during combustion condenses; NCV (lower heating value) excludes it. Knowing when they are equal aids in comparing fuels fairly. Given Data / Assumptions: Definitions: GCV counts condensation heat of water; NCV does not. Water arises from fuel moisture and from hydrogen combustion. Concept / Approach: When a fuel contains hydrogen (or moisture), combustion generates water. If that water remains vapor, NCV is lower than GCV by the latent heat of vaporisation. If no water is produced (no hydrogen and zero inherent moisture), there is no latent heat term, and GCV equals NCV. Step-by-Step Solution: Recognise the difference GCV − NCV ≈ mass of water produced * latent heat.Set water production to zero by eliminating hydrogen and moisture.Conclude: with zero hydrogen/hydrogen compounds (and negligible moisture), GCV = NCV. Verification / Alternative check: Pure carbon (dry) is a classic example: C + O₂ → CO₂ yields no water, thus GCV ≈ NCV. Why Other Options Are Wrong: Zero ash or low carbon does not remove the water term. “Under no circumstances” is false; dry hydrogen-free fuels are counterexamples. Oxygen content does not directly set the latent heat difference. Common Pitfalls: Attributing the GCV–NCV gap to ash or nitrogen; forgetting the role of fuel moisture alongside chemically bound hydrogen. Final Answer: If the fuel contains no hydrogen or hydrogen-bearing compounds

Question 11

Natural gas composition — Which constituent contributes the most to the heating value of typical natural gas?

Accepted Answer

Introduction: Natural gas is a valuable fuel whose calorific value is mainly determined by its combustible hydrocarbons. Identifying the key contributor is essential for combustion calculations and gas quality control. Given Data / Assumptions: Typical pipeline natural gas composition is predominantly methane, with smaller amounts of ethane, propane, butanes, CO₂, N₂, and traces of others. Heating value arises from combustibles; inerts lower the value. Concept / Approach: Methane typically comprises 80–95% of natural gas on a volume basis and has a high heat of combustion. Therefore, even though heavier hydrocarbons have higher heats per mole, methane’s dominant mole fraction makes it the principal contributor to overall calorific value. Step-by-Step Solution: Identify major component: CH₄ is the bulk of natural gas.Note that CO₂ and N₂ are inerts (no calorific contribution).CO and H₂ may be present in trace amounts; their contributions are minor in natural gas.Conclude CH₄ is the primary source of heating value. Verification / Alternative check: Gas quality specifications and tariff billing (therms) closely track methane number and its mole fraction, underscoring methane’s role. Why Other Options Are Wrong: CO₂ and N₂: inerts, reduce CV. CO and H₂: limited presence in natural gas streams; overall impact is small compared to methane. Common Pitfalls: Overemphasising heavier hydrocarbons due to higher per-mole heats while ignoring their lower fractions. Final Answer: Methane (CH₄)

Question 12

Low-temperature carbonisation — Which statement best describes the products and aims of low-temperature carbonisation of coal (about 500–700 °C)?

Accepted Answer

Introduction: Carbonisation temperature controls coke properties and by-product distribution. Low-temperature carbonisation (LTC) differs substantially from high-temperature carbonisation (HTC) used in metallurgical coke making. Given Data / Assumptions: LTC temperature range roughly 500–700 °C; HTC about 900–1200 °C. Focus on dominant product purpose and relative yields of gas and tar. Concept / Approach: LTC yields a porous, smokeless “semi-coke” suited for domestic or industrial heating (not metallurgy). Because reactions are milder, LTC typically generates relatively high tar yields and a modest gas volume. HTC is required for strong metallurgical coke and gives higher gas volumes but lower tar yields. Step-by-Step Solution: Identify the objective: LTC → smokeless fuel, not BF coke.Compare by-products: LTC → more tar, less gas; HTC → more gas, less tar.Select the statement that aligns with these trends: option (a). Verification / Alternative check: Typical data: gas yield LTC ≈ 120–200 Nm³/ton (lower); HTC can be several hundred Nm³/ton (higher). Tar yield LTC ≈ 10–14% vs. HTC ≈ 2–5%. Why Other Options Are Wrong: (b) Metallurgical coke requires HTC for strength and low volatiles. (c) and (d) invert gas/tar trends. (e) Incorrect; LTC intentionally recovers valuable tar and other condensables. Common Pitfalls: Assuming all carbonisation is for metallurgical coke; mixing yield trends between LTC and HTC. Final Answer: It is mainly used to produce smokeless domestic coke (semi-coke) with significant tar by-product

Question 13

High-temperature carbonisation — What is the principal effect of free moisture present in coal during high-temperature carbonisation in coke ovens?

Accepted Answer

Introduction: Moisture content in the coal charge influences heat transfer, volatile evolution, and secondary reactions during coke oven operation. Understanding its role aids in by-product recovery quality and oven scheduling. Given Data / Assumptions: Scenario: high-temperature carbonisation (metallurgical coke making). Presence of free (non-bound) moisture in the coal charge. Interest: effect on volatile products and process time. Concept / Approach: As the charge heats, free moisture evaporates, absorbing latent heat and locally lowering peak temperatures where volatiles emerge. This moderating effect can reduce thermal cracking of vapours at hot surfaces (coke and oven walls), helping preserve valuable by-products (e.g., tar fractions). However, the need to drive off this moisture typically increases, not decreases, overall coking time. Step-by-Step Solution: Recognise evaporation of free moisture absorbs heat (endothermic), moderating local temperatures.Lower local temperature → less secondary cracking of volatiles → protection of products.Time impact: moisture removal adds a drying stage, lengthening coking time rather than reducing it. Verification / Alternative check: Operationally, higher-moisture coal increases heat duty for drying and can extend push times; by-product composition often shifts toward heavier fractions due to reduced cracking. Why Other Options Are Wrong: (a) Opposite of practice; drying time adds to the cycle. (c) Moisture tends to suppress dusting/agglomerate fines during charging, not increase loss. (e) Coke strength is governed mainly by rank, maceral balance, ash chemistry, and HTC conditions; mere moisture does not “temper” to strengthen. (d) Not applicable since (b) is correct. Common Pitfalls: Confusing effects on by-product cracking with effects on oven throughput; assuming moisture is always detrimental to by-products. Final Answer: It helps protect volatile products from cracking by moderating temperatures near hot coke and oven walls

Question 14

Chimney design — The natural draught (draft) produced by a chimney primarily depends on which factors?

Accepted Answer

Introduction: Natural draught provides the pressure head to move combustion gases without fans. It is created by density differences between hot flue gases and the cooler ambient air column surrounding the chimney. Given Data / Assumptions: No mechanical draft devices (purely natural draught). Steady operation; uniform ambient conditions along chimney height. Concept / Approach: The pressure difference (draft) arises because the column of lighter hot gas inside the chimney exerts a lower hydrostatic pressure than an equal-height column of cooler, denser outside air. The draft is approximately proportional to the height and the density difference, which in turn depends on gas temperature and composition. Step-by-Step Solution: Express draft head ΔP ≈ g * H * (ρ_air − ρ_gas).Note dependence on H (height) and densities (set by temperatures).Conclude that both density difference and height control natural draught. Verification / Alternative check: Stack effect equations and design charts for chimneys explicitly use H and temperature (density) to compute draft and volumetric flow. Why Other Options Are Wrong: (a) or (b) alone are incomplete; both factors act together. (d) Contradicts the physical basis of the stack effect. (e) Moisture affects density slightly but is not the primary sole factor. Common Pitfalls: Ignoring ambient temperature variations; neglecting frictional losses which reduce the effective draft available to equipment. Final Answer: Both (a) and (b)

Question 15

Coal washeries — The calorific value of coal middlings produced during coal washing is typically around how many kilocalories per kilogram (kcal/kg)?

Accepted Answer

Introduction: Coal washing separates run-of-mine coal into clean coal, middlings, and rejects. Middlings are often used as a lower-grade fuel for captive power or industrial boilers, so knowing their approximate calorific value is important for utilization planning. Given Data / Assumptions: Middlings are intermediate-density fractions with higher ash than clean coal but lower than rejects. Typical Indian washeries and similar operations are implied. Concept / Approach: As ash content increases, calorific value decreases. Clean thermal coal may be 5000–6500 kcal/kg; middlings tend to be in the mid-range (often around ~3500–4500 kcal/kg), while rejects can be much lower. Step-by-Step Solution: Recognise middlings as mid-quality fraction.Recall typical CV range for middlings ≈ 3500–4500 kcal/kg.Select the closest listed representative value: 4000 kcal/kg. Verification / Alternative check: Power plant fuel-blending practices and washery reports often list middlings near this value, adjusting for local geology and washery cut-points. Why Other Options Are Wrong: 1000 and 2500: Too low; characteristic of high-ash rejects. 6000 and 8000: Too high for middlings; in the range of cleaned coal or premium grades. Common Pitfalls: Assuming middlings are nearly as good as clean coal; ignoring site-specific ash and moisture variations that shift CV by several hundred kcal/kg. Final Answer: 4000

Question 16

Safety and fouling — Why are fuel gases that contain significant hydrocarbons (e.g., coke oven gas) generally not preheated before combustion?

Accepted Answer

Introduction: Preheating gaseous fuels can improve combustion efficiency, but gases rich in hydrocarbons (like coke oven gas) bring risks. Understanding thermal cracking and fouling mechanisms is crucial for reliable firing systems. Given Data / Assumptions: Fuel contains light hydrocarbons and higher unsaturates/aromatics. Preheating involves high wall temperatures in recuperators or heaters. Downstream equipment is sensitive to fouling. Concept / Approach: Hydrocarbon cracking is a temperature-driven process where larger molecules decompose into smaller fragments and elemental carbon (soot) on hot metal surfaces. This carbon deposits inside preheaters and exchangers, raising pressure drop, impeding heat transfer, and potentially causing hot spots. Therefore, practice avoids preheating hydrocarbon-rich fuels; instead, mixing with air at ambient or using limited preheat under controlled conditions may be preferred. Step-by-Step Solution: Recognise composition: hydrocarbon-rich fuel → cracking prone.Assess thermal environment: hot surfaces → soot formation risk.Conclude main reason: fouling/choking from carbon deposition. Verification / Alternative check: Plant experience shows increased maintenance and exchanger plugging when preheating such fuels; design guidelines often prescribe limits or recommend on-burner steam injection to mitigate soot in some cases. Why Other Options Are Wrong: (b) and (c) are incorrect; preheating does not inherently reduce calorific value or flame temperature before burning. (d) While explosion risks exist with any fuel handling, the principal, routine reason to avoid preheat is cracking/soot fouling. (e) Sulphur removal is unrelated to simple preheating. Common Pitfalls: Assuming preheating is universally beneficial; ignoring material balance of carbon deposition that progressively blocks passages. Final Answer: Because hydrocarbons can crack on hot surfaces, depositing carbon soot that fouls and chokes heat-transfer passages

Question 17

Fuel reactivity for steam–carbon gasification (C + H2O → CO + H2): Among the following solid carbonaceous materials used in industry, which one is generally the most reactive toward steam at comparable conditions?

Accepted Answer

Introduction / Context: Gasification converts solid carbon (C) and steam (H2O) into synthesis gas (CO + H2). The rate of the steam–carbon reaction is strongly influenced by the reactivity of the solid fuel or char. Understanding which solid carbon materials are more reactive helps in designing gasifiers, optimizing temperature, and selecting feedstocks for efficient syngas production. Given Data / Assumptions: Reaction of interest: C + H2O → CO + H2 (endothermic, steam–carbon reaction). Comparative evaluation at similar temperatures where intrinsic carbon reactivity matters. Materials compared: blast-furnace coke, low-temperature coke (semi-coke), anthracite, sub-bituminous coal, graphite. Concept / Approach: Carbon reactivity correlates with microstructure, surface area, porosity, amount of disordered carbon, and catalytic mineral matter (alkalis, iron, etc.). Lower-rank coals and chars made at lower heat-treatment temperatures retain more functional groups and disorder, making them more reactive toward steam than highly ordered carbons (anthracite, graphite) or strongly carbonized cokes (metallurgical coke). Step-by-Step Solution: Rank typical reactivity trend: graphite ≪ anthracite ≪ high-temperature coke (BF coke) < sub-bituminous coal-derived char ≲ low-temperature coke.Low-temperature coke (semi-coke) has higher volatile-derived porosity and less structural order, enhancing active sites for steam attack.Therefore, among the options, low-temperature coke is generally the most reactive toward C + H2O. Verification / Alternative check: Laboratory thermogravimetric studies and industrial experience in gasifiers show faster conversion rates for low-temperature chars relative to metallurgical coke or anthracite at the same temperature. Catalytic ash constituents in lower-rank materials further promote reactivity. Why Other Options Are Wrong: Blast-furnace coke: highly carbonized, dense, and less reactive. Anthracite: very ordered carbon, among the least reactive. Sub-bituminous coal can be reactive, but its raw form must devolatilize first; its resulting char is typically less reactive than dedicated low-temperature coke. Graphite: the least reactive due to high crystallinity. Common Pitfalls: Assuming higher fixed carbon always means higher reactivity; ignoring the role of microstructure and ash catalysis; comparing raw coal directly to char without considering devolatilization effects. Final Answer: Low-temperature coke (semi-coke/char made at lower coking temperatures)

Question 18

Coal-tar products: Which of the following is not obtained by distillation of coal tar in a by-product recovery plant?

Accepted Answer

Introduction / Context: High-temperature coal carbonization produces coke, coke-oven gas, and coal tar. Valuable aromatics are recovered from these streams by cooling, scrubbing, and fractionation. Some compounds are primarily recovered from coal tar, while others are recovered from the gas phase using wash oils. Distinguishing their origins is important for by-product plant design and operation. Given Data / Assumptions: Source streams: coal tar (liquid by-product) and coke-oven gas (gaseous by-product). Typical coal-tar distillation fractions include light oils (phenolic), middle oils (naphthalene), creosote oil, and pitch; higher polycyclic aromatics like anthracene are enriched in heavier tar cuts. “Benzol” commonly refers to a benzene/toluene (and related light aromatics) mixture recovered from gas via wash oil, not from the tar still. Concept / Approach: Coal tar is distilled to yield specific fractions rich in phenolic compounds, naphthalene, creosote oil, anthracene oil, and finally pitch. Meanwhile, “benzol” (benzene/toluene/xylene range) is typically absorbed from the coke-oven gas using a wash oil and then stripped—this is a gas-treatment operation rather than tar distillation. Step-by-Step Solution: List tar distillation products: phenol (light oil), naphthalene (crystallizes from middle oils), creosote oil (middle–heavy), anthracene (heavy fractions), pitch (bottoms).Identify benzol’s usual source: absorption from coke-oven gas (not from tar stills).Hence, the correct choice for “not obtained by tar distillation” is benzol. Verification / Alternative check: Process-flow diagrams for by-product recovery show parallel trains: tar decanting/distillation for heavy aromatics and pitch; gas cooling/scrubbing for ammoniacal liquor and light aromatics (benzol). This separation by stream origin is standard practice. Why Other Options Are Wrong: Phenol, naphthalene, creosote oil, and anthracene are classic coal-tar derivatives obtained via fractional distillation and crystallization. Common Pitfalls: Assuming all aromatics come from tar; forgetting that the lightest aromatics are preferentially recovered from the gas by absorption rather than from the tar column. Final Answer: Benzol (benzene/toluene mixture recovered from coke-oven gas)

Question 19

Dry gaseous fuels: The presence of which component in a dry gaseous fuel does not contribute to its calorific value, and may even dilute it?

Accepted Answer

Introduction / Context: Calorific value reflects the heat released upon complete combustion of a fuel. For gaseous fuels, the composition—fractions of combustible species versus inerts or oxidants—determines usable heating value and flame characteristics. Understanding which components contribute and which do not is essential for burner tuning and energy balance calculations. Given Data / Assumptions: “Dry” gaseous fuel excludes water vapor for the analysis. Combustibles include H2, CO, CH4 and higher hydrocarbons. Oxygen present within the fuel acts as a built-in oxidizer/diluent rather than a fuel. Concept / Approach: Only combustible constituents generate heat by oxidizing to final products (CO2 and H2O). Noncombustible or already-oxidized components either do not release heat or can lower the adiabatic flame temperature by dilution. Oxygen in the fuel stream is not a fuel; it reduces the required external air but does not add to calorific value itself. Step-by-Step Solution: Identify energy-bearing species: H2, CO, hydrocarbons → positive contributions.Assess oxygen: not a fuel; contributes no heat of oxidation.Conclude: oxygen does not add to calorific value and can reduce volumetric heating value by dilution. Verification / Alternative check: Bomb-calorimetry or calculated heating values confirm that including oxygen in the mixture lowers the net heating value per unit volume compared to an equivalent inert-free, oxygen-free stream with the same combustibles. Why Other Options Are Wrong: Hydrogen, CO, and hydrocarbons (carbon) are the main energy contributors in gas fuels. Sulphur compounds (e.g., H2S) are combustible, though undesirable for corrosion and emissions; they do contribute heat upon oxidation to SO2. Common Pitfalls: Confusing net heating value effects with stoichiometric air requirements; overlooking safety impacts of oxygen-enriched fuel streams. Final Answer: Oxygen

Question 20

Coal petrography vocabulary: A coal containing a very high percentage of durain (the dull, hard lithotype rich in inertinite) is commonly termed ________ coal.

Accepted Answer

Introduction / Context: Coal is described using macroscopic lithotypes: vitrain (bright), clarain (banded), durain (dull), and fusain (fibrous). These reflect maceral composition and depositional history, which in turn affect caking, coking, and combustion behavior. Identifying a coal dominated by durain helps anticipate its performance in carbonization and gasification. Given Data / Assumptions: Durain: dull, hard lithotype, inertinite-rich, poor caking. Vitrain: bright, vitrinite-rich, better caking behavior. Terminology uses historical industrial names: “splint coal” is associated with high durain content. Concept / Approach: Coal with abundant durain tends to be hard, dull, and non-caking; it fractures splintery—hence the name “splint.” Such coals are less suitable for metallurgical coke but can be used for domestic fuels or gasification where caking is not required. In contrast, “bright coal” refers to vitrain-rich material with better coking tendencies. Boghead/cannel coals are algal or sapropelic and are distinct from durain-rich banded bituminous coals. Step-by-Step Solution: Match lithotype to industrial term: durain → splint coal.Eliminate terms tied to different lithotypes: bright coal (vitrain), boghead/cannel (sapropelic, hydrogen-rich).Choose “splint coal” as the standard designation for durain-rich coal. Verification / Alternative check: Petrographic references and coke-oven practice consistently associate splint coal with poor caking and a high proportion of dull bands (durain). Why Other Options Are Wrong: Bright/vitrain-rich coals are the opposite texturally; boghead and cannel describe sapropelic coals formed from algal material; “non-banded” does not specifically indicate high durain content. Common Pitfalls: Confusing maceral groups with lithotypes; assuming “bright” refers to calorific value rather than appearance and petrography. Final Answer: Splint coal (durain-rich, hard, non-caking)

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