3.2.2. Chemical Properties

Chemical energy is stored in biomass in two forms: Volatiles and fixed carbon. VM is defined as the released gas by heating and FC is the mass remaining after this release, excluding the ash and moisture contents (MC).

The content of volatiles also influences the ignition temperature which is the temperature above which combustion reactions become self-sustaining. In general, the ignition temperature is lower for higher VM contents.

The composition of pellets described by their PA was carried out using a mu ffle furnace following the standards [20–22]. The energy developed during the combustion of RS and RH pellets was evaluated through the HHV. Table 7 lists the PA, UA, and HHV (calculated by Equation 2) of RS and RH pellets.


**Table 7.** PA, UA, and HHV of RS and RH pellets.

Similar compositions were found for both feedstocks. A slightly higher value of volatile matter for RH pellets led to a slightly higher heating value. The ash content, main objective of LIFE LIBERNITRATE, was higher in RS pellets identifying them as a better choice in terms of material efficiency. Comparing the composition before and after densification, slightly higher volatile contents were found for pellets with respect to raw straw (VMraw = 57.5% [27]). They also presented higher HHV (HHVraw = 11.6 MJ/kg [27]) which indicated an increased energy emission during the combustion due to densification. On the contrary, the ash content slightly decreased with respect to raw materials (Araw = 18.8% [27]). This suggested that a higher quantity of pellets will need to be processed in order to achieve the same quantitative efficiencies. HHV values were found to be lower than commercial wood pellets for both feedstocks (HHVwood = 19.5 MJ/kg [28]). An increase in these values could be achieved by mixing them with other available residues (HHVpersimmon stems = 17.3 MJ/kg [29] or HHVappletrees = 17.2 MJ/kg [30]). That way, the increase in the quality of the final pellet would permit using these residues with low calorific values in energy recovery processes. Sulfur and nitrogen contents remained in a very low percentage (below 0.3% wt and 2.5% wt, respectively). This low sulfur and nitrogen content results in low NOX and SOx emissions and contributing to minimize environmental damage.
