## 3 State-of-the-Art
### Agricultural Residual Biomass
#### 3.2.4 Palm Kernel Shells  
The industry associated with the production of palm oil is of the utmost importance in agroforestry sectors in tropical countries, and also the one with the highest productivity (2020). Palm oil is a very broad-spectrum feedstock with uses ranging from food and biofuel production to cosmetics, soap, or detergents (2020). In fact, this constant demand for palm oil, given its versatility as a raw material, has also led to the expansion of planted forests of _Elaeis quintensis_ coming into conflict with native forests in different parts of the world, as shown by numerous studies. For example, [PERSON] et al. (2016) presents the ecological impacts of this expansion in Indonesia, while [PERSON] et al. (2020) assess the long-term changes in the landscape caused by the expansion of palm cultivation in the Brazilian Amazon (2018, 2020). The use of palm oil in the production of biofuels, mainly liquids, is widely covered in the literature, as shown by the available publications, such as the works of [PERSON] et al. (2008), by [PERSON] et al. (2011), by [PERSON] and [PERSON] (2002), by [PERSON] et al. (2011), by [PERSON] et al. (2001), or by [PERSON] et al. (2009) (2018, 2020, 2020). However, the palm oil extraction process also results in a set of residues, including oil palm trunks (OPT), oil palm fronds (OPF), empty fruit bunches (EFB), palm pressed fibers (PPF), palm kernel shells (PKS), and palm oil thousand effluent (POME), which have been studied over the years, in order to find a solution for their elimination and/or recovery (2020)[PERSON] The use of thermochemical conversion pyrolytic technologies has already been addressed such as by [PERSON] et al. (2013), [PERSON] et al. (2014), [PERSON] et al. (2018), [PERSON] and [PERSON] (2011), or by [PERSON] and [PERSON] (2016) (2016, 2017, 2018, 2019, 2020). The use of direct combustion has also been widely studied in the works of [PERSON] et al. (2020), [PERSON] and [PERSON] (2013), [PERSON] and [PERSON] (2012), or by [PERSON] and [PERSON] (2015) (2016, 2017, 2018, 2019). Currently, the use of residues from the palm oil industry, mainly PKS, is widely used in power generation, with a steady market installed in Southeast Asia, for supplying biomass to countries such as Japan and South Korea (2018, 2019, 2020). [PERSON] Biomass Thermochemical Conversion Technologies  
Biomass thermochemical conversion technologies are processes wherein, by manipulating the temperature with an atmosphere poor or without oxygen, biomass undergoes structural changes that improve its energy properties, increase its grindability, and upgrade its physical storage capacity, due to an increase in hydrophobicity, and resistance to biotic agents (2018, 2019). As previously mentioned, there are several works on the thermochemical conversion of biomass residues of the most different provenances. However, the upscaling of conversion from laboratory to industrial production scale is an unchartered territory, since, currently, there are no large-scale operative production units, which allow confirmation of the promising experimental data made available in recent decades. Some examples are the works by [PERSON] (2020) presenting a study on the grindability of agroforestry wastes, aiming its use as an alternative to coal in coal-fueled power plants, by [PERSON] et al. (2015), which present a study on the grindability and combustion behavior of coal and torrefied biomass blends, by the study from [PERSON] et al. (2019), that analyzes the improvement of energy density and grindability of wood pellets by dry torrefraction, or by the study from [PERSON] et al. (2020), that analyzes torrefraction in a perspective of pre-treatment of biomassfor eliminating halogenated products such as chlorine [11; 24; 117; 118]. The principles of thermochemical degradation can be also utilized for assessing the chemical structure of biomaterials, through instrumental analysis of the products released by biomass pyrolysis.