**1. Introduction**

Energy efficiency is a basic pillar when it comes to achieving a certain degree of social and environmental sustainability, guaranteeing an adequate level of energy security [1]. In this sense, the European Union has set achieving greater efficiency in the development of renewal energies as a priority objective [2,3], committing to increase the exploitation of renewable energy by 20% [4].

Under this premise, the research of fuels as an alternative to crude oil and coal has led scientists to analyze materials that come from agriculture and forest environments [5–7]. Among the biofuels obtained from these systems, lignocellulosic materials such as wood chips and pellets take on an important role [8–10], as they can be used in combustion boilers to produce heat in both industrial—especially in oil mills—and domestic applications [11–13]. Improved cogeneration systems have also been developed to simultaneously produce heat and electricity [14,15]. Another very relevant application is the use of lignocellulosic materials in pyrolysis processes [16–18]. The primary biofuels used in small-scale combustion systems commonly have a woody origin, and as the demand for these biofuels grows, the pressure on forest exploitation will increase, with possible negative environmental consequences, which is why it is important to find new sources from which to obtain biofuel [4].

Ignorance regarding the energetic properties of the lignocellulosic materials from agricultural environments has led to numerous investigations determining the calorific value, ash, and elemental composition of the wood of different crops, such as the orange tree [19–21], the olive tree [8,20,21], the almond tree [19,20,22], the apple tree [23], the vine [8,19,20], and even herbaceous plant remains from greenhouses [24,25]. The heating or calorific value is one of the most important aspects related to the use of biomass, as it expresses the energy content of the biomass fuel and is a key parameter that has been widely used for the development of calorific power prediction models based on elemental, proximal and structural composition [26–30], although unfortunately the accuracy of the correlations based on those analyses are generally not very high [31].

In fact, just as the quality of the fruit varies according to the cultivation techniques used, the energy properties of the wood can be affected by the growing conditions and agricultural practices, such as the variety or rootstock selection or the fertilization.

Considering the lack of accurate biomass standardization, especially in relation to physicochemical, process and environmental parameters, the study and choice of raw materials for achieving better process efficiencies is not devoid of difficulties [32]. If the agricultural practices for obtaining materials influence their composition, obviously they will also do so in their energetic properties. Therefore, a proper characterization is required for the adequate use of the wastes obtained for biofuel uses.

The aim of this work was to demonstrate this hypothesis on the wood of almond tree (*Prunus dulcis* (Mill.)), which is one of the most important crops in both the Mediterranean region and also the Californian coast, which generates a huge amount of available biomass for energy uses [22], and has potential calorific powers superior to other common sources of biomass such as the almond shell, the olive pomace from oil mills, or the pressed grape waste from the wine industry [32]. Two varieties were tested, Marcona and Vayro, with two rootstocks and different fertilizations. The novelty offered by this study consists of the introduction of new parameters, such as rootstock selection and fertilization, in the study and classification of biomass products.

If the hypothesis is true, it would be necessary to consider those practices that improve the obtained products for food production, on the one hand, and on the other, the energetic material coming from the pruning waste. The use of both resources could significantly contribute to regional and national bio-economies in the future [9,14].
