Flexible Hybrid Process for Combined Production of Heat, Power and Renewable Feedstock for Refineries
Flexible Hybrid Process for Combined Production of Heat, Power and Renewable Feedstock for Refineries
Managing seasonal energy supply and demand for heat and power in Europe
A flexible combined heat, power and fuel production concept, FlexCHX, is being developed for managing the seasonal mismatch between solar energy supply and the demand for heat and power characteristic of northern and central Europe. The process produces an intermediate energy carrier (Fischer-Tropsch (FT) hydrocarbon product), which can be refined to transportation fuels using existing refineries. The FlexCHX process can be integrated into various combined heat and power (CHP) production systems, both industrial CHPs and communal district heating units. In the summer season, renewable fuels are produced from biomass and hydrogen; the hydrogen is produced from water via electrolysis that is driven by low-cost excess electricity from the grid. In the dark winter season, the plant is operated only with biomass in order to maximise the production of the much-needed heat, electricity and FT hydrocarbons. Most of the invested plant components are in full use throughout the year with only the electrolysis unit being operated seasonally. The catalytic reformer plays a key role in this process by converting tars and light hydrocarbon gases into synthesis gas (syngas) and by bringing the main gas constituents towards equilibrium. Developmental precious metal catalysts were used, and an optimal reformer concept was established and tested at pilot scale. Reforming results obtained at pilot gasification tests with commercial nickel catalysts and with the developed precious metal catalysts are presented.
CHP production technologies using various wood residues and agricultural biomasses are commercially available at different sizes (1, 2). The state-of-the-art CHP technologies have been under severe financial stress in changing European markets characterised by a rapid addition of variable renewable energy (VRE) capacity and stagnating electricity demand (3). Consequently, many new thermal generators are currently designed to produce only hot water for heating purposes instead of CHP (4, 5). As a result, there is a clear need for new flexible district heating and CHP production solutions in Europe that can maintain economic feasibility under increasing VRE penetration.
On the other hand, advanced transportation biofuels have been the focus of intensive development since early 2000, but industrial deployment has been postponed (6–8). One fundamental reason for this is the attempt to reach satisfactory economics by exploiting economies of scale; which leads to extremely large-scale plant concepts (>300 MW) that are eventually deemed too risky by the investors. Large-scale plants also suffer from incomplete utilisation of byproduct heat, as it is difficult to find such large heat consumers who could effectively exploit the heat supply. Thus, the biomass utilisation efficiency of stand-alone plants rarely exceeds 55% lower heating value (LHV) even with the best available technologies (8).
In response to the growing share of solar and wind power in the energy systems and consequent need for converting surplus electricity into storable form, power-to-gas (P2G) and power-to-liquids (P2L) concepts have recently been suggested for managing the temporal mismatch between solar energy supply and heat and power demand (10). However, the simple P2G and P2L concepts producing, for example, synthetic natural gas (SNG) or methanol from excess electricity and CO2 suffer from poor round-trip efficiency (typically <40%) when the final product after storage is once again converted to electricity (11). In addition, annual operation times for these plants, including fuel synthesis, are low especially in the northern European countries, typically only ca. 2000 h year−1. As a solution to this problem hybrid systems have been suggested, where electrolysis technology is used to boost the biomass gasification and chemical synthesis plant (12, 13).
This paper deals with the experimental development activities related to one promising hybrid production concept, FlexCHX, illustrated in Figure 1 (14). This process produces heat, power and an intermediate energy carrier, FT wax, which can be refined to transportation fuels using existing oil refining equipment. In the summer, renewable fuels are produced from biomass and hydrogen; the hydrogen is produced from water via electrolysis that is driven by low-cost excess electricity from the grid. During the dark, winter season, the plant is operated with just biomass in order to maximise the production of much-needed heat, electricity and FT wax.
In principal, the FlexCHX process can be realised at large scale using pressurised fluidised-bed gasification developed in Finland (14–16) or at smaller size range using the new staged fixed bed (SBX) gasifier developed in an ongoing European Union (EU) Horizon 2020 project (17, 18). In both gasification systems, the feedstock is gasified at moderate temperatures to generate a tar-containing raw gas, which is filtered at high temperature, and led to the catalytic reformer, where tars and hydrocarbon gases are reformed and the yields of hydrogen and carbon monoxide are increased. After final cleaning, the gas can be led into chemical synthesis units producing intermediate products, which can be refined to transportation fuels or chemicals using large-scale industrial units.
One of the most significant issues in syngas purification is the fate of light hydrocarbons (mainly methane, C2 hydrocarbons and benzene) and ‘tars’ produced during gasification. These hydrocarbons can constitute a significant proportion of the overall carbon content of the gas, and their conversion to syngas is therefore essential to improve syngas yield, as well as prevent poisoning of FT catalyst and plant fouling downstream of the gasifier. Technologies already exist for trapping tars at low temperatures such as water scrubbing and solvent wash, although these techniques will not address lighter hydrocarbons, and some tars with low dew points. A more elegant solution, utilised in the FlexCHX process, is the high temperature reforming of hydrocarbons to syngas immediately downstream from the gasifier.
In the FlexCHX project, previous knowhow of VTT, Finland, on catalytic reforming technology is combined with the catalyst knowhow of Johnson Matthey, UK. VTT’s technology with combinations of zirconia, noble metal and nickel catalysts has already been demonstrated in fluidised-bed gasification applications aiming to synfuel applications (15, 21). In FlexCHX project, this existing expertise is used to design an optimised reformer reactor for the raw gas of the pressurised SXB gasifier. The primary reduction of tar content already in the gasifier and on the filter cake together with new impurity tolerant catalysts developed by Johnson Matthey in the project will offer new, more efficient and robust design alternatives for the process. The platinum group metal (pgm) catalysts provide a potential for improved conversion efficiency to be achieved already at lower temperatures.
In addition to the reforming of tars and hydrocarbon gases, the catalytic reformer has another key role in the FlexCHX process in bringing the main gas components (carbon monoxide, water, CO2 and hydrogen) towards equilibrium. The H2:CO molar ratio should be close to two in the FT synthesis, while it is considerably lower in the raw feed or gas entering the reformer. During the heating season, the gasifier is operated with a mixture of steam and oxygen as gasification agents and the raw gas contains still typically 40% steam. This steam is then further consumed by reforming reactions and the main gas components approach the equilibrium of water gas shift reaction (Equation (i)):
The target molar ratio of H2:CO can be achieved by controlling the steam feed of the reformer. During summer season, less steam is used while CO2 is separated from the syngas and recycled back to the gasification process. In this case, the CO2 is consumed by reforming reactions and the equilibrium of the water gas shift reaction is pushed towards high carbon monoxide contents. This makes room for the use of additional electrolysis hydrogen. This concept could not be realised without the catalytic reformer that on one hand consumes steam and CO2 and on the other hand catalyses water gas shift reaction.
This paper is focused on the pilot scale development of catalytic reformer as part of the FlexCHX process. Results from four pilot test campaigns are presented and discussed.
2.1 Gasification Pilot Plant
The schematic process diagram of the gasification pilot plant is shown in Figure 2. Biomass is gasified in a SXB gasifier, where biomass feedstocks are fed to the top of stage one and a fixed-bed is created from the biomass charcoal and ash at the bottom of the reactor. Primary gasification agents, mixtures of air, oxygen, steam and CO2, are fed through a distributor system to the bottom of the bed, where oxidation and gasification reactions take place in a similar manner as in commercial updraft gasifiers (19, 20). The gasification and pyrolysis gases produced in stage one flow to the second stage of the gasifier, where secondary gasification gases are introduced through a specially designed catalytic system. Major part of tars and light hydrocarbon gases are decomposed in the second stage and the gas temperature is raised from 300–500°C to the target outlet temperature of 750–900°C.
After leaving the gasifier, the raw gas is led via the first gas cooler into the filter unit and the filtered gas is then reformed in a two-stage catalytic reformer. Finally, the gas is cooled to 200–400°C and the pressure is reduced close to ambient by the pressure control valve. Produced gas is led to the boiler, which is connected to the district heating network of Espoo, Finland. A slipstream of the gas is taken after the pressure letdown valve for the bench-scale ultra-cleaning unit and FT called the mobile synthesis unit (MOBSU).
The catalytic reformer has two stages, both of which are realised with fixed beds filled with granular catalyst material. The reformer is operated autothermally, and the required heat for the endothermic reforming reactions is provided by oxidation reactions. Mixtures of oxygen, nitrogen and CO2 are fed to both reformer stages. This staged reforming of filtered gasification gas has been previously developed and tested at VTT for fluidised-bed gasifier systems (21). The reformer was constructed with internal reactor made of heat resistant steel mounted inside a refractory lined pressure vessel. The inner reactor had electrical heaters to compensate heat losses and to assist in preheating.
In the pilot test campaigns, two different catalyst loadings were tested as shown in Figure 3. In the first campaign the reformer was loaded with a new batch of the same commercial nickel catalysts as were used in the previous fluidised-bed tests (15, 16). Whole bed volumes were filled with the same material (marked as A1). In the second test campaign, precious metal development catalysts of Johnson Matthey were used in combination with nickel catalysts. In the first bed commercial and robust nickel catalyst A2 from Johnson Matthey was used as guard bed in front of catalyst B, which was developed for tar reforming. The second bed was loaded with the previously used commercial nickel catalyst (A1) followed by precious metal catalyst C developed specially for methane reforming. Robust nickel catalysts were used in first layers, where added oxygen reacted with gas components resulting in locally high temperatures. The applied method of feeding oxygen with high velocity towards the catalyst beds, so that it does not have time to react in the empty gas space before the catalyst beds, is considered to play a key role in avoiding soot formation especially in the first bed as described in (22). The principal aim of this staged reformer concept is that most of the high-molecular-weight tars and part of C2 hydrocarbon gases are reformed already in the first bed, whereas the second bed is used for reforming of benzene and methane as well as for finalising the tar conversion.
In the SXB pilot plant final gas cleaning of reduced sulfur compounds, trace halides, nitrogen species such as ammonia and hydrogen cyanide as well as residual tars and benzene, is realised using the slip stream gas cleaning unit decribed in (26). The process is divided into the operations involving purification and the compression steps.
The pilot plant has two main sampling points for collecting sample gas for online gas analysers, micro gas chromatography for sampling of tars and nitrogen and sulfur species. The first sampling point is located after the filter, the second after the pressure let down valve. The applied analytical methods are described in a detail in (23–26).
2.2 Reformer Catalysts
2.3 Gasifier Feedstocks
Table II presents the averaged results for the proximate and ultimate analyses of the feedstocks used in the SXB test campaigns. Four pilot test campaigns were realised using different wood-based and agricultural derived feedstocks. Photographs of the feedstocks are presented in Figure 5.
3. Results and Discussion
Four test campaigns summarised in Table III were realised at the SXB pilot plant by the end of March 2020. The pilot plant was operated continuously without any interruptions in these test runs. Each test was started by preheating the plant at first in hot air and then by combusting wood in the gasifier reactor. This took 24–30 h, before the gasifier was switched from combustion to gasification mode. The flue gases from preheating periods were also led through the filter and reformer, which were gradually heated up as well. When the gasifier was turned from combustion to gasification, feeding of oxygen and nitrogen mixture to both beds was started and the catalyst beds were gradually heated to the target operation temperatures. Measurements were carried out in 3–20 h long periods, during which the mass flow rates of input streams were kept as constant as possible. Elemental mass balances and performance indicators of the reformer were calculated for the set point periods based on average measuring results.
One typical challenge of tar reforming has been soot formation (23), which over time decrease the catalyst activity and increase the pressure drop so that finally the reformer must be oxidised to remove the soot. In these pilot tests, the pressure drop of the reformer stayed constant at all set points reflecting just the changes in gas flow rate and operation temperature. As an example of the variation, Figure 6 shows the measured pressure drop in test run 20/11. Evidently, the applied pre-reforming taking place in the second stage of the gasifier together with the applied method of oxygen feeding into the reformer prevented soot formation reactions from taking place.
The main measured results and calculated performance figures for selected four set points are presented in Table IV. Set points SXB 19/34B and 19/34E are tests where the reformer was loaded with nickel catalysts and set points SXB 20/11A and 20/11D are for the second reformer loading including the development catalysts B and C. Two of the set points (19/34B, 20/11D) represent operation with clean biomass with very low sulfur content and two set points (19/34E, 20/11A) are for bark gasification. The hydrogen sulfide contents of gas were typically ca. 20–30 ppm and 100 ppm respectively.
The tar concentrations measured after the filter unit were in the range 6.0–6.1 g m−3 with clean wood and 7.1–8.2 g m−3 for bark gasification. The benzene content varied in the range 9.7–13.1 g m−3 at these set points. Volumetric concentrations and volume flow rates in this article are presented in the conditions of standard temperature and pressure (STP) defined as 273.15 K and 101.325 kPa. Finally, methane and C2–C5 hydrocarbon concentrations in the reformer inlet were in the range 6.3–7.1 vol% and 1.2–1.5 vol% respectively. The C2–C5 hydrocarbons consisted mainly ethylene and ethane, which represented over 95% of these light hydrocarbon gases. These inlet concentrations of hydrocarbon gases and tars are of the same order of magnitude as determined for pressurised steam-oxygen blown fluidised-bed gasification of same feedstocks (15, 27). Consequently, similar hot gas filtration and catalytic reforming solutions can be applied for both gasifier types and the reforming results obtained in this study are applicable for fluidised-bed gasifiers as well. At these set points, the raw gas was cooled before filtration to 500–600°C in order to remove alkali-metal vapours during filtration as described in (28, 29). With wood residues, this gasification process could also be realised without cooling the gas before filtration, as is assumed in the process evaluation studies presented in (17).
In the first reformer stage, the gas temperature was raised by partial combustion reactions from 512–536°C to the maximum-targeted temperature, usually in the range 930–1000°C. The oxygen feed rate was controlled to ensure a reasonable temperature variation range at each target set point. Once endothermic steam and CO2 reforming reactions occurred, the temperature decreased. The oxygen feed used in the first reformer stage was of the same order of magnitude in 2019 tests with nickel catalysts and in the 2020 test with the development catalysts.
The inlet temperature of the second reformer stage varied at these set points in a narrow range of 748–779°C. The second stage was also operated by controlling the oxygen feed so that the maximum temperature occurring close to the top of the catalyst bed was kept in the target range. As before, the temperature decreased as a result of reforming of remaining tars and part of methane occurred. The higher efficiency of the first reformer stage in 2020 tests can be seen indirectly from higher maximum temperatures and from the fact that clearly lower oxygen feed rates were needed in the second stage than were needed with the nickel beds. The higher efficiency of the second catalyst stage is clearly linked to the lower outlet methane contents as well as in 120–150°C lower gas outlet temperature.
The space velocities for both reformer stages are calculated for the whole catalyst bed volumes including both the nickel and precious metal sections in the 2020 tests. In the 2020 test, the share of nickel and precious metal catalyst in the first stage were 10% and 90% and in the second stage 40% and 60%. The gas hourly space velocity (GHSV) is given in Table IV both in actual average temperature and pressure and in standard STP (273.15 K, 101.325 kPa). The molar volume of ideal gas (22.41 dm3 mol−1) is used in converting molar flows to volume flows. In the first stage, the gas volume flow is calculated as a sum of inlet raw gas flow and flow rate of added oxygen and nitrogen. The final gas flow after the reformer is used in calculating the GHSV for the second catalyst bed.
The achieved conversion efficiencies were calculated based on average measuring results and mass balances for each set point. The results are shown in Figures 7 and 8. Methane conversion with the nickel catalyst in 2019 tests was close to 50% with both clean wood and bark. In the 2020 tests with the development catalysts, clearly higher methane conversions were achieved: 75.6% with clean wood and 66.1% with bark. The conversion of C2–C5 hydrocarbon gases was complete in the 2020 tests, while with the nickel catalyst used in 2019 tests the reformed gas contained 0.05–0.1% of C2 hydrocarbon gases despite higher reformer outlet temperature. Tar conversions with the nickel catalyst were 99.3% with clean wood and 97.4% with bark. In the 2020 tests, tar conversions were complete and only some trace concentrations of 2.7 mg m−3 could be found from the reformed gas. No significant differences could be found in benzene conversions determined with the nickel catalyst and with the development pgm catalysts. This may be due to the higher operating temperature of the second reformer stage in the 2019 experiments. It can be expected that benzene conversions can be increased by increasing the operation temperature of the second reformer stage.
One of the challenges of gas cooling, compression and final cleaning is the deposition problems caused by heavy tars. The conversion of heavy tars was almost complete at all set points but again the results obtained with the development catalyst in 2020 were even better as can be seen in Figure 9, which shows the measured inlet and outlet concentrations of tar components which have higher molecular weight than naphthalene.
The increase in sulfur content of the raw gas, when changing from clean wood to bark, had already clear effects on methane and benzene conversions with both tested catalyst set ups. According to our previous experiences with nickel catalysts (15, 30) this deactivation is reversible at least in the concentration range of 10–150 ppm typical to gasification of clean wood, bark and forest residues. All conversion efficiencies were further reduced significantly, when the feedstock was changed from bark to sunflower husk, which contained roughly five times more sulfur than bark.
In general, the developed catalytic reformer technology is able to convert tars and hydrocarbon gases into syngas and to bring the gas composition towards equilibrium of the water gas shift reaction. Thus, the reformer seems to fulfil both key roles it has in the hybrid biofuel production concept FlexCHX. The residual tar contents are almost negligible, which makes it possible to remove them by activated carbon simultaneously with bulk sulfur removal in the first guard bed. High methane conversion and low outlet temperature achieved with the development pgm catalyst is beneficial for the overall process efficiency and makes it possible to design concepts where the tail gases of FT synthesis are recycled back to the gasification process.
Stable reformer operation with no signs of soot formation could be achieved with both reformer loadings tested in the 2019 and 2020 test runs. Evidently, the overall method of tar control applied in the SXB gasification process has been successful. The very high primary tar content typical to updraft gasifiers is reduced in the secondary gasification zone to similar levels as achieved in fluidised-bed gasifiers. Dust particles are efficiently removed by metal filters, which makes it possible to apply fixed-bed reformer designs. The resulting raw gas could then be efficiently reformed in the two-stage fixed bed reformer.
When the reformer was operated in a way that the maximum temperature in both beds was kept stable by controlling the oxygen feed rate, the difference in the activity of reformer catalysts could be seen in the gas outlet temperatures. With the more active pgm catalysts used in 2020 tests, the outlet temperature from the reformer was 120–150°C lower than in the 2019 test runs. Higher tar conversions could be achieved by the development catalysts already with lower outlet temperature than were achieved with the commercial nickel catalysts.
Methane conversions with the nickel catalysts used in 2019 tests were of the order of 40%, while with the development catalysts used in 2020 methane conversions were in the range 71–87% with clean wood and 48–68% with bark. The major part of C2–C5 hydrocarbon gases were reformed in all test runs. The reformer performance with woody biomass was very good, while in the test runs with high-sulfur sunflower husk; the conversions were lower than targeted. The solution could be higher operation temperatures or lower space velocities for feedstocks having high sulfur contents.
Ammonia decomposition was rather modest in these test runs. Evidently, the operation temperatures were too low, and the catalyst selection was not efficient for ammonia decomposition. In future tests, the effect of increased temperature will be studied. Alternatively, the reformer can be designed to be realised with three beds, where the final bed would be dedicated for ammonia decomposition. Alternatively, ammonia can be scrubbed from the syngas by simple acid scrubbing but if the concentration is high, the acid consumption becomes large.
The next steps in the project are to utilise these experimental results to help make conceptual design for an industrial scale FlexCHX plant. The design will incorporate a full technoeconomic assessment for this hybrid production concept of biofuels and heat. Further pilot gasification tests will also be carried out in order to optimise the catalyst loadings and the performance of the catalysts under different conditions is studied with simulated gases in the laboratory.
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FlexCHX project has received funding from the EU’s Horizon 2020 research and innovation programme under Grant Agreement No. 763919.
FlexCHX is an EU Horizon 2020 project, which develops a flexible and integrated hybrid process combining electrolysis of water with gasification of biomass and catalytic liquefaction. FlexCHX is a three-year project (2018–2021) with almost €4.5 million in EU funding and a consortium of 10 partners.
The project consortium comprises 10 entities from four different EU countries: three research organisations: VTT (Finland), Lithuanian Energy Institute (Lithuania) and DLR (Germany); five industry participants: Enerstena (Lithuania), Johnson Matthey (UK), Neste Engineering Solutions (Finland), Kauno energija (Lithuania) and Helen Ltd (Finland); and two small and medium-sized enterprises (SMEs): INERATEC GmbH (Germany) and Grönmark (Finland). The project is coordinated by VTT. The consortium of the FlexCHX project combines chemical engineering, power plant technologies, construction and engineering knowledge as well as business understanding.
FlexCHX LINK http://www.flexchx.eu/
Esa Kurkela is a senior principal scientist, Principal Investigator MSc (Tech), at VTT Technical Research Centre of Finland. He has 40 years of professional experience in the gasification technologies. He specialises in gasification of biomass, waste, peat and coal; fluidised-bed and fixed-bed gasification; hot gas cleanup and new high-efficiency power production and fuel synthesis systems. He has been the coordinator of several EU and national projects and will be the Coordinator of the FlexCHX project.
Minna Kurkela MSc (Tech) is a Senior Research Scientist. She has over 25 years of experience in gasification and gas cleaning research and development and black liquor pyrolysis research at VTT Technical Research Centre of Finland. She works with project management and practical implementation of scientific research.
Christian Frilund works as a research scientist at VTT Technical Research Centre of Finland. He has over five years of experience on catalytic gas cleaning and gas treatment.
Ilkka Hiltunen MSc (Tech), is a research Team Leader of the Thermochemical Conversions Research Team at VTT Technical Research Centre of Finland. He has over 15 years of experience on gasification technologies and their different applications. He has excellent experience on industrial cooperation projects and pilot-scale activities.
Ben Rollins is a Research Scientist who has been working at Johnson Matthey since 2016. The primary focus of his research has been on heterogeneous catalysis of light hydrocarbon reactions including steam reforming and the oxidative coupling of methane. Through this work he has collaborated on several successful EU projects in the Horizon 2020 and LIFE programmes.
Andrew Steele is a Principal Scientist. He has been working at Johnson Matthey since 2001. His background is in catalyst science and reactor engineering; with a focus on utilisation of renewable energy. He has extensive experience in project management for several EU projects. He was a laureate for the 2006 Descartes Prize for Collaborative Scientific Research for Hydrosol 1.