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 Densified products Washing Therefore, pre-treatment steps are applied in order to upgrade the fuel quality and facilitate an energetic use of these difficult biomass feedstock with the highest possible efficiencies. Biomass raw materials and residues Preparation (e.g. milling, crushing) Torrefaction Thermo-chemical conversion HydrothermaI carbonisation Compaction (pelletisation, briquetting) Torrefied products Advanced biofuels Hydrothermal products Steam explosion Steam exploded products Fig. 2. Conversion routes from raw biomass to bioenergy carrier. Adapted from [11]. 2.2 Biomass pre-treatment The majority of available biomass feedstock for bioenergy has properties not desired in a fuel such as low energy density, high moisture content, poor grindability, and high concentrations in alkaline and chlorine that lead to corrosion and deposit issues in biomass furnaces. To overcome these issues, upgrading problematic feedstock with the help of pre-treatment technologies are investigated in the Biofficiency project. F i g u r e 2 shows the general conversion pathway from a biomass raw material to an upgraded bioenergy carrier. Bioenergy carriers can be processed via different conversion routes. Most commonly, the collected biomass undergoes a preparation step (i.e. milling or crushing) before being compacted to pellets or briquettes. However, the feedstock can also undergo a thermochemical conversion or bio-chemical conversion that aims to ameliorate fuel properties. In the Biofficiency project three pre-treatment technologies have been investigated: Torrefaction (Torr), hydrothermal carbonisation (HTC) and steam explosion (SE). In torrefaction, the biomass is heated in absences of oxygen at temperatures around 200-320 °C. The structure of the biomass is changed in such a way that the material becomes more brittle and hydrophobic. In addition, the carbon content of the torrefied biomass is increased leading to a higher energy density of the material [6]. To extract problematic ash components from the feedstock, a washing step can be conducted prior or after the torrefaction treatment. A pre-treatment by steam explosion involves process steaming of biomass at elevated pressure (1 to 20 bar) and temperature (160-280 °C) followed by a release of the hot and softened biomass to a lower pressure [7]. The expanding steam partly breaks the structure of the biomass, which is origin of the wording “steam explosion”. Steam exploded biomass material is durable, water resistant, easy to grind and has a higher energy density compared to the raw material. In HTC, biomass is suspended in water and heated to temperatures around 180-300 °C. Pressure is applied to keep water in the liquid phase. During the treatment, the material undergoes a similar transformation as during natural coalification, only much faster [8]. Similarly to torrefaction, this transformation yields a solid that has a higher energy density, is more brittle and more hydrophobic. In comparison to other thermal treatment methods, HTC allows direct conversion of wet biomass without any pre-drying of the feedstock. Another advantage arises from a pre-treatment in water: Species active in corrosion, slagging and fouling such as chlorine and alkali metals are often present as water-soluble compounds that can be removed with the process water. A comparison overview of the three different pre-treatment technologies is presented in Ta b l e 2 . During Biofficiency, over 10 different biomass feedstocks from all classes presented in Ta b l e 1 were pretreated by torrefaction, washing and torrefaction, hydrothermal carbonisation and steam explosion. The behaviour of inorganic components during pre-treatment could be determined by various experiments in lab- and pilotscale. Washing followed by torrefaction removes about 90-95 % of Cl, 50‐80 % K, 30‐60 % S and 30 % P, from the low-grade biomasses. Post-washing (washing after torrefaction) seems to be a viable route to upgrade dry-type biomasses. It was shown, that equal salts extraction efficiencies were achieved as for pre-wash, at decreased energy costs. However, for a combination of torrefaction and washing, wastewater must be treated (e.g. by anaerobic digestion) at a cost. HTC removes about 20-90 % of Cl, 60-95 % K, 40‐85 % S and 20-60 % P, from the lowgrade biomasses during upgrading. The technology is especially suited for wet-type biomasses. Currently, the technology still suffers from large amounts of wastewater generated and large associated costs for wastewater treatment attached. Per ton of produced HTC biomass, approximately 2 m 3 of waste water are generated. Different Tab. 2. Overview over the three pre-treatment methods explored during Biofficiency. Method Torrefaction Steam explosion Hydrothermal carbonisation Short description Process conditions Impact on fuel quality Impact on ash properties Thermal treatment in oxygendeficient atmosphere T = 200-320 °C p = atmospheric T ≈ minutes Steaming of biomass at high pressures, followed by explosive decompression T = 160-280 °C p = up to 20 bar T ≈ seconds to minutes Thermal treatment in water at elevated temperatures and pressures T = 180-300 °C p = 20-100 bar T ≈ minutes to hours Energy densification, increased hydrophobicity, increased grindability, facilitated pelletisation, better storability Torrefaction: • increased ash content • no compositional changes Torrefaction+Washing: • decreased alkali and chlorine content • higher ash melting temperatures • no compositional changes in ash • slightly decreased ash melting temperature • decrease of alkali and chlorine content • yields high Si ashes • higher ash melting temperature 64
VGB PowerTech 7 l 2020 The Biofficiency Project | Part 1 Tab. 3. Overview of the main systems employed in biomass combustion. Combustion Feedstock Live steam temperature Grate firing Fluidised bed firing (CFB/ BFB) Pulverised fuel firing Solid biomass, e.g. wood chips, agricultural waste, straw, waste wood, Fuel moisture content between 15 and 65 %, fuel size 50 mm and more (no grinding necessary) Solid biomass, e.g. wood chips, biomass pellets, agricultural and forestry residues such as straw, Fuel moisture content up to 60 %, fuel size 3-150 mm -EN ISO 17225-1 P100 (no grinding needed) Grindable fuels, e.g. wood/biomass pellets, fuel moisture content up to 30 % (roller mills) or up to 70 % (hammer mills with hot flue gas recirculation for drying in the mill), fuel size after grinding < 1 mm (roller mill) or < 2 mm (hammer mill) idcas for the further utilisation and disposal of HTC process water exist, i.e. anaerobic digestion, application as fertiliser, ultrafiltration or wet oxidation. All these technologies for process water treatment are still under development. The Biofficiency project has taken first steps towards a thorough characterisation of process water from HTC. Steam explosion was found to be well suited to treat woody and straw like biomasses. Yet, no significant changes in the mineral contents during upgrading were observed after a steam explosion treatment. Consequently, the technologies advantage lies in the energy densification and improved physical properties of the produced material. A washing procedure after SE would be considerable part of the process, when improving biomass quality and elemental composition of ash is a major topic. Overall, all upgrading techniques improved heating values, hydrophobicity, grindability, resistance to biological deterioration and decreased corrosion potential. Additionally the Biofficiency project contributed to enhance market readiness of pre-treatment technologies. Mass- and energy balances for the pre-treatment technologies were performed, efficiencies mass- and energy yield were determined. It was found, that for a cost efficient production of biofuels mostly feedstock that come with a gate-fee or CO 2 credits should be considered as well as feedstocks were the Up to 540 °C Up to 600 °C Up to 600 °C Plant sizes Up to 150 MW th Up to 800 MW el 50-1,100 MW el presented pre-treatment technologies offer decreased disposal costs for a waste stream. During the Biofficiency project, the first industrial-scale steam explosion facility for black pellet production using Valmet’s steam explosion biomass pre-treatment plant was ordered and will be built in France. Once commissioned, its success will allow the global roll-out of a new biofuel industrial sector in France, Europe and the rest of the world. Additionally, also washing and torrefaction was demonstrated successfully on pilot-scale and is ready for industrial scale-up. Furthermore, the Biofficiency project showed that the use of difficult, previously unused feedstock can be enabled by pretreatment. Demonstration experiments confirmed that steam-exploded spruce bark can be used in large scale CFB and PF boilers. Washed and torrefied straw combustion in fluidised bed and pulverised fuel furnaces showed promising results with less additives needed compared to raw materials. HTC leaf litter showed promising results with no additives needed in labscale combustion tests. 2.3 Biomass combustion technologies for electricity generation 2.3.1 Overview of biomass firing systems When it comes to the combustion of solid biomass, three technologies have emerged in the past decades: Grate, fluidised bed and pulverised fuel firing. A schematic comparison of these, with two variations of fluidised bed firing, is shown in F i g u r e 3 . During grate firing, the solid biomass is put on a grate, where it is slowly pushed through the combustion chamber while combustion air is supplied from below the grate. This technology, however, usually delivers lower efficiencies at smaller plant sizes. The design of grate-fired units in turn lead to lower strain on the plant and lower risk of corrosion together with high resiliency to fuel quality fluctuations, making this type of firing system popular with very difficult and inhomogeneous fuels. Fluidised bed combustion units (FB) use a large amount of air to “fluidise” a solid mixture of fuel and bed material on top of grid nozzles. The strong movement inside the bed leads to a good mixing of the fuel, as well as good heat transfer properties. These types of plants are built in larger sizes (up to 800 MW el for coal, currently up to 300 MW el for biomass, increasing) and offer higher efficiencies. Fluidised bed boilers can be separated into two designs based on the fluidisation velocity: bubbling fluidised beds (BFB) and circulating fluidised beds (CFB). Pulverised fuel (PF) combustion units are fired with small pre-milled particles. The fuel gas mixture is injected via nozzles into the combustion chamber and burnt with combustion air. Similar to fluidised bed units, these boilers deliver high steam temperatures and large plant sizes leading to high efficiencies. New plants using this technique with single unit capacities of up to 1,100 MW el are in operation for coal combustion. The two large power plants investigated as part of the Biofficiency project (Avedøre Unit 2 and Studstrup Unit 3) are former PF coal-fired power plants retrofitted to fire biomass. In the Biofficiency project, the two most efficient power plant types (FB and PF) were investigated at different scales. The following chapters provide a brief overview of the results gained in each area. As part of one work package, a reference plant of 300 MW th Flue gas out a) b) c) d) Flue gas out Flue gas out Flue gas + fly ash out Fuel in Air in Ash out Fuel in Air in Fuel in Air in Fuel + air in Fuel + air in Ash out Ash out Bottom ash out Fig. 3. The four available firing systems for the combustion of solid biomass: a) Grate firing, b) bubbling fluidised bed (BFB), c) circulating fluidised bed (CFB), and d) pulverised fuel (PF). 65
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