An overview of the Solid Oxide Fuel Cell (SOFC) operated with biomass, in the residential sector in the UK

An overview of the Solid Oxide Fuel Cell (SOFC) operated with biomass, in the residential sector in the UK

Abstract- The present report involves the studies the integration of two renewables energies for the energy generation. Specifically, the Solid Oxide Fuel Cell (SOFC) technology and the fuel produced from the biomass gasification process. Furthermore, it considers the energy analysis of the hybrid system and its challenges. At the same time, it incorporates the UK regulations associated with it is functioning in the domestic area. Moreover, it includes market projections and applications.

The benefit of using the biomass as fuel is that the feedstock is a renewable source. As a result, it allows declining wastes and greenhouse gas emissions. The comparable way, the positive attributes of the SOFC technology make the integration of biomass-SOFC a clean alternative of energy production due to the cogeneration capacity of the hybrid system. Nevertheless, the challenges of this system are thermal expansion and corrosion phenomenon in the SOFC components, and impurities elements resulting in the gasification process of biomass, that under control situation is possible to apply this technology with a significant performance.

Keywords: Biomass, Solid Oxide Fuel Cell, Hybrid system, Gasification process

1. Introduction

The SOFC-biomass system is composed of two renewables technologies. The first one is the fuel cell, which is considered a clean energy generation method due to low emission of carbon monoxide and the long potential expectancy, between 40000 and 80000 hours [1]. In an equivalent, the biomass is processed to produce bioenergy, which involves converting the biomass feedstock into useful products, such as electricity, heat and liquid fuel by thermo-chemical processes with carbon neutral [2].

The biomass processed by gasification technology converted into gaseous fuel at high temperatures (around 600-1000°C) in a thermal decomposition process. In general, the feedstock used in this technology includes municipal solid waste, agriculture residue, commercial and industrial waste. At the same time, the potential products that can be obtained from bioenergy process by gasification are the stationary power, heat and power station fuel. The detail associated with the biomass energy process described in Figure 1 [3]

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Figure 1: Scheme of the biomass energy process

The operating system of the Solid Oxide Fuel Cell (SOFC) starts with the oxygen ions that travel from the cathode to solid electrolyte (solid non-porous electrolyte, usually YSZ). Here reacts with the hydrogen gas at the anode electrolyte interface producing water and electrons. Then the electrons then travel back to the cathode via an external electrical circuit generating electrical power [4]. The fuel cell process illustrated in Figure 2.

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Figure 2: Operating system of Solid Oxide Fuel Cell [5]

The incorporation of the components of biomass into SOFC involves understanding the electrochemical reaction by integration methane (CH4) and carbon dioxide (CO2) into the fuel cell system. Specifically, the conversion process is the gasification biomass, which generation has higher electrical efficiency than biomass combustion, between 50 to 60% [6]. In most of the cases, the gasification biomass worked with internal combustion engines.

2.1. Technology description

The first scenario of the hybrid system involves the obtaining of the useful fuel for the SOFC. As a result, the process starts with the selection of the biomass feedstock than to the gasification process. From there get the gaseous fuel, which is the final fuel product used on the fuel cell. However, firstly the syngas must be clean from impurities, such as alkalis and particles that can affect the global efficiency of the hybrid system. Finally, the SOFC produce electricity and heat from different processes inside the fuel cell [7]. Specifically, the hybrid process involves the conversion of chemical energy into electrical and heat energy, through the hydrogen/hydrocarbons & oxygen to electricity. In this case, the cell works like a battery but instead of storing the energy is distributed it for use straight away [8]. These stages explained in Figure 3.

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Figure 3: Energy cycle of hybrid system SOFC-Biomass

Usually, the SOFC technology is possible to apply in auxiliary power, electric utility and distributed generation [9]. The efficiency associated with electricity and the global generation is around 40% and 80%, respectively [10]. However, the efficiency in the cogeneration system has a range of 60-85% [11]. Additionally, the power level of SOFC technology is between 0.1-100MW [12].

The integration of biomass into an SOFC process established by the conversion of a combustible mixture of gases produced in a gasifier under a thermodynamic model, shown in Figure 4. In general, the level of impurities components has a direct relationship with the biomass type and the grade of humidity of the waste, such as urban and industrial waste, wood residues, forestry waste and others [8]. The impurities resistance level of the Ni/GDC anodes is higher compared to Ni/YSZ anodes [6]

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Figure 4: Integration of gasification-SOFC system [13].

Figure 5 describes the operational process of reversible SOFC and their interaction with diverse renewables sources. Particularly, the syngas produces from biomass gasification is composed of hydrogen, carbon monoxide, carbon dioxide, methane, water vapour and impurities components. However, it is crucial to clean the fuel from impurities (e.g. alkali compounds) in various temperatures to avoid the efficiency reduction. At the same time, there are two scenarios in this cycle, where the SOFC use the fuel from the biomass to produces energy and then the electricity generated provides portable power, transportation power or stationary power [8].

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Figure 5: Incorporation of gasification biomass into SOFC process [8]

2.2. Potential market and application sector

Overall, fuel cells have stationary, transport and portable application. Which is conditioned to the operating temperature range. However, the SOFC operated from biomass fuel can be applied in the production of gas and electricity (stationary use) but not suitable for transportation and portable applications (described in Figure 6). The stationary use involves an internal combustion engines conversion of other fuels. In general, this divided into two combinations, such as the heat-power and cooling-power [14].

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Figure 6: Application and fuels of fuel cell technologies [14]

Regarding the stationary installations, there are two classes, the primary and auxiliary. The power sources for such facilities can be in homes and office buildings, industrial sites, military installations and self-sufficient remote applications. [14]

At the same time, the fuel cell with higher investment in Europe is PEM technology, with around 150 USD million in the year 2016. However, the SOFC shows considerable projections for the year 2024, with approximately 300 USD millions of investments. Additionally, this type of fuel cell expects a rise in almost 50% during the period 2016-2024. The detail illustrated in Figure 7.

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Figure 7: European investment market of the fuel cell by typology [15]

The development of the fuel cell market is related to the level of commercialization of their technology. Figure 8 describes the trend of the fuel cell, which is the stationary application due to the significant rise projections, with approximately 50% more yield for the year 2026. Additionally, this category represents the highest global market, with around 80% of the application. In a second place, transportation projects 40% of participation in the fuel cell market. Lastly, the portable application expects the lowest global demand.

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Figure 8: The global market proportion of the fuel cell by application. Modified from [16]

The initiative of a feed-in tariff for low carbon in the residential sector is an example of stationary applications in the UK, which involves the generation up to five kW sold to the grid from the British homeowners. In an equivalent way, the company AFC Energy in North East of England has developed a fuel cell system that allows capture and storage around 500MW of carbon, with the integration of gasification combined of 300MW [17]

2.2. Opportunities, benefits and/or challenges

One of the most important benefits of SOFC is highly global efficient, between 60% and 80% which the efficiencies are independent of size. At the same time, this type of technology has zero carbon emitters due to the waste product is water, so there is lower emission [11]. Figure 9 shows the electrical performance of diverse types of technologies. The fuel cell has around 60% of efficiency and the output power range of 50W and 1MW, considering elevated operating temperature (from 600°C to 1000°C) [7] . In contrast, the energy conversion with gas turbine simple cycle has lower performance, with around 30% of efficiency.

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Figure 9: Efficiency and power output of Fuel cell [12]

On the other hand, the parts of the SOFC system can be moved, allowing the design of varied sizes (from W to MW). Furthermore, it is suitable for cogeneration system due to the waste heat is used as fuel for energy generation [18]. At the same time, the availability of natural sources allows considering the biomass as potential fuel (by the gasification process) due to there is a balance between the energy demand and storage capacity. Involving elevated and efficient power generation [19]

The hydrogen production has a significant effect on the performance of the SOFC stack. The syngas produced from the gasification of biomass can generate around 13% of H2 and the average of stack voltage is 36V [4]. The detail illustrated in Figure 10.

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Figure 10: Stack voltage produced by the syngas in a SOFC [4]

One of the most challenges of SOFC- biomass system is the thermal expansion in the material of SOFC, which makes difficult the fabrication of elements. Moreover, the metal stack components are affected by corrosion phenomenon, which is directly related to the density of the materials, especially in the design of interconnection in the fuel cell. At the same time, it requires a long start-up time and limits shutdowns. As a result, the cycling process is longer [20]. Furthermore, the SOFC technology needs higher materials quality and complex system configuration. Regarding the applications of the SOFC system, this is not suitable for transportation and small portable uses due to the fragility of the structure [21]

Another relevant point is the level of impurities components resulting in the gasification process of biomass, such as tar, sulphur, chlorine, nitrogen and alkali elements. Those dregs can obstacle the SOFC anode pores due to the size of particulates in the producer gas. Producing lower performances in the energy generation

2.2. Government regulatory, policy or guidance alignment

According to the commitment of Energy Act, 2004 generated by the UK government establish the promotion of the microgeneration of residential energy with the objective of decline the environmental damage and increase sustainable energy. As a result, the integration of technologies of SOFC with micro-CHP (Micro combined heat and power) and using biomass waste derived syngas, has the intention of supply the high energy demands by the UK residential sector [19]

Normally, around 68 to 82% of the thermal energy consumed in a residential building in the UK belong to hot water and space heating. The role of SOFC technology is fundamental in thermal performance due to the high heat recovery from the exhaust gas and operating temperatures (above 500°C), improving the overall CHP efficiency [19]

The UK regulations of fuel cell installation in residential zones (2006) organised by the law of Building Regulations. Specifically, Scotland has the building regulations (2004) that include the specification about the fuel cell installations, conservation of fuel and power, electrical safety, combustion appliances and fuel storage [22], The analogous way, the hydrogen fuel cell installations describes the regulation of the supply of machinery (regulations19,20,21), the electrical equipment (regulations 199418) and electromagnetic compatibility (regulations 200617). The hydrogen facilities legislation involves the pressure and equipment evaluation, particularly ATEX Directives11,23. In addition, the protective system designed, manufactured or sold use in potentially explosive situations based on the EPS Regulations13 [22]

On the other hand, the fuel of SOFC is produced in the gasification process of wastes (biomass) by a thermo-chemically process. According to the regulation of biomass, the UK has an available considerable number of potential sources of biomass, such as wheat, oilseed rape, sugar beet, forestry waste, straw and waste oils. However, the UK has not the expertise in the processing of biomass on a big scale [23]

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