The Pellet Handbook

The Production and Thermal Utilization of Biomass Pellets

Gerold Thek, Ingwald Obernberger

• Editorial: Routledge
• Año: 2012
• ISBN:
  9781136539916
• Edición: 1
• Categoría: Ciencias biológicas
• Categoría: Ecología

Biomass pellets are a suitable fuel type for a wide range of applications, from stoves and central heating systems up to large-scale plants, and with practically complete automation in all these capacities. This handbook, written and edited by experienced professionals from IEA Bioenergy Task 32 in cooperation with Bios Bioenergiesysteme GmbH, Graz, Austria, other IEA Tasks and external experts, is the first comprehensive guide in English language covering all pellet related issues, as illustrated by the following list of topics covered by the book:

• international overview of standards for pellets
• evaluation of raw materials and raw material potentials
• quality and properties of pellets
• technical evaluation of the pellet production process and logistic aspects of pellet supply
• safety and health aspects for pellets during storage, handling and transportation
• technological evaluation of pellet furnace technologies and future developments
• economic and ecological evaluation of the pellet production process
• economic and ecological evaluation of pellet use in small-scale furnaces in the residential sector
• overview of international pellet markets and market developments
• international case studies for the use of pellets for energy generation
• latest trends concerning research and development in the pellet sector.

Extensively illustrated and packed with practical knowledge, this is the ultimate reference for anyone involved in or affected by this burgeoning industry. It addresses all the players of the pellet market, ranging from raw material producers or suppliers, pellet producers and traders, manufacturers of pellet furnaces and pelletization systems, installers, engineering companies, energy consultants and end users.

1	Introduction

The drawbacks of biomass as a fuel alternative to coal, oil or gas are attributed mainly to its low energy density, high moisture content and heterogeneity. The problems of conventional biofuels can be lessened or even prevented altogether by the use of pellets with consistent quality - low moisture content, high energy density and homogeneous size and shape. Consistent fuel quality makes pellets a suitable fuel type for all areas of application, from stoves and central heating systems up to large-scale plants, and with practically complete automation in all these capacity ranges. It was not until such a homogenous biofuel with regard to shape and size was introduced to the market that the development of fully automatic biomass furnaces for small-scale applications with a similar user comfort as modern oil or gas heating systems was possible. Wood pellet combustion technologies ranging from pellet stoves up to large-scale plants are explained in detail and case studies of all possible applications for energy generation are presented. The standardisation of pellets has made a major contribution to their success. International environmental obligations lead the large district heating and power producing companies to convert their large coal burning plants to the use of solid biofuels including hog fuel, chips, bark, agro-material, briquettes and pellets. In many cases these plants were originally set up to burn pulverised fuel and the only fuel suitable for the large infrastructure already in place has been pelletised material that can be ground to powder before being injected into the furnaces in the same way as coal.

The focus of this book lies on the production and energetic utilisation of wood pellets, starting from the raw materials via the production process, characteristics and combustion technology up to ecological and economical considerations. Whenever it is relevant, pellets made of herbaceous biomass (e.g. straw pellets) are taken into account and explicitly noted. If it is just “pellets” that are discussed, wood pellets for energetic utilisation are meant. There is a strong demand for knowledge with regard to optimised or innovative production technologies and improved logistics, distribution systems and combustion technologies. This book addresses all the players of the pellet market, ranging from raw material producers or suppliers, pellet producers and traders, manufacturers of pellet furnaces and pelletisation systems, installers, engineering companies, energy consultants up to the end users. It should contribute to a further increase of pellet utilisation within the energy sector by the appropriate distribution of information.

The production of the pellets is labour intensive and commercially challenging. Still, recent years have seen advancement in machinery and processes used for the transformation of low grade biomass to high grade solid biofuels. Pellets can be packaged and transported in a variety of forms such as consumer bags, jumbo or big bags, containers, railcars, wrapped pallets, flat bed trucks, tanker or silo trucks and ocean bulk carriers. They can also be stored in readily available flat storages as well as vertical silos. Year-round pellet supply can therefore be realised by reasonable and efficient fuel logistics.

Consistent fuel quality allows for easier transport and greater transportation distances. Increasingly large volumes of pellets are moving across the globe in bulk, which, in general, demands a comprehensive set of testing standards. In the course of this book, physio-chemical characteristics of both raw materials and pellets are evaluated in order to estimate the suitability of certain raw materials for pelletisation on the one hand (on the basis of the relevant standards and on requirements posed by the pelletisation technique) and the combustion behaviour on the other hand.

Safety and health concerns in pellet production, handling and storage are given major priority. All biomass, like any biological material, is subject to decomposition as a result of microbial activities in combination with chemical oxidation resulting in self-heating. During the decomposition, the biomass emits non-condensable gases such as CO, CO₂ and CH₄ as well as condensable gases such as hydrocarbons. These off-gassing phenomena are a cause of concern during transportation, handling and storage. Also, most solid biofuels are relatively brittle and disintegrate when exposed to attrition and impact resulting in fines. Part of the fines is very small in size and easily becomes airborne as dust. This dust is highly explosive, with wood dust being the material that causes more fires and explosions in the industry than any other material.

Regarding the acceptance of pellet heating systems, the high investment costs as compared to oil or gas heating systems were and still appear to be unfavourable. However, full cost calculation is necessary for a valid economic evaluation, which was carried out within the framework of this work for seven different residential heating systems. Possible investment subsidies that can be attained for heating systems are of relevance and hence are taken into account in the full cost calculation. The results show that pellets can be favourably compared to oil, however, gas heating systems are more economic under present framework conditions. If external costs caused by environmental impacts such as health damage, damage to flora and fauna and damage to buildings as well as climate and safety risks are taken into consideration, pellets have clear benefits.

Market conditions in different countries can vary widely. In Sweden for instance, pellets are used in medium- and large-scale systems for the most part, whereas in Austria pellets are chiefly used in small-scale furnaces. Comparisons of international framework conditions for the use of pellets are carried out and presented.

As pellets have become an internationally and intercontinentally traded good, international trade in addition to the socio-economic impacts of pellet production and utilisation need to be addressed. The strong growth of the pellet market requires an examination of the available raw material potential and consideration of alternative raw materials. In some cases, it may be more reasonable to produce high quality wood chips with defined quality criteria instead of cost and energy intensive pelletisation. Dry wood shavings have been and still are the preferred choice for pelletisation; wet sawdust from sawmills is also suitable for pelletisation and is available in great quantities, though it requires an increased number of processing steps. Other possible raw materials for pelletisation include wood chips from diverse wood processing steps, short rotation crops, log wood or bark. However, the use of these raw materials requires even more processing steps with regard to pre-processing compared to the use of wet sawdust (chipping, coarse grinding, separation of foreign matter, bark separation). For pellets containing bark and raw materials containing bark, special consideration should be given to their utilisation in small-scale furnaces due to higher ash content. The use of herbaceous biomass as raw material for pelletisation should also be carefully evaluated because of the higher ash content and high nitrogen, sulphur, chlorine and potassium contents. These elements can cause problems regarding corrosion, deposit formation and emissions.

Whether and to what extent national framework conditions concerning energy policy have an effect on pellet utilisation is analysed in detail. Sweden is mentioned as one example where high CO₂ and energy taxes boosted the consumption of biofuels. In Austria, the changeover to modern biomass heating systems as well as their installation in new buildings is supported by investment subsidies. How big an influence these subsidies have on the increasing number of pellet heating systems requires detailed investigation, especially considering the fact that the investment costs of pellet heating systems are still much above those of comparable oil heating systems.

Despite rapid pellet market development, there is still a lack of information in the public domain. In countries such as Austria, Germany or Sweden, where the use of pellets is already quite common, this is not pronounced. In countries where the pellet market is at the beginning of its development, such as Canada or the UK, there still is considerable lack of information. A lot of potential pellet users may not know about the existence and potential benefits pellets can offer. The supply of information by appropriate marketing campaigns is regarded as one of the major tasks to fulfil in the establishment of new markets and, as a result, help new markets to benefit from the experience of “old” markets. International exchange of knowhow is thus especially important and should be supported.

Ecological arguments against the use of pellets are often brought forward from the oil sector. Energy consumption for pellet production and higher emissions of some air pollutants are often stated as arguments against pellets. Especially the much argued and controversially fought discussions about fine particulate emissions have often been linked to wood furnaces since they cause more particulate emissions than oil or gas heating systems. Detailed investigation into a number of promising approaches sheds some light onto the issue. So do the current R&D trends that are concerned with the reduction of fine particulate emissions of biomass furnaces.

In general, current R&D trends are presented with regard to pellet production, further standardisation activities, and the production of pellets from herbaceous biomass and woody short rotation crops, the improvement of pellet quality, the development and promotion of small-scale pelletisation systems, and the phenomenon of self-heating and self-ignition in raw material and pellet storages. In the area of pellet utilisation there are activities concerning the further reduction of fine particulate emissions and the utilisation of pellets made of herbaceous biomass. Innovative pellet furnaces are being developed such as micro-scale furnaces with very low heating capacities for use in low energy houses, optimised combinations of the solar and pellet heating systems, flue gas condensation, multi-fuel boilers as well as micro-CHP (combined heat and power) systems. For the development of new pellet furnace technologies, computational fluid dynamics (CFD) simulation is employed to an increasing degree, whereby development risks and the number of required prototypes and test runs can clearly be reduced.

2	Definitions and standards

Pellets are a solid biofuel with consistent quality – low moisture content, high energy density and homogenous size and shape. In particular the residential pellet markets demand high quality pellets, as pellets are predominantly used in small-scale furnaces in this sector. This fact is accommodated in many countries worldwide by the existence of different standards for pellets. The different national standards and quality regulations attempt to control pellet quality in ways that, in part, differ greatly from one another. Above the national standards, work on European standards for solid biofuels has been done in recent years, which will lead to the publication of a series of European standards from 2010 onwards and consequently to a harmonisation and better comparability of pellets on an international basis. Above all, work on ISO (International Organization for Standardization) standards for solid biofuels has been in progress since 2007 and will lead to international standards within a few years. The ISO standards will finally replace all European EN standards.

The standards demonstrated in the following sections aim at the utilisation of pellets in small- scale furnaces below 100 kW_th, in which high standards are required to safeguard fully automated and trouble free operation. Apart from that, there are other quality classes in some countries, so-called industrial pellets, which are intended to be used in furnaces of larger than 100 kW_th and have lower quality requirements. With the new European standard for pellets, two additional quality classes besides the top quality class A1 are now also standardised, i.e. classes A2 and B. Class A2 might also become a relevant standard for pellets to be used in the residential heating sector as soon as pellet heating systems adapted to this class are available on the market (adaptation will be necessary due to the higher ash content). Pellets according to class B represent industrial pellets to be used in applications above 100 kW_th. Class B means that a standardised quality exists for the first time for this type of pellets. What makes them different from the higher quality pellets is that larger diameters, higher ash, nitrogen, sulphur and chlorine contents and lower NCVs are allowed. Industrial pellets are adapted to the requirements of large-scale users and are relatively low-cost (as compared to the high quality pellets for small-scale users). It must be explicitly stated at this point that industrial pellets should not be used in small-scale furnaces as this could lead to serious malfunctions of the systems.

Besides product standards for pellets and related analysis and quality assurance standards, there are also standards and certification systems for pellet transport and storage in the residential heating sector, both on national and European levels. Furthermore, not only is the fuel quality assured by respective standards, but furnaces using pellets as a fuel are also standardised and probably regulations for small-scale heating systems based on the European ecodesign directive will already be in force from 2011 onwards.

All these issues are discussed and described in the following sections.

2.1	Definitions

In this section, the terminology for pellets that is used in this book is described. It is partly derived from the respective standards.

2.1.1

General definitions

The term pellet stands for “a small round mass of a substance” [1]. A pellet is thus normally a small round mass, mostly made of compressed material, of a spherical or cylindrical shape. Usually, the word is used in the plural as pellets are normally not used singularly but as a bulk of material.

Various products and materials can be pelletised to be used thermally or still as a material as shown by the following list:

•Pellets made of iron ore are preliminary products in iron produ...