4.2. Physicochemical Characterization
As can be observed in
Table 2 (proximate analyses), the values obtained allow us to differentiate between two large groups of materials, scrubs and pruning of forest-based conifers and the remains of agricultural kiwi and vine pruning.
The forest-based materials have lower ash content values (≈1.1%) than the agricultural pruning remnants (≈2.5%). These ash content values are an important constraint on the production of quality solid biofuels. The values obtained suggest that it will be necessary to work both in the pretreatment processes and in the incorporation of chemical additives in order to reduce ash content values and the associated problems in valorization processes (mainly risk of sintering), something confirmed by other authors that previously worked with these type of biofuels [
29,
30].
Referring to moisture content at the time of collection, the material with the lowest average moisture content was heather (38.6%) followed by vine pruning (44.7%) and gorse scrub (45.8%). On the other side, forest pruning (48.2%), broom (51.2%) and pruning kiwi (57.9%) had the highest values. The results obtained suggest the need for drying in all cases.
In relation to the net calorific value of each material, it was first analyzed with a humidity content of 10%, as this moisture is a reference value for the production of densified solid biofuels such as pellets and briquettes. In the case of forest material, this value was between 17.28 and 17.87 MJ/kg, always above the minimum value required for the manufacture of pellets for domestic use (requirement > 16.5 MJ/kg) [
28]. However, in the case of agricultural material, average values of net calorific value at 10% moisture are always lower than this requirement, which are 15.40 MJ/kg in the case of pruning kiwi and 16.13 MJ/kg in the case of vine pruning [
29,
30].
With regards to the calorific value net to the collection humidity, pruning kiwi presents the lowest value with 5.23 MJ/kg. In the upper part, heather scrub had a result of 11.43 MJ/kg. The other materials are in the range of 8.33−9.49 MJ/kg.
The concentrations of N, S and CI in different biofuels are of major importance because they can cause gaseous emissions during combustion processes. Although the formation of these emissions also depends on other parameters such as excess oxygen and CO concentration in the flue gas, higher concentrations in the biofuel are the most important influencing variable for increasing the gaseous emission level [
31].
According to elemental analyses performed (
Table 3), it is necessary to point out the higher amount of nitrogen of studied biomasses respect to the reference fuel, something claimed by some other authors that have previously studied similar biofuels [
30,
32]. It is especially remarkable the high nitrogen content of broom scrub (values between 1.38−1.41%). Consequently, the release of nitrogen pollutants mainly as NO
X could be expected upon combustion [
7,
33].
The chlorine content of samples analyzed is moderately higher than reference fuel in all cases, and gorse scrub was the sample with the highest value (0.1726%). Thermochemical valorization with fuels with a high chlorine content can cause corrosion, slagging and fouling in downstream piping and equipment, apart from cause HCl formation [
31,
34]. However, combustion processes carried out with similar biofuels have shown generally low HCl emissions [
35,
36].
The rest of parameters considered in these analyses are close to that showed by the reference fuel except sulphur, which was resulted to be slightly higher in studied biomasses [
30,
32]. This compound may be responsible for corrosion and pollutants formation (SO
2) [
31,
37]. Some other authors claimed [
35,
36] that the released fractions of this compound during the combustion of similar biofuels resulted in only moderated SO
2 formation.
Finally, regarding to inorganic elements (
Table 4 and
Table 5), it is particularly important to evaluate the quantity of those elements that can have a role on the ash melting (Na, K, P, Ca, Si, Mg) [
38]. The higher the content in alkaline earth oxides regarding alkaline, the higher the sintering temperature and the lower the risk of sintering of each sample [
39].
As can be observed in
Table 4, the proportion of alkaline earth metals regarding alkaline is higher in reference fuel, which suggests that sintering problems may occur during thermochemical processes developed with studied biomasses [
40]. An exhaustive control of the thermochemical valorization processes should be carried out [
31,
38].
With respect to minority elements (
Table 5), toxic elements such as Hg, Cr and Zn are especially important due to their role on particulate matter emissions. They may also cause problems with ashes reutilization [
38]. No relevant differences were observed in those elements’ concentrations between NVBHP and the reference fuel. The most significative deviation is related to the Zn content, which was considerably higher in pruning vine, pruning kiwi, broom scrub and forest scrub. Measuring the possible emission of particles and checking the leachate of the ashes would be advisable. Also remarkable are the higher Cu values of agricultural biofuels with respect to the other biomasses and with respect to the reference fuel.
Even though the Zn and Cu results were higher than expected, all the studied fuels are within the ranges specified in the standards for wood pellets in Spain (UNE-EN ISO 17225-2:2014). No significant amounts of heavy metals were detected so the use of the ashes for other applications or their easy disposal in landfills could be feasible.
4.3. Valorization Tests
Figure 3 shows that as confirmed by other authors [
41,
42], the greater the temperature difference between the hot and the cold source, the greater the electrical power generated by the module studied. In the tests conducted, 4.05 kW was the maximum electrical power reached in the case of using pine pellets. A value of 3.63 kW was obtained when vine pruning pellets were employed. In this case, the difference between the powers reached was due to the fact that the temperature difference between sources was higher on the day that the pine pellets were used (76 °C), while the difference reached on the day of the test with the vine pruning pellets was 72 °C.
Regarding the electrical performance achieved by both fuels (see
Figure 4), the tendency obtained is the same as in the case of the power, that is, it increases the greater the temperature difference between the hot and the cold source, as establishes the Carnot Theorem for any thermal machine. The maximum efficiency obtained is about 10% in the case of using pine pellets and 8% when the fuel used is vine pruning pellets while these values are reduced up to 6% (pine pellets) and a 5% (vine pruning pellets) when the temperature difference between the sources is lower. Again, the difference observed is due to the fact that the difference in temperature between sources was greater on the day when the tests were carried out with pine pellets.
It should be noted that the small differences that can be observed both in the power values and in the performance values obtained at the same temperature difference with the two materials used are due to the fact that, even if the temperature difference in absolute value is the same, more promising values will be obtained when the ORC is closer to the design conditions (this is T hot source = 100 °C, cold source T = 15 °C).
Finally and according to
Table 6, using the ORC module used in the present investigation, cogeneration efficiencies close to 97% can be achieved, demonstrating its suitability for energy recovery from the residual biomasses studied.