Forest Bioenergy

This chapter introduces and outlines the book “Forest Bioenergy: From Wood Production to Energy Use”, dedicated to biomass, currently the most commonly used renewable energy source, which contributes to 10% of the worldwide energy supply. The majority of bioenergy comes from woody biomass, which is mainly converted into heat (mostly in households, followed by industries). Its conversion to power is also relevant, while the production of transport biofuels is a promising pathway. Modern bioenergy presents numerous advantages: it has a renewable, versatile, local and distributed nature; it helps increase energy security and meet the rising global energy demands; it easily substitutes for fossil fuels; and it presents potential environmental and economic benefits. Carbon sequestration and storage are among the several environmental services provided by forests. The amount of biomass they produce, and consequently, their bioenergy potential, is highly variable. Forest plantations provide the highest bioenergy yields per unit area, while in forest systems managed for other purposes, factors such as stand structure affect residual biomass generation. Assessing and monitoring biomass, along with determining bioenergy potentials, are essential tasks, often based on mathematical models that vary in complexity and span different spatial and temporal scales, frequently with associated cartography.

Forest biomass used for energy or biofuels can be sourced directly from land-use systems, indirectly from wood-based industries or recovered from other human activities outside the forest sector. The former, referring to primary biomass from forests, includes organic products or residues derived directly from living or recently dead trees or other forest vegetation. It constituted nearly half of the world’s harvested forest biomass in 2021 and holds particular importance in the Global South, where traditional biomass remains a vital energy source for many people. Besides direct wood fuel, secondary wood residues represent another substantial source of forest bioenergy. These organic residues, such as wood chips, sawdust or black liquor, are generated by the industries processing wood, especially primary forest industries. A large amount of these residues is well-suited for further material use and energy generation. However, wood suitable for energy is not solely generated by forest-based industries. Various other activities use wood products that eventually reach the end of their usable life and are discarded, such as wood waste from construction or demolition, furniture waste or end-of-life pallets and packaging used to transport goods. This chapter presents and characterises the different woody biomass streams that can provide feedstock for energy.

Trees and stands store large amounts of biomass, but this storage is dynamic in time and space. It depends on the species, stand structure, silvicultural systems, and silvicultural practices. Furthermore, interactions between the trees in the stands and forests and disturbances result in biomass variability. The forest systems biomass estimation sometimes does take into account this variability. Additionally, all harvests remove biomass to a smaller or larger extent from the forest systems. Their sustainability is dependent on the amount and biomass components removed. The biomass exports are related to the management goals and the harvest type. Overall, stem biomass exports have smaller impacts than whole tree harvest on the sustainability and resilience of forest systems. However, forest residues removal can be done to maintain the forest system sustainability as long as biomass components richer in nutrients are maintained, at least partially, in the forest systems.

Energy plantations have been gaining importance in the supply of biomass for energy purposes, due to their high yield in short timeframes. These forest systems also enable to reduce the pressure in other forest systems to provide biomass for energy, in particular those under protection and conservation status. This chapter reviews the state of the art of energy plantations and their yields. It addresses the selection of species, density, rotation, harvest cycles, site selection, management practices, harvesting, biomass yields, and their estimation. Overall, there is a wide set of species and management options that can be used in energy plantations. Similarly, there is a large variability in yields, that vary between and within species, due to site, density, rotation, harvest cycles, and management. Though there are many studies, further research is needed on yield optimisation, rotation length, harvest cycles, management practices, and harvesting.

Models are abstractions that enable to assess and predict forest stands variables. Two broad methods to estimate biomass were defined. The direct method, the most accurate, has the disadvantage of resulting from destructive sampling. Inversely, the indirect method uses a variety of mathematical methods, with forest inventory, remote sensing, and ancillary data as explanatory variables. The accuracy of the biomass models is dependent on data acquisition precision and accuracy as well as on the model’s uncertainties. Moreover, model accuracy is also dependent on species, individual tree biomass partitioning, stand structure, region, and spatial and temporal scales. This chapter overviews the data sets and mathematical methods used for modelling biomass and their uncertainties. Overall, the performance of the forest biomass functions is linked to its ability to accommodate the variability inherent to forest data and to make biomass assessments, monitoring, and predictions with the best possible precision and accuracy and the smallest bias.

The diversity of species, tree allometry, and stand structure makes modelling forest biomass a challenge. At tree level diameter at breast height, and height are the most frequently used explanatory variables. Yet, other variables that encompass the variability in tree allometry due to species, stand structure, competition between trees, and site allow better performances of the biomass models. Similarly, at area level, the biomass functions have large variability in the data and explanatory variables used for modelling. This is due to the differences in species, stand structure, and their correlation with the remote sensing data. The combination of different remote sensing data sets from passive and/or active sensors linked with ancillary data enabled to improve models’ performance. Furthermore, a wide set of mathematical methods have been used to capture the stands and forests diversity and variability and accommodate it in the models to improve predictions. Overall, the wide range of biomass models corresponds to a continuous need to develop biomass functions that enable assessing, monitoring and predicting total or per component biomass.

Biomass is a highly versatile and reliable source of firm, renewable energy, capable of generating heat, power and various biofuels. The technologies used to convert biomass into fuels or energy can be broadly divided into two categories: biochemical and thermochemical. Biochemical pathways for forest biomass conversion into fuels still face techno-economic challenges, requiring further research to make them economically attractive. In contrast, thermochemical conversion processes, including gasification, pyrolysis and combustion, are well suited for forest biomass conversion, with several technologies having reached a fully commercial stage. Combustion, the most common and mature thermochemical pathway, converts forest biomass into heat, power, or combined heat and power. While traditional, inefficient and polluting methods are still used for burning forest biomass, modern, cleaner, and more efficient combustion technologies are available and in use. Some pathways based on gasification and pyrolysis are also commercially viable, providing solid, liquid and gaseous biofuels. These options offer versatility across combustion systems, heat engines, fuel cells and synthesis applications. This chapter provides a comprehensive overview of forest biomass as an energy source, covering processing technologies, technology readiness levels, fuel characteristics and pre-treatment methods. It emphasizes the potential and challenges associated with using forest biomass for sustainable energy production.

Biomass is an important source of energy in the residential sector and meets a significant proportion of the energy needs of one-third of the world’s population. In many low- and middle-income countries, particularly in rural areas, it is still used in a traditional way and provides the basic energy needs of the population, such as cooking and water and space heating. In many poor regions where forests are abundant, wood, of all the possible biomass resources, is the dominant fuel. It can be obtained at low or no monetary cost and burns in simple and inexpensive equipment. However, the consequences of this traditional use of biomass are several: indoor and outdoor pollution, impacts on health, pressures on forest resources and increased burden on women and children. Developments in residential wood fuel energy technologies are driven by the need for higher efficiency and fewer environmental impacts. As a consequence of such developments, today, highly efficient and cleaner equipment is used to provide energy from forest resources, but still mainly in high-income countries. This chapter reviews the use of energy from forest biomass in the residential sector and the different available conversion technologies, from the traditional to the most advanced ones.

The industrial sector, the world’s largest energy consuming end-user, is a major greenhouse gas emitter. It heavily relies on fossil fuels, with only a small contribution from renewables, and of these, only biomass (mainly primary solid biofuels) is not marginal at a global scale. Several factors contribute to the limited adoption of renewables within the industry. The sector’s extraordinary diversity and complexity make a one-size-fits-all solution impossible. Industrial energy consumption varies significantly among different sub-sectors and even within each sub-sector, depending on production composition and industrial processes. Energy-intensive industries typically consume substantial amounts of process heat, while non-energy-intensive ones tend to rely more on electricity. Given the importance of energy-intensive industrial sub-sectors, finding solutions to decarbonise process heat is crucial. Process heat encompasses various applications, technologies, energy sources, temperatures and delivery methods. There is substantial demand for high-temperature process heat (>500 °C), with only a limited number of renewable energy options available, including bioenergy. Bioenergy holds the potential to contribute to the decarbonisation of industry but requires tailored solutions for each sub-sector and context. This chapter presents key commercially available biomass heat production systems, which vary in configuration, technologies and scale, with similarities to district heating systems, also discussed.

Despite intensive efforts to decarbonise the power sector and the growing contribution of renewables to global electricity generation, fossil fuels, especially coal, continue to dominate as the most commonly used energy sources in this sector. The power industry accounts for a substantial portion of the world total energy supply and remains the largest contributor to CO₂ emissions. In 2020, renewable energy sources accounted for 28% of the electricity generation, with only 2% of the electricity produced derived from biofuels. Despite this relatively small share, the role of bioenergy in the power sector holds the potential to contribute to grid stability, a critical factor as the share of intermittent renewables in the energy mix increases. Additionally, co-combustion of biomass in coal power plants offers a cost-effective means of reducing carbon emissions, particularly in regions heavily reliant on coal. Several commercial technologies are available for converting biomass into electricity. While the efficiency of dedicated biomass-to-electricity plants is relatively low, combined heat and power systems that utilise waste heat achieve significantly higher overall efficiencies. The choice of technology depends on factors like capacity, efficiency and economic viability. This chapter provides an overview of commonly used conversion technologies for power generation from solid biomass.

This is the last chapter of the book entitled “Forest Bioenergy: From Wood Production to Energy Use”. The preceding chapters have covered the sources and distribution of forest biomass, its availability for different stand structures, and the methodologies and tools employed for its estimation. Furthermore, the diverse pathways for converting forest biomass into energy and biofuels have been explored, along with the associated technologies for biomass-to-energy conversion in the residential, industrial and power sectors. Forest biomass is a highly versatile source of energy, holding the potential to play a central role in a sustainable energy future. However, its use should guarantee the continued sustainability of forest stands, forests and the array of products they offer. It is equally essential to identify and prioritise the most effective pathways for the conversion of forest biomass into energy. A large body of work has been done; however, improvements can be achieved with whole system approaches that can accommodate the variability of forest biomass and conversion technologies and enable a comprehensive characterisation of the entire system. Accurate and precise statistics are of primordial importance for the whole system analysis.