Solid Biofuels for Energy

In 2006, the total biomass contribution to primary energy consumption in the European Union was 86.6 million tons oil equivalent (Mtoe). The main share of 66.4 Mtoe was provided by solid biomass, with the remainder provided by biogas, transport biofuels and renewable solid municipal waste [1]. The bioenergy supply potential has recently been assessed at global level by (among others) the IPCC, US EPA, World Energy Council, Shell, IASA and the Stockholm Environment Institute [2, 3]. Estimates of the share of biomass in the future global energy supply range from below 100 EJ/year to above 400 EJ/year in 2050, compared to a global primary energy consumption of 420 EJ for the year 2001 [4]. One of the major reasons for the large ranges observed is that studies differed widely in their estimates of land availability and energy crop yields, and, to a lesser extent, the availability of wood and residue resources. Studies at the European level also deliver widely ranging results. Conservative results on the total biomass potential come from the EEA study [8]: how much bioenergy can Europe produce without harming the environment? It estimates a total bioenergy potential from agriculture, forestry and waste of almost 300 Mtoe in 2030. Of this, 142 MTOE will come from agriculture only which is obtained from 19 million hectares of agricultural land. This is equivalent to 12% of the utilised agricultural area in 2030. The purpose of this chapter is to provide a perspective on solid agricultural biomass feedstocks in EU27 and to summarise relevant data that influence the availability and future supply of these feedstocks for energy and fuel production. To achieve this, the chapter is structured in sections that aim to provide a series of concise answers to key questions arising regarding biomass availability and supply:

The technical committee developing the draft standard to describe all forms of solid biofuels within Europe (CEN/TC 335) has published 27 technical specifications for solid biofuels. The two most important are classification and specification (CEN/TS 14961) and quality assurance (CEN/TS 15234). Now these technical specifications are upgraded to full European standards (EN). Both these standards will be published as multipart standards. Part 1 – General requirements of EN 14961-1 includes all solid biofuels and is targeted for all user groups. The classification of solid biofuels is based on their origin and source and biofuels are divided to four sub-categories: (1) Woody biomass, (2) Herbaceous biomass, (3) Fruit biomass, and (4) Blends and mixtures. The quality tables were prepared only for major traded forms. Parts 2–6 are product standards, which are targeted for non-industrial use. Non-industrial use means fuel intended to be used in smaller appliances, such as in households and small commercial and public sector buildings. In the product standards all properties are normative and they are bound together to form a class, for example A1, A2, and B. Although these product standards may be obtained separately, it should be recognized that they require an understanding of the standards based on and supporting EN 14961-1. This chapter concentrates on Part 1 of EN 14961, which was published in 2010. The remaining five product standards are being drafted and are at the voting stage, with an expected publication date within 2010.

This investigation explores the reasons for and technical challenges associated with co-combustion of biomass and coal in boilers designed for coal (mainly pulverized coal) combustion. Biomass-coal co-combustion represents a near-term, low-risk, low-cost, sustainable, renewable energy option that promises reduction in effective CO₂ emissions, reduction in SO_x and often NO_x emissions and several societal benefits. Technical issues associated with cofiring include fuel supply, handling and storage challenges, potential increases in corrosion, decreases in overall efficiency, ash deposition issues, pollutant emissions, carbon burnout, impacts on ash marketing, impacts on SCR performance and overall economics. Each of these issues has been investigated and this presentation summarizes the state-of-the-art in each area, both in the US and abroad. The focus is on fireside issues. While each of the issues can be significant, the conclusion is that biomass residues represent possibly the best (cheapest and lowest risk) renewable energy option for many power producers.

Solid Recovered Fuels (SRF) are solid fuels prepared from high calorific fractions of non-hazardous waste materials intended to be fired in existing coal power plants and industrial furnaces (CEN/TC 343, Solid Recovered Fuels, 2003). In other frameworks, these types of fuels are referred to as refuse or waste derived fuels. They are composed of variety of materials of which some, although recyclable in theory, may be in forms that made their recycling an unsound option. The use of waste as a source of energy is an integral part of waste management. As such, within the framework of the European Community’s policy-objectives related to renewable energy, an approach to the effective use of wastes as sources of energy is outlined in documents like the European Waste Strategy. Within the scope of the European Demonstration Project, RECOFUEL, SRF co-combustion was demonstrated in two large-scale lignite-fired coal boilers at RWE in Germany. As a consequence of the high biogenic share of the co-combusted material, this approach can be considered beneficial following European Directive 2001/77/EC on electricity from renewable energy sources (directive). During the experimental campaigns, the share of SRF in the overall thermal input was adjusted up to 15%. The measurement campaign included boiler measurements in different locations, fuel and ash sampling and its characterization. The corrosion mechanisms and rates were analysed and monitored by dedicated corrosion probes. The scope of this chapter covers the characterisation of SRF using the pre-nominative technical specifications of CEN and the status of the standardization activities. Additionally this chapter summarizes some of the experiences gained from co-firing of SRF and biomass in large scale demonstration plants. These include handling and pretreatment of the SRF, milling corrosion, emissions behaviour, and the quality of solid residues.

Unlike pulverized coal, biomass particles are neither small enough to neglect internal temperature gradients nor equant enough to model as spheres. Experimental and theoretical investigations indicate particle shape and size influence biomass particle dynamics, including essentially all aspects of combustion such as drying, heating, and reaction. This chapter theoretically and experimentally illustrates how these effects impact particle conversion. Experimental samples include disc/flake-like, cylindrical/cylinder-like, and equant (nearly spherical) shapes of wood particles with similar particle masses and volumes but different surface areas. Small samples (320 μm) passed through a laboratory entrained-flow reactor in a nitrogen atmosphere and a maximum reactor wall temperature of 1,600 K. Large samples were suspended in the center of a single-particle reactor. Experimental data indicate that equant particles react more slowly than other shapes, with the difference becoming more significant as particle mass or aspect ratio increases and reaching a factor of two or more for particles with sizes over 10 mm. A one-dimensional, time-dependent particle model simulates the rapid pyrolysis process of particles with different shapes. The model characterizes particles in three basic shapes (sphere, cylinder, and flat plate). With the particle geometric information (particle aspect ratio, volume, and surface area) included, this model simulates the devolatilization process of biomass particles of any shape. Model simulations of the three shapes show satisfactory agreement with the experimental data. Model predictions show that both particle shape and size affect the product yield distribution. Near-spherical particles exhibit lower volatile and higher tar yields relative to aspherical particles with the same mass under similar conditions. Volatile yields decrease with increasing particle size for particles of all shapes. Assuming spherical or isothermal conditions for biomass particles leads to large errors at most biomass particle sizes of practical interest.

Fluidised bed combustion (FBC) technology was developed in the 1970s in order to exploit the energy potential of high-sulphur coals in an environmentally acceptable way. The FBC technology was soon expanded for biomass and other low-grade fuels, which have typically large variations in fuel properties. The benefit of the FBC is the large amount of bed material compared with the mass of the fuel (98 vs 2%) and, thus, the large heat capacity of the bed material that stabilises the energy output caused by variations in fuel properties. Moreover, by selecting reagents as bed material and controlling the bed temperature, the emissions of pollutants can be controlled. In the last two decades, rapid progress has been achieved in the application of FBC technology to power plants up to intermediate capacities, caused by the increasing demands for fuel flexibility, stringent emission control requirements, stable plant operation and availability. Especially concerning the fuel range; there is a definite trend to widen the range of biomass fuels and waste fractions. The aim of this chapter is to review critically the technical requirements of biomass and/or waste combustion in FBCs, the operational problems, the needs for emissions control and the ash handling issues.

It is predictable that energy demand will greatly increase in years to come, due to the continuous growth of world population, together with the quest to improve living standards. CO₂ emissions are hence expected to increase significantly. Gasification is a mature technology for energy production that permits an easier separation of CO₂ for its storage. As modern societies are producing everincreasing amounts of wastes with negative impact on the environment, new technologies have been developed to co-gasify these wastes either with coal or alone, thus resolving a serious problem of waste disposal. Wastes gasification reduces the dependence on fossil fuels and co-gasification with coal could provide the benefit of security in fuel supply, as the availability of wastes and biomass fuels could vary from region to region and show seasonal changes. Gasification experimental conditions and technologies and syngas cleaning methods are key issues for the production of a clean gas that could find a wide range of applications. This chapter will concentrate on syngas end-uses, focusing on new ones, like gas turbines or engines in IGCC, synthesis of methanol, ethanol and dimethyl ether, Fischer–Tropsch synthesis, and hydrogen production. The role of gasification in CO₂ sequestration will also be discussed.

Renewable micro-CHP systems are a combination of micro-CHP technology and renewable energy technology, such as biomass gasification systems or solar concentrators. The integration of renewable energy sources with micro-CHP allows for the development of sustainable energy systems with the potential for high market penetration; a cost-effective and reliable heat and electricity supply; and a highly beneficial environmental and economical impact on a pan-European scale. The purpose of this chapter is to present results from the European coordination action project MICROCHEAP that intended to bring together industrial specialists and research experts to focus entirely on renewable micro-CHP technology, co-ordinate and steer research in this field, and highlight the most promising technologies with the highest potential for market penetration in existing and future market conditions. The chapter discusses the state of the art technological options in the field of renewable micro-CHP with biofuels with regards to technology, cost, and environmental impacts and presents a market survey concerning the possibility of future penetration of the technology in Europe. The results will provide a coherent overview of the basic technological options for renewable micro-CHP with biofuels and will provide an insight to the market trends within Europe and projected future market scenarios, taking into account cost estimations for various micro-CHP technologies, feedstocks, and electricity and fuel prices in Europe.

This chapter gives an overview of the main ash formation and deposition mechanisms for various relevant biomass fuels, also in blends with selected coals, in pulverised-fuel (PF) boilers. The chapter is divided into three sections. In the first, a general outline of the ash formation mechanisms is given. The second section gives a review of experimental and analytical techniques for the (lab-scale) characterisation of fuels, with the emphasis on the ash-forming elements contents and fate during the combustion. Fuel reactivity and burnout, devolatilisation behaviour, N-release and slagging and fouling propensity are discussed. Also, a detailed overview of the experimental conditions and their relevance for the existing as well as the future technologies is given. Further an outline of diagnostic techniques for the in-boiler characterisation of slagging and fouling is issued. In the third section, key ash-formation phenomena are discussed for various pure biomass fuels and selected typical coals. This is done on the basis of exemplified results, generated with the techniques discussed in the foregoing section. For slagging and fouling this is also backed up with data from full-scale diagnostic measurements.