Simple carbon emissions from fuel and fire calculations
Define a constant for greenhouse gases GHG to avoid hard-coding them in separate functions:

GHG = ('co2', 'ch4', 'n2o')  # greenhouse gases



get_fuel_emission_factor function.
The for loop with accumulation is concisely replaced with builtin sum function on generator expression:

def get_fuel_emission_factor(fuel):
    """Get fuel total CO2 emission factor."""
    factors = FUEL_FACTORS[fuel]
    return sum(factors['energy_factor'] * factors[f'{gas}_emission_factor'] / 1000
               for gas in GHG)

fuel_to_co2 function.
Inline variable technique can be applied:

def fuel_to_co2(fuel, quantity):
    """Get total tCO2 emission by kL of fuel."""
    return get_fuel_emission_factor(fuel) * quantity