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The added value of torrefaction
This project was carried out parallel to another SenterNovem project called BIOCOAL known under project number 2020-02-12-14-013 (Bergman et al., 2005b). This project particularly focussed on the experimental evaluation, process synthesis and economic evaluation of torrefaction, but without densification. The required torrefaction experiments for the present work were to a large extent combined with those of the BIOCOAL project. Also, the design and process evaluation done in the BIOCOAL project were of direct use, so that the experimental work of this project could be focussed mainly on the pelletisation of torrefied biomass. During the experimental work, the emphasis was on determining the effect of torrefaction on the pelletisation process. This was done by using different biomass feedstock, which were torrefied under different conditions (temperature and time). Subsequently, for a certain torrefied biomass several densification experiments were carried out to also determine the effect of the main densification conditions (temperature and pressure) on the produced TOP pellets. With respect to product characterisation, the produced pellets were evaluated on the most important properties, which are the pellet density, calorific value and durability (mechanical strength and water resistance). As a laboratory press was used (a modified Pronto-Press), also pellets of untreated biomass feedstock were made (reference pellets) to see the effect of torrefaction on pelletisation. Before the experimental programme was conducted, the conceptual design of the TOP process was largely performed to match the experimental programme optimally (experimental design) to the structure of the TOP process. The conceptual design was then completed on the basis of the experimental results. The design data on torrefaction already available at ECN was combined with the design data obtained from the experimental results of the presented work to evaluate the technical feasibility of the process. This included the design of selected unit operations and the estimation of the net process energy efficiency. The economic evaluation comprised the estimation of the required total capital investment and the total production costs. Furthermore, a production analysis was performed to have an indication of the possible advantages of TOP pellets with respect to transportation and co-firing at existing coal-fired power stations. This part of the work was conducted in collaboration with a pellet producer in the field. The techno-economic evaluation of the TOP process was done using conventional pelletisation as the (state-of-the-art) reference. Background biopellets
Biopellets (or wood pellets) offer many more attractive properties in comparison to untreated biomass. With respect to heating value, grindability, combustion nature, storage, transport and handling, biopellets are in many cases the superior fuel. Particularly their high (energy) density and uniformity has proven to be the basis for a relatively new and boosting pellet market. Their usage as heating fuel in the domestic market has increased strongly, especially in scattered areas such as the Nordic countries and Austria. Compared to untreated biomass, biopellets have a relatively high heating value and in combination with the high bulk density this allows small combustion units (domestic application: pellet stoves) and cost savings in handling and transportation. Biopellets are less vulnerable to biological degradation as they are dry, so that periods of storage can be longer (Lehtikangas, 1999). But also large volumes of pellets are nowadays produced for the large-scale generation of heat and power, in order to replace coal with sustainable energy resources. With respect to large-scale biomass co-firing in coal-fired power stations, biopellets proved to offer a solution to grinding issues existing for untreated biomass (e.g. wood chips). Largeparticle biomass feedstock are difficult to grind in the existing coal mills due to their tenacious and fibrous nature. Biopellets are already composed of small particles and in a coal mill they are readily disintegrated (crushed) to these original particles. In countries such as the Netherlands, the large-scale production of power from biomass in existing coal-fired power stations can only be established though the import of biomass. This requires transportation of large volumes of biomass to the Dutch harbours from all over the world (e.g. Canada, Brazil, South Africa). Here, biopellets with their high (volumetric) energy density are an interesting fuel. Despite the strong development of the biopellet market over the last decade, research is still ongoing to improve the biopellet properties. This mostly concerns their durability and biological degradation. The durability of biopellets can be interpreted as resistance against water and moisture uptake and the mechanical resistance against crushing and dust formation. Generally, when exposed to water, snow, moisture or condensed water, biopellets rapidly swell and disintegrate to the original feed particles (and volume: original mass density). To prevent this they need to be stored in a dry and possibly conditioned place. Additionally, special precautions to the handling and transportation need to be taken (Alakangas and Paju, 2002). Biological degradation of biomass is decreased after pelletisation, but can still occur. The biopellets are dry and that inhibits degradation processes, such as fungal growth and microbial activity. The effect of these phenomena on the biopellet properties can be dramatic. Especially a decrease of the mechanical durability and variations in uniformity can be the result after changes of the biological, physical, and chemical properties (Lehtikangas, 1999). Besides, storage can become hazardous due to temperature development. As pelletisation mainly consists of physical operations, the feedstock quality is crucial in meeting the desired biopellet quality standards. Pellet uniformity is difficult to establish as the sources for quality variations are numerous. There are large differences between softwoods, hardwoods, between different tree species, and between different parts of the trees. Moreover, climatic and seasonal variations affect feedstock properties, as well as the length of the storage period and the type of storage (Lehtikangas, 1999). Sawdust and planer shavings (cutter shavings) are the most favoured feedstock for pelletisation. These are often uniform and are low in mineral content so that high combustion quality can be established. This especially concerns domestic applications, which do not include advanced technology for emission reduction. Softwood is preferred over hardwood, since the lignin content of softwood is higher. Lignin is one of the main biomass polymers and acts as binding agent. The more lignin, the higher quality of the pellet and the milder the densification conditions can be. Also bark is a good feedstock for biopellets. It gives a high calorific value pellet, but it contains more pollutants. Therefore, it is mainly suitable for large-scale applications that typically comprise gas clean up, such as power stations. Pelletisation of fresh biomass is more difficult and, according to Alakangas and Paju (2002), pellets produced thereof are not available commercially. Their durability is poorer and they are much more vulnerable to biological degradation. It is one of the threats of pelletisation that it is practically limited to sawdust and cutter shavings as economical feedstock. With that in mind, the pellet market is closely related to the wood-processing industry and coupled to its economic nature. This may lead to future feedstock shortages when the pellet market continues to boost. The added value of torrefaction
Torrefaction is a thermochemical treatment of biomass at 200 to 300 Â°C. It is carried out under atmospheric conditions and in the absence of oxygen. In addition, the process is characterised by low particle heating rates (< 50 Â°C/min). During the process the biomass partly decomposes giving off various types of volatiles. The final product is the remaining solid, which is often referred to as torrefied biomass, or torrefied wood when produced from woody biomass. Figure 2.1 provides a typical mass- and energy balance of torrefaction. Typically, 70% of the mass is retained as a solid product, containing 90% of the initial energy content (Bioenergy, 2000). 30% of the mass is converted into torrefaction gases, but contains only 10% of the energy content of the biomass. Hence a considerable energy densification can be achieved, typically by a factor of 1.3 on mass basis. This example points out one of the fundamental advantages of the process, which is the high transition of the chemical energy from the feedstock to the torrefied product, whilst fuel properties are improved. This is in contrast to the classical pyrolysis process that is characterised by an energy yield of 55-65% in advanced concepts down to 20% in traditional ones (Pentananunt et. al., 1990). In the 1930â€™s, the principles of torrefaction were first reported in relation to woody biomass and in France research was done on its application to produce a gasifier fuel (Bioenergy, 2000). Since then the process received only attention again when it was discovered that torrefied wood could be used as a reducing agent in metallurgic applications. This led to a demonstration plant, which was operated during the eighties, but was dismantled again in the beginning of the nineties of the last century (see also Bergman et al., 2005b). During the last five years, torrefaction has received attention again, but now as pretreatment technology to upgrade biomass for energy production chains (co-combustion and gasification). During this recent period new process concepts have been proposed and are under development. No commercial torrefaction production plant is operated at the moment and its development is to be considered in the pilot-phase (Bergman et al., 2005b). The key-property that makes torrefied biomass attractive for co-firing in existing coal-fired power stations is its superior grindability compared to untreated or fresh biomass. After torrefaction biomass has lost its tenacious nature and partly its fibrous structure (Bergman et al, 2005a). Through torrefaction, biomass becomes more alike coal and so its size reduction characteristics. Besides, the devolatilisation during torrefaction results in an increase of the calorific value on mass basis, as the reaction products are rich in oxygen (e.g. H2O, CO2, acetic acid). Biomass is completely dried during torrefaction and after torrefaction the uptake of moisture is very limited. This varies from 1-6% depending on the torrefaction conditions and the treatment of the product afterwards. The main explanation of the hydrophobic nature of the biomass after torrefaction is that through the destruction of OH groups the biomass loses its capability of hydrogen bonding. Moreover, unsaturated structures are formed which are non-polar. It is likely that this property is also the main reason that torrefied biomass is practically preserved and biological degradation, as often observed for untreated biomass, does not occur anymore. The most reactive biomass polymer during torrefaction is hemicellulose. After torrefaction it has reacted completely to alternative char structures and volatiles. Most of the weight loss can be contributed to hemicellulose with the effect that torrefied biomass mainly consists of cellulose and lignin. Hence the lignin content has increased. Although torrefaction leads to increased energy density on mass basis, during torrefaction only little shrinkage can be expected so that the volume of produced torrefied biomass is decreased only slightly. From experimental analysis (reported in Bergman et al. 2005b) the density of torrefied biomass is ranging from 180 to 300 kg/m3 or generally 10-20% lower than the used feedstock (when dried). Despite the higher calorific value, the volumetric energy density is not improved (typically 5 GJ/m3). Torrefied biomass is more brittle of nature compared the biomass it was derived from. This is crucial for establishing the desired grindability, but has the drawback of decreased mechanical strength and increased dust formation. Consequently, torrefaction and pelletisation can be very complementary when considering the pros and cons of their resulting products. From the pelletisation viewpoint, the implementation of torrefaction within the pelletisation process offers theoretically solutions to the problems encountered with the durability and biological degradation of biopellets. Torrefaction can potentially be applied to a wide variety of biomass (softwood, hardwood, herbaceous, wastes) so that the range of biomass feedstock for biopellets can be enlarged seriously. From the torrefaction viewpoint, the implementation of pelletisation within the torrefaction process subsequently offers solutions to the drawbacks of torrefied biomass, such as the low volumetric energy density and dust formation. Synergy effects through the combination of torrefaction with pelletisation have earlier been recognised by Reed and Bryant (1978). They researched simultaneous torrefaction and densification at a temperature up to 225 Â°C and found that the densification process was enhanced. The compaction pressure required for densification could be reduced with a factor of 2 to achieve the same pellet quality as if produced under typical pelletisation conditions. Also the energy consumption needed for densification could be reduced by a factor of 2, whilst the pellet density and the calorific value increased significantly. Importantly, they also explored temperatures in the range of 250-300 Â°C, but encountered heavy devolatilisation during compression. Koukios (1993) also investigated the effect of simultaneous torrefaction and densification of biomass. Apparent biomass densities exceeding 20 GJ/m3 were observed for straw, olive kernels and waste wood (softwood). Also, Koukios (1993) observed slight devolatilisation during densification, but this was limited probably due to the low temperatures applied. In Japan, where biomass resources are far away from the urban areas, the combination of torrefaction and densification (again simultaneous) is under investigation to reduce transport volumes of biomass (Honjo et al., 2002).
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