# Use of Invasive Acacia Biomass to Produce Biochar and Solid Biofuels
## 1 Introduction  
The utilization of waste and invasive plant biomass represents a crucial possibility for reducing CO\({}_{2}\) emissions, mitigating the greenhouse effect, and addressing global warming. This approach not only creates additional value in waste and/or invasive plant biomasses, but also increases economic activities in rural areas, where these materials are mainly located, promoting population growth and sustaining local economies, particularly through the production of value-added products like biofuels or biochar. _Acacia_ spp. was introduced in Spain and Portugal over one hundred years ago, principally to control coastal erosion, and is linked to the production of sticks and posts for vineyards and fruit trees [1]. However, this species has spread and is now threatening the native flora, becoming an environmental issue. Nowadays, it is considered an invasive species in Spain [2, 3, 4]. Its distribution area is unclear, but some studies indicate a substantial increase in recent decades when comparing the last two Spanish National Forest Inventories [5, 6], suggesting an increase in its dominance. Of particular significance are _Acacia dealbua_ (with altitudes between 1100 m and 280 m) and _Acacia melanogaster_ (with altitudes between 710 m and 250 m) and, to a lesser extent, _Acacia longifolia_. Specific locations have also been found in the Iberian Peninsula with forest cover of _A. farnesiana, A. salicina, A. mernnsii, A. decurrens,_ and _A. retinodes_[6, 7]. Managing these invasive species is a complex challenge, as they tend to dominate certain areas due to their invasive capacity and resilient ability to resprout, particularly following fires and clear cutting without adequate management [8, 9]. Harvesting these species to obtain added-value products could generate returns to offset the costs associated with their management. Therefore, it is crucial to propose strategies for the sustainable utilization of biomass from invasive acacias, transforming them into a renewable raw material that provides societal benefits with the potential to replace other materials, especially when seeking to avoid petroleum-derived products. Furthermore, the use of invasive plantshelps deal with their management or eradication. This is of significant interest since added value could be achieved by starting with residual biomass from invasive plants, contributing to the enhancement of sustainable management practices in protected areas. Studies of the potential of using _Acacia_ spp. as a raw material for bioenergy sources through biofuels, like biriquettes or pellets, and for biochar shows great potential but its suitability needs to be checked [10; 11]. Among the currently available technologies, mechanical densification and thermal transformation constitute important alternatives for the recovery of this biomass and also allows for adequate adjustments to obtain different products and different properties. As a result of these processes, bioproducts (biochar) and solid biofuels like biriquettes and pellets can be produced. Pyrolysis is the thermal decomposition of the biomass under inert atmosphere conditions (absence of oxygen), with temperatures usually over 300 \({}^{\circ}\)C, and is the most common method to produce biochar. This thermal process also converts the initial waste biomass into other sources, such as oil and gas [12]. The preparation methods for biochar are mainly divided into different classifications: torrefaction [13; 14], pyrolysis [13; 15], hydrothermal carbonization-HTC [16], and microwave carbonization [17]. Within the pyrolysis process, slow, fast, and very fast systems can be distinguished [18; 19; 20], where the times and/or temperatures reached in the pyrolization process are employed as variables.