Thermochemical comparison of fuels

This tutorial compares the thermal behavior of three fuels of interest in energy systems, using Glenn.jl:

We will visualize $C_p(T)$, $S^\circ(T)$ and the sensible enthalpy change $\Delta H(298.15\,\mathrm{K} \to T)$ over a temperature range relevant to combustion.

Resolving the identifiers

get_available_species with exact_match=true performs a case-insensitive exact lookup — "CH4" returns only methane, not longer hydrocarbons.

using Glenn

FUELS = Dict(
    "CH4"    => "Methane (natural gas)",
    "C2H5OH" => "Ethanol",
    "C3H8"   => "Propane (LPG)",
)

function resolve_id(calc, name, phase="gas")
    species = get_available_species(calc, name, exact_match = true)
    for s in species
        if s.phase == phase
            return s.id
        end
    end
    error("Species '$name' ($phase) not found")
end

calc = Calculator()
ids = Dict(name => resolve_id(calc, name) for name in keys(FUELS))

for (name, sid) in ids
    println("  ", rpad(name, 8), " -> id ", sid)
end

  C3H8     -> id 393
  CH4      -> id 296
  C2H5OH   -> id 370

Collecting properties over 300–2000 K

We use get_properties_range to evaluate all temperatures at once. The 300–2000 K range spans from ambient conditions up to typical flames.

temperatures = collect(300:50:2000)

data = Dict()
for (name, sid) in ids
    results = get_properties_range(calc, sid, temperatures)
    Ts = [r.temperature for r in results]
    cp_vals = [r.cp for r in results]
    s_vals = [r.s for r in results]
    dh_vals = [
        calculate_enthalpy_change(calc, sid, 298.15, T) / 1000.0
        for T in Ts
    ]  # kJ/mol
    data[name] = Dict(
        "T"  => Ts,
        "cp" => cp_vals,
        "s"  => s_vals,
        "dh" => dh_vals,
    )
end

println("Properties collected for: ", join(keys(data), ", "))

Properties collected for: C3H8, CH4, C2H5OH

Specific heat $C_p(T)$

\[C_p\]

rises with temperature as more vibrational modes become active. Larger molecules (ethanol, propane) have higher $C_p$ because they have more degrees of freedom.

using Plots
gr()

p1 = plot(
    title  = "Molar specific heat at constant pressure",
    xlabel = "Temperature (K)",
    ylabel = "Cp  (J·mol⁻¹·K⁻¹)",
    legend = :topleft,
    grid   = true,
)
colors = palette(:default, length(data))
for (i, (name, d)) in enumerate(data)
    plot!(p1, d["T"], d["cp"], label=FUELS[name], lw=2, color=colors[i])
end
p1

Sensible enthalpy $\Delta H(298.15\,\mathrm{K} \to T)$

This is the heat required to warm 1 mol of fuel from 298.15 K up to $T$ — a central quantity in energy balances for preheating and heat recovery (HRSG).

p2 = plot(
    title  = "Sensible enthalpy relative to 298.15 K",
    xlabel = "Temperature (K)",
    ylabel = "ΔH  (kJ·mol⁻¹)",
    legend = :topleft,
    grid   = true,
)
hline!(p2, [0.0], color=:gray, lw=0.8, label=nothing)
for (i, (name, d)) in enumerate(data)
    plot!(p2, d["T"], d["dh"], label=FUELS[name], lw=2, color=colors[i])
end
p2

Numerical summary at reference points

Direct comparison of $C_p$ and $S^\circ$ at three temperatures of interest.

using Printf
targets = [300, 1000, 2000]
println(rpad("Fuel", 22), " ", lpad("T (K)", 6), " ", lpad("Cp", 10), " ", lpad("S°", 10))
println("-"^50)
for (name, d) in data
    for T in targets
        i = findfirst(x -> x == Float64(T), d["T"])
        if i !== nothing
            @printf("%-22s %6d %10.3f %10.3f\n",
                FUELS[name], T, d["cp"][i], d["s"][i])
        end
    end
    println()
end

Fuel                    T (K)         Cp         S°
--------------------------------------------------
Propane (LPG)             300     73.955    270.770
Propane (LPG)            1000    174.613    417.339
Propane (LPG)            2000    221.955    556.442

Methane (natural gas)     300     35.760    186.591
Methane (natural gas)    1000     73.676    248.330
Methane (natural gas)    2000    101.442    309.447

Ethanol                   300     65.593    280.996
Ethanol                  1000    142.689    404.550
Ethanol                  2000    178.201    516.949

close(calc)

Reading the results

• Ethanol and propane, being larger molecules, show higher $C_p$ and $S^\circ$ than methane across the whole range.
• The sensible enthalpy grows almost linearly at high temperatures, reflecting the plateau of $C_p$.
• These data feed energy balances in combustion chambers, gasifiers and power cycles.