## 2 Nature and Characteristics of _A. altissima_
### Utilization in Bioenergy Production  
There has been an increasing interest in the potential use of _A. altissima_ as a biomass feedstock for energy production. The fact that this species is characterized by a high growth rate and can grow in a great range of conditions makes it a potentially valuable feedstock for the production of biofuels or other bio-based products. A. altissima wood indeed seems appropriate to be used in the production of high-quality firewood, compared to that of white oak, walnut, and birch species. This quality, of course, refers to older trees since the wood of younger trees that has been grown quickly is less energetically dense and more brittle, can be easily split, while it may present slightly higher ash contents. It generally acquires a high calorific value (18,040.17 \(\mathrm{J}\cdot\mathrm{g}^{-1}\)) and relatively low ash levels [66]. [PERSON] and [PERSON] [66] concluded that _A. altissima_ wood and bark marked high heating values and low ash content compared to _Robinia pseudoaccia_ and other species, making those materials to be considered suitable for pellets production. Specifically, this wood species was found to contain 0.5-0.69% ash, which means that it can be used as a raw material for the production of pellets intended to be used for residential applications. The wood can be classified in the best quality class of pellets (ENplus-A1) and meets the requirements of EN14961-2 for very low ash values (<0.7%) [66]. _A. altissima_ wood and bark pellets showed comparable physical and mechanical properties to pellets made from other commonly used wood species, indicating their potential for use as a feedstock in pellets production. [PERSON] et al. [97] also reported high potential of using _A. altissima_ and _R. pseudoaccia_, as feedstock for pellet production, due to high energy density and low ash contents. It was noted that the use of invasive species as feedstock can provide a solution to the problem of invasive species management, as well as contribute to the development of sustainable energy systems. It has also been used for coal production in many countries. [PERSON] et al. [98] investigated the potential of _A. altissima_ for use in co-firing with coal in power plants and found that it had similar combustion characteristics to other biomass feedstocks and could potentially be used as a renewable energy source in co-firing applications. [PERSON] et al. [27] focused on the production of biodiesel from _A. altissima_ (Mill.) seed oil using a green and recyclable potassium hydroxide activated _A. altissima_ cake and cadmium sulfide catalyst. The authors reported that the use of this species seed oil as a feedstock for biodiesel production is advantageous due to its non-edible nature and availability. It was mentioned that the use of green and recyclable potassium hydroxide-activated _Ailanthus_ cake and cadmium sulfide catalyst is eco-friendly and sustainable. The optimal conditions for biodiesel production were found to be a molar ratio of 1:9 for oil to methanol, a catalyst loading of 1.0 wt.% based on oil weight, a reaction temperature of 60 \({}^{\circ}\)C, and a reaction time of 2 h. The produced biodiesel met the international standard specifications for biodiesel. A promising approach for the sustainable production of biodiesel using _A. altissima_ (Mill.) seed oil and a green and recyclable potassium hydroxide activated _Ailanthus_ cake and cadmium sulfide catalyst was presented, though it is crucial to note that cadmium sulfide is a toxic material, and its use should be carefully monitored to prevent any negative environmental impacts. Additionally, the impact of graphene oxide nanoparticles, a novel fuel additive, on the engine performance and emission characteristics of a diesel engine was studied by [PERSON] et al. [99]. Their results demonstrated that environmentally beneficial fuels may be produced using diesel fuel, graphene oxide (GO) nanoparticle additives, and _A. altissima_ biodiesel.