PRODUCTION AND GASIFICATION OF WASTE PELLETS

Transcription

1 UNIVERSITY OF BORÅS SCHOOL OF ENGINEERING PRODUCTION AND GASIFICATION OF WASTE PELLETS Farokh SAHRAEI-NEZHAD Sara AKHLAGHI-BOOZANI This thesis comprises 30 ECTS credits and is a compulsory part in the Master of Science with a Major in Energy and Material Recovery, 120 ECTS credits No. 6/2010 i

2 PRODUCTION AND GASIFICATION OF WASTE PELLETS Farokh Sahraei, Sara Akhlaghi, x080157@student.hb.se, akhlaghi.sara@yahoo.com Master thesis Subject Category: Technology University of Borås School of Engineering SE BORÅS Telephone Examiner: Supervisor, name: Supervisor, address: Professor Tobias Richard Professor Tobias Richard University College of Borås, School of Engineering SE , BORÅS Client: Borås Energi och Miljö AB, Date: Keywords: Gasification, Municipal solid waste treatment, Renewable Energy, Syngas, Waste treatment, Sustainable waste management, Waste Pellet ii

3 Abstract Thermo-chemical processing of waste materials including gasification is considered as an effective waste treatment method in modern communities. Through this process, Green House Gases are significantly reduced alongside the emergence of value added products (renewable fuels), sustainable development targets are met. In this study, the gasification of municipal solid waste materials has been investigated. Syngas is a gasification product which is made up of carbon monoxide, hydrogen and methane. Syngas can be used for power production, automotive fuel and production of chemicals. Product gas through gasification is considered as a sustainable alternative transport fuel for petroleum based fuel which can improve the domestic support of energy for the independent countries. In the gasification process, the energy content in carbonates material like municipal solid waste is converted into a gas phase fuel. The conversion is affected by several parameters such as gasification agent, operating temperature, reactor design, heating method, moisture content and particle size of feedstock. It has been found out that gasification performance; product gas yields, chemical composition and heating value of product gas are significantly impacted by the gasification agent and operating temperature of the gasifier. The kinetic mechanism of the gasification reaction and energy balance have been applied in order to design the gasifier and to predict the operational behavior of the process. Also the shrinking un-reacted particle model (SUMP) has been selected for waste particles gasification modeling. By this way, designed parameters and optimal conditions of a gasification process are achieved. This study presents the correlation between the reactor temperature, size of the fed particles and the particles residence time. The findings of this study shows that a waste gasifier can be an indirect heated, atmospheric pressure, bubbling fluidized bed, steam gasifier which can be connected to a bubbling fluidized bed waste boiler. The gasifier would be run at temperature of 650 C with steam as gasifying agent and fluidized media by 1.84 ton/ton of feed supported by a connected boiler with circulating sand with flow rate of ton/ton of feed. The residence time of the particles has been found to be around 318 second. Accordingly, the combination of waste gasifier and waste boiler can be considered as an efficient method for waste treatment process to produce renewable energy. iii

4 Acknowledgements We hereby acknowledge to numerous persons in contributing and assisting the work of this thesis. Firstly we would like to sincerely acknowledge our supervisor and examiner professor Tobias Richards for his valuable guidance and constant support during this research project. The project is exists because of his vision and his guidance in right direction. We are truthfully grateful to Borås Energi och Miljö AB staff especially Mr. Per Karlsson and Ms. Pauline Salomonsson Lindberg at Reyavarket site, as well as Mr. Claes Ranweg and Mr. Hans Skoglund at Sobacken site of Borås Energi och Miljö. Their supports have been very helpful in many ways. We would also like to thank Dr. Claes Breitholtz and Margareta Lundberg in research and development department of the Metso Power Company and Mr. Ove Johanzon at Sodra Cell Varo gasification plant in Varbarg (Sweden) for their technical support and useful advice. Finally our program director at school of engineering Dr. Peter Therning is gratefully acknowledged for his support. Farokh Sahraei Nezhad Sara Akhlaghi Boozani iv

5 Contents Abstract... 3 Acknowledgements... 4 List of tables... 7 List of charts... 8 List of figures... 9 Abbreviations Chapter 1, Introduction Aim of this project Methods and materials Background review Gasification products and application Syngas market Syngas market terend Feedstock of gasification Incineration versus gasification of waste Process and general comparison Emission and pollutants Pollutants Ash and slag, residues handling Gas clean up procedure Economic Benefits of gasification Drawback of gasification of waste Chapter 2, Feedstock preparation Production of waste pellet Benefits and drawbacks of pelletization of waste materials Pelletization steps Size reduction steps Metal separation Drying Pelletizing Cooling Chapter 3, Gasification Gasification of municipal solid waste Gasification main steps Drying Pyrolysis and de-volatilization Gasification reactions Key factors for gasification of waste Gasification agent Temperature Pressure Moisture Heating rate Heating method Feedstock heating value Waste particle size and preparation steps v

6 Ashes Pollutants level Gas utilization Types of gasifier Fixed bed gasifier Fluidized bed gasifier Entrained flow gasifier Kinetic of waste pellet gasification Selecting a modeling for waste particles gasification Residence time of the particles Heat required for gasification of waste Pellet The assumptions Chapter 4, Cost estimation of the project Chapter 5, Results and Discussion Pre-treatment of gasification waste feedstock by pellet production Gasification of pelletized waste in gasifier combined to the boiler Cost estimation of the project: Conclusions Further works References vi

7 List of tables Table 1. The contributions of authors Table 2. Analysis of various gasification feedstock Table 3. Effect of temperature on gas composition of MSW gasification [2, 45, 47, 48] Table 4. Effect of heating method on heating value of product gas[2, 3] Table 5. Summarized characteristics of different gasifier [7, 8] Table 6. Experiential models for char gasification kinetics Table 7. Summarized assumptions, formulas of estimating residence time Table 8. Residence time of particles at different temperatures Table 9. Summarized the heat balance calculations Table 10. Operational data of boilers in Borås Energy Plant Table 11. Product gas composition at reactor temperature of 650 C [64, 65] Table 12. The carbon consumption for the reactions paths considered for assumption Table 13. Summarized energy demand/supply calculations Table 14. Design parameters of the gasifier Table 15. Summary of design parameters of the gasifier Table 16. Purchased cost of equipment Table 17. Price list of equipment vii

8 List of charts Chart 1.Worldwide gasification capacity by products, data taken from [3] Chart 2. World Market of Gasification GW th per year. [3] Chart 3. (a), (b) Effect of gasification agent on composition and heating value of product gas. [42, 45] Chart 4. The syngas main components as a function of temperature. [2, 48] Chart 5. Effects of pressure on gas composition at 1000 C[3] Chart 6. Average heating value of gasification feedstock Chart 7. Effects of gasification temperature on residence time Chart 8. Bed material at different temperatures Chart 9. Energy demand considering assumption Chart 10. Bed material and steam flow rate at different temperatures Chart 11. Effect of moisture content of feed on sand flow rate Chart 12. Effects of gasification temperature on gasifier bed volume viii

9 List of figures Figure 1. (a), (b) Biomass pellet, (c), (d) Refused derived fuel pellet (RDF) [23] Figure 2. A Hook shredder [28] Figure 3 (a), (b). Mechanism of milling of waste material [29] Figure 4 (a), (b). Pelleting mechanism in flat die pellet mill [32] Figure 5. Pelleting mechanism in ring die pellet mill[25] Figure 6. Schematic of biomass gasification[1] Figure 7. Summary of biomass gasification reactions[1] Figure 8. Updraft gasifier [1] Figure 9. Downdraft gasifier [1] Figure 10. Schematics of bubbling fluidized bed [48] Figure 11. BFB gasifier [7] Figure 12. Schematics of circulating fluidized bed[48] Figure 13. CFB gasifier [7] Figure 14. Entrained flow gasifier [14] Figure 15. Single char particle conversion models Figure 16. Schematic of waste pellet production Figure 17. Flow diagram of the waste pellet production (made by Super Pro) Figure 18. Trap-door and steam collector of waste boiler in Borås Energi ach Miljö site Figure 19. Bottom ash screw and ash elevator in Borås Energi ach Miljö site Figure 20. Bottom ash screw Borås Energi ach Miljö site Figure 21. Rejected bottom ash (larger than 2mm) ix

10 Abbreviations Y H2 Y CO Y CH4 Cross section of the cylindrical or cubic gasifier Total reacting surface area per unit of mass Reference state of the conversion Heat Capacity Diameter Activation energy ) Structural profile Particle Height Gas enthalpy kinetic coefficient of the reaction Enthalpy Mass of carbon in waste pellet sample Mass flow rate of feed Mass flow rate of bed material Mass flow rate of steam Order of reaction Pressure Heat duty Gas constant Char reactivity per unit of surface reacting Char reactivity of a waste pellet sample Temperature Steam velocity Minimum steam velocity Volume Carbon conversion at time t Carbon composition of waste (% wt. dry basis) Percentage of Hydrogen in raw product gas Percentage of Carbon monoxide in raw product gas Percentage of methane in raw product gas Heat of reaction Density Volumetric flow rate Residence time x

11 BFB CFB CPM DME EF FB FT Diesel GHG HHV HRSG IGCC LHV MSW PMSC PMSP RDF SMR SNG SUCM SUPM UCM Bubbling Fluidized Bed Circulating Fluidized Bed California Pellet Mill Dimethyl Ether Entrained Flow Fixed Bed Fischer-Tropsch Diesel Green House Gases High Heating Value Heat Recovery Steam Generator Integrated Gasification Combined Cycle Low Heating Value Municipal Solid Waste Progressive Model with Shrinking Core Model Progressive Model with Shrinking Particle Model Refused Derived Fuel Steam Methane Reforming Synthesis Natural Gas Shrinking Un-reacted Core Model Shrinking Un-reacted Particle Model Uniform Conversion Model xi

12 Table 1. The contributions of authors Chapter Part of the project Contributor s Page 1 Introduction Farokh Sahraei 13 1 Gasification products and application Farokh Sahraei 14 1 Syngas market Farokh Sahraei 15 1 Feedstock of gasification Farokh Sahraei 16 1 Incineration versus gasification of the waste Farokh Sahraei 17 1 Benefits and drawback of gasification of waste 2 Feedstock preparation 2 Benefit and drawback of pelletization of waste materials 2 Pelletization steps 3 Gasification 3 Gasification reactions 3 Key factors for gasification of the waste 3 Types of gasifier 3 Kinetic of waste pellet gasification 3 Heat required for gasification of waste Pellet and design parameters 4 Costs estimation 5 Results and discussion Farokh Sahraei 19 Farokh Sahraei 21 Sara Akhlaghi Farokh Sahraei 22 Sara Akhlaghi 23 Farokh Sahraei 28 Farokh Sahraei 29 Farokh Sahraei 31 Sara Akhlaghi 36 Sara Akhlaghi Farokh Sahraei Sara Akhlaghi Farokh Sahraei Farokh Sahraei 52 Farokh Sahraei Sara Akhlaghi 53 5 Conclusions Farokh Sahraei Sara Akhlaghi 61 xii

13 Chapter 1, Introduction People are not going to stop producing waste. Thermo chemical processing is an effective way to treat municipal solid waste material and it has achieved a great attraction in modern communities. Furthermore, gasification of waste materials has the potential to offer a major impact on the ability for communities to meet the sustainable development targets[1]. Gasification of the waste is considered as a renewable and sustainable method in an effective and environmental friendly way. It can meet the targets to reduce Green House Gases at the first rank[2], and produces a valuable renewable fuel which can improve demand of the sustainable energy present to different sectors of society. In the gasification process, the energy content in the waste materials is converted into a gas phase fuel [3]. The conversion is affected by several parameters where one of the important is the preparation of the feedstock to achieve increased conversion efficiency. Pellet production as an option for pretreatment of waste, has highly influence on waste gasification. Gasification of waste offers numerous benefits with few drawbacks. It has the potential to increase the overall efficiency of electricity generation compared to conventional waste incineration[1]. On the other hand, excluding some of the disadvantages for waste incineration process; waste combustion has a slightly higher thermal efficiency than gasification [4-6]. Aim of this project The aim of this project is a theoretically investigation of the possibilities to improve the efficiency of the waste boilers by waste gasification. Consequently in this research project the focus is to use waste pellet (mainly municipal solid waste) as raw material to the gasification process. The study has been carried out to assess the feasibility of gasifying feed as homogeneous structure as possible in order to fulfill the renewable fuel production, and also sustainable management of waste materials adapted to the city of Borås (Sweden). Methods and materials This investigation has been done based on literature review, some calculations; and analysis of the results of those calculations which were carried out. The data used for cost estimation of the project was achieved through connection with the related industries. 13

14 Background review Society always needs to use fuels for different purposes. The first fuel that human used was wood for heating their homes. Wood was also used in the form of charcoal and later the coke was used instead of the charcoal. When the population increased the shortage of this fuel became a problem so the needs of using fuel from other sources become important. At the end of the eighteenth century gas was produced and used from coal. In the twentieth century, the gas usage became more significant in the market particularly for heating [2, 3, 7]. Gasification is a technology that becomes more important when humans faces with the lack of fossil fuels and also the damages from the burning of these fuels, because fossil fuels like coals produce CO 2 and the emission is one probable causes of global warming. Hence for the emission of CO 2 some pretreatments need to be done on the coal or other feedstock like biomass. The advantages of using gasification is that CO is produced instead of CO 2, and also the energy content of the products are increased and they can be used in other application such as high efficiency power generation in gas turbine, fuel production and chemical manufacturing[8, 9]. In the period of the usage of gasification technology increased. It then halted for a couple of decades before the interest was reactivated in the 1980 s, particularly for environmental problems such as global warming[8]. Some landmarks in the early history of gasification are: Coal gas first patented for lighting in 1804 [9]. Westminster Bridge (London) illuminated with town gas light on New Year s Eve using wooden pipe for gas delivery in 1813[9]. Baltimore, Maryland becomes first U.S city to light street with town gas in 1816 [3]. Town gas lightning in factories replaces candles and lanterns, making the night shift possible and enabling the industrial age in 1800s[9]. Gasification products and application The product from the gasification process is a product gas mainly includes carbon monoxide (CO) and hydrogen (H 2 ) and less percentage of methane (except for nitrogen, water and CO 2 ). When the concentration of CO and H 2 is high, the product is named syngas. Syngas is colorless, odorless and very flammable[2]. The application of syngas could be as following: Electricity production Syngas and product gas can be fired in gas turbines and gas engines to generate electric power. For the gas turbine application, high level of gas cleaning is required in order to remove particles, alkali, sulphur and tar in comparison with gas engine. The efficiency of a gas turbine is higher compared to a gas engine [ 3]. In addition, electricity production of syngas is more efficient than directly combustion in a boiler to produce heat for the electricity production purposes. They also can be used as fuel in Integrated Gasification Combined Cycle (IGCC) to further increase the electricity production. 14

15 Automotive fuel Bio-based automotive fuels like synthetic fuels including methanol, ethanol, DME, FT-diesel, synthetic natural gas (SNG) and hydrogen are all alternatives as automotive fuels. The produced gas from gasification of waste is more appropriate for SNG application considering its methane percentage. Also syngas is more suitable for Fischer-Tropsch products (gas to liquids) and methanol because of high percentage of CO and H 2 [1, 10, 11]. Production of chemicals The hydrogen content in syngas can be used to produce a wide range of chemicals, ammonia and fertilizers. Hydrogen can react with nitrogen available in the air to form ammonia (NH 3 ). Some other products are lube oils, and waxes[2]. The CO content could be used to produce renewable plastics[1, 4]. Syngas market According to statistic data from the energy market, syngas are increasingly being used for different applications. Worldwide gasification capacity is shown in Chart 1 by different products. As it can be seen, there is significant demand in the world for power production using syngas at the first rank and chemical and automotive fuel in the second rank [12]. Chart 1.Worldwide gasification capacity by products, data taken from [3] Syngas market terend Nowadays, market demand for gasification products has a rapid growth in the world. Based on the Simback report in 2007 (Chart 2), gasification planned capacity had an increased by 110 GW th per year in 2010 and by 150 GW th in Thus syngas as one of the energy carrier can be one of the major players in energy market in the future. [3] Chart 2. World Market of Gasification GW th per year. [3] 15

16 Feedstock of gasification A wide range of carbon content feedstock can be converted to syngas through the gasification process. The feedstock should have a high ratio of carbon-to-nitrogen, quite little sulfur, and low moisture content. According to heating value, the feedstock can be divided into two main categories which are high heat values feedstock such as coals, oil, black liquor, lignin, and low heating value materials like municipal solid waste (MSW), biomass, petroleum coke (pet coke), high-sulfur fuel oil, and any other low grade carbon content feedstock [1, 2]. The chemical composition of the feedstock might be the main limited factor of selecting the feed for gasification process. The chemical compositions of the feed have great effect on the quality of the product gas, design of the process and gas clean up procedure. In Table 2, there are some comparisons between different feedstock s and their produce gas composition in different plants. It also shows differences between properties of coal, biomass and waste materials as following: 1. Coals have higher heating value and carbon content than biomass and waste material. 2. MSW and coal have more ash and sulfur in comparison with biomass. 3. MSW has lower oxygen content, higher nitrogen and ash content than biomass since oxygen content of biomass is higher than coal except in Anthracite coal. 4. MSW has high alkaline and chlorine content which leads to produce corrosive gas. In contrast, chlorine content of biomass is rather low except the forest biomass materials coast close to the sea however they have rather high alkali content which must be reduced before a gas turbine application. 5. MSW is non-uniform in comparison with coal and biomass. MSW has different physical properties, densities and morphologies that makes it difficult feed for gasification[4]. The municipal solid waste was compared for energy content, chemical composition and water content using proximate analysis, with the same of biomass and coal in Table 2. The analyses (C, H, O, N and S) are made on dry ash material. Table 2. Analysis of various gasification feedstock Feedstock Anthracite coal [2, 3] Lignite coal [2, 3] Brown coal[2, 3] Biomass [2, 3] MSW[13] MSW[14] Region Ruhr, Germany Dakota, USA Rhein, Germany Typical Biomass Borås, Sweden Asian Countries LHV MJ/kg Carbon % mass Hydrogen % mass 16 Oxygen % mass Nitrogen % mass Sulphur % mass Water % mass Ash % mass

17 Incineration versus gasification of waste Waste incineration, as one of the current options for waste management, has a great of interest in the most modern cities. Incineration of the waste is a method for turning the waste material to electrical energy and heat. The other option is gasification which is an advanced method for converting waste to energy [15]. The common mistaken is claimed that gasification is just another name for incineration [16]. Those two methods have advantages and drawbacks. The main differences of both methods are discussed in the following. Process and general comparison During the waste incineration process, partial or total oxidation of carbonations matters is take place. Heat is released from incineration whereas gasification is a process whereby organic materials are decomposed through thermal cracking into useful products as well as more valuable gases used for different purposes[1]. Gasification offers the highest controllability whereas incineration offers higher heat application efficiency [2, 6]. Emission and pollutants Pollutants The incineration process is designed to maximize the conversion of feedstock to CO 2 and H 2 O whereas gasification is designed to maximize the conversion of feedstock to CO and H 2. Both processes convert carbonaceous materials to gases but the composition of gases before cleanup is different. The gasification resulting gas consists mainly of H 2, CO, H 2 S, NH 3, and particles while the composition of flue gas from the combustion process are CO 2, H 2 O, SO 2, CH 4 and particulates[2, 17]. Combustion processes operate with excess oxygen or air. It means that the combustion agent has to be added to waste as input and the waste burns. The result of these sorts of chemical reactions are mainly heat and CO 2 and H 2 O which can emitted to the environment while, the air emission can raise the level of Green House Gases and other air pollutants. The gasification processes operate with a limited amount of oxygen[2]. Syngas is burned with significant little NO X emissions[6]. So emission of NO x will be decreased when produce gas is burned in gasification of waste compared to incineration. Release of sulfur and nitrogen oxides in the atmosphere are led to acid raining[6]. In case of using syngas for chemicals these acid-rain are not produced but sulfur may be entered with feedstock into the gasifier and is converted to H 2 S, also nitrogen in the feed is converted to diatomic nitrogen (N 2 ) instead of NH 3. The toxic and carcinogenic pollutants like dioxin and furan may be produced during organic materials combustion. In gasification process there is no production of furans and dioxins because the gasification process is performed at high temperature so furan or dioxin will be cracked that caused precursors in the feedstock. The other reason is 17

18 related to control amount of oxygen in the gasifier which restricted formation of free chlorine from HCl [2, 6]. Ash and slag, residues handling Solid residues of both processes are different. Particles from gasification are char and inert slag but bottom ash and fly ash are leftover of combustion. The leftover of gasification at low temperature is char which has value and can be sold. Char consists of un-reacted carbon and the mineral matter existed in the waste. The major utilization of char is as a source of activated carbon. Activated carbon may be used for decolorization and waste water treatment [1, 6]. High temperature gasification above the melting point of mineral matters is resulting of formation of glassy state non-hazardous and inert slag. Those not vaporized molten slag are suitable for use in fields of road and construction industry and also as an abrasive materials used for sand blasting[2]. In other hands, about the 30% of input to the waste incineration is converted to the ash[18] which is consists of high amount of mineral matter and less amounts of unreacted carbon. Those sort of by products must be safely treated and disposal as hazardous waste[16]. Therefore, combustion as a waste treatment method is makes a solid waste but a sort of hazardous solid waste. Gas clean up procedure Economic In gasification cleanup of the syngas is performed in order to use that as a fuel, chemicals and energy sourced but treated flue gas from combustion will be discharged to the atmosphere. As mentioned, the sulfur contents in gasification feedstock are recovered as a byproduct in form of sulfur or sulfuric acid. In contrast sulfur content in combustion fuel converts to SO 2 and must be removed from the flue gas. Cleaned syngas is mainly consisted of H 2 and CO but cleaned fuel gas consists of CO 2 and H 2 O [1]. A cleaned syngas free from sulfur and nitrogen oxides is ready for different applications such as combusted in a gas turbine to produce electric power or it can be burned in a steam boiler to produce hot water or steam. The syngas can be used as a base for new chemicals like automotive fuel. Although the product gas without any cleaning may use for gas fired steam boiler combined with steam turbine to produce electricity [2]. The steam cycle has a higher operational reliability but requires higher investments. The gasifier engine has a higher efficiency but lower reliability [5, 6]. Gasification of waste using heat recovery system has an increscent up to 50% in electricity output in comparison with waste incineration process [4, 5]. 18

19 Benefits of gasification 1. Flexibility in wide range of feedstock Ability to process a wide range of carbonaceous feedstock and convert them to the syngas among high heat values feedstock such as coals, oil, wood and wide range of low-value carbon content material like municipal solid waste, biomass, agricultural wastes, petroleum coke (pet coke), high-sulfur fuel oil, refinery residuals, refinery wastes, hydrocarbon contaminated soils, and any other carbon content feedstock [2]. This range of flexibility increases the economic value of these resources and decreases costs by providing industry with a broader range of feedstock options [1, 2, 19]. 2. Flexibility in wide range of product The syngas which is produced by gasification process can be converted into numerous valuable products, ranging from electricity, heat, steam to fuels in gas and liquid phase (FT Diesel), chemicals such as Acetic anhydride (Acetic Acid), methanol, ammonia, Fertilizer (Urea) and hydrogen[19]. 3. Low cost of cleaning equipment The application of gasification technology is produced less volume of flue gas in the process. So it reflects in a decreased size and cost of equipment related to off-gas cleaning system and increases opportunities for added revenues [2, 20]. 4. Clean products gasification process has the ability to remove contaminants in the feedstock and produce a clean syngas product. 5. Almost zero emissions The system based on gasification process can meet the strictest environmental regulations pertaining to emissions of sulfur oxide SO x (sulfur dioxide SO 2 ), particulate matter, and toxic compounds other than coal contaminates such as mercury, arsenic, selenium, cadmium, etc.. Further, gasification provides an effective means of capturing and storing or sequestering carbon dioxide (CO 2 ), a greenhouse gas. The carbon dioxide produced during gasification is present at much higher concentrations and at higher pressures than in streams produced from conventional combustion, making them easier to capture[2, 16]. The vision is to convert synthesis gas into pure hydrogen using the waster gas shift reaction and use the hydrogen as an ultra-clean fuel with an exhaust gas of nothing but water [7]. 6. Energy security By making better use of available and domestic biomass renewable rich energy resources such as biomass and MSW, gasification can decrease dependency on petroleum, fossil fuel and other imports energy sources[6]. High efficiency by combined with power cycle The systems based on gasification process can be integrated with other processes and technologies for power production, mainly combustion, gas turbines cycle and also 19

20 solid oxide fuel cells[1]. The integrated systems are significantly efficient, and result in more value from each unit of raw material and feedstock[19]. Waste incineration plant consumes high quality fuel for its operation; it is possible to apply gasification process as the first stage of thermal treatment to minimize the consumption of auxiliary fuel [20]. Accordingly, the combination of waste gasification and waste incineration has the potential to increase the overall efficiency of both waste gasifier and waste boiler compared to conventional waste incineration process. Drawback of gasification of waste Waste gasification technology is still in a premature stage and not common technique for MSW. That is the major drawback of gasification process. Numbers of problems associated with gasification for waste materials are as following. 1. Tar formation, gas reforming and gas cleaning. The gas reforming procedure can be one of the complex and expensive stage of the process as well as the gas cleaning steps of hot product gas[4]. 2. Not fully conversion of inorganic content. Inorganic content of waste materials are not fully converted at rather low temperature less than 800 C and lower content of oxidizing agent therefore, all materials may be not reacted. But in case of combine waste gasifier and waste combustor that is not matter, whatever is not reacted in gasifier will react in the combustion unit but the gas yield will be less[19]. 3. Alkali content of waste materials at high temperature gasification. At temperature higher than 700 C, alkali metal is problematic when it reacts with chlorine gives sodium chlorine which is corrosive[19]. 20

21 Chapter 2, Feedstock preparation All type of biomass among waste material is required preparation process because of variety in physical, chemical and morphological characteristics[1]. The different characteristics of municipal solid waste materials make a necessity to pretreatment the feedstock for gasifier. Pelletization is one of the pretreatment methods for make the feeds uniformed. When the materials in small size are compressed into cylinder shape they called pellets. Pellet is produced of wide ranges of biomass materials and used for different purposes. Finland and Sweden are the two leading countries in pelletizing technology in Europe [21, 22]. The process of pellets manufacturing was first developed for the livestock feed industry [23]. Special focus here is on municipal solid waste as feedstock for gasification process. The property of this waste stream varies with lifestyle, season, location, trends of sorting of the waste materials which make them heterogeneous. The waste streams are varied in size, type, shape, density. Therefore, they behave differently when they are entered to the reactor. And prediction of reactor condition is become difficult. For an efficient gasification process, a homogeneous waste mixture and uniform structure and density is required, thus control of the system will be much better [24-27]. Production of waste pellet Production of biomass pellet is common. Although a wide range of biomass and agricultural residues are pelletized for different thermal proposes. However when the feedstock is waste material, the process is more difficult as mentioned earlier. Steps of production of waste pellet include size reduction, metal separation, drying, pelletizing, cooling, storage and transport. Figure 1, illustrates pelletized biomass and RDF (refused derived fuel) respectively. The major composition of RDF is plastic and paper. a b c d Figure 1. (a), (b) Biomass pellet, (c), (d) Refused derived fuel pellet (RDF) [23] 21

22 Benefits and drawbacks of pelletization of waste materials The benefits of pelletization can be explained as following: 1. It can provide the homogenous feed, which makes easier controlling of the flow rate of a reactor, prediction of conditions, and controlling of the system. 2. Less storage place is needed. Waste materials are compressed as pellets, required less space compare to untreated waste [24]. 3. There is a limitation on keeping untreated municipal solid waste in the waste bag more than 3 days when they are collected at the waste collection center because of the risk of firing and bacterial pollutant. This is especially important in the summer season when the heat demand of the city is lower than available waste at the plant. By waste pelletization, it is possible retained them more for the next demand. 4. Pelletization of waste material is efficient way of loading, unloading and handling them from waste collection center to the plant. Hence the cost of transportation and material handling would be decreased. 5. Amount of extra dust in the feedstock could be decreased and the feed becomes more stable as a result of pelletization. 6. Distribution of the waste and odor problems in the environment would be controlled better by waste pelletization [21]. Drawback of pelletization is that when the density by compaction is increased, the active surface area will be decreased then reaction rate will be decreased and the reaction time will be increased. 22

23 Pelletization steps Size reduction steps If the dimension of input waste material is larger than a certain range, size reduction step is required [20]. This range for most of the pellet mills is 30-50mm [25]. The size reduction process can involve one or more steps like grinding, shredding, crushing or milling. A shredder has the ability to significantly reduce the size of the large waste materials with a maximum size of 2700 mm[28]. A Hook shredder used in Sobacken recycle center, Borås, Sweden is Figure 2. Figure 2. A Hook shredder [28]. Size reduction is typically performed in hammer mills (Figure 3) which is appropriate for waste material with rather low energy cost compares to other size reduction method [29]. It can reduce the size of the material to 80mm. Hammer mills use rotating hammers or knives to provide an extremely high hammer tip speed, as well as a minimum clearance between the hammer and the screen which determines the size of the outgoing material. A hammer mill includes a rotor with rods for the hammers and a motor for rotating the rotor. As the rotor turns, the hammers are free to swing on the rods and the waste materials are fed into the hammer mills and are crushed. This gives an energy efficient grinding process[30, 31]. a b Figure 3 (a), (b). Mechanism of milling of waste material [29]. 23

24 Metal separation All sorts of ferric and non-ferric metals must be detected and are removed from the waste stream via magnetic separation. Separated metals can be recycled. The reason is that even a very small amount of metal could destroy the pellet mill equipment. Drying To achieve a good quality of the pellet, the moisture content of the feedstock is one of the important factors. The moisture content of the waste materials is rather high and it has influenced on gas composition and the energy balance of the process[20]. From gasification point of view, high moisture content is required more energy for evaporation. On the other hand, depending on the process design and the desired produced gas, controlling the amount of moisture is necessary. It should be in the range of 10-15%. One of the reason for this range is, when the wet produced pellet is dried, some cracks are appeared in the pellet due to lack of water and the pellet becomes fragile [32]. In addition, if the moisture content is less than that range, the pellets are denser and loosed their quality, also in the pelletizing step, the friction between the feedstock and the dies in the pellet mills increase and could cause blockage of the holes in the dies [21, 22, 33, 34]. To achieve the desired moisture content, different dryers can be used. There are a lot of dryers that can be classified in different categories. Based on methods of heat transfers, they can be conduction, convection, radiation and electromagnetic field [35, 36]. About 85% of industrial dryers are convective [35]. In convective dryers, drying medium and materials have direct contact with each other but in conductive dryers materials dry with indirect heating [35, 36]. They can also be continuous or batch based on the mode of operation [36]. Another category can be based on the vessels that used for drying the materials such as tray, fluidized bed, rotating drum, pneumatic spray [35]. Also different sources can be used for providing the necessary heat in the form of hot air, flue gas and superheated steam for evaporating the moisture from the materials [36]. In case of municipal solid waste as a feedstock, some emissions release during the drying, due to volatile organic compounds. Usually when the temperature of the feedstock is more than 100 C, these emissions happen[34]. Another matter is the risk of firing in the dryers due to the explosion of combustible gases that might release during the drying. If the high temperature and enough oxygen exist explosion can be happened. When the concentration of oxygen is more than 10%, it could be dangerous [34]. Many factors can be influenced on the selection of the dryers. For example depending on the desired quality of the product, the size of materials that used, the energy demand is needed for drying, the safety of the dryers and impacts of them on the environment, and the residence time of the dryers [34-36]. The residence time of continuous dryers can be varied from few minutes to two hours [35]. As mentioned above most of the dryers are convective, also most of them are continuous with atmospheric pressure and hot air as a medium drying [35, 36]. 24

25 Rotary cascade dryer is one of the common dryer that used in the commercial and industrial scale for drying biomass. This dryer consist of cylindrical shell with the diameter between 1m to 6 m inside the shell. There are some vanes that elevate materials and flood them through the cylinder. This dryer has a little slope that materials can move through the cylinder while rotating [34]. For instance, the Vandenbroek Company uses a rotary drum dryer for drying municipal solid waste. Drying medium is hot air that co-currently contacts with the materials. This dryer has multi-pass system which consists of 10 drying passes. The temperature of the hot air is between C. The temperature of the final product is not to be exceeding 90 C due to the possibility of firing in this temperature[37]. There are also other dryers that can be used for drying biomass. Perforated floor bin dryers are usually used in small scale with batch system that is appropriate for feedstock like grains. The bed conveyor dryer is similar to the perforated dryer but with a continuous system feeding. In this type, the feedstock is carried out on the conveyor with bands and the drying medium is fed by fans in the dryer [34]. Fluidized bed steam dryers are more advanced in comparison with the other dryers mentioned and can be used for drying e.g. biomass. They are expensive but in large scales they can be appropriate [34]. Pelletizing Production of pellet consists of compaction and densification of crushed waste in the form of pellets. This part is the most energy demanding part. For having the better quality of the pellets, such as hardness and stability, sometimes in order to attaching the feedstock binders are added[26]. Increasing the feed temperature decreases the energy demand, however temperatures higher than 100 C, cause more emissions of volatile organic compounds [38]. Two common pellet mills are the flat die pellet mill and the ring die pellet mill [26, 37, 39, 40]. The flat die pellet mill, Figure 4, consist of two main parts: roller and perforated die. The roller parts are made from alloy metals which are tight (severe) and cannot be broken easily. This part can have one or more (two or three) pair of rollers. The perforated die parts include holes in order to compress and compact the material and make the pellet. The diameter of the holes could be changed due to different requirements. The materials are pressed between the rollers and the flat die and come out from the holes in the die part in the form of pellet [32, 39]. a b Figure 4 (a), (b). Pelleting mechanism in flat die pellet mill [32] 25

26 In the ring die pellet mill, Figure 5, materials are pressed between the roller and the ring die into the holes on the ring die. On the outside of the ring die a stationary knives cut the pellets to the desired length [26, 37, 40]. Figure 5. Pelleting mechanism in ring die pellet mill[25] In the following there are some comparisons between these two pellet mills that can be affected on the selection of them for different feedstock. 1. The diameter size of the ring die pellet mill is limited due to the limitation of the ring die mould, so the pressure is limited, but in the flat die there is no limitation of diameter size from moulds, so in this type by increasing the diameter size the press power of the rollers are increased, also the service life of the machine can be extended [39]. 2. The material in the ring die pellet mill is distributed by high-speed rotation of the roller that has a centrifugal distribution. The materials are non-uniform fed, but in the flat die the materials are entered the equipment vertically by their own weights and they are equally distributed [39]. 3. The ring die has high-speed rotation with high rate of damaging of the materials when they are exiting but the flat die has low-speed rotation with low breakage rate [39]. 4. The ring dies pellet mill are usually used for high bulk density materials but flat die can handle rather low bulk density materials[41]. 5. The capacity of the ring die is much bigger than the flat die. 6. The ring die has a complex structure in comparison with the flat die, so flat die usually used in home or small workshops[26]. 7. In the ring die pellet mill ease of maintenance and operation, only straight forces on the die which makes the machine more efficient in comparison with flat die. Also less wear parts even on rollers and dies is another advantages of ring die pellet mill [19]. 8. Investment cost in ring die pellet mill is lower than flat die[41]. 26

27 Cooling The cooling process is a very simple procedure where ambient air sometime is cooled by a coolant which is flowed over the hot pellets. According to the pelletization procedure, the temperature of the produced waste pellet is between 80 to 100 C [25] and the moisture content is about 15% which makes the pellet slightly forgeable. Cooling is needed to reduce the temperature to 5 C higher than ambient temperature and also about 3 % - 5 % reduction of the moisture [42]. Therefore the final moisture content of produced pellet would be in range of 10-12%. The reasons are that high amounts of moisture in the produced pellets lead to deformation of the pellet when they are dried and caused low stability during handling and transportation. In other hands, high amounts of energy are needed to evaporate the waste in the feed materials within gasification process. There are two basic structures for coolers, horizontal and vertical. The flow in both is counter current. This means that the airflow is in the opposite direction of the feed [42, 43]. In the horizontal cooler, pellets are conveyed on a perforated steel mesh by a transmission belt where cool air stream passes through. In the vertical type, pellets fall by gravity into a chamber through where air is sucked upwards by a fan [42]. The cooler is based on the air flow through columns of hot and moist pellets, which result in evaporation of moisture from the pellets. For moisture evaporation, heat is taken from the pellets leading to cooling of pellets. To achieve the necessary cooling effect, air pressure and volumetric flow rate of coolant, length of column and the residence time will play an important role [35]. 27

28 Chapter 3, Gasification Gasification in general is a process to extract a gas from mixed carbonates materials. In other words, gasification is a process that involves a series of reactions to convert carbon-based material in form of liquid and solid into a gaseous combustible product called syngas or synthesis gas. Syngas is mainly composed of carbon monoxide CO and hydrogen H 2 [1, 2]. Gasification of municipal solid waste Municipal solid waste (MSW) materials have a rather high carbon content and heating value resembled to biomass material [13] as indicated in Table 2. The analyses (C, H, O, N and S) are made on dry ash material. In MSW gasification, the energy content in solid waste is converted to energy rich gas fuel. The remaining material parts that could not be converted in to syngas, such as metal, glass, rock and concrete are vitrified to produce s slag [1, 43]. Some factors have influenced on carbon conversion rate and product gas yields. Factors like density, size and shape of the waste particle, reactor temperature, type and amount of gasification agent, design of gasifier and heat transfers rate [1, 44]. Gasification main steps Drying As the first step of gasification feedstock is heated below temperatures of 200 c and the water content in the fuel is released by evaporation. Pyrolysis and de-volatilization Wet feedstock + Heat Dry feedstock + Steam When dried feedstock is exposed with high temperature in the range of 300 C to 700 C [1] pyrolysis and de-volatilization of feedstock are takes place [4]. Dry feedstock + Heat Char + Volatiles Components In this step, non-condensable volatile components of feedstock are released when the material is heated and cracked. They form pyrolysis gases and the remaining solid material is called char and its organic part consists mainly of carbon. The composition of products gas is depended upon temperature and methods of heat support to the rector [1]. During direct heated gasification, thermal decomposition, the light hydrogen rich volatile hydrocarbons components are release in presents of insufficient amount of oxygen. Then a gas product and solid carbon (char) and also other components like tars, phenols and hydrocarbon gases are formed [1, 4, 20]. In indirectly heated gasification, steam is used as a heating source but steam plays an important another rule as hydrogenation agent. If hydrogen is added to the system formation 28

29 of hydrogen and carbon monoxide will be accelerate. It means that steam plays as a catalytic for gasification reactions at relatively low temperatures. Formation of methane is promoted by the hydrogenation process within the gasification reactor at the same low temperature. [2] The amount of volatile components are varies. It is depending on the source of the raw materials and contains mainly H 2 O, H 2, N 2, O 2, CO 2, CO, CH 4, H 2 S, NH 3, and C 2 H 6. In addition to these compounds, there are also a slight amount of unsaturated hydrocarbons such as acetylenes, olefins, aromatics and Vanillin Syring aldehyde Conifer aldehyde, Whiskylactone, tars and char. The Figure 6, represents the different steps during biomass gasification as well as the products of each reaction [1, 2, 4]. Figure 6. Schematic of biomass gasification[1] Gasification reactions The following stoichiometric reactions take place in a gasifier when oxygen is present [1, 43]. (1) C + O2 CO2 ( H= kj/mol Exothermic) (2) C + 1/2O2 CO ( H= kj/mol Exothermic) (3) H2 + 1/2O2 H2O Inorganic materials in the char + Heat Slag Depending on the gasification process, the reactions are carried out in presence of insufficient amount of oxygen. Most of the supplied oxygen into the gasifier is consumed by the reactions (1), (2) and (3). Results of char combustion are ash, un-reacted inorganic material. They can be melted into liquid slag. Slag can be solidified and formed clinker [2]. In the combustion step, CO 2 and H 2 O are formed when the syngas is burned to supply the necessary thermal energy for the endothermic reactions. By combustion of char carbon, the amount of necessary energy for the gasification reactions is directly provided. In some cases, the required energy is supplied indirectly by combusting fuels separately from outside the gasifier[2, 4]. 29

30 The principal gasification reactions (4) and (5) are water-gas reactions which are endothermic and accrue at high temperatures and low pressures. (4) C + H2O = CO + H2 ( H=131.3 kj/mol. Endothermic) (5) C + 2H2O = CO2 + 2H2 Reaction (6) is known as the Boudourd reaction, which is endothermic and is much slower than the combustion reaction (1) at the same temperature in the absence of a catalyst[2]. (6) C + CO2 = 2CO ( H= kj/mol. Endothermic) Boudouard reaction (7) C + 2H2 = CH4 ( H= kj/mol. Exothermic) Reaction (7) mainly is an exothermic and very slow reaction which is called hydrogasification. (8) CO + H2O = H2 +CO2 ( H= kj/mol. Exothermic) Reaction (8) is called water-gas shift reaction, is important especially where H 2 production is desired. Optimum yield is obtained at low temperatures (up to 260 C) in the presence of a catalyst. It is noticeable that the pressure has no effect on increasing hydrogen yield[1]. (9) CH4 + H2O = CO + 3H2 ( H= kj/mol. Endothermic) Reaction (9) is called steam methane reforming (SMR) reaction which is proceeds very slowly at low temperatures in the absence of catalysts. Also during the reactions (4) through (9) the heat necessary for drying the solid fuel, breaks up chemical bonds, is provided and resulted in the raising in temperature of the reactor to make gasification process. The following chemical and thermal reactions may take place (10) C + H2O = 1/2CH4+1/2CO2 Reactions (4) to (10) are equilibrium reactions which mean that they can go either way. Reaction (10) is relatively thermal neutral, suggesting that gasification could proceed with little heat input but methane formation is slow relative to reactions (4) and (5) unless catalyzed. Summary of gasification reactions are shown in Figure 7[1]. Figure 7. Summary of biomass gasification reactions[1] 30

31 Key factors for gasification of waste The following parameters as key factors for gasification of waste materials have significant impact on gasification performance, chemical composition and heating value of product gas. Gasification agent The gasification agent has a significant effect on the system performance and product gas composition[45]. According to the gasification process, one of the medium substances such as air, oxygen, steam and CO 2 can promote the gasification process. Due to high percentage of nitrogen in the air, gasification of biomass with air have rather high amount of nitrogen in product gas contains e.g. 47.5% by volume dry basis, and low heating value approximately 5.5 MJ.Nm 3. In case of oxygen, the product syngas contains low amount of nitrogen e.g % by volume dry basis depending on the purity of the oxygen, but the oxygen separation from air has a high energy demand and thus is associated with a high cost. Early experimental works by different researchers confirm that gasification of solid fuels like biomass and MSW with pure steam can improve the heating value of the syngas from ~15 MJ/Nm 3 up to 30 MJ/Nm 3 [45, 46] by reducing the nitrogen content in the syngas depending on purity of the steam and ratio of steam to oxygen[46]. When steam is used as gasification agent, a significant raise is observed in yield of hydrogen and carbon monoxide and methane as indicated in Chart 3[11, 45]. Steam agent can provide heat which is needed to maintain the gasifier temperature. Required steam can be generated through a boiler or a heat recovery steam generator (HRSG). Effect of gasification agent on heating value and average composition of the produced gas of the Güssing biomass gasifier is presented in Chart 3[47]. The Güssing biomass gasifier plant is working at C with a feed composition of 50% C, 6% H, 44% O 2 (% wt. dry basis) and 20% moisture content [20, 24, 45, 47]. (a) (b) Chart 3. (a), (b) Effect of gasification agent on composition and heating value of product gas. [42, 45] 31

32 Temperature The effect of reactor temperature on gasification is a key factor of the process. The higher temperatures promote the reaction rate, yield of the produced gas, hydrogen content and the complete conversion of char. The lower temperatures promote methane formation, char, tar also volatile content [2, 11]. At increasing temperatures less methane is formed as well as less concentration carbon dioxide. As it is observed in Chart 4 and Table 3, the content of CO is decreased from 500 C to 650 C and then increased again at 900 C. It means that the maximum concentration of carbon monoxide and hydrogen can be obtained at temperature of 900 C. Also, gradually increased temperature may lead to increase of hydrogen and high heating value of product gas at equilibrium condition for biomass gasification. Table 3. Effect of temperature on gas composition of MSW gasification [2, 45, 47, 48]. Temperature C H 2 mol % CO mol % CO 2 mol % CH 4 mol % HHV MJ/m Chart 4 shows the yield of the main components of synthesis gas as a function of temperature from municipal solid waste gasification [2, 48]. Chart 4. The syngas main components as a function of temperature. [2, 48] Consequently, the gasification temperature can control the concentration of the desired product such as methane, hydrogen or carbon monoxide. However, it must be considered that the reaction kinetic, fuel composition and eventual catalytic material also have a high impact when the optimum operation conditions are determined[49]. 32

33 Pressure Operating pressure of a gasifier is selected according to process requirement and products applications. For instance, the pressure of syngas used for ammonia production should be in range of bars. Also applied gas turbine for power production needs the product gas in the range of 20 bars. Therefore, gasifier may operate at the same pressure [3]. Effects of different pressures on the composition of product gas at 1000 C are shown in Chart 5. As it indicates, increasing pressure makes a gradual increase in CH 4 and CO 2, and slight decrease in CO and H 2 contents in product gas. Chart 5. Effects of pressure on gas composition at 1000 C[3] In industrials application of syngas for power or chemicals production, when a high pressure is desired, it is preferable that the gasifier is pressurized to reduce energy consumption, for reducing equipment size, and decreasing agglomeration of the ashes inside the reactor [1]. It is noticeable that majority of gasifiers are working in the sub atmospheric pressure range since in case of leakage, the outside air will move into the reactor, therefore no potential hazardous gases will come out. Furthermore, applying high pressure generally has influences on reactor materials, gasifier joints, isolation issues and feedstock feeding equipment which is lead to an increase on the cost of a rector and the whole system. Moisture The moisture level has various impacts on reactions temperature and composition of syngas as well as on the energy balance of the process. Appropriate range of moisture content is recommended 10-15% [4]. Heating rate The effect of the heating rate can be seen on volatiles separation during the pyrolysis and the de-volatilization step. A higher heating rate significantly increases a higher porous of char in the steam gasification of biomass as a result of the volatile matters has been rapidly released. Consequently, the increase of the reaction rate of char, as well as volatile yield and also the conversion of biomass can be expected. On the other hand, low heating rate allows char particles to react with volatiles[2]. The heating rate doesn t influence the elemental composition of char[50]. 33

34 Heating method The gasification process involves a series of endothermic and exothermic reactions, so the required heat should be supplied. Gasifies are classified as autothermal or allothermal depending on how the heat is supplied. In the autothermal method or direct heated gasifier, the necessary heat is provided by exothermic reactions paths through a partial oxidation of the feedstock inside the reactor. But in case of allothermal or indirect heated gasifier, the heat needed is provided by an external source. The heating value of product gas by allothermal method is greater than autothermal as it is shown in Table 4 [2, 3, 24]. Table 4. Effect of heating method on heating value of product gas[2, 3]. Producer gas Gasification Agent HHV of Product Gas (MJ/Nm 3 ) Aautothermal (Direct heated ) gasification Air 4-6 Pure oxidation gasification O Allothermal (Indirect heated) gasification Steam Feedstock heating value The heating value in general term refers to the amount of heat releases from the combustion reaction which is indicated by weight unit for solid fuel (MJ/kg) or volume unit for gas fuel (MJ/Nm 3 ). There are high and low heating values for the fuels: The High Heating Value (HHV) is the amount of heat released from the complete combustion reaction where the energy released during water condensation is taken into account[51]. The Low Heating Value (LHV) is the amount of heat released from the combustion reaction where the energy released during water condensation is not taken into account[51]. In other word, it is the heat released from the combustion reaction where the water produced is steam. Heating value of feedstock depends on the amount of moisture and combustible organic material [14]. In Chart 6, the heating value of different biomass and coal is presented. As it indicates, the highest value is related to the coal and the rest which are corresponded to the biomass is in the range of (MJ/kg) [2, 3, 51]. Chart 6. Average heating value of gasification feedstock 34

35 Waste particle size and preparation steps Type of gasifier determines density, size and shape of the waste particle. They influence the heat transfer within the gasifier bed. For instance, in case of an entrained flow reactor the feedstock has to be in range of hundreds of µm and for a fluidized bed reactor, it should be in the range of a few mm, larger particles are accepted in fixed bed gasifier[4] however, the higher ranges lead to un-treat feed particles in the process[1, 44]. Ashes The composition and melting point of the ash have great impact on ash behavior particularly at high temperatures, and on the accumulation rate in the reactor. Moreover the amount of ashes can be influenced on the ash discharge system and type of the gasifier[52]. Pollutants level Pollutants can be classified into tar, particles and heavy metals. Level of tars is one of the limiting factors in the gasification process. Tars refer to condensable organic or inorganic compounds which are present in raw product gas. Tars make fouling or inactivating of catalytic filters[49]. The particles like mineral substances in fuel, un-reacted solid materials and part of fuel entrained with gas products, outcome as pollution. Pollutants can be removed by cyclones, filters or scrubbers. Feedstock like MSW has quite low level of heavy metals content. If they are present in high amount, they must be removed through e.g. filtration[20]. Gas utilization In order to utilize gasification produces, gas cooling and gas cleaning procedures are required. The quality of syngas is depended on mentioned gasification key factors and applied gas cleanup technology as well as feed composition. Cleaning process from the producer gas stream involves removing unwanted components including particulates, alkali, tars, sulfur, and ammonia. A gas cleaning system may consist of cyclone separation, gas cooling system, low temperature gas cleaning, high temperature gas cleaning, acid removal, sulfur recovery, CO 2 removal, gas reforming and Fischer Tropsch synthesis. Obtained product gas through biomass gasification can be upgraded to hydrogen-rich synthesis gas. The synthetic gas can be further converted to liquid (Fischer Tropsch synthesis) or gaseous fuels and chemicals, including fuels such as methanol, dimethyl ether (DME), and synthetic diesel. However, the raw product gas contains both gas and particle impurities which can have negative affects to both catalysts and hot-gas filters [53]. 35

36 Types of gasifier Based on the reactor design they are fixed bed, fluidized bed and entrained bed gasifier. In the following the types of the gasifier will be explained [1, 20]. Fixed bed gasifier Updraft gasifier In this simple type of gasifier (Figure 8), the feedstock is fed from the top of the gasifier and passed through the different zones in the gasifier such as drying, pyrolysis, oxidation and reduction zones. Gasification agent is injected from the bottom. The produced gas leaves the gasifier from the top in the converse direction of the feedstock. One of the advantages of this type is high thermal efficiency due to internal heat exchange that gives a low temperature of the exit gas. Another advantage is that rather large size of the feedstock in range of mm can be used. The main drawbacks of this type are high amounts of tars and pyrolysis products which they are not combusted therefore, for power production application expansive gas cleaning is required [1, 20, 54]. Downdraft gasifier The feedstock in the downdraft gasifier is fed from the top (Figure 9), but the produced gas is exit from the bottom in the same direction of the feedstock. Gasification agent injects from the top or from the sides of the gasifier. In this type the feedstock passes the zones mentioned above in the updraft gasifier with some different in the locations of the zones. The content of tar in the produced gas is relatively low and it is appropriate for engine applications. When gas passes the oxidation zone small ash particles enter to the gas and caused high dust and ash content of the gas, also increased the temperature of the exiting gas which is lead to the low efficiency of the gasifier and that is the drawback of this gasifier. There is some limitation for the feedstock used for this type. For example the moisture content should be less than 25%, also in the range size of mm[1]. Heat transfers between feedstock and medium agent in fixed bed gasifier is not optimal due to different zones that are in these types. Also they are usually used in small scale plant for production of power and heat [1, 2, 54]. Figure 8. Updraft gasifier [1]. Figure 9. Downdraft gasifier [1]. 36

37 Fluidized bed gasifier Using fixed bed gasifier have limitations like high ash content, hot spots and limitation in using small particles due to blocking and increasing the pressure drop. To overcome the problems of fixed bed gasifier and use gasifier in large scale plant, fluidized bed gasifiers were developed. The bed materials behave as a fluid in contact with medium agent which is steam, air or oxygen to increase the reaction rates and heat transfers. Usually sand is used as bed material to provide good contact with the feedstock. The different zones in fixed bed gasifier cannot be distinguished here due to compact mixing of the materials. The temperature is rather similar in the whole bed and can be controlled in the range of C [1, 2]. Particles are become suspended and fluidized at minimum fluidization velocity (µ mf ) by means of the inlet velocity of medium agent µ 0 which is an important parameter for designing the fluidized bed gasifier[55, 56]. In comparison with fixed bed gasifier, the fluidized bed gasifier has several benefits. The fluidized bed gasifier has high heat transfers and reaction rates due to the fast mixing and also they have identical temperature in the bed without hot spots. The fluidized bed gasifier can accept different particles size with different shapes. Also these types of gasifiers have drawbacks. They have rather high content of dust and tar in produced gas. They have also low conversion efficiency due to their operating temperature range. The fluidized bed gasifier requires power consumption for fluidizing media and they have complex operation. There are two types of fluidized bed gasifier according to the condition of the suspension when it is small is related to bubbling fluidized bed and when it is high related to circulating fluidized bed[1, 56]. Bubbling fluidized bed gasifier Bubbling fluidized bed (BFB) gasifier (Figure 10 and Figure 11) is commonly used in industry. In the structure of this gasifier, the freeboard above the bed is separated from the fluidized bed reaction zone. Fluidization rate in this type is in the range of 2-3m/s[55, 57] which is low and avoids from some of the fines entrainment [1, 58]. BFB gasifier is possibly the lowest capital cost option among the advanced waste gasification technology. Figure 10. Schematics of bubbling fluidized bed [48] Figure 11. BFB gasifier [7]. 37

38 Circulating fluidized bed gasifier In the circulating fluidized bed (CFB) gasifier (Figure 12 and Figure 13), there is no separation between the freeboard and fluidized bed reaction zone. There is just a small bed left of the material. The fluidization rate in this type is in the range of 5-10 m/s which is higher than bubbling fluidized bed that lead to particles exit with produced gas. The particles are removed from the produced gas by means of a cyclone also char particles are recycled to the gasifier in order to improve the char conversion rate [58]. Figure 12. Schematics of circulating fluidized bed[48] Figure 13. CFB gasifier [7]. Entrained flow gasifier The feedstock for entrained flow gasifier (Figure 14) has to be very small and pulverized. The common feedstock for entrained flow gasifier is coal due to its slurry property. For biomass becomes difficult and costly due to the required pretreatment for pulverizing them. Operational conditions for this type of gasifier are at high temperature range of C and high pressure between bar, short residence time approximately 1 second with large capacity more than 100 MW.[1, 20] High temperature and pressure improve the heat transfers between the feedstock and medium agent also avoid from the present of methane and tar in produced gas. But leads to decreasing the thermal efficiency of the gasifier due to the high temperature of the produced gas. The product gas should be cooled before used for power generation. That is the reason why it is more efficient to convert the produced gas in chemicals instead of power applications[20]. Figure 14. Entrained flow gasifier [14] 38