Feasibility study for a 300kW pilot scale hydrogen production process from wood biomass: The case of Sandviksverket, Växjö

(1)

Feasibility study for a 300kW pilot scale hydrogen

production process from wood biomass

The case of Sandviksverket, Växjö

Thesis

Author: Derick Samjeila Ewang Supervisor: Professor Michael Strand Examiner: Professor Michael Strand Term: 20VT

Subject: Bioenergy Technology Level: Masters

Course code: 4BT04E

(2)

(3)

Abstract

Historically, fossil fuel sources (coal, oil and natural gas) have played significant role in meet global energy demand. As global population continue to rise, more and more energy resources are needed and there is a continuous depletion of these energy resources. The prices of these fuels have often fluctuated over the years and this has greatly affected the economy of several nations. The use of these fossil-based fuels has had enormous impact on the environment. This is as a result carbon dioxide and other greenhouse gases emissions. Many nations are continuously showing great interest and efforts to work together to address some of these issues, for example, the global response to combat climate change (Paris Agreement), which has been ratified by over 188 states. There is a growing interest in the technological development of renewable sources like biomass, in the production of heat, electricity and synthetic fuels. The renewable energy consumption target by the European union, stands at 32%, with a minimum of 14% in the road and transport sector. Sweden as a member state, has made tremendous progress in reducing its dependence on fossil fuels and increase its use of renewables (particularly biomass) over the years. Sweden has a target to reduce its CO2

emission in the transport sector by 79% and an independence of fossil fuels in its vehicle fleets by 2030.

The thermochemical conversion of biomass to hydrogen, to power hydrogen fuel cell vehicles, has been suggested as one route to achieve this target.

Biomass has high volatile content and the kinetics and stoichiometry of thermochemical conversion is very complex. This study evaluates the feasibility of a 300 kW hydrogen pilot scale production unit from biomass (wood pellet) in Växjö. Mass and energy balance calculations were performed, mainly in the first two reactors, in the envisage design and the lower heating value of the product gas was determined. Data that were used for this study was gotten from the plant in Växjö, literature survey of related systems and tools that were used to facilitate the calculations include Microsoft Excel and HSC chemistry for equilibrium calculations.

The result of mass and energy balance analysis on the 1MW fuel feeding system, showed that, the flow rate of sand required for the pyrolysis process (700°) is 2610 kg/h, and the energy of pyrolysis equals 217 MJ/h. Partial burning of the pyrolysis gas in the secondary reactor produces product gas consisting of mainly CO, CO2 H2 and H2O with volume percent equal 47.08%, 6.94%, 30.29% and 15.69% respectively. The calculated lower heating value of this gas is 9.04 MJ/Nm³when pure oxygen was used in partial burning of pyrolysis gas and approximately 6.0 MJ/Nm³when air was used for burning.

The 1MW fuel (wood pellet) feeding via thermochemical conversion has a production potential of 229 kW of hydrogen gas. Based on the parameters considered in this thermochemical process, the calculated Cold Gas Efficiency equal 59.7% and a Carbon Conversion Efficiency equal to 70.3%.

Key woods: Hydrogen, Biomass, pyrolysis, gasification, combustion.

(4)

Acknowledgments

First and foremost, I wish to express my most sincere gratitude to my supervisor Professor Michael Strand for his continues guidance and availability throughout this project work. Your sincere and timely response to all my questions facilitated the progress of this work which I deeply appreciate.

I am forever grateful for being a recipient of Linneaus University Scholarship Scheme. I wouldn’t have been able to pay the tuition fees Linneaus University.

Thank you very much for the tuition fee reduction scholarship. I will do my best to be a good ambassador of Linneaus University.

Am forever indebted to every member of my beloved family (The Ewang’s and Nyambi’s). Thank you for your prayer, love, moral and financial support.

I could come this far because you guys believe in me. To my late grand mom I know you are with your creator. Thank for the legacy you left behind.

To my best friend and prayer partner (Prudencia Makebe), I love you very much.

To the all my classmates, especially Alice and Nasrin. I will forever cherish the time we spent together. Thank you, guys, for the corporation.

To all the teachers and staffs of the Department of Build Environment and Energy Technology, I say thank you for you for the sacrifices. You made learning easy and my stay in Sweden memorable.

(5)

1 Introduction

Fuels consumption over the years has evolved from solids such as wood to coal, an then a parallel use of crude oil, which is preceded by today’s increasing use of natural gas. The state of science and technology has often guided the usage and requirement of energy carriers needed for industrial development. Fossil fuels have been the main energy source used by mankind to meet the growing global energy demand [1].

It should be highlighted that, the primary energy consumption according to the International Energy Agency (IEA), is expected to increase by 1.6% per year and the fossil fuels (oil, natural gas and coal) reserves supplying a greater part of the world energy needs are declining. Studies from the Energy Research Centre of the Netherlands (ECN) has shown that the global oil production might decline within 30 years [2]. Mounting to the problem of securing energy supply is the CO₂ emissions resulting from fossil fuels combustion, which causes global warming.

The need to address climate change issues and reduce fossil fuel use and its related carbon dioxide emission has been the driving force of the Paris Agreement. Despite withdrawal by the United States of America from this agreement, this United Nations Framework Convention on Climate Change (UNFCCC), which brought all nations with a common cause of setting ambitious goals, with the aim to keep global temperature rise below 2 °C, is however the strongest international framework for the development of sustainable alternative technologies and promotion of the use of renewable energy sources across the globe [3].

The substitution of fossil fuels with hydrogen, could be one of the ideal methods to enhance the decarbonization of global economy [1]. Hydrogen is an essential intermediate in the chemical industry and as of 2019, annual global production of hydrogen stands at about 70 million tonnes, with a market valued of USD 117.49 billion. According to the International Energy Agency (IEA) statistics, this is equivalent to 14.4 exajoules (EJ), around 4% of global final energy. Hydrogen production is anticipated to be 122.58 million tonnes and its market value is expected to reach US$191.8 billion by 2024 [4].

About 95% of all production of hydrogen is from fossil fuels (natural gas and coal) and these processes are often associated with huge greenhouse gases (GHGs) emissions. While the other close to 5% is hydrogen which is a by- product from chlorine production through electrolysis and a small fraction of H2 is generated from the electrolysis of water, a process which uses electricity to split water [5]. Currently hydrogen production from renewable sources such as wind, solar or through thermochemical and biological routes are not significant. However, hydrogen from renewable energy sources is a potential efficient and environmentally friendly secondary energy carrier to meet the

(9)

world's growing energy demand. The use of hydrogen as fuel is a vital step of not only decarbonizing our present energy systems (i.e. reducing GHG emissions, particularly CO2) but hydrogen could also be use directly as feedstock for further syntheses and in electricity generation and storage [6].

1.1 Why hydrogen as a transport fuel in Sweden

In Sweden, municipalities and industries have been showing strong interest in increasing the market share of hydrogen and the use of the fuel cell technology.

Their aim is to substitute fossil fuels with suitable sustainable alternatives.

Sweden as member of the European Union is actively working to meet the EU climate change plans of a 20% cut in emissions of GHG by 2020 (compared to the 1990 levels), an introduction of an average 20% share of renewable in the European energy mix, and an energy consumption cut of 20% by it member states [7].

To meet EU’s ambitions to increase the share of renewables, reduce emissions and increase energy efficiency, the Swedish government together with its opposition parties jointly work to ensure a smooth and feasible transition its energy system. The expected outcome is to reduce emissions and ensure economic and environmental sustainability. The Swedish national energy strategy has an environmental target in the transport sector to reduce its carbon dioxide emissions by 70% and an independence of fossil fuels of the vehicle fleet by 2030 [8]. Sweden is more ambitious and has as long-term target a net zero emissions of GHG by 2045 and a negative emission thereafter. In addition, the nation strives for a 100% renewable electricity production by 2040. To achieve these targets and to domestically secure for supply of alternative fuels, one of the suggested routes is through hydrogen production from thermochemical conversion of biomass.

1.2 Location of project

This study entails the feasibility of a 300 kW hydrogen production test facility from biomass, in Växjö. Växjö is one of Swedish municipalities with much experience in the field of renewable energy and very sound environmental management policies. The municipality of Växjö has been internationally recognized for its “Environmental Programme”, and the city is known for its slogan " The Greenest city in Europe". This has attracted visitors from all over the world, with many wanting to learn and emulate the city’s sustainable development strategies [9].

1.2.1 Motivations and challenges faced by the municipality of Växjö Political commitment and unity

The unanimous agreement by the local politicians to stop the use of fossil fuels in 1996 has led to some of the municipality success. The municipality is working on its very challenging environmental goal of becoming “Fossil fuel free Växjö” by 2030 [9].

(10)

Tremendous success has been recorded so far, for example the city cogeneration plant (Sandvik) as of December 2019 is proud of its 100% fossil fuel free district heating, district cooling and electricity. Figure 1 presents a comparison of CO2 emission from fossil-based sources per resident in Växjö, used in transport, heating and in electricity. Emissions have decreased by about 58% since 1993(compared to 2017 data) and this is mainly due to the use of biomass for heating and electricity production and other renewables.

Nonetheless, comparing the 1993 and 2017 data, the total carbon dioxide emissions attributed to transport is still very significant and shown in figure 1.

There is a need to address this issue and the production of hydrogen by gasification of biomass is a very important area to exploit if Växjö is to meet its target of becoming fossil fuel free by 2030 [10].

Figure1. Carbon dioxide emissions per resident in Växjö (tons) 1.3 Objectives

This study focuses on the thermochemical routes (pyrolysis and partial burning of the pyrolysis gas) for hydrogen production from woody biomass. The aim of this study is to provide details of the technical aspects required for the design of a 300 kW, hydrogen pilot scale test facility, via thermochemical conversion processes (of woody biomass), gas upgrading and cleaning in the Sandvik cogeneration plant in Växjö.

The task performed and study limit, in order to achieve the objectives, include the following:

§ Performing a mass and energy balance of the pyrolysis and the secondary tar cracking reactors, based on calculated fuel feeding rates and selected process parameters;

§ Determining the heating value of the product gas and;

0 20000 40000 60000 80000 100000 120000 140000 160000 180000

Transport Heating Electricity

Tonnes of fossil CO2

1993 2017

(11)

§ Calculating the Cold Gas Efficiency and Carbon Conversion Efficiency of the process.

2 Bioenergy energy use in Sweden

Sweden desire to ensure a long-term energy security has been at the forefront on the country’s push for its bioenergy expansion. Before the first oil crisis in 1973, Sweden had an 80% dependence on imported fossil fuels (mainly oil).

Oil rationing as a result of the crisis and the sharp increase in oil prices throughout the decade, resulted in significant economic toll. This cause the oil- importing countries to switch from oil to other sources of energy. During this period, the strong political push for nuclear power was also questioned. This led to the development of the bioenergy sector as an alternative to enhance energy security i.e., the switching of oil heating plants with wood-based fuels for district heating.

Today, the Swedish energy sector (particularly the transport sector), still largely depend on oil imports coming from the North Sea (Norway) and Russia. Oil imports nevertheless still entail energy dependency and is still a strain on the Swedish economy [11].

2.1 An overview of emissions by sector in Sweden

Figure 2 shows the historical trend of emissions calculated in carbon dioxide equivalence from various sectors in Sweden between the 1990-2017. It is observed that, emissions from the transport sector dominates and the emission in 2017 is lower than that in 1990. This decrease from 1990 to 2017 is as a result of the development and use of more energy efficient cars, and also the increasing use of biofuels in the transport sector.

(12)

Figure 2. Evolution of greenhouse gas emissions in various sector in Sweden from 1990-2017

The emissions from energy industries is mainly from electricity and heat and heat production and emission fluctuations over the years are due to weather conditions, which influences the need for heating. Emissions from manufacturing industries and construction have decreased since from the 90’s and this decrease is due to the switch from oil to biomass. An important example in this sector is the pulp and paper industry. The slight decrease in emissions in the agriculture sector during this period is as a result of less livestock and smaller amounts of fertilizers being used. Emissions from Industrial processes and product use is from mainly fluorinated GHGs (e.g. in cooling systems). Emissions in this category fluctuates with level of production and there has been a decrease in the emissions in the chemical industry thanks to enhanced production technologies [12].

2.2 Bioenergy potential in Sweden

Swedens quest for an alternatives to imported oil and nuclear power in the 1970s and 1980s, has promoted research on renewable energy and energy efficiency. A broad research programme was initiated around the 1980’s involving the state energy company, new energy agency and research institutes. Bioenergy was one of the major research areas and study focus on investigating the potential of biomass, its methods of harvest and sustainable use. The scientific basis of the political decisions to promote bioenergy use was fostered by the results of the special expert commission appointed by the Swedish government in 1990. This expert commission investigated the

(13)

potential of biomass from forestry and confirmed large potential in their report.

Also, previous research had shown the large energy potentials of residues from final felling and from thinning operations. This is thanks to the availability of technology to harvest these residues, and a clear positive synergy between harvesting of industrial wood and increased use of biomass for energy purpose [11].

2.3 Environmental concerns and carbon tax

Even though energy security concerns motivated Sweden’s bioenergy development, another vital push has been environmental issues. Since from the 1960s, environmental policy had been a growing subject in Swedish politics. This was further highlighted when Sweden hosted the 1972 United Nations environmental summit in Stockholm. This summit raised issues such as air and water pollution, acidification from emissions of sulphur and nitrous oxide, and concerns on the limited supplies of food and materials. The mounting pressures due to climate issue moving to the forefront further led to the Rio summit in 1992 and the negotiated Kyoto Protocol in 1997.

The implementation of carbon tax in 1990 by the Swedish parliament was part of a big tax reform. Carbon tax went into effect in 1991 and the industry sector had a lower tax rate than those of households and the service sector. Over the years, it has been increased many times, up till 2018 where the tax rate is the same for all sectors of the Swedish economy outside the European Union Emission Trading Scheme. Sweden has the highest carbon tax globally.

Swedish carbon tax is charged or levied per tonne of CO2 emissions on the different fossil fuels (heating oil, propane, fossil gas and coal) as in figure 3.

Source: Swedish government, Ministry of Finance, and Svebio, 2018

(14)

Figure 3. Carbon tax in Sweden, from 1991-2018 (EUR per tonne of CO2) The most important factor that has promoted bioenergy and made it more competitive with fossil fuels in Sweden has been carbon tax. This continue to be important as the prices of oil have weakened and might remain weak over the longer period. Meanwhile, there has been increasing volumes, improvement of harvesting techniques and conversion technology. These has greatly reduced biomass cost, and making it competitive with fossil fuels, even without incentives in many applications [11]. The prices of heating oil and pellets for residential use in Sweden from 2006 to 2016 is shown in figure 4.

Figure 4. The Influence of carbon tax on heating oil versus pellet prices in Sweden (Swedish krona-SEK/ Megawatt hour-MWh) [13].

The green line shows the price for pellets, while the darker bars shows market price for oil excluding taxes, and lighter bars show the additional oil price as a result of carbon tax. Carbon tax has nearly doubled the price of heating oil for this period, making wood pellets competitive. From figure it is also observe that the pellets cost is less than that of heating oil during the period considered (excluding carbon tax). This difference in prices was very small in some years.

However, in other to motivate the switching of fuel from heating oil to biomass, the price difference needs to be significant enough, so as to recover the investment cost of new boilers, fuel storage systems and equipment handling [11].

(15)

2.4 Experience with wood for heating in Sweden

Most district heating systems in Sweden are run by municipalities and the country has had a well-developed district heating system since in the 1970s.

The rebuilding or installation of most combined heat and power (CHP) plants within municipalities were boosted by government incentives aimed at enabling the switch from fossil fuels to renewable fuels. These investment grants and carbon taxation facilitated the rebuilding of heat plants or installation of CHP plants in large as well as in smaller cities and towns.

Figure shows the Swedish Bioheat Map i.e. heat plants and grids that uses biomass and biogenic waste for district heating purposes. Larger cities, usually have more than one heat plant to supplying the grid and wood is mostly used as fuel in these plants. However, waste plants are used in biger cities with capacities large enough to support the high cost of advanced flue gas cleaning and need for the cost effectiveness of all year-round operation. Also, there is market for the hot water generated in summertime, as well as for district cooling by these utilities [11].

In recent years, biomass and municipal waste has substituted fossil fuels in almost all district heating in Sweden. Considering 2015 for example as shown in Figure 5, biomass accounted for roughly 60% of the fuel for district heating, while about 15% was from the use of municipal waste. This combined accounts for about three-quarters of Sweden’s district heating needs.

Source: Swedish Energy Agency (2017)

Figure 5. Fuels used for district heating in Sweden, 1970-2015 (TWh)

(16)

2.5 A broad political support

Bioenergy use and development in Sweden has gain strong public support, and is being promoted, thanks to a combination of factors already discussed (Sweden’s energy security concerns, environmental pressures, CO2 tax, renewable electricity certificate requirement system and the nation’s experience in the efficient use of wood resources from forests). At the national scale, this support is being manifested through effective forest governance and long-term general incentives such as carbon tax. In addition to this, there is also support at local levels, for example some municipalities have implemented measures to encourage the use renewable energy technologies and energy efficiency [11].

2.6 Share of energy use in Sweden

Sweden has achieved much success as a result of the policies it has implemented so far to meet the country energy needs. Bioenergy, mainly woody biomass (wood chips, firewood, waste wood, pellets, briquettes and Logging residues i.e. branches, tops, stems) is used to meet much of these needs. Wood has a number of advantages over fossil fuels: its renewable resource; readily available in a country like Sweden with a well-structured and sustainable managed forests; it is carbon neutral; relatively lesser amount of sulfur and heavy metals emissions.

The advantage of using biomass as fuel is that it is considered a carbon dioxide neutral fuel. This is because the amount of CO2 that is released during burning of biomass, is equals to the amount that is taken from the atmosphere during growth of the biomass. Furthermore, fuels obtained from biomass, for example hydrogen, methane, Fischer Tropsch (FT) diesel and methanol, have the potential to become a CO2 negative fuel. This is because a share of the biomass carbon is separated as CO2 during the process of production and can therefore be sequestrated. Such process are very attractive methods for reducing the level of greenhouse gases emissions in the atmosphere.

Sweden leads the EU and is one of the world leaders in the deployment of renewable energy. These measures have not only reduced the entire nation’s greenhouse gas emissions and but also a significant reduction of the nation’s dependence on imported oil. As presented in figure 6, as of data from 2016, bioenergy accounted for 37% of final energy use in Sweden, while renewable energy (mainly hydro power, wind power and ambient air to heat pumps) provided a share total of 54%. Hence, bioenergy use meets more than one- third of Sweden energy needs [11].

(17)

Source: Svebio, based on data from Statistics Sweden and Swedish Energy Authority, 2017.

Figure 6. Energy use in Sweden, 2016 2.7 Swedish Forest Industry

Approximately 67% of land surface of Sweden is covered with forest, making Sweden one of the richest European countries in forests. This is equivalent to 28.4 million hectares of forest of which about 22 million hectares actively managed for different uses. Close to half of this forest area is owned by 300,000 private owners, normally with some relatively small holdings.

Products from the forest includes timber, wood fuel and felling residues. The Currently mean growth rate of Swedish forests stands at 5.1 cubic meters per hectares per year. All forest owners in Sweden are obliged to sustain wood production taking into consideration: biodiversity; recreational needs enhancement; protection soil and waters and mitigation of climate change.

Figure 7 shows a representation of the share of different wood found in the Swedish forest sector while figure 8 shows million cubic meter of wood flow in the Swedish market in 2011[14,15].

(18)

Source: Swedish Forest Agency

Figure 7. Share of the various types of wood found in the Swedish forest sector [15]

Figure 8. Million m³ of wood flow in the Swedish market [15]

The forest industry in Sweden and its by- products provide the major source of bioenergy used in the various municipalities. It should be noted that another important driver for wood products is the construction sector. The attributes that favors the use of wood in the construction sector is thanks to the fact that wood is renewable, ecological, environmentally friendly and climate-smart material and it is also durable. The increasing demand of wood for energy purposes have however strengthens the supply side and increase the profitability of forest industry.

2.8 Uses of hydrogen

Four main uses of hydrogen production worldwide is for ammonia production 50%, refinery applications 22%, methanol production 14%, reduction

(19)

processes 7% and the remaining 7% spread to other consumers. Some reasons accounting for the worlds increasing demand for hydrogen production include:

the need for more ammonia and methanol; more hydrogen is needed for hydro- desulfurization processes due to more stringent environmental regulations promoting the production sulfur free products and the growing interest of the use of hydrogen as an energy carrier[5,16]. There has been increasing demand in the use of hydrogen as fuel in transport sector. Thanks to breakthroughs in fuel cell technology and the quest of many nations to develop economies which are independent of fossil fuels.

2.9 Fuel cell electric vehicles (FCEV) – a long term solution To secure our mobile lifestyle, fuel cell electric vehicles (FCEV) are a long term solution to the growing shortage and the rising cost of energy resources.

Automotive industries considers FCEV as one major approach towards a sustainable and affordable mobility for the general public. This has been a driving for development of hydrogen powered cars by companies such as Daimler, Ford, GM/Opel, Honda, Hyundai/ Kia, Renault, Nissan and Toyota.

The development of these cars has motivated other stakeholders to start the building of commercial network of hydrogen refuelling stations.

Advantages of such initiatives include:

i. Reduction of GHGs emissions ii. Energy storage

One major challenge faced by the global energy systems is the availability of economic energy storage technologies. By comparing to other energy storage solutions, hydrogen has larger scale energy storage capacity and over longer periods of time. This makes it storage to be a valuable complementary solution to expensive investments in new grids systems.

iii. Job opportunities

Since the use of hydrogen in the transport sector is very promising, it is inevitable that there will be an increase in the labor force at local, nation and international levels. This will be as a result of investments made by organizations and nations in the development of hydrogen and fuel cells technology.

2.10 Vehicles fuelled by hydrogen

Hydrogen can be used to power FCEVs and with high energy efficiency. This fuel is environmentally friendly as water is the only local emissions. The operating principle of the FCEV involves a principal component i.e. the fuel cell which converts hydrogen and oxygen in an electrochemical process to electricity. The electricity produced by the fuel cell is used in an electric engine to drive the power train or propels the vehicle. In order to control the fuel cell and manage the power supply to the engine, a small battery is used. Comparing to vehicle designs using battery (battery-powered electric vehicles) as the only mode of energy storage, compressed hydrogen achieves more energy storage

(20)

and thus a longer driving range. In addition to these, there is no lengthy recharging time as with battery-powered electric vehicles. The fuel cell continues to work as long as it has hydrogen and oxygen flowing into it. Fuel cell electric powertrain is suitable for various types of vehicles. Present limitations include energy storage and power density in the fuel cells, hence limiting use to lighter vehicles rather than heavy long-distance trucks. FCEVs present in today’s market can travel about 700km on a single tank while refueling in minutes as opposed to being recharged in several hours.

Furthermore, adding to the environmental advantages of hydrogen fuel cell technology over conventional fossil fuel engines, the energy efficiency of fuel cell in the range of 40 %-60 %, compared with the average energy efficiency of 25 % for petrol engine.

Hyundai ix35 and Toyota Mirai were the first FCEVs that was recently manufactured, with handling and performance similar to that of a conventional vehicle. The range of this vehicle varies between 500-594 km and have a refuelling time of three minutes [17,19]. The price of ix35 in Sweden is SEK 629 000 (excl. VAT), while that of Toyota Mirai is USD 56 934 (SEK 486 000) (market not specified) [18]. Three Hyundai ix35 FCEVs are currently used in Sweden. There is a possibility of a significantly reduction of the prices of these vehicles as a result of improvement in research and development of the manufacturing processes of these vehicles as well as with economies of scale. However, most of the large car manufacturers plan to release FCEV models in the near future.

In Sweden, the Nordic Hydrogen Corridor (NHC) initiative which has as aim of providing zero emission transport solutions based on hydrogen, is now managed by Hydrogen Sweden (Vätgas Sverige) and includes vehicles, hydrogen production process and distribution. Two new partners in addition to the car producers Hyundai and Toyota are Statkraft (largest producer of renewable energy in Europe) and Everfuel (green hydrogen fuel distributor).

In Sweden, about 40 different cities and municipalities are interested in hydrogen-based zero emission transport and the NHC-partners are working with the most promising locations in order to ensure sufficient demand before taking investment decisions and moving to execution phase. An increase in the use of FCEVs within the European Commission has been projected as an important step towards reaching an emission free transport sector. This is because hydrogen will cause no GHG emissions as it is produced from renewable energy [20].

2.11 Fuel cell vehicles and hydrogen fueling stations as of the end of 2018

Globally, the number of FCEVs exceeded 12,900 by the end of 2018. This represents an 80% increase in 2018. Approximately 46% of these vehicles are in the United States, followed by Japan with 23% and China with around 14%.

A greater majority of the vehicles are passenger cars in most parts of the world

(21)

while the Chinese stock predominatly commercial vehicles. A more detailed picture is presented in figure 9.

Figure 9. Global proportion of FCEVs, 2018

There are 376 refueling stations in operation worldwide. The three countries with the highest Hydrogen Refueling Stations (HRS) included: Japan with 100 refueling; Germany having 60; and the U.S with 44 refueling stations [22].

Figure 10. Number of hydrogen refueling stations by country

(22)

The EU targets a minimum of 747 hydrogen refueling stations by 2025. Also, the Hydrogen Mobility Europe project plans to place more than 1,400 fuel cell cars to customers and deploy 49 hydrogen refueling stations across Europe by the end of 2020.

By the year 2030, the Hydrogen Council [22] has projected that 1 in 12 cars in Germany, Japan, South Korea and California (USA) would be powered by hydrogen. This globally accounts for about 10-15 million cars and 500,000 trucks. While by 2050, hydrogen will power more than 400 million cars, 15- around 20 million trucks, and approximately 5 million buses globally.

2.12 Projections of Freight activity or heavyduty transport

Heavy duty vehicles accounts for about 47% of CO2 emissions from land based mobility and approximately 8% of the total global CO2 emissions. Also, it has been projected that freight activity (ton-km) will double by 2050 and renewable hydrogen is the most promising zero-emission fuel for heavy vehicles in the nearest future [23].

2.12.1 Volvo Group and Daimler Truck AG joint venture for large- scale fuel cells production

These companies aim to lead in the development of sustainable transportation. To achieve this, they intent to develop, produce and commercialize fuel cell systems for heavy-duty vehicle applications and other related automotive and non- automotive uses. This initiative aligns with the Green Deal vision of sustainable transport and a carbon neutral Europe by the year 2050.

According to Martin Daum, Chairman of the Board of Management Daimler Truck AG, the use of fuel cells are one important way of meeting the growing need of a CO2-neutral transport fuel.

In the last two decades, Daimler through its Mercedes-Benz fuel cell unit has built significant expertise and combining both company’ experience will be a turning point in bringing hydrogen fuel cell powered trucks and buses onto our roads.

The chemical energy of hydrogen is converted to green electricity which is used to power electric trucks used in long-haul operations, and this complement battery electric vehicles and renewable fuels. According to Martin Lundsted, CEO and president of the Volvo Group the realization of this vision requires support and contributions of other companies and institutions also need to support and contribute companies and institutions to develop and establish such needed fuel infrastructure.

Even though both companies continue to compete in all other areas of business and operate as an independent and autonomous entity, the advantages of this 50/50 partnership between Daimler Truck AG and the Volvo Group include:

reduction of development costs for both companies; speed up market introduction of fuel cell systems for heavy-duty transport and demanding long distance applications [24].

(23)

3 Methods of Hydrogen production

Projected H2 demands require looking for alternative routes for future hydrogen production, currently dominated by non-renewable sources.

Generally, renewable hydrogen can be produced through electrochemical, biological and thermochemical processes.

Electrolysis of water is the most important electrochemical method which attracts a lot of interest and the electricity must be generated by a sustainable energy source to split water molecules into hydrogen and oxygen. However, fluctuations in the output of renewable electricity from wind power and photovoltaic energy systems have created a growing need for energy storage systems. Hence, converting electricity into chemical energy (hydrogen) by means of electrolysis represents a promising complementing technology as the H2 generated can be either stored or reconverted into electricity during times of an undersupply [25].

Biologically or photo-biologically methods for hydrogen production employs different microorganisms over a series of metabolisms. Examples of hydrogen producing metabolic pathways include: Biophotolysis of water using green algae, photo-fermentation, dark fermentation, biological water gas shift reaction, and hybrid systems. The hydrogen- producing enzymes catalyzing such biological hydrogen production processes include; hydrogenase and nitrogenase. These methods have the advantage of operating under ambient temperature and pressure, in addition to the use of renewable feedstock or solar energy. It should be noted however that, the state-of-the-art of these technologies is at laboratory scale and practical industrial applications still need to be demonstrated [26, 27].

The state-of-the-art for industrial scale H2 production is through thermochemical routes, based mainly on fossil fuels. However, renewable hydrogen through thermochemical processes can be achieved using biomass as feedstock. Conversion technologies used for the generation of hydrogen from hydrocarbons such as biomass and fossil fuels involve pyrolysis, gasification and reforming processes. The products of these thermochemical processes is synthesis gas, consisting mainly of hydrogen and carbon monoxide. Synthesis gas obtained may then be subjected to other downstream processes in order to produce pure hydrogen.

3.1 Thermochemical conversion of biomass

Biomass, a product of the process of photosynthesis, is a major form of solar energy conversion and accumulation. It is a clean and renewable energ source.

Energy production from biomass through thermochemical conversion processes, will not produce lots of harmful Sulphur dioxides (SO2), oxides of nitrogen, and other pollutants, in addition to the fact that CO2 emissions is almost zero. Globally, it has been estimated that, biomass energy will account

(24)

for about 50 % of the global energy consumption by 2050 [28]. Biomass as a fuel contains up approximately 70 % or more volatile raw material properties, making it very suitable for application in thermochemical technologies such as gasification [29].

3.2 Biomass gasification

The technology of biomass gasification is a thermo-chemical process which convert solid biomass fuels such as wood, crop residues, sewage sludge and municipal wastes into a fuel gas. This fuel gas or product gas could be used could be used for applications such as electricity power generation, heating purposes and in the synthesis of other fuels or chemical products. The chemical reaction in the gasification process involves a partial oxidation reaction, which converts the solid biomass to gaseous fuel, using an air-fuel ratio less than 1 at specific temperature and pressure. It should be noted that pure oxygen or steam could also be used as gasifying agent in this process. The chemistry of this process is very complex. The final product obtained depend on the process parameters and many different chemical reactions occur at high temperature and other process conditions like the specific equivalence ratio (air fuel ratio), pressure and catalyst used in the process [30]. The process of biomass gasification can be categorised into three steps.

§ Upstream processing steps: This includes, biomass reduction size, drying and preparation of gasifying agents);

§ Gasification process steps: Involving mainly the pyrolysis or devolatilization and gasification reactions, and;

§ Downstream process steps: Includes the various technologies used in producer gas clean-up, upgrading, reforming and gas utilisation [31].

The biomass gasification process step is the heart of the process. Biomass is thermo-chemically converted to energy rich combustible gaseous product in controlled conditions, using different gasifying agents, for example air, O2 steam and CO2 or different proportions of a combination of these gasifying agents. Unlike the process of combustion in which the biomass oxidation is completed in one-step, the process of solid biomass gasification undergoes a series of physical transformation and chemical reactions within the gasifying reactor our gasification units. An illustration of theses processes are shown in Figure [32].

3.2.1 Drying

In the drying process, the moisture content of the solid biomass evaporates, and theses leaves a dry biomass. Drying occurs in the temperature range between 100–200 ^oC and the solid fuel in this drying process doesn’t

(25)

decomposed, since temperature is not high enough to cause chemical reactions. The steam releases in these processes may later contribute in chemical reactions such as the WGS.

3.2.2 Pyrolysis

Pyrolysis process is simply defined as the thermal decomposition of biomass in an inert or oxygen-free atmosphere. Pyrolysis is an endothermic process and solid biomass when exposed to elevated temperature in the gasifier, and in the absence of a gasifying agent yields mainly three types of products: volatiles, which include mainly permanent gas, condensable gas and solid residue or char [33].

Figure 11. A schematic presentation of the pyrolysis process of biomass and it products

In thermochemical conversion of biomass, pyrolysis precedes the gasification process and the temperature range for the release of these volatiles can vary between 350-700^oC. In this first step, there is devolatization of the volatile materials and thermal breakdown of weaker chemical bonds found within the large hydrocarbon molecules in the solid biomass. The composition of the low temperature volatile vapours consist of gaseous species such as carbon dioxide, carbon monoxide, hydrogen , methane and other lighter hydrocarbons. Also present are large condensable molecules (comprising of phenol and acids) called primary tars. These primary tars are highly oxygenated compounds, a characteristic which makes it highly reactivity.

Finally, there is also the solid residue or chars a material consisting of mainly carbon and ash. It should also be noted that if gasifying agent is present and at relatively elevated temperatures i.e. 700-850 ^oC, secondary gas-phase reactions such a cracking, reforming, combustion, and CO shift, of primary tars occurs and these reactions produce combustible gases and secondary tars consisting of phenolic compounds and olefins. Furthermore, at even higher temperatures (850-1000^oC), tertiary conversion of secondary tars, producing

(26)

polyaromatic hydrocarbons (PAHs) occurs and also some soot formation soot observed at this reaction conditions [34, 35].

3.2.3 Combustion

Combustion within the gasification environment involves partial oxidation of a portion of the char and/or volatile products with oxygen (limited amount) to produce CO2, CO, H2O and the heat required to sustain the various gasification reactions.

3.2.4 Gasification

Gasification process involve the partial oxidation by the gasifying agent at high temperatures (600-1500^oC) of the char residues, pyrolysis tars (primary, secondary and tertiary tars) and pyrolysis gases to produce a producer gas. The composition of the producer gas consists of mainly hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and traces of lighter hydrocarbons (ethylene, ethane and propane). In the presence of incomplete reactions of the product gas may contain char and tar [36].

Figure 12. Schematic representation of the three processes in thermochemical biomass conversion [32].

It should be noted as earlier stated that during the gasification process step, several different thermal processes are taking place. However, depending on process conditions, different endothermic and exothermic chemical reactions take place in the gasification units(s). Given that gasification reactions are

(27)

reversible, establishing the direction of these reaction and its conversion requires knowledge reaction kinetics thermodynamics. Thermodynamic equilibrium of gasification reactions imposes a high effect on the thermal efficiency and the composition of the producer gas.

3.3 Technologies of biomass gasification

The technology of Biomass gasification has become one of the most efficient and practical methods of biomass energy conversion process and has attracted lots of research interest. There are basically three types of well-developed and commercially available gasifiers. They include fixed bed, entrained flow and fluidized bed (FBG) gasifiers. In recent years, research scholars has paid much attention on the development dual fluidized bed biomass gasification technology. This is a very promising technology with potentials to significantly contribute in meeting the global increase in energy demand and climate change challenges.

These gasifiers differ in their design details and this may depend on the gasification agent (air, oxygen, steam or mixture), pressure which may be either atmospheric or pressurized, and the mode of heat supplied (autothermal or allothermal). The product gas composition and its heating value is directly affected by the type of gasification agent that is being used. For example, the LHV of product gas air-blown gasification of biomass is in the range of 4-7 MJ/Nm³ (contains approximately 50% nitrogen), while that of gasification of oxygen and steam have LHV of 10-15 and 13-20 MJ/Nm³respectively [37,38].

3.3.1 Moving bed gasification

Moving or fixed bed gasifiers are design base on two type of concepts i.e.

updraft and downdraft. In the case of updraft gasifier, the fuel (biomass) is feed from the top of the reactor and it moves downwards under gravity via the drying, devolatilization, gasification and oxidation zones. In this design, the gasification agent is supplied from the bottom, while the product gas is collected at the top. Thus, it is an autothermal gasification process involving heat generated by chemical reactions during gasification. Advantages of the updraft gasifier includes it simplicity, high char conversion, high overall efficiency and flexibility to size of fuel particles and the moisture content. A major disadvantage of this design is the high tar content of product gas since it leaves the reactor near the pyrolysis zone. Gasification medium is not always, in counter-current flow with the fuel as in downdraft gasifiers.

Biomass and gasifying agent flow in same direction(co-current) via the drying, devolatilization, oxidation and gasification zones. Tar concentration is much lower compared to updraft gasifier as the gas passes through the hottest oxidation zone before leaving the reactor. The disadvantage of this design include, lower energy efficiency, greater amount of particulate matter

(28)

entrained in product gas, and fuel used for this design requires that its moisture content be less than 25% This technology is limited to smaller scales and with fuel feed size ranging from 6-50 mm [39].

3.3.2 Fluid bed gasification

This is an autothermal gasification process which occurs inside a fluidized bed reactor, with either silica sand or catalytic particles being used as bed material.

In the design of this reactor, size distribution of bed material is very important as too large particles i.e. >10 mm are not fluidized, whereas too small or fine particles are lifted out of the bed. Pure O2 and steam are often used as oxidizers in order to avoid the dilution of syngas by N2, in case where air was used as oxidizing agent. Fluidization enable proper mixing between bed materials, fuel particles and oxidizing agent and this promotes heat and mass transfer. Due to the proper mixing in the fluidized bed, there is an even distribution of the inert bed material, ash, unconverted char and fuel particles, inside the bed. A disadvantage with this is the reduction carbon conversion efficiency as a result of unconverted char and fuel particles removed together with ash from the gasifier [36]. It should be noted the fluidization regime changes with increasing fluidization velocity and there are three different process regimes for fluid bed gasifiers. This consist of stationary or bubbling fluidized bed (BFB) with velocity in the of 0.5-2 m/s, circulating fluidized bed (CFB), velocity in the of 2-5 m/s or transport reactor. Also, FBG are very sensitive to sintering and agglomeration of the ash/bed materials, hence they are generally operated below 1000 °C i.e. below the softening temperature of fuel ash so as to avoid bed agglomeration problems. This low gasification temperature results to syngas having large amount of higher hydrocarbons and tars [39].

3.3.3 Entrained flow gasification

In entrained flow gasifiers (EFG) the fuel being feed and the oxidant are in co- current flow. Residence time within EFG is in the order of a few seconds and for this reason EFG requires much smaller particles, higher gasification temperature to attain complete fuel conversion, compared to the other type of gasifiers. Oxygen is generally used as the oxidizing agent in other to achieve the high temperatures desired. A benefits with EFG is the conversion of CH4 and higher hydrocarbons such as tars, due to the high process temperature in the gasifier. Hence, EFG produces syngas with the highest quality [39].

Besides, this technology is fuel flexible as most gasifiers operate in the slagging range, implying the removal of fuel ash from the gasifier as a smelt.

3.3.4 Dual fluidized bed(DFB) gasification

DFB gasification is based on the use of two fluidized reactors i.e. one for gasification and the other for combustion [40]. The forms of the gasifier includes a bubbling fluidized bed, a circulating fluidized bed, a two-stage

(29)

fluidized bed, a U-shaped fluidized bed, moving bed or down-bed etc.[41].

The two interconnected reactors are separated with loop seals, which allows the transport of bed material between the reactors and also preventing cross- contamination of gases between the two reactors. Steam is used as the fluidization and gasification agent in the gasifier.

The principle of the dual fluidized bed gasification process is shown in Figure 13. The system reactor consists of mainly two parts: an endothermic gasifier and an exothermic combustion furnace, where the processes of drying, pyrolysis, gasification and combustion of the biomass takes place. The first part is a bubbling fluidized bed gasification reactor, and the other consist of a circulating fluidized bed combustion reactor. Since the gasification reactions are endothermic, the heat required for these reactions are supplied by circulating hot bed materials i.e. either sand or olivine particles from the combustion reactor [42]. The bed materials transfer heat from the combustion unit or reactor to the gasification units where the volatiles are released. Air is usually used as oxidizer in combustion reactor and steam in the gasification reactor resulting in high heating value of the syngas due to avoidance of syngas by N₂ dilution. The residual char after the biomass gasification process is transported from the gasification unit to the combustion unit along with the bed materials which is further heated in the combustion unit by the burning of char particles. This design improves the rate of carbon conversion and the system thermal efficiency [43]. The main disadvantage of DFB is the high yield of larger hydrocarbons and tars, produced as a result low gasification temperature (around 850 °C). Compared to O₂ blown entrained flow gasification, the investment cost of DFB gasifiers is relatively lower since there is no O₂ production plant.

Figure 13 The principle of dual fluidized bed (DFBG) gasification process.

Heat

Char + sand Combustion

uint

Gasification unit

Fuel +Air Fuel

Flue gases (CO2 , N2 ,O2, H2O)

Water vapour Circulation of bed

material

Flue gases

(CO2 , CO, H2 CH4

H2O)

(30)

3.4 Product gas upgrading and cleaning process

The state of the art unit operations used for upgrading and cleaning product gas in order to improve hydrogen yield include the following

3.4.1 Water gas shift

To increase H2 yield and lower the content of CO generated in product gas, a WGS unit is used. The WGS reaction shown below is a well-established technology used industrially to produce hydrogen or setting CO/H2 ratio of synthesis gas.

CO+H2O↔H2 +CO2 ∆H= −41.2kJ/mol

WGS reaction converts carbon monoxide and steam into hydrogen and carbon dioxide. The equilibrium equilibrium constant of this reaction decreases with temperature. This implies high conversions are favored by low temperatures, as presented in Figure 14 [5].

Figure 14. The Variation of equilibrium constant (Kp) for WGS reaction with temperature [5].

At industrial scale, WGS unit usually consist of one or more fixed bed reactors.

For relatively low CO conversion in order to adjustment CO/H2 ratio for synthesis a by-pass, high temperature (HT) WGS stage is used, while in the case of H2 production, the product gas stream is treated in 2 or 3 steps at gradually lower inlet and outlet temperature, this is to attain a high CO conversion.

Many different catalyst are used in order to attain sufficient economic reaction rates. The use of Fe-Cr-based catalysts are suitable for HT WGS steps. High temperature stages operate adiabatically and the inlet gas temperature from 350 to 550 °C, with space velocities ranging from 400 to 1 200 per hour.

Operating pressure will depends on the requirement of the plant. (Liu et al., 2010) The advantages of Fe-Cr-based catalysts is that they are robust against

(31)

sulfur poisoning. H2S is often observed in the product gas of steam biomass gasification[44].

Low temperature (LT) WGS stage (around 200 °C) uses Co-Mo or Cu-Zn- based catalysts. It should be noted that Co-Mo catalyst are resistant to the presence of sulfur components, while Cu-Zn-based catalyst is very sensitive to sulfur poisoning [5], hence, requires sulfur removal.

3.4.2 Technologies for hydrogen separation

Pressure swing adsorption (PSA) and membrane based processes are used the separation technologies used. However, PSA is used when high purity (> 99%) is required.

In PSA process the gas molecules are physically bound onto a solid adsorbent material. Interaction between gas and adsorbent depends on the component of the gas component, its partial pressure, adsorbent type, and temperature. This state-of-the-art gas separation process is widely use in different commercial scale applications, i.e. for example in hydrogen production, upgrading of biogas and in air separation [45]. A simplified flow chat for the PAS process is shown in figure 15.

Figure 15. Simplified flowchart of a PSA process.

In this process, the compressed feed is successively fed into various adsorber vessels. The regeneration of the of the vessels are carried out by lowering the pressure, while flushing with the product (raffinate) at high-pressure. Low- pressure product (adsorbate) containing the contaminants of the feed may be reused in other upstream or downstream processes. The main components of product gas and their adsorption strength on activated carbon is given by the following relation: CO2 > CH4 > CO > H2. (Liu et al., 2010) This implies

(32)

CO2 is preferably adsorbed on activated carbon and is better removed from feed gas stream compared to for example H2. Activated carbon is a very good adsorbent for pure hydrogen production. Methods aiming to reach the fuel cell grade H2 production through PSA of product gas from DFB biomass steam gasification plants has been carried out in DFB plants in Güssing and Oberwart, Austria. In their experiments with lab-scale PSA unit (using activated carbon as adsorbents) a H2 recovery of about 80% was achieved [44,46].

3.4.3 Dust filters

Dust separation in from the product gas in commercial gasifiers is achieved by the use of bag house filters. In order to ensure the durability of the bag house filter product gas needs to be cooled down below 200 °C. Char residue particles which are entrained from gasification reactor together with product gas are collected in the filter. Fly char may be returned internally (e.g. into the combustion reactor in a DFB system) or it may be discharged from the system as waste stream.

Dust particles from the product gas may also be separated using electrostatic precipitators (ESP). ESPs are more advantageous in the cases of the removal of smaller dust particles. Particles separation from product gas stream using the force of an induced electrostatic charge. Certain commercial scale fluidized bed air gasification reactors employed the use of ESPs. It should be noted that, the high carbon content of fly coke from gasification processes, makes ESPs problematic as downstream cleaning. Wet ESPs on the other hand are suitable alternative for cooling and condensing, in order to remove aerosols of solids or liquids[47].

(33)

4 METHODOLOGY

The areas that were covered in this methodology section include: a presentation of the design layout of the gasification process for hydrogen production which is to be installed in the Sandvik plant; the investigated system boundary of study; data collections from literature and assumptions made in the mass and energy balance calculations; some simulation calculations that were performed; The tools that were used for the calculations include Microsoft excel and HSC Chemistry for equilibrium calculations.

4.1 Process overview

The entire process flow and layout is presented in figure16 and figure respectively. The investigated system boundary for the study focus, on the pyrolysis and tar cracking reactors as shown in figure16.

Figure 16. overview of the hydrogen production process in Sandvik plant For the pilot test facility to be installed in Sandvik, wood pellets are to be used as fuel and commercial wood pellets is assumed to contain about 8% moisture in this study. For a 1MW feeding system, the wood pellets are to come in

gas

Pyrolysis/gasification Tar cracking

Fuel

dust

S- hydrolysis

H2S HCL

NH3

HC N

Reformin g

HT WGS LT WGS

Metanisatio n PSA

Compresio

n H2

Carbon residue Drying

O2/air

Temp. ^oC

1500

1000

500

0

(34)

contact with the bed material (sand) at 700 °C. In the pyrolysis reactor, there is simultaneous drying and devolatilization of the wood pellets. The sand and the and the char that are produced from this process, are carried back into the combustion or the boiler unit. The pyrolysis vapor which consist of the condensable and non-condensable gases which are considered in this study, flows into the secondary unit or tar cracking reactor. In this reactor, part of the pyrolysis vapor is burn by controlling the inflow of oxygen. This process increases the reactor temperature. The high temperature in this reactor enables the conversion of both the larger and the light organic fractions in the pyrolysis vapor and hence improves the quality of the product gas. The final product gas is pass through several upgrading and cleaning operation to obtain pure hydrogen. A summary of the operating principle of the two gasifiers is presented in figure 17.

Figure 17. Operation principle of envisaged H2 production test facility to be installed in Växjö.

4.2 Fuel conversion rate

Wood pellets are to be used as fuel for the gasification process. They are commercially available. The value of the moisture content and lower heating value LHVfuel of the wood pellets used in this study are approximated 8% and 18.5 MJ/kg respectively. To obtain a gasifier capacity Pgasifier of 1MW, the fuel conversion rate ṁfuel (kg/h) was calculated using equation (1).

ṁfuel = ^!!"#$%$&'

"#$_%(&) (1) Char + sand

Combustion unit

Pyrolysis reactor

Fuel +Air

Fuel + moisture Flue

gases (CO , N

Circulation of bed material

Secondary tar cracking

reactor Pyrolysis

vapor Condensable

+ Non- condensable

gases

Product gas cleaning

& upgrading O₂/air

(35)

The lower heating value of biomass (LHVfuel MJ/kg) was calculated according to Boie formula

LHVbiomass =34.835xC +93,870xH -10.800xO +6280xN + 10.465xS (2) Where xC, xH, xO, xN, and xs are mass fraction of the elements C, H, O, N, and S that make up the composition of the fuel considered in this study [48].

4.3 Drying and devolatilization

4.3.1 Drying

The moisture content of the wood pellet considered in this study is 8%. This moisture content is very good in order to achieve generate a high heating value product gas. We assume in this study as the wood pellets enter the primary pyrolysis reactor, the water bound (moisture) in the wood pellets irreversibly and instantaneously changes to the gas phase due to the high temperature of the incoming bed material(sand). Given that biomass is a very complex mixture of organic materials, with major elements being carbon, hydrogen, oxygen, nitrogen and Sulphur, data from ultimate analysis of biomass that was used to determine the chemical formula of wood and the heating value of the gas did not consider the impact of the nitrogen and Sulphur content of the fuel.

4.3.2 Devolatilisation

The devolatilization process is normal a very complex phenomenon as a result of the numerous chemical and physical transformation occurring rapidly and simultaneously. It should be noted that the production of primary tar from the pyrolysis reactor is measurable and this study relied on experimental measured values at the required pyrolysis temperature.

Data from proximate and ultimate analysis were vital in order to calculate the yield and the composition of the devolatilization products. Figure 18 shows the 3 major competing reactions of the devolatilization process.

Figure 18. simplified representation of the devolatilization process

Wood Tar

(oil) Gas

Volatiles

Char Fixed carbon

(36)

It was assumed that the char yield (in weight %) produced in the pyrolysis step, takes the value of the value of the fixed carbon from the proximate analysis of wood pellets. Furthermore, it is also assumed that the total hydrogen and oxygen content of the wood pellet is released in this devolatilization step and the volatiles that was considered in this study include a mixture of CO, CO2, H2, H2O, CH4, C2H4 and primary tar.

4.4 Mass and energy balance of the pyrolysis reactor

Mass and energy balance calculations were performed on the pyrolysis and tar cracking steps, based on data collected from literature studies and assumptions considered in the gasification process design.

Parameters that were taken into consideration in order to develop the process mass balance include:

Proximate and ultimate analysis data (table 1 ) of wood pellets which is to be used as input fuel. Data from this fuel analyses provided the necessary information for calculating the input stream mass balance on molar basis. Also, experimental data of the measured gases from similar studies were used to ease the calculations.

Table 1. Wood pellets analysis taken into consideration for mass and energy balance [50].

Proximate analysis Value

Moisture (% wt.) 8

Ash (% wt.) ^0.50

Volatiles (% wt.) ^80.57

Fixed Carbon (% wt.) ^18.94

Ultimate analysis

C (wt %) 49.8

H (wt %) 6.1

O (wt %) 43.6

N (wt %) 0.16

S (wt %) 0.005

Lower heating value (MJ/kg) 18.5

4.4.1 Chemical formula of biomass

Despite the complexity to determine the chemical formula of biomass, several approximations have been imposed to generalize its chemical formula. The method used to determine the chemical formula of the fuel is based on the approximations of the generalized chemical of biomass. This method utilizes

Feasibility study for a 300kW pilot scale hydrogen production process from wood biomass: The case of Sandviksverket, Växjö