# Feedback Characterization for Enhanced Heat Recovery from Composing Processes: A Review
## 3 Results and Discussion
### Energy-Related Properties  
This section delves into the data analysis of the LHV, VS, MC, C content, and TOC across plant-based, animal-derived, and non-agricultural feedstock sources in order to evaluate their suitability for heat recovery from aerobic processes. These properties have a significant impact on the effectiveness of energy recovery driven by microbial activity and decomposition processes. Specifically, the low heating value (LHV) of the feedstock is a key factor in understanding its potential for heat production, as it represents the energy content available for release during aerobic processes. While literature on the direct correlation between LHV and heat output in composting is limited, it is essential to highlight that LHV serves as an indicator of the suitability of feedstocks for energy recovery. Statistics on these parameters for different feedstock types are presented in Figure 4 and Table 5.  
Figure 4: Statistical comparison of different feedstocks: (**a**) LHV, (**b**) VS, (**c**) MC, (**d**) C content, and (**e**) TOC. The LHV of composting feedstocks is a parameter of great interest due to the potential use of these feedstocks for energy recovery purposes. Out of all feedstocks, plant-based materials have the highest LHV with a mean value of 14.79 MJ/kg TS but with significant variation (CV = 0.39), ranging from 6.12 MJ/kg TS in soft garden waste [29] to 28.30 MJ/kg TS in woodchips enriched with biochar and inoculants [30]. This variability underscores that certain plant-based feedstocks, particularly those combined with biochar, could enhance heat recovery due to the fact that they have a higher energy potential. The positive effects of biochar in increasing composting efficiency are due to its ability to shorten the composting period and increase the rate of biodegradation of organic materials [30]. On the other hand, the LHV of animal-derived materials is quite lower, with a mean value of 9.72 MJ/kg TS and a range of 9.62-9.82 MJ/kg TS. However, the current sample size for animal-derived materials (n = 2) is insufficient to draw robust conclusions regarding their LHVs. To improve the reliability and applicability of these findings, further data collection in these material categories is necessary. No studies involving non-agricultural feedstocks and composting were found in our analysis. As already mentioned, mixed waste cannot be considered for the production of compost. Thus, the absence of data on this type of feedstock is not a major issue for this study. Non-statistically significant differences in the mean values of LHV for plant-based and animal-based feedstocks were found, suggesting that the suitability of each feedstock may depend on specific energy recovery goals rather than on LHV alone. VS are a key measure of the organic matter in feedstocks, which include elements of organic materials within composting feedstocks that decompose to generate energy as heat. Since the potential of energy recovery is often higher with higher VS content, it is an important value in determining the feedstock suitability in composting systems. Plant-based materials reported a mean VS content of 772.34 g/kg TS with considerable variability across sources. The VS content ranges from 327.20 g/kg TS in soft garden waste [29] to 960.00 g/kg TS in the organic fraction of MSW [31]. This variability is reflected in a coefficient of variation of 0.27, with some plant-based materials that would be more efficient in energy recovery but some that would offer less energy potential. Such variation in plant-based feedstocks highlights the importance of selecting high-VS sources, as some, due to lower organic content, may not yield consistent energy output in heat recovery applications. In contrast, animal-based materials have a more consistent VS profile,  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  
with 785.70 g/kg TS mean VS content, which is slightly higher but with a narrower span (737.60 to 857.00 g/kg TS) than plant-based materials [12; 22].