The basic idea behind the Uniform Wood Energy Terminology (UWET) is to create a suitable framework for the identification of the amount and type of wood energy flowing from different supply sources to meet end user needs. Thus the fuel or product used to transport energy is the basic parameter to be accounted and properly classified. Either in commercial or non-commercial terms, these fuels should always be considered goods or commodities that are valuable and capable of meeting demand effectively.
Figure 1 presents a conceptual view of wood energy systems, showing how wood energy physically flows to satisfy demand. The figure introduces the three different supply sources: nature (forests and trees-outside forests), wood industries and society. The user's side is also presented, shared among the main demand sectors. Additional considerations regarding wood energy trade complete the picture.
As a complement to the general approach presented in Figure 1, a detailed scheme for classifying biofuels is presented in Table 2. The prime consideration behind the classification suggested in Table 2 is to recognize the basic site where biomass production takes place; in other words, to distinguish whether the biofuel was connected to forest, agricultural or municipal activity. Moreover the inclusion of a group on the use of agrofuels aims at distinguishing classical biofuels (generally related to forest exploitation) from later ones, basically oriented towards annual or pluri-annual plantation.
The groups on the supply side concern important sub-divisions which clearly identify the origin of biofuels. On the end user side, the variety of fuels that can be produced for each group can be observed, thus allowing for data comparison and verification on both supply and demand. The far right column of Table 2 lists the different types of primary, secondary and even tertiary fuels which can be used for heat, electricity and power generation. Secondary and tertiary fuels are often derived from raw biomass produced from various supply sources following the application of relatively complex transformation processes. Definitions of the main terms employed are provided in Section 5.2.
Figure 1. Wood fuel balance scheme, from supply source to end user
Table 2: Biofuel classification scheme
Production side, supply |
Common groups |
Users side, demand examples |
5.2.1 Energy
· Renewable energy: consists of energy produced and/or derived from sources infinitely renovated (hydro, solar, wind) or generated by combustible renewables (sustainably produced biomass); usually expressed in energy units and, in the case of fuels, based on net calorific values.
· Bioenergy (or biomass energy): covers all energy forms derived from organic fuels (biofuels) of biological origin used for energy production. It comprises both purpose-grown energy crops, as well as multipurpose plantations and by-products (residues and wastes). The term by-products includes solid, liquid and gaseous by-products derived from human activities. Biomass may be considered as one form of transformed solar energy.
· Bioenergy balance: quantitative summary data on biomass energy production and consumption represented in an energy balance table and diagram. A bioenergy balance represents an overview of production and consumption of primary and secondary biofuels for a specific area, country or region. All values must have the same units and the same prefix to indicate its magnitude (tera, peta, giga, etc.). Energy balances should cover all the primary and secondary energy sources, showing clearly the non-energy use of such sources. In the cases where biofuels are utilized for industrial processes, such as in the pig-iron industry, their use should be clearly indicated and consideration given to all the previous processes involved.
· Wood energy (forest energy): all energy derived from primary and secondary solid, liquid and gaseous biofuels derived from forests, woodlands and trees. Wood energy represents the energy produced after combustion of woodfuels, such as fuelwood, charcoal, pellets, briquettes, etc., corresponding to the net calorific value (NCV) of the fuel.
5.2.2 Bioenergy sources
Biofuels: organic primary and/or secondary fuels derived from biomass which can be used for the generation of thermal energy by combustion or by using other technology. It comprises both purpose-grown energy crops, as well as multipurpose plantations and by-products (residues and wastes). The term "by-products" includes the improperly called solid, liquid and gaseous residues and wastes derived from biomass processing activities. The main definitions employed in UWET include three types of biofuel: woodfuels, agrofuels, and municipal by-products.
A. Woodfuels
Include all types of biofuels derived directly and indirectly from trees and shrubs grown in forest and non-forest lands. The definition of forest used in the FAO Forest Resource Assessment 1990 (FAO Forestry paper 124, p7) is rather broad and includes lands with a minimum crown cover of 20% in developed countries and 10% in developing countries. Woodfuels also include biomass derived from silvicultural activities (thinning, pruning etc.) and harvesting and logging (tops, roots, branches, etc.), as well as industrial by-products derived from primary and secondary forest industries which are used as fuel. They also include woodfuels derived from ad hoc forest energy plantations.
According to origin, woodfuels can divided into three groups: Direct woodfuels, Indirect woodfuels, and Recovered woodfuels, as presented in Figure 1 and defined as follows.
· Direct Woodfuels: consists of wood directly removed from Forests (natural forests and plantations; land with tree crown cover of more than 10% and area of more than 0.5 ha); Other Wooded Lands (land either with a tree crown cover of 5-10% of trees able to reach a height of at least 5 m at maturity in situ; or crown cover of more than 10% of trees not able to reach a height of 5 m at maturity in situ, and shrub or bush cover); and Other Lands to supply energy demands and includes both inventoried (recorded in official statistics) and non-inventoried woodfuels. Direct woodfuels can be directly burned or are converted into another fuel, such as charcoal, pyrolisis gases, pellets, ethanol, methanol, etc.
· Indirect Woodfuels: usually consists of industrial by-products, derived from primary (sawmills, particle boards, pulp and paper mills) and secondary (joinery, carpentry) wood industries, such as: sawmill rejects, slabs, edging and trimmings, sawdust, shavings and chips bark, black liquor, etc.. Indirect woodfuels can be directly burned or are converted into another fuel, such as charcoal, pyrolisis gases, pellets, ethanol, methanol, etc.
· Recovered Woodfuels: refers to woody biomass derived from all economic and social activities outside the forest sector, usually wastes from construction sites, demolition of buildings, pallets, wooden containers and boxes, etc., burned as they are or transformed into chips, pellets, briquettes, powder, etc.
Table 3. Woodfuel importance
Supply side (sources) | ||||
Commodities (wood energy vectors) |
Direct Woodfuels |
Indirect Woodfuels |
Recovered Woodfuels | |
Fuelwood |
XXX, E |
XXX, NE |
XXX, NE | |
Charcoal |
XXX, E |
XX, NE |
X, NE | |
Black liquor |
XXX, NE |
|||
Other (methanol, ethanol, pyrolitic gas) |
X, NE |
X, NE |
X, NE | |
Importance: XXX : very important XX : important X : less important or in technological development |
database E : currently estimated NE: currently non evaluated | |||
As regards the commodities to be considered in wood energy accounting, woodfuels can be divided into four types of products: fuelwood, charcoal, black liquor and other, defined as follows.
· Fuelwood (or firewood): includes "wood in the rough" in small pieces (fuelwood), chips, pellets and/or powder derived from forests and isolated trees, as well as wood by-products from the wood products industry and from recovered woody products. They preserve essentially the original structure of wood and can be used either directly or after some conversion to another woodfuel, as charcoal. When needed, fuelwood can be prepared (without major chemical-physical transformations) into more convenient products, such as chips and pellets.
chips: wood that has been deliberately reduced to small pieces from wood in the rough, or residues suitable for energy purposes.
wood pellets: can be considered a fuel derived from the auto-agglomeration of woody material as the result of a combined application of heat and high pressure in an extrusion machine.
· Charcoal: refers to a solid residue derived from the carbonization, distillation, pyrolysis and torrefaction of wood (from the trunks and branches of trees) and wood by-products, using continuous or batch systems (pit, brick and metal kilns). It also includes charcoal briquettes.
charcoal briquettes: made from wood-based charcoal which, after crushing and drying, is moulded (often under high pressure), generally with the admixture of binders to form artefacts of even shapes.
· Black liquor: is the alkaline-spent liquor obtained from the digesters in the production of sulphate or soda pulp during the process of paper production, in which the energy content is mainly derived from the content of lignin removed from the wood in the pulping process.
· Other woodfuels: includes a broad range of liquid and gaseous fuels derived from fuelwood and charcoal basically by pyrolitic or enzymatic processes, such as pyrolisis gases, ethanol, methanol, products of growing interest but to date not as important as energy commodities.
The present importance of different types of woodfuels, as well as the availability of data in FAO STAT current tables are summarized in Table 3.
B. Agrofuels
Fuel obtained as a product of agriculture biomass and by-products. It covers mainly biomass materials derived directly from fuel crops and agricultural, agroindustrial and animal by-products.
_ Fuel crops: describe species of plants cultivated on fuel plantations or farms to produce raw material for biofuel production. The fuel crops can be produced on land farms (manioc, sugar cane, euphorbia, etc.), on marine farms (algae) or in fresh water farms (water hyacinths). The land-produced fuel crops can also be classified under: sugar/starch crops, oil crops and other energy crops.
sugar/starch crops: are crops planted basically for the production of ethanol (ethyl alcohol) as a fuel mainly used in transport (on its own or blended with gasoline). Ethanol can be produced by the fermentation of glucose derived from sugar-bearing plants (like sugar-cane) or starchy materials after hydrolysis.
oil crops: cover oleaginous plants (like sunflower, rape, etc.) planted for direct energy use of vegetable oil extracted, or as raw material for further conversion into a diesel substitute, using transesterification processes.
other energy crops: include plants and specialized crops more recently considered for energy use, such as: elephant grass (Miscanthus), cordgrass and galinggale (Spartina spp. and Cyperus longus), giant reed (Arundo donax) and reed canary grass (Phalaris arundinacea).
_ Agricultural by-products: are mainly vegetal materials and by-products derived from production, harvesting, transportation and processing in farming areas. It includes, among others, maize cobs and stalks, wheat stalks and husks, groundnut husks, cotton stalks, mustard stalks, etc.
_ Agro-industrial by-products: refer to food processing by-products, such as sugar-cane bagasse, rice/paddy husks and hulls, coconut shells, husks, fibre and pith, ground nut shells, olive pressing wastes, etc.
_ Animal by-products: refer to dung and other excreta from cattle, horses, pigs, poultry and, in principle, humans. It can be dried and used directly as a fuel or converted to biogas by fermentation.
biogas: is a by-product of the anaerobic fermentation of biomass, principally animal wastes by bacteria. It consists mainly of methane gas and carbon dioxide.
In the future, also more attention will be given to the definition of different types of agrofuels which so far have received marginal attention; not only regarding the terminology used but also for the development of improved data bases.
C. Municipal by-products
Refer to biomass wastes produced by the urban population and comprise two types: solid municipal by-products and gas/liquid municipal by-products produced in cities and villages.
_ Solid municipal biofuels: comprise by-products produced by the residential, commercial, industrial, public and tertiary sectors that are collected by local authorities for disposal in a central location, where they are generally incinerated (combusted directly) to produce heat and/or power. Hospital waste is also included in this category.
_ Gas/liquid municipal biofuels: correspond to biofuels derived principally from the anaerobic fermentation (biogas) of solid and liquid municipal wastes which may be land-fill gas or sewage sludge gas.
For easy reference, Table 4 provides brief definitions of the main terms adopted.
Table 4: Definition of Biofuel Classifications Proposed
1st level |
2nd level |
Brief definition |
|
Woodfuels |
Direct Woodfuels |
Wood used directly or indirectly as fuel, produced for energy purposes | |
Indirect Woodfuels |
Mainly solid biofuels produced from wood processing activities | ||
Recovered Woodfuels |
Wood used directly or indirectly as fuel, derived from socio-economic activities outside the forest sector | ||
Wood-derived fuels |
Mainly liquid and gaseous biofuels produced in forest activities and the wood industry |
||
Agrofuels |
Fuel crops |
Growing plants for the production of biofuels |
|
Agricultural by-products |
Mainly residues from crop harvesting and other kinds of by-products from agricultural activities left in the field | ||
Animal by-products |
Basically excreta from cattle, horses, pigs and poultry | ||
Agro-industrial by-products |
Several kinds of materials, produced chiefly in food processing industries, such as bagasse and rice husks | ||
Municipal by-products |
Solid and liquid municipal residues | ||
This present version is far from perfect and some weak points and overlapping areas still remain which will need clarification in the future. However, in light of recent technical developments in the bioenergy sector, some terms commonly used in current literature and bibliography have been eliminated. In fact, terms such as non-commercial energy have simply been eliminated and the previous definition of biofuel, which used to refer to biomass processed to obtain ethanol, esters, etc., has been revised in order to give the most adequate and realistic definition. It should be noted that the term energy plantations is presently being used to refer to both forest and agro-energy plantations. In this new classification, energy plantations will refer to forestry energy plantations; while agricultural plantations will simply be called energy crops. On the other hand, new terms are proposed such as agrofuels, meaning the biofuels deriving from non-forest activities, which include energy by-products from animal breeding, agriculture and agro-industries. As a general rule, it is proposed that the term wastes and residues be replaced by by-products.