Chemistry of fuels and energy Knowledge-Based Download PDF

Questions

1. 1.

   What is the fundamental thermodynamic difference between the higher heating value (gross calorific value) and the lower heating value (net calorific value) of a chemical fuel?

   • A) The higher heating value accounts for the latent heat of vaporization released by condensing the water produced during combustion, whereas the lower heating value assumes the water remains in the vapor phase.
   • B) The higher heating value includes the thermal energy released from oxidizing atmospheric nitrogen to nitrogen dioxide, whereas the lower heating value assumes inert nitrogen.
   • C) The higher heating value is measured strictly under constant pressure conditions, whereas the lower heating value represents energy output under constant volume in a bomb calorimeter.
   • D) The higher heating value subtracts the energy absorbed during the thermal cracking of fuel molecules, whereas the lower heating value measures the total uncorrected enthalpy change.
2. 2.

   In the production of second-generation bioethanol from lignocellulosic feedstock, what is the primary purpose of the pretreatment step prior to enzymatic saccharification?

   • A) To directly ferment monomeric pentose and hexose sugars into ethanol and carbon dioxide using yeast enzymes.
   • B) To disrupt the recalcitrant lignin-hemicellulose barrier and increase the accessibility of cellulose fibers to cellulase enzymes.
   • C) To catalytically dehydrate aqueous ethanol into ethylene for subsequent polymer synthesis.
   • D) To separate and purify the dilute ethanol broth through fractional and azeotropic distillation.
3. 3.

   In the chemical processing of fuels to generate high-purity hydrogen from synthesis gas, which statement correctly describes the function of the water-gas shift reaction?

   • A) It reacts methane directly with oxygen at high temperatures to yield equimolar amounts of carbon monoxide and water.
   • B) It reacts carbon monoxide with steam to produce additional hydrogen gas while oxidizing carbon monoxide to carbon dioxide.
   • C) It thermally cracks long-chain alkanes into light alkenes and hydrogen gas in the absence of a chemical oxidant.
   • D) It catalytically hydrogenates carbon dioxide with hydrogen gas to synthesize liquid methanol fuel.
4. 4.

   Which of the following statements correctly describes the primary chemical process and products involved in the transesterification of vegetable oils to produce biodiesel?

   • A) Long-chain fatty acids undergo thermal cracking at high temperatures to yield short-chain alkanes and alkenes.
   • B) Triglycerides react with a short-chain alcohol in the presence of a catalyst to yield fatty acid alkyl esters and glycerol.
   • C) Unsaturated carbon-carbon double bonds in plant lipids are catalytically hydrogenated to produce solid paraffin waxes.
   • D) Simple hexose sugars derived from plant cellulose are anaerobically fermented by yeast to yield bioethanol and carbon dioxide.
5. 5.

   Which of the following statements correctly compares the combustion chemistry and environmental impact of hydrogen fuel with conventional hydrocarbon fuels?

   • A) Hydrogen fuel produces carbon monoxide and water vapor as its primary direct combustion products, making it less toxic than burning methane.
   • B) Hydrogen has a higher volumetric energy density at standard temperature and pressure than liquid hydrocarbons, requiring significantly smaller storage tanks.
   • C) The combustion of hydrogen releases more carbon dioxide per mole than octane because of its higher molar mass.
   • D) Hydrogen yields only water vapor as its direct carbon-free combustion product and possesses a higher gravimetric energy density than gasoline.