WOOD PROCESSING FOR ENERGY USE

Forest biomass has been used as an energy source since ancient times. Since then, several ways of using them have emerged, along with technologies to improve their energy quality. One can cite genetic improvement, thermal transformation through pyrolysis for charcoal and torrefied biomass production, and mechanical transformation through compaction, to produce pellets and briquettes and chipping for the production of chips. However, it is somehow difficult to find articles on these topics that are clearly and objectively presented, making it difficult to read them. The objective of this work was to search data on the ways of processing forest biomass and solutions for the better use of this biomass and its energy use. Therefore, Google Scholar was used as a database from which articles already recognized and others with less impact were obtained. The following search words were used: Eucalyptus, Pinus, wood chips, pellets, briquettes, charcoal, and torrefied wood. To filter the results obtained, the articles that appeared as the most relevant were selected first, then filtered for articles with less than five years from publication, and those at less than two years of publication. Next, the selected articles went through a verification of the data contained in them, and the necessary information was removed from each, which were the species, immediate analysis, extractives, HCV, etc. These data were organized in tables according to the type of processing, prioritizing the values of greatest interest in each analysis, along with the appropriate references. It was observed from the data obtained that the results are compatible among different researchers in their analyses. For samples processed without thermal treatment, the initial characteristics of the wood are maintained, and when going through pyrolysis or torrefaction, these characteristics are changed.


INTRODUCTION
Energy demand is constantly on the rise in the world, where most energy consumption is based on non-renewable sources, such as oil, coal, and natural gas, which generates several environmental and economic issues. Many countries have been searching for reducing energy based on these fuels, especially in the industrial sector because of the great environmental and social pressures (MACIEL, 2020).
It is in this scenario that renewable sources, such as wood, by-products, and biomass residues represent important systems for energy supply (DIAS JUNIOR et al., 2017;MACIEL, 2020). Biomass is any organic, non-fossil material that has chemical energy contained in its interior, which includes all aquatic or terrestrial vegetation, organic waste, agricultural waste, animal manure, and other types of industrial waste (VIDAL; HORA, 2011;AREIAS et al., 2020;BERNARDO et al., 2021).
In general, dendroenergy is lignocellulosic energy biomass and its by-products, especially on a renewable basis. LORA, 2003;DIAS JUNIOR et al., 2017) Brazil has the advantage of having a great potential for biomass production as it is one of the countries with the greatest abundance of renewable energy in the world (BORGES et al., 2016).
The current forest sector is the result of investments by companies that developed technologies and pioneered cultivation systems for various forest species. This sector has been generating an increase in the Brazilian economy in the last century, and Brazil has large and vast forests of native species and planted forests, where the Eucalyptus and Pinus genera stand out LERAYER, 2008).
As exotic trees in Brazil, Eucalyptus and Pinus adapted well to the climate and soil where they were introduced, where Pinus is most planted in the southern region, while eucalyptus is planted in almost the entire Brazilian territory (VECHI; JUNIOR, 2018;STUPP et al., 2017).
Although eucalyptus and Pinus are the species most used for forestry, other species such as rubber, acacia, teak, and paricá are also cultivated, (IBÁ, 2019).
Because of the rise in the use of biomass, new ways of its processing are being studied. The energy transformation of wood biomass is based on physical, chemical, thermochemical, and biological processes. The suitability of each process depends more on the existing infrastructure and market conditions than on the conditions of each process (VIDAL; HORA, 2011;BERNARDO et al., 2021).
The vast majority of transformations search at increasing HCV. The Higher Calorific Value is defined as the amount of heat that is given off by the complete combustion of the fuel considering the water of the products in the liquid phase, with the combustion products at room temperature (NEIVA et al., 2018).
Given the need to contribute to a large number of academics, researchers, and laypeople engaged in solutions for better use of forest biomass, the objective of this work was to search for data on ways to benefit from forest biomass and solutions for better use of this biomass and its energy use.

MATERIAL AND METHODS
In bibliographical research, the database chosen for the study is of great importance. In this work, we chose to use the Google Scholar database, which is a free-access database and with a wide variety of articles, theses, and dissertations in its portfolio. As search keywords, the following were used: eucalyptus, pinus, wood chips, pellets, briquettes, charcoal, and torrified wood. To filter the results obtained, the articles that appeared as the most relevant were the first to be selected, then the articles at less than 5.5 years of publication (from 2021 to 2016) were filtered and then to less than 2 years of publication (2019-2021). Then, the selected articles were checked for the data contained in them, obtaining the necessary information such as the species, immediate analysis, extractives, HCV, and others.
The data were organized in tables according to the processing, prioritizing the values of greatest interest in each analysis, and the respective references.

Wood chips
Wood chips are a renewable resource made up of sheared chips obtained from wood logs, which are mostly used to produce energy in ovens and boilers. They have good energy characteristics and better performance regarding their flow in silos (DINIZ, 2014).
The wood logs are transformed into chips through mechanical work, where they are cut through the blades of a chipper, starting the formation of wood chips (BUSQUIM, 2019;SOARES, 2016).
In order to classify only the chips with the recommended sizes, they go through a selection process, where chips larger than the ideal particle sizes are separated, while the finer chips are sent to the stockyard or directly to the energy generating source (BUSQUIM, 2019;SOARES, 2016, DINIZ, 2014. The greater specific surface and greater reactivity of wood chopped in chips, compared to firewood logs, may increase the efficiency of different biomass utilization systems (COSTA et al., 2010;DINIZ, 2014).
Few results were obtained in the search for "chip". Table 1 gathers those that, when compared, demonstrate that the analyzed variables vary greatly regardless of the species.

Pellets
The evolution of policies concerning energy in developed countries, associated with the demand for renewable sources such as forest biomass, has been driving the growth in the production of wood pellets in the world since the beginning of the 2000s (QUÉNO et al., 2019). Once the pellets have become commodities traded around the world, Brazil has been naturally appointed as one of the main countries in this segment due to its favorable climatic characteristics for the production of forest biomass (GARCIA et al., 2017a).
Pellets result from the biomass densification process, that is, the physical transformation of the particulate lignocellulosic material into a solid biofuel (PROTASIO et al., 2015).
They can be produced using wood-industry residues, chips, and branches in the form of small particles. This raw material is processed in stationary industrial plants at high pressure and temperature, where they are compressed into small cylinders from 6.0 to 10.0 mm in diameter and up to 30 mm in length (GARCIA et al., 2013;QUÉNO et al., 2019;DIAS et al., 2012). Unlike wood and chips, where the amount of holocellulose, lignin, and extractives are of great importance, for the processed wood for energy use, other parameters are also considered, namely: fixed carbon content, volatile content, and ash content.
The compaction temperature plays a very important role in the final properties of the product and energy consumption during compaction. The application of pressure by biomass particle compaction equipment favors different binding mechanisms (DIAS et al., 2012).
The greater the amount of fixed carbon, the greater the combustion rate of the particle (FROEHLICH; MOURA, 2014).
Ash content is the percentage in the mass of ash after the complete burning of the briquette or pellet. Most biomass residues have low ash content (DIAS et al., 2012).
Density is an important parameter in compaction: the higher the density, the higher the energy/volume ratio. In addition, high-density products are desirable regarding transport, storage, and handling (DIAS et al., 2012).
They must be produced with low moisture content (less than 10%), allowing high combustion efficiency (GARCIA et al., 2013).
The global demand for these products grows exponentially because they are low-carbon energy resources and used in countries that need to reduce their greenhouse gas emissions (GARCIA et al., 2018a). The production of pellets has been consolidating in the last ten years particularly as fuel for burning in ovens that feed the aviaries. This modernization can be seen in Table 2 where the works have been published for almost a decade. Similar behavior can also be seen in the results of the performed analyses.

Briquettes
Briquetting is a very efficient way to concentrate the available energy into biomass. This fact is exemplified in the consideration that 1 m³ of briquettes contains at least five times more energy than 1 m³ of wood (QUIRINO; BRITO, 1991;YAMAJI et al., 2013).
The advantages of compacting agricultural and forestry residues are operational, logistical, energetic, and environmental (DIAS et al., 2012). Initially, any plant biomass can be used in the production of briquettes. Currently, the agricultural, forestry and industrial process residues are the most used materials (YAMAJI et al., 2013).
Briquettes are direct alternatives for firewood and wood chips in many applications, including residential, industrial, and commercial use (DIAS et al., 2012). So far, briquetting is the least known and little mentioned form. Despite the existence of studies since the 1980s, very little knowledge was passed on, and because of that, this technology has not evolved. It can be seen in Table 3 that the analyses performed over time resulted in varied values.

Charcoal
One use of the wood, the production of charcoal, has always occupied and still occupies, a prominent position in the main reforestation companies in Brazil (BOTREL et al., 2007).
Charcoal is an important wood by-product obtained through a process known as pyrolysis, where in the complete or partial absence of oxygen, the molecules are broken resulting in a new material with different characteristics from the original ones (FROEHLICH; MOURA, 2014).
Charcoal represents an excellent raw material for the Brazilian steel industry due to its behavior as fuel and reducer, high purity, low production cost, and for being an environmentally correct product when coming from planted forests BRITO, 2007). The physical-chemical properties of charcoal are affected by the intrinsic characteristics of the raw material and the carbonization parameters (ARAÚJO et al., 2018). In determining the quality of charcoal, one of the fundamental properties to be analyzed is the fixed carbon content. The higher this content, the better the quality of the  BARRICHELO, 1982), as shown in Table 4.

Torrified wood
Torrefaction emerges as a way of diversifying the supply of biofuels. This process consists of a thermal treatment of the biomass at lower temperatures, ranging from 200ºC to 300ºC, generating an intermediate material between wood and charcoal (OLIVEIRA, 2011;KLAFKE;TRIERWEILER, 2018).
The fundamental objective of torrefaction is to concentrate biomass energy in a product formed in a short time, allowing the retention of higher calorific value volatiles in the product itself (SILVA, 2013).
This process also results in a more efficient logistics of transporting and storing biomass, as these costs are related based on the volume of material, and torrefaction reduces this volume as it can be seen in Table 5, resulting in an economically favorable treatment (MALAGUTTI; ASCENÇÃO, 2019). The most common types of small-scale reactors used in torrefaction are convection, fluidized bed, rotating drum, and microwave reactors (JUNIOR; ALVES; TORRES, 2017).
It could have been observed that at chipprocessing, all initial characteristics remain unchanged at the end of the process, as it is just a process of cutting the raw material. Pelletizing and briquetting promote agglutination of the raw material, which increases its density, but it does not change its chemical properties. On the other hand, torrefaction and pyrolysis after the physical and chemical characteristics of biomass in order to reduce the amount of volatile materials present and increase the fixed carbon content. These changes lead to an increase in the HCV of the charcoal and the torrefied wood generated. As an example, one can mention the species Eucalyptus Grandis. This species has HCV values of 4295kcal kg -1 (Table 1).
Once they go through pelletization processes, the contents of carbon, volatiles, and ash are 10.72, 82.31, and 6.95, respectively (Table 2). After the torrefaction process, mean values of 26.43, 73.36, and 0.2 were found, with a HCV of 5314 kcal kg -1 (Table 5). Also, after a pyrolysis process, these values varied with means of 82.34, 15.43, and 2.24, shown in Table 6.

CONCLUSIONS
• It was observed that despite the immense number of tree species on our planet, the vast majority of the studies focus on the once established species of Pinus and Eucalyptus. This is explained by its great adaptability to the most varied regions, its use in planted forests, its rapid development, and good physicochemical characteristics, which are essential for use in heat or energy generation systems. However, in the last years, new studies have been carried out concerning native species such as ipe, angico, cinnamon, and fruit trees in energy forest systems, where these species are planted instead of foreign species, enriching the local fauna and flora. In parallel to this, nonwoody species are studied for energy use, with emphasis on grass, oleaginous and saccharific species. These may contemplate a new study highlighting its history, importance, and characteristics.
• It was observed that, as the temperature increases in thermochemical processes, the fixed carbon content also increases, and a reduction occurs in values of the volatiles, which are burned during the process. It was also observed a rise in the HCV values, where charcoal has values with an increase of up to 100% compared to other forms of processing.
Because the briquetting and pelletizing process does not alter the chemical structures of the wood, these have very similar values in all performed analyses. The keywords used in this work were adequate as expected, and several results on ways to improve wood were found in the literature. Charcoal has a greater number of works about it compared to other processing methods such as briquetting and pelleting. However, these are the best choice for energy use, economy, and sustainability, for many situations due to their less waste, ease of transport, and cost-effectiveness. A pattern was observed as different authors studied the same species, and it is possible to state that the methods used, although not always the same, reached very similar results, demonstrating high reproducibility and reliability.