Combustion and Fossil Fuels

Enthalpy of Combustion (Calorific Value)

  • The enthalpy of combustion (\Delta_cH) or calorific value (C.V.) is the heat energy released when a compound undergoes complete combustion with oxygen.
    • Products of complete combustion include CO2 and liquid water (H2O).
    • If nitrogen is present, N_2 is also formed.
  • Gross (Upper Level) Values: Include the latent heat of condensation for steam formed during combustion.
  • Net (Lower Level) Values: Exclude latent heat and are important for the operation of boilers and burners.
  • Units:
    • For solid and liquid fuels: Typically in MJ \cdot kg^{-1}.
    • For gases: Often expressed volumetrically (MJ \cdot m^{-3}) at 15^\circ C and atmospheric pressure (101.3 kN/m²), which are standard temperature and pressure (STP) conditions.

Bomb Calorimeter

  • Used to measure the heat of combustion at constant volume.
  • Components:
    • Strong body casing and tightly fitted lid of "bomb"
    • Sealable O_2 inlet
    • Water bath
    • Insulated wall
    • Electrical heater for water bath (Not shown)
  • For Gases: A dynamic setup replaces the static system, where gas is burned at a known rate, and heat is transferred to water flowing at a constant rate through the calorimeter.

Combustion Calculations (Solids, Liquids)

  • Step i: Calculate Heat Transferred to Water
    • Heat = weight{water} \times SpecificHeatCapacity{water} \times TempRise
    • Units: weight in kg, specific heat capacity in kJ/kg/K, temperature rise in °C or K.
  • Step ii: Determine Weight of Water
    • weight{water} = actualWeight{bomb} + equivalentWeight_{calorimeter}
    • The equivalent weight calibrates for other calorimeter design factors that contribute to the heat transferred such as efficiency and heat from firing wire.
  • Step iii: Correct for Heat Loss
    • If information is provided, consider cooling (heat loss) of water and correct the measured temperature rise.
  • Calculate Gross Calorific Value
    • Gross \; \DeltacH = \frac{heatTransferred}{weight{sample}}
    • Units: MJ \cdot kg^{-1}

Net Calorific Value Calculation

  • Calculate the net value from the gross value.
  • For 1 kg of sample, calculate the total weight of water formed during combustion.
  • Sources of water:
    • Water produced by fuel combustion.
    • Water from any organic H2 gas present in the sample reacting with O2.
    • Moisture content already present in the fuel sample.
  • TotalWeight{water} = weight{moisture} + (\frac{18}{2} \times weight{H2 \; in \; fuel})
  • Calculate Heat Loss
    • HeatLoss = totalWeight{waterReleased} \times latentHeat{steamCondensation}
  • Calculate Net Calorific Value
    • Net \; \DeltacH = Gross \; \DeltacH - heatLoss (per kg of sample)

Determining ΔcH for Gases

  • Step i: Burn gas at a known flow rate, and transfer heat to water flowing at a constant rate through the calorimeter.
  • Step ii: Correct the volume of gas burned per unit time to standard conditions (T = 15^\circ C, P = 101.3 kN/m²) using the Gas Law.
  • Step iii: Calculate heat transferred to water.
    • Units: MJ \cdot m^{-3}
  • Step iv: Calculate Gross Calorific Value
    • Gross \; \DeltacH = \frac{heatTransferred}{volume{gasBurned(corrected)}}

Net ΔcH Calculation for Gases

  • Calculate the heat lost from not allowing water to condense:
    • HeatLoss = weight{water(per \; m^3 \; gas \; burned, \; standard \; T \; and \; P)} \times latentHeat{steamCondensation}
  • If the composition and amount of gas are known (e.g., CH4, H2), the weight of water produced can be calculated from the combustion equation.
    • Example: CH4 + O2 \rightarrow CO2 + 2H2O (2 moles/volumes of water are produced from one of methane).
    • 1 mole of gas occupies 22.4 dm^3 at STP.
  • Calculate Net Calorific Value:
    • Net \; \DeltacH = Gross \; \DeltacH - heatLoss (for 1 m³ of gas)

Comparison of ΔcH Values

  • Comparison of Fuel values in MJ \cdot kg^{-1} vs MJ \cdot L^{-1} [Liquid form]:
    • H_2: 142
    • CH_4: 55.7, 23.4*
    • Ethane: 51.9, 28.0
    • Propane: 51.0, 28.9
    • Octane: 48.1, 33.5
    • LPG: 46.1, 26
    • C2H5OH: 29.7, 23.5
    • Petrol: ~44, ~32
    • Diesel: ~43, ~36
    • Coal: ~35
    • Carbon: 34
    • Peat: ~16-20
      *Energy/mass vs energy/vol
  • Low MW compounds have a low density.
    • Storage volume is most important for transport.
  • Lower H/C ratio = lower energy/gram

Chemicals from Fossil Fuels

  • Fossil fuels are valuable raw materials for producing many chemicals.
  • Petrochemicals:
    • Much of today’s organic chemical industry is based on chemicals derived from petroleum.
    • Examples: synthetic fibers, pharmaceuticals, fertilizers, and plastics.
  • Oil Usage:
    • Distillation, cracking, and reforming to produce a wide range of products.
  • Coal and Natural Gas Usage:
    • Production of synthesis gas and synthetic liquid fuels (alternative to oil for high-quality diesel, LPG).

Major Starting Points for Chemical Production

  • Methane (steam reforming).
  • Ethane (cracking).
  • Propane (cracking).
  • Naphtha fraction of crude oil (catalytic reforming).
  • Coal (gasification).
  • Important chemical intermediates:
    • Synthesis gas.
    • Ethene (ethylene).
    • Propene (propylene).
    • Benzene.
    • Toluene.
    • Xylene.
    • Methanol

Synthesis Gas (Syn Gas)

  • Mixture of CO and H_2.
  • Important intermediary for producing various chemicals.
  • Can be used as a fuel directly in gas turbines.
    • Lower C.V. than natural gas but burns with greater efficiency due to lower operating temperatures.
  • Production of SYN GAS (see NOTE 5)
    • Steam reforming of CH_4.
    • Gasification of coal.
      *Solar energy
      *N.B. Reforming refers to altering the structure of a hydrocarbon but keeping the number of C atoms the same

Syn Gas Reactions

  • CO + 3H2 \rightleftharpoons CH4 + H_2O (Methanation, reverse of steam reforming).
    • Catalyst: Ni/Al2O3 or Raney Ni.
    • Highly exothermic; lower T (~300^\circ C), higher P.
    • Use directly as a fuel: Lower combustion T and better efficiency but C.V. < ½ of CH_4 and CO poisonous.
  • CO + 2H2 \rightleftharpoons CH3OH (Methanol synthesis).
    • Catalyst: Cu.
    • Multiple uses.
      *Hydrocarbons, transport fuels
      *Catalyst: Fe or Co
  • Different H_2/CO stoichiometry is required for these reactions, which can be controlled via the water gas shift reaction and purified.

Conversion of Natural Gas to Methanol

  • Key Steps:
    • Co-Mo catalyst converts sulfur in the gas stream to H_2S, which reacts with ZnO and converts to elemental S.
    • Ni catalyst (steam reforming) is poisoned by S.
    • Cu catalyst (MeOH synthesis) is very efficient (99%) and operates at a lower temp.
  • Methanol synthesis is important as it provides a springboard to the generation of a large range of valuable chemicals.

Fischer-Tropsch Synthesis

  • Initially developed by Germany during WWII and interest is increasing again as petroleum stocks become more expensive for transport fuel production.
  • Typically coal gasification is used to produce syn gas
  • Straight-chain alkanes:
    • nCO + (2n)H2 \rightarrow CnH(2n+2) + nH_2O (\Delta H = -231 kJ/mol)
    • Very exothermic reaction and temperature control is very important.
    • Fe catalysts help increase H_2 content via water-gas shift reaction but are less active than Co as a catalyst so a mixture of both is usually used.
    • Many reaction conditions and additional catalysts are used to optimize the production of linear/branched, high/low MW, alcohol/alkane production.
  • Alcohols:
    • nCO + (2n)H2 \rightarrow CnH(2n+1)OH + (n-1)CO_2

Catalysts

  • Catalysts are crucial; understanding the complex series of steps that can occur on a catalyst surface is very much an active field of research.
  • The selectivity and enhanced reaction rates of a particular catalyst need to be determined. *Metals (e.g. Fe, Pt, Cu, Ni, Mn)
    • strongly adsorb and dissociate H_2 - important in hydrogenation / dehydrogenation.
    • adsorb CO, and other molecular species, C=C etc
  • Acidic oxides (zeolites, Si/Al2O3)
    • Bronsted acid sites promote carbocation formation
      *Cracking, alkylation, isomerization, reforming rxns
      *Metal oxides & Metal sulphides (MoS_2, Co, CuO, ZnO)
    • Desulfurization; Oxidation

Catalyst Specific Surface Area

  • Specific surface area = total surface area per unit of mass (e.g., m^2/g).
  • the conversion of raw materials into fuels and chemical feedstocks involves catalysis to some extent.

Catalyst Particle Surface Area Calculation

  • Problem: Calculate the surface area (SA) of spherical catalyst particles with a 50-micron diameter, given the catalyst material has a density (ρ) of 3 g/cm³.
  • Mass of 1 particle (MP) = (4/3) \cdot \pi \cdot (d/2)^3 \cdot \rho
  • Area of 1 particle (MA) = 4 \cdot \pi \cdot (d/2)^2
  • No. of particles in 1 g = 1 / MP
  • SA = (1/MP) * MA = 6 / (ρ * d)
  • SA = 6 / (3 \times 50 \times 10^{-4}) = 400 cm^2/g = 0.04 m^2/g
    *Further exercise
  • Use the same approach to work out the value of SA for cubic particles size, r.
  • What would be the SA for nanoparticles of the same material with 50 nm diameter?

Metallic Nanoparticles for Catalyst Investigations

  • Many different shapes and sizes of metallic nanoparticles are being developed for catalyst investigations
    *Others: Core-shell, stars, cups, cages, prisms, nanopores, clusters rods
  • Shapes: cubes, spheres, wires, dogbones, tubes

Young's Invention

  • James “Paraffin” Young (1811 - 1883) invented (and patented!) a method for producing liquid fuels and waxes from coal.

Petroleum Analysis - Molecular Composition

  • • The molecular composition of Petroleum (crude oil) is a complex mixture of hydrocarbons.
  • • Principal components: n- and iso-alkanes, cycloalkanes, aromatics (benzene, alkylbenzenes, napthalenes), NSO’s (compounds containing heteroatoms)
  • Elemental composition varies over a more narrow range (82-87% C, 12-15% H, plus O, N, S) than coal but many molecular isomers present (with variation increasing with number of C atoms).
  • • Low inorganic content, ash < < 1%
  • • H/C ratios ~ 1.5 – 2 (c.f. coal < 1)

Petroleum - Specific Gravity

  • Current Worldwide Consumption estimates are ~2 L a day per person!! Expressed as °API gravity, provides an indicator of aromatic character and molecular weight

Initial Rapid Assessment of Petroleum Type Based On

  • • Distillation products
  • • Elemental analysis (H/C ratio, %S) high S - sour; low S - sweet
  • - light vs heavy
  • - paraffinic (alkanes) vs aromatic/napthenic
  • °API decreases with increasing MW and aromaticity
  • • Additional characterisation tests for refined products such as petrol, diesel described later e.g. octane, cetane numbers.

API Gravity Calculation

  • API gravity = [141.5 / (Specific gravity) ] – 131.5
  • Specific gravity = density liquid / density water (measured at 15.6^\circ C, 60 F)
  • Light oils: API > 30^\circ e.g. hexane 83^\circ
  • Medium: ~ 20 - 30^\circ e.g. benzene 30^\circ
  • Heavy crude: ~ 10 - 20^\circ
    *Extra Heavy: API < 10^\circ e.g. oil sands (Canada) / bitumen
  • For water, specific gravity = 1; API = 10^\circ
  • Look at the effect of S on the API value:
    *75° (C-C bond length = 0.154 nm)
    *42° (C-S bond length = 0.208 nm)
  • The increased molecular volume is offset by the increase in mass replacing on –CH_2– by S. i.e. mass increases more than volume on S substitution - higher density. Average MW, Fuel Density, Greater aromaticity with lower H/C ratio

Petroleum Refining - Key Stages

  • Key stages in refining
  • 1. Distillation of crude oil and separation into fractions
  • 2. Thermal and catalytic cracking
  • 3. Heteroatom removal (HDS, HDN - see pollution control later)
  • 4. Reforming/Hydrogenation
  • An additional desalting step is often performed before distillation by mixing oil and water to dissolve minerals, NaCl crystals etc and then the water layer is drawn off.

Role of Structure on Boiling Point (b.p.)

  • Toluene 111^\circ C
  • Benzene 80^\circ C
  • Napthalene 218^\circ C
  • n-pentane 36^\circ C
  • neopentane 9.5^\circ C
    *Distillation Bubble cap: vapour liquid
  • Vaporisation, condensation and reboiling occurring over various stages
  • Promotes liquid/vapor contact with vapour forced to bubble through.
  • In a distillation column, 15-30 plates takes a “cut” or fraction at a particular temperature.

Distillation of Crude Oil

*Fractions

  • • LPG (C3-C4)
  • • Gasoline/Petrol (C5-C9) 30-150°C
  • • Jet fuel (C8-C{16}) 150-205°C
    *Gases at >400 °C
  • • Naphtha 30-200°C
  • • Gas Oil 200-350°C
    *Steam at > 400°C
  • • Vacuum Gas Oil 350-450°C
  • • Paraffin/kerosene (C6-C{16}) 150-250°C
  • • Diesel fuels (C9-C{20}) 180-350°C
  • • Heating oils
  • • Bunker fuel (C{20} - C{80})
  • • Waxes
  • • Asphalt, Bitumen, Lubrication oils > C_{80}

Demand Distribution

*Refining stream demand distribution

  • Gases (C1-C5): Natural gas, LPG
    *Naphtha/gasoline (C6-C{10})
  • • Aviation gasolines, gasoline
  • • motor gasolines, light weed
  • • control oil, dry cleaning, ATHTASolvents
    *Refining stream demand distribution
  • Kerosene/jet fuels (C{11},C{12}): Diesel fuels, jet fuels, illuminating and stove oils, light fuel oils
  • Light gas oil (C{13}-C{17}): Gas turbine fuels, diesel fuels, cracking feedstock, furnace oil
  • Heavy gas oil (C{18}-C{25}): Cracking feedstock, fuel oils
    *Lubricant Fraction(C26-C38)
  • Gear and machinery lubricating oils,
    *cutting and heat-treating oils
  • *Residue(>C38)
    *Refinery and losses (roof,watertreat)

Cracking in Petroleum Refining

  • Typically, much less than 50% of crude oil boils below 350^\circ C – need for refining to increase yield of more commercially valuable products (products in gasoline range).
  • A key reaction in petroleum refining is cracking (breaking C-C bonds to produce hydrocarbons of lower molecular weight).
  • Endothermic reaction carried out at relatively high temperatures.
  • Two reaction types: thermal and catalytic.

Thermal Cracking

  • Bond Cleavage - Free radical reaction, 800^\circ C, not selective.
  • Used to produce C2-C4 alkenes from naphtha fraction.
  • Increase petrol octane number.

Catalytic Cracking

  • Catalysts are zeolites, 600^\circ C, better selectivity.
  • Obtain C5-C{20} compounds from heavier oils rather than < C_4 species.
    *Complex Al-Si-O stoichiometry
  • Pores < 2 nm
  • Heteroatomic bonds (C-S, C-O, C-N) also need to be broken; see discussion on POLLUTANTS later.

Catalytic Cracking Mechanism

  • Bronsted acid site on catalyst transfers H^+ to alkene molecule and creates weak C-C bond.
    *Undergoes carbocation rearrangements (see REFORMING next slides)
    *Stability of bonds being broken 3° > 2° > 1° isomerization occurs (radical species do not tend to do this)
  • In zeolites:
    *Network of {SiO_4}^{4-}
  • tetrahedra with partial exchange of Si^{4+} with Al^{3+} ions in network.
  • With Al/Si ratio changes, counter cations are also present to balance charge (e.g. Na^+) and the overall stoichiometry is also important in controlling the zeolite pore size.
    *Replacing Si with Al creates an extra negative charge which is stabilized by a proton (& other counterions)
  • This creates strongH^+ donation sites on the zeolite catalyst surface.
    *Hydrogen abstraction by catalyst site
  • Catalyst site can also play a role in abstracting H from alkanes to create C^+.