What is the significance of heating value?
Note that the mentioned values are molar heats of combustion. Propane is a larger molecule than methane and it is therefore obvious that more energy per mole is released.
If we replace the molar amount by molar mass, the resulting specific heat of combustion has the opposite relation:
$$\begin{align}\text{Methane:}&amp;&amp;\frac{\pu{890kJ/mol}}{\pu{16g/mol}} &amp;=  \pu{55.6kJ/g}\\[0.5em]
\text{Propane:}&amp;&amp;\frac{\pu{2200kJ/mol}}{\pu{44g/mol}}&amp;=\pu{50kJ/g}\end{align}$$
For the enviromental aspect of $\ce{CO2}$ emissions, the relevant value is the heat of combustion per mole of carbon atoms. It is $\pu{890 kJ/mol}$ for methane, but $\pu{2200 kJ/mol}$ for propane, which is roughly $\pu{730 kJ/mol}$ for carbon. For methane, both numbers are the same, as a propane molecule contains 3 carbon atoms, while a methane molecule just 1.
So methane has a an advantage as energy source, as it has higher energy production per the same $\ce{CO2}$ emission.
PLA is partially already oxidized, therefore it has significantly lower energy content per 1 kg of produced $\ce{CO2}$, compared to  PP.
PLA has an energy content of $\pu{16.3MJ/kg}$
PLA has structure [-O-CH(CH3)-CO-]n, with basic unit containing 3 C + 4 H + 2 O
$\pu{1 kg}$ of PLA contains 36/72 kg of carbon
PLA has an energy content of $\pu{16.3 * 72 / 36 MJ/kg} \simeq \pu{32.6 MJ/kg of C}$ .
PP has an energy content of $\pu{46.4MJ/kg}$
PP has structure [-CH2-CH(CH3)- ]n, with basic unit containing 3 C + 6 H
$\pu{1 kg}$ of PP contains 36/42 kg of carbon
PP has an energy content of $\pu{46.4 * 42 / 36 MJ/kg} \simeq \pu{54.1 MJ/kg of C}$ .
To get the same energy by the burning of the stuff,
PP releases $54.1/32.6 \simeq 1.66$ times less $\ce{CO2}$ than PLA.
Or, with the same emission of $\ce{CO2}$, PP releases 1.66 times more energy than PLA.