VGB POWERTECH 7 (2020) - International Journal for Generation and Storage of Electricity and Heat

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The Biofficiency Project | Part 1 VGB PowerTech 7 l 2020 Tab. 5. Overview on biomass ashes selected for Biofficiency. Full-scale unit Capacity / MW el Biomass fuel Combustion system Rodenhuize 150 MW Wood pellets 1 Wood pellets 2 Wood pellets 3 Wood pellets 4 of different techniques is important. For example, with XRF analyses, the presence of carbonate, hydrate, hydroxide and UBC must be taken into account. There may be some differences in the ashes produced in laboratory-, bench-, pilot-, and full-scale boilers. In small scale, the fuel can be more homogeneous than in fullscale, which consumes several truckloads of biomass per hour. On the other hand, the fly ash from the pilot-scale combustors may contain significant amounts of stainless steel corrosion products such as Cr and Ni. At full-scale plants, the biomass fuels are combusted at temperatures about between 850 °C and 1,500 °C depending on the combustion technology, and include processes of evaporation and condensation of compounds of Na, K, Cl, S, P, etc. Evaporation occurs at the high combustion temperatures and condensation upon cooling of the flue gases. At higher combustion temperatures, there is more evaporation of K, Na, Cl, S, P, which condensate on the ashes upon cooling of the flue gases, and hence more salts are present in the fly ash. Fly ashes produced at higher combustion temperatures (PF) differ markedly from those produced at lower combustion temperatures (CFB), not only regarding salt content, but also, regarding reactivity at temperatures above 850 °C. This was demonstrated by TGA/DTA/DSC analyses, which show exothermic reactions with CFBC fly ashes at temperatures above 850 °C. Problematic components regarding exact and repeatable analysis are often B, Cl and F, which require further attention. Additives Avedøre 2 400 MW Wood pellets PF Bituminous coal fly ash Herning 78 MW Wood chips Grate none Pulp&paper mill power plant PF none none none none 87 MW Bark CFB none Pilot-scale unit Fuel input / kW th Biomass fuel Valmet pilot plant VTT pilot plant TUM pilot plant 4000 kW 50 kW 200 kW Bark EFB EFB and coal Bark Torrewashed bark Torrewashed bark Torrewashed straw Straw Bark SE bark SE bark Combustion system CFB CFB PF Additives none none none none none Sulphur Kaolin Kaolin none Kaolin Coal fly ash With increasing combustion temperature, more Cr(VI) is anticipated. This is critical, since Cr(VI) is known to be carcinogenic and toxic. Consequently, more work on the aspect of Cr(VI) in ash is required. By water-leaching of the biomass fuel i.e. in pre-treatment, fly ashes will contain less Na, K, Cl and S, and hence less salts will be present in the fly ashes. 2.4.2 Novel applications for biomass ash In the course of the project existing valorisation options for biomass ashes such as use as fertiliser, filler materials or construction materials were identified. However, many of these applications can be considered as “creative” land filling, at least by authorities and NGOs. Some of these applications are not specific for biomass ash. New potential applications were identified that exlude applications including land filling and applications that are not specific for biomass ash. –– Leaching, with recovery of valuable elements and compounds, especially P and K –– Application in traditional ceramics and traditional glass –– Application in geo-polymers –– Recycling elements back to the soil, for limiting depletion –– Application as or in fertiliser –– Application in calcium silicate bricks In the project these proposed valorisation options were evaluated. Due to the new EU fertiliser regulation 2019, the application of biomass ashes in top-soils seems to be restricted. Biomass ashes will most probably exceed limits for allowed concentrations of toxic heavy metals. On the other hand, the use of biomass ash in construction materials seems promising, especially in calcium silicate blocks and geo-polymers. To achieve a higher added value in this field of application, biomass ash should be used as binder rather than as filler material. It could potentially replace limestone, which is currently mined and calcined with high fossil CO 2 emissions. Fly ash from wood and bark were tested in these novel applications during the Biofficiency project. It was demonstrated that ashes from biomass fuels can be used in construction materials, like fired bricks, calcium silicate blocks and geo-polymers (see F i g u r e 6 ). Calcium silicate materials and geo-polymers with high compressive strengths of more than 50 MPa, even more than 100 MPa, were obtained. Additionally also the feasibility of the use of biomass ash in top soils was investigated Fig. 6. Fired bricks made from two different Dutch clays (Maas and Waal), and with 0, 5 and 25 wt.-% wood fly ash from a PF boiler. 68
VGB PowerTech 7 l 2020 The Biofficiency Project | Part 1 by leaching experiments. It was observed, that leaching of some elements depends on the pH-value and/or the presence of carbonate. While some nutrients such as potassium or sodium leached well, others like Cu, Zn and P were not leached, which indicates that these elements are not available for plants, at least on the short term. Besides, the amount of heavy metals being leached in relation to leached potassium, which must be considered when potassium is recovered as fertiliser, was found to be too high. Pre-treatment or the application of additive to the combustion process also influence the quality and the quantity of the biomass ashes obtained. For some applications of the biomass ashes, this can be very beneficial. For example, the use of aluminum silicate as combustion additive increases the amount of fly ash and should be beneficial for application in geo-polymers. 3. Conclusion Three prominent pre-treatment technologies for solid fuel conversion were investigated in the Biofficiency project: torrefaction, steam explosion and hydrothermal carbonisation. The project confirmed that all upgrading techniques improve heating values, hydrophobicity, grindability, resistance to biological deterioration and decrease corrosion potential. The fate of inorganic elements was investigated, showing that HTC and torrefaction combined with washing can significantly lower chlorine and alkaline levels in the treated fuels. Thereupon pre-treatment enables the use of difficult previously unused feedstock as biofuels. To produce market-competitive fuels by pre-treatment, either CO 2 credits should be awarded to pre-treated fuels or feedstocks with a gate fee should be treated. With experiments in three different pulverised fuel combustion test rigs, the project confirmed that the use of the additives kaolin and coal fly ash reduces the fine particle concentration in the flue gas significantly. Reduced amount and changed chemical composition of fine particles decreases the risk of deposition formation. When using additives the deposits contained significantly less chlorine, decreasing consequently the corrosion potential and abating fast deactivation of denox catalysts due to potassium poisoning. The use of additives during biomass combustion enables the use of difficult previously unused feedstock as biofuels. In FB combustion, new insights to combustion behaviour of challenging fuels and their implications on ash-related challenges were gained. Washing and torrefaction pretreatment can upgrade biomass characteristics and ease the availability issues. However, the benefits depend on the fuel. The dosage of kaolin can be decreased by washing straw from ~9 to ~3 wt.-% from dry fuel mass flow. The upgrade of fuel by washing will decrease additive costs by 60 %. The share of challenging biomass can also be increased via co-firing with coal. Steam temperatures ober 600 o C with challenging biomass require better materials than exist today. This would need superheater material R&D but furthermore improved turbine technology to enable over 600 °C steam values. Chemical analysis of the wide variety of biomass ash produced from fluidised bed, grate and pulverised fuel fired boilers was carried out. The properties of biomass ash were found to be dependent on fuel composition, combustion technology and the use of additives. Consequently, also the valorisation option for ashes are affected. Different ways of valorising biomass ash were tested, including application as a fertiliser or as an alternative binder or filler material in concrete and bricks. The results are very promising, especially for applications in the construction material industry. Acknowledgement This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 727616. All project outputs are available free of charge on the EU Commission’s CORDIS database as deliverables. The authors would like to thank project partners Liisa Clemens (Mitsubishi Hitachi Power Systems Europe), Pedro Abelha (Netherlands Organisation for applied scientific research TNO), Hanna Kinnunen (Valmet), Patrik Yrjas (Åbo Akademi), Flemming Frandsen (Technical University of Denmark), Frans van Dijen (Engie), Katariina Kemppainen (Metsä Fibre), Despina Magiri-Skouloudi (National Technical University of Athens) and Bo Sander (Ørsted) for contributing immensely to the results that have been summarised in this article. 4. Abbreviations and Acronyms CFB Circulating Fluidised Bed CHP Combined Heat and Power DSC Differential Scanning Calorimetry DTA Differential Thermal Analysis FB Fluidised Bed HTC Hydrothermal Carbonisation NGO Non-Governmental Organisation PF Pulverised Fuel SE Steam Explosion TGA Thermogravimetric Analysis Torr Torrefaction UBC Unburned Carbon XRF X-Ray Flourescence 5. References [1] Lukas Sulzbacher JR (2011) Heating and cooling with biomass – Summary report – D6.1: EUBIONET III: 49 p. [2] (2019) Brief on biomass for energy in the European Union. [Publications Office of the European Union], [Luxembourg]. [3] Jori Sihvonen SE (2016) How much sustainable biomass does Europe have in 2030? https://www.transportenvironment.org/ publications/how-much-sustainable-biomass-does-europe-have-2030. [4] Hupa M., Karlström O., Vainio E. (2017) Biomass combustion technology development – It is all about chemical details. Proceedings of the Combustion Institute 36(1): 113–134. doi: 10.1016/j.proci.2016.06.152. [5] Jenkins B., Baxter L.L., Miles Jr. TR et al. (1998) Combustion properties of biomass. Fuel processing technology 54(1-3): 17-46. [6] (2005) Torrefaction for biomass upgrading. [7] Joronen T., Björklund P., Bolhàr-Nordenkampf M High quality fuel by steam explosion. Proceeding from the European Biomass Conferance, 14-18th of May 2017. [8] Funke A., Ziegler F. (2010) Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels, Bioprod. Bioref. 4(2): 160-177. doi: 10.1002/bbb.198. [9] Spliethoff H. (2010) Power Generation from Solid Fuels, 1. Aufl. Power Systems. Springer-Verlag, s.l. [10] James A., Thring R., Helle S. et al. (2012) Ash Management Review – Applications of Biomass Bottom Ash. Energies 5(10): 3856-3873. doi: 10.3390/en5103856. [11] Thrän D., Billig E., Brosowski A. et al. (2018) Bioenergy Carriers – From Smoothly Treated Biomass towards Solid and Gaseous Biofuels. Chemie Ingenieur Technik 90 (1-2): 68–84. doi: 10.1002/ cite.201700083. l FIND & GET FOUND! POWERJOBS.VGB.ORG ONLINE–SHOP | WWW.VGB.ORG/SHOP 69
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