Forestry products as renewable energy sources

Abstract

The world's energy needs are growing, and fossil fuel resources are becoming less available, and it is certain that, in some period of time, they will disappear. In order to reduce the dependence on non-renewable energy, renewable energy has been increasingly used since the end of the last century. The most important source of renewable energy is biomass, and wood biomass is the most represented. The myth of cheap and easily available energy disappeared in the 70s of the last century after the great energy crisis. Then, it has become clear that the future could not be built on a single source of energy. The greatest success in the development of techniques and technologies for the utilization of wood biomass was achieved by the Scandinavian countries, which managed to put the traditional forestry sector in the function of biomass production for general use.Today, biomass energy production plays a key role in the strategies of many countries, with the aim of climate change mitigation. At the Paris Climate Conference in December 2015, 196 countries adopted a binding global climate agreement. The governments of the participating countries have agreed to keep the average global temperature increase below 2 °C until 2100, compared to the pre-industrial period. Thanks to the insistence of small island countries that are the first to be hit by climate change, the agreement states that an effort will be made to limit the temperature increase to 1.5 °C (United Natons, UN 2015). Renewable energy consumption in the XXI century recorded a growth of 38.5%. Wood biomass for energy production is found in various forms (piece of wood, chipboard, pellets, briquettes, etc.), and about 50% of the energy produced from biomass is used in industrial plants, for heating residential and business premises, cooking, etc. A large number of EU countries will achieve the planned renewable energy quotas by 2020, and the tasks are set by 2030. They include a minimum of 40% reduction of greenhouse gas emissions, compared to level in 1990, providing a minimum of 27% of renewable energy and a minimum of 27% improvement in energy efficiency (European Pellet Council 2019). In May 2014, the Government of the Republika Srpska published an Action Plan for the utilization of renewable energy sources. The Action Plan defines the share of renewable energy in the total final energy consumption in heating, cooling, electricity and transport. Energy production and consumption fromwood biomass are not in accordance with the potential. In order for more raw materials to be offered to the market, it is necessary to create preconditions for the application of potential, more efficient, technological solutions of wood biomass utilization in the conditions that characterize the forests of Republika Srpska. The problems that forestry faces with are related to low forest openness and outdated mechanization. Technological solutions are adapted to the utilization of technical roundwood and firewood intended for plants for mechanical and chemical wood processing. The new approach implies the application of technology that will enable the use of all categories of wood in accordance with sustainable management. New technological solutions should be technically applicable, environmentally friendly and economically profitable. There is a great variety of work technologies in the world, depending on the purpose of the forest and the orographic characteristics of the terrain. Energy raw material from the forest is intended for heating plants, power plants, CHP plants (Combined Heat and Power), pellet and briquette plants. Firewood is used in power plants, heating plants, CHP plants, pellet and briquette production plants, dry distillation plants, coal production plants and small pieces of wood. In addition to firewood, there is wood residue and shrubby vegetation in the forest, which are used as raw materials for energy production. The wood residue in the forest is found in different forms and comes from different categories of wood. We can divide it into forest residue from stumps, from tree stem and from branches. The share of residue in relation to the gross volume of the tree differs for deciduous and coniferous tree species, as well as for stands of different quality. The wood residue after cutting and the bushy vegetation are intended for plants for the production of heat and electricity. The main problem when using wood residue from felling is its spatial arrangement. In such conditions, the costs of production (collection, loading and transport) are high, so the question of cost-effectiveness of use arises, given the low market value of the product. Within forest utilization, there are several methods of making wood assortments that differ in terms of the place of production and the degree of work mechanization. The basis for distinguishing the methods are the terrain characteristics, the stand characteristcs and the phase of forest care. On flat and slightly sloping terrains, there is a greater possibility of choosing the means used in the production process in relation to hilly and mountainous terrains. In hilly and mountainous areas, such as those in Republika Srpska, the use of wood residue is much more complex. The characteristics of the terrain have the greatest influence on the efficiency of work. In these conditions, it is possible to apply various methods and systems of work. Having in mind the advantages and disadvantages of all methods of making wood assortments that are known so far, none of the described methods is effective when using wood residue fromforests in hilly and mountainous conditions. The solution can be in the application of the half-tree lenght method of broadleaves with crown parts, which was developed with the aim of more efficient utilization of wood residue from deciduous forests in hilly and mountainous areas. The use of wood residue from coniferous stands in hilly and mountainous areas refers to the application of modern work technologies. The cutting and production of wood assortments is done with chainsaws, and then the transport of whole trees by cable car, on which a crane with a harvester head is mounted. The brushwood that remains in the timber yard is used as raw material for power plants or heating plants. There is a possibility that the branches will be chipped directly at the timber yard, transported as compact to a larger timber yard, then chipped or baled, and then the bales will be transported to the energy production plant, where the chipping is performed. In addition to modern technology, there is a conventional technology that involves collecting branches and then transporting. This technology is technically applicable, but economically questionable. The organization of work on chip production is mainly subordinated to chipping. The position of the chipper within the entire work system determines the structure of the material for transport and the independence of other means from the work of the chipper. Transportation of wood biomass from timber yard to consumers makes up half of the operating costs in the supply chain. In forestry, different modes of transport are used by trucks, railways and ships (barges). The most widespread is the transport of biomass by trucks. The use of wood biomass from forest ecosystems is exposed to various risks, and some of them are: erosion, loss of organic matter, soil compaction, landslides, chemical pollution of soil and aquatic ecosystems, soil acidification, occurrence of insects and pathogens, etc. With the increase in energy production from wood biomass, and especially production from wood residue after felling, different opinions have emerged on the justification of utilization. The negative impact of the utilization of wood residue from the forest on the content of nutrients in the soil can occur in a shorter or longer period, and in some situations after several decades. In order to avoid the negative effects of wood residue use, especially to prevent environmental risk, the Finnish guidelines recommend the use of residue only in forests on high-quality soils, while leaving 30% of the wood mass of the branch. It is assumed that the amount of carbon emitted into the atmosphere by the combustion of wood biomass, as well as that caused by the decomposition of wood biomass, accumulates during plant growth. Using the entire mass of the tree over a longer period of time can lead to a reduction in carbon stocks, but at the same time the use of fossil fuels is reduced, so the result is positive in the long run. The development of production capacities for the use of biomass and the production of chips provides the employment of a part of the population, mostly from rural areas. Employment opportunities in the bioenergy sector are higher, compared to other sectors that use renewable energy sources. Having in mind the benefits of using wood biomass for energy, many countries have taken measures to stimulate the production of energy from wood biomass. Stimulation measures were reflected in the enactment of laws and other legal acts, which encourage the production and promote the use of biomass. The use of wood biomass for energy in Republika Srpska is not at a satisfactory level, despite the fact that great progress has been made in recent years.