low carbon - hydrogen economy, fossil fuels, and biomass (Q1)

different hydrogen production methods

• steam reforming of methane

• coal gasification

• water electrolysis (green hydrogen)

• biomass and waste food

steam reforming of methane (SMR)

• most common method of producing hydrogen, but it relies on fossil fuels (natural gas)

• reaction 1(steam reforming): CH4​+H2​O→CO+3H2

• reaction 2 (water-gas shift): CO+H2​O→CO2​+H2

• alternative (partial oxidation): CH4​+0.5O2​→CO+2H2

SMR: reaction 1(steam reforming)

CH4​+H2​O→CO+3H2

• highly endothermic (+206.4 kJ/mol)

• uses Ni catalyst

• at 700°-1100°C

SMR: reaction 2 (water-gas shift)

CO+H2​O→CO2​+H2

• mildly exothermic (-41.2 kJ/mol)

• uses Fe/Cr oxide catalysts

SMR: Alternative (Partial Oxidation):

CH4​+0.5​O2​→CO+2H2​

• exothermic (-35.7 kJ/mol)

alternative to steam reforming (autothermal reforming)

• combines SMR, water-gas shift, and partial oxidation into single reactor

• energy neutral - heat generated by partial oxidation matches heat required for steam reforming

Coal Gasification

• coal undergoes devolatilisation to form gaseous mixture and solid char residue

• it is then gasified into CO and H2

• environmental issues: releases severe pollutants, including heavy metals (Mercury, Cadmium, Zinc), NOx, SOx, and poisonous CO

Coal gasification: reactions

• partial oxidation of char: C+0.5O2→CO (exo)

• steam reforming of C: C+H2O→CO+H2 (endo)

• steam reforming of C: C+2H2O→CO2+2H2 (endo)

• water-gas shift: CO+H2O→CO2+H2 (exo)

Water Electrolysis (Green Hydrogen)

• splitting of water into hydrogen and oxygen using electric current

• Reaction: 2H2O→2H2+O2

• anode:

• cathode:

• Green opportunity: if electricity used to power electrolysis comes from renewable sources, hydrogen produced with no greenhouse gas emissions and no reliance on fossil fuels - truly sustainable hydrogen economy

Biomass and Waste Food

• biomass = biological material that is carbon neutral (wood, crops, food waste, animal residues, industrial waste)

  • carbon neutral = absorbs CO2 when growing and releases it when burned

• biological conversion (fermentation)

• thermochemical (the carnol process)

biomass and waste food: biological conversion (fermentation)

• microorganisms (e.g enzymes/archaea) convert glucose into ethanol

• Equation: C6H12O6→2C2H5OH+2CO2

Biomass and waste food: thermochemical (the carnol process)

• methane → methanol fuel

• Step 1 (thermal decomp): CH4​→C+2H2

• Step 2 (methanol synthesis): CO2​+3H2​→CH3​OH+H2​O

• overall:

2CO2​+3CH4​→2CH3​OH+2H2​O+3C

Hydrogen purification methods:

chemical methods

• selective methanation

• selective oxidation

physical methods

• pressure swing adsorption (PSA)

• palladium membrane separation

Selective methanation

• uses Co, Fe, or Ru catalyst to convert CO back into methane

• CO+3H2→CH4+H2O

• Drawback: highly exo (-206.1 kJ/mol) and consumes valuable H2 just produced

Selective Oxidation

• uses catalyst to selectively promote CO+0.5O2→CO2 over combustion of H2

• Thermodynamics favour CO reaction at lower temps

Pressure Swing Adsorption (PSA)

• removes impurities based on their Mr at high pressures

• using beds of zeolites, silicia and carbons

Palladium Membrane Separation

• H2 molecules hit the Pd membrane

• disassociate into monatomic hydrogen

• diffuse through microscopic gases between metal atoms

• recombine into H2 on the other side

• +: yields 99.9999% pure H2

• -: palladium is very expensive

why must hydrogen be purified?

Hydrogen produced via SMR or gasification contains 0.2% to 3% CO. CO poisons Platinum catalysts used in fuel cells, must be reduced to incredibly pure levels (1-10 ppm)

hydrogen storage issues

• hydrogen has tiny molecular size and a negative Joule-Thompson coefficient

• meaning leaks can easily escape and self-ignite

Physical hydrogen storage

• stored as highly compressed gas in reinforced cylinders or cryogenic liquid hydrogen

• liquid spills can cause severe explosive burns

hydrogen safety: protocols

• Testing tanks/equipment rigorously to prevent leaks.

• Installing extra safety valves.

• Designing equipment to withstand shocks, vibrations, extreme temps.

• Using H2/O2 leak detectors.

• Keeping fuel cell supply lines physically separated from other equipment

Alternative fossil fuels and biofuels

• the biodiesel cycle

• shale gas (hydraulic fracturing/fracking)

The biodiesel cycle

• Reaction (Transesterification):

  • Triglyceride + 3 Alcohol ←> Alkyl esters + glycerol

  • conditions: requires catalyst (NaOH/KOH) and is heated to 160 °C

• +: renewable, biodegradable, higher flashpoint (safer to store), higher lubricity prolongs engine life), significantly reduces Particulate Matter (40-60%) and Co (10-50%) compared to petroleum diesel

• -: poor quality biofuel damages engines, SVO is too viscous to use directly, ‘food vs fuel’ supply conflicts

Shale gas (hydraulic fracturing/fracking)

• natural gas trapped in impermeable shale rock deep underground

• well is drilled horizontally

• mixture of water, sand, chemicals pumped in at 1500 lbs per square inch to fracture rock and release gas

• +: increases energy security, creates jobs, natural gas burns cleaner than coal/oil

• -: requires lots of water, risks polluting freshwater aquifers with heavy metals/chemicals, high-pressure fracturing can trigger minor earthquakes

biodiesel cycle diagram

hydraulic fracturing

palladium membrane diagram