Energy Engineering - Chemical Processes for energy vectors

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Complete course

ORGANIC CHEMESTRY (15.09) Chemistry of carbon compounds Hydrocarbons are the main components of fossil fuel, they are made mainly of carbon and hydrogen. Biomass is more complex than hydrocarbon since it also contains oxygen . Tetravalency of carbon: carbon have the ability to form four bonds. Paraffin (or alkane): it is an acyclic saturated hydrocarbon. In other words, it co nsists of hydrogen and carbon atoms arranged in a tetradic structure in which all the carbon –carbon bonds are single . The general form of paraffins is %�*6�>6 - Methane ( %*8): it has 109,5° between bonds. - Ethane ( %6*:) - Propane ( %7*6?6��?8�� ND: number of double bonds NT: number of triple bonds THERMODYNAMIC STABILITY The Francis diagram plots the Gibbs free energy of formation of the hydrocarbon starting from C and H2 versus the temperature. In other words, is the Gibbs free energy of the reaction starting from C and H and producing the hydrocarbon . Thus, Francis diagrams expresses the stability of the hydrocarbons with respect to its components. In particular, it is possible to notice that the GFE (i.e. instability) is incre asing with temperature (except acetylene). When the GFE is negative it means that the hydrocarbon is more stable than its component , the opposite is true for positive GFE. Temperature is a thermodynamic driving force for the reaction of cracking (for examp le steam reforming). Trivially, larger molecules with more carbons have higher GFE but it is also true that using the ratio GFE/#carbons the trend is the same and moreover the curves are closer and closer with higher carbon atoms. Since GFE/#carbons is a parameter indicating the change of stability of the single carbon depending on the molecule in which this carbon is present, the higher the number of carbons, the higher the thermodynamic driver of cracking (and lower temperature th reshold for cracking). )�4 O r and therefore -�� P s there is a thermodynamic driver for the reaction. It can be seen that fully saturated HC ( paraffins ) are more stable than unsaturated HC at low temperature while the opposite is true for high temperatu re Lessons 21 September (1:20) CHARACTERIZATION FACTORS (21.09) There are several technical/practical parameters which re -managed physical properties and are used in the oil industry. Usually, all these characteristics (properties and composition) are all included in the crude oil assay . The assay provides critical inf ormation on the suitability of crude oil for a particular refinery and estimating the desired product yields and quality. It also indicates how extensively a given crude oil should be treated in a refinery to produce fuels that follow environmental regulat ions. API GRAVITY Specific gravity (SG) brings information about the relative density of crude oil with respect to water. It is defined as the density of the liquid material at 60°F (15.6°C) divided by the density of liquid water at 60°F (0.999 g/cm3). In the early years of the petroleum industry, the American Petroleum Institute (API) adopted the API gravity (°API) as a measure of the crude oil density. The API gravity is calculated from the following equation: �#2+ L svs
�w 5) 59� Fsus
�w The higher the A PI, the lower the density. This derived parameter allows to exalt even small differences in the specific gravity. Liquid hydrocarbons with lower SGs have higher API gravities. The API of crude oils varies typically between 10 and 50, with most crude oils falling in the range of 20 -45. Using API gravity, the conventional crude oils can be generally considered: - Light with �#2+ P ur - Medium with ur P �#2+ P tt - Heavy with �#2+ O tt (hard to refine) The heavy (high -density) crude oils tend to have high concen trations of aromatic hydrocarbons, wh er eas th e light (low -density) crude oils have high concentrations of paraffinic hydrocarbons. The relationship between °API and specific gravity is not linear. Therefore, the °API gravity of crude blends cannot be calculated by linear averaging of the component °APIs but in practice averaging is admitted. VISCOSITY Viscosity is a physical property of a fluid that describes its tendency/resistance to flow . Power requirement to transport (e.g., to pump) a fluid depen ds strongly on the fluid’s viscosity thus viscosity is related to the cost of transportation. � L (@ 5R Interestingly, the viscosity of liquid decreases with increasing temperature , while viscosity of gases increases with increasing temperature. Among p etroleum products, viscosity constitutes a critically important characteristic of lubricating engine oils. Viscosity of liquids is usually measured in terms of kinematic viscosity , which is defined as the ratio of absolute (dynamic) viscosity to absolute d ensity ( ν = μ/ ρ). Values of kinematic viscosity for pure liquid hydrocarbons are usually measured and reported at two reference temperatures, 38°C (100°F) and 99°C (210°F) in cSt. However, there are different standards. K-FACTOR It is way of classifying a crude oil according to its paraffinic, naphthenic, intermediate or aromatic nature . -��� � L 6> 57 5) 59� � 6> : volume, or mean average normal boiling point in R (degree Rankine) 5) 59� �: specific gravity at 15.6°C (60°F). To calculate K UOP or K W, volume average boiling point (VABP) or mean average boiling point is used, respectively. Depending on the value of the Watson characterization factor, crude oils are classified as paraffinic (Kw = 11 -12.9), naphthenic (Kw =10 -11), or aromatic (Kw  ^ @C E AD The net reaction rate is an intensive property defined as: DN L DN> FDN? L F s =D8 D@D0� D@DP L F s >D8 D@D0� D@DP L F s @D8 D@D0� D@DP L F s AD8 D@D0� D@DP L Where D8 is an extensive property such as ( reactor volume, catalyst volume, catalyst weight, catalyst internal surface …). The reaction rate depends on three parameters: DN L DB:D6
�D2
�DT�; Something also the contact time but this only affects the overall conversion not the reaction rate. Usually , the effect of temperature is separated from the that of composition: DN L DG:D6; �DB:DL�
�DL�
�
�; where DG:D6; is the rate constant or rate coefficient and it is derived from the Arrhenius equation. The only case in which the effects of pressure and composition is known a priori is the one of elementary reaction. A reaction is considered elementary if it occurs in a single step that cannot be divided into further sub steps . DN L DN> FDN? L DG>DL��DL�p FDG?DL��DL�� Only for elementary reaction the reaction order s equal the stochiometric coefficients. The rate constants depend on the tempe rature according to the Arrhenius equation : DG:D6; L DG4exp @?���� A Where DG4 is the pre -exponential factor (generally assumed to be independent on T) and it is correlated to the number of active sites available on a specific catalyst. And D'� is the activation energy which represents the energy barrier that needs to be surmounted to make the reaction happening (it fixes the slope of the exponential) . POWER LAW It is empirical approach which d escribes the influences of partial pressure on the reaction rate. From the mathematical point of view, it consist s of finding the polynomial expression that fit the experimental data . DN> L DG>DL���DL���DL��� (same result for the backward reaction) Where DJ�
�DJ�
�DJ� are reaction orders (positive, negative, integers, fractional) of reactants and s (speci e not present in the stoichiometry ), they differ from the stochiometric coefficient s, and they are adaptive number that must be determin ed experimentally. The limitations of the empirical approach are: - In most of the cases the power low doesn’t work since the expression is not a ble to describe the data . - Even when the power low works , it can note predict what will happen outside the interval of experimental data . The power law doesn’t allow extrapolation and the results are only valid within a narrow range of conditions . FORMAL MODELS (OR MECHANISTIC MODELS) They are more robust , are valid for almost all the reaction and even they allow extrapolation, but they required assumption of the steps of the reaction therefore are more difficult to build . FLUID CATALYTIC CRACKING (FCC) (14.10) As all the cracking processes, the products are in liquid, solid and vapour phase . In this case, the process is design in order maximize the liquid fraction for gasoline production (more or less a third of the components of the gasoline are FCC products). It is an endothermic process thus heat must be provided. Gasoline products mainly consist on ? With respect to the thermal cracking, the catalytic one presents a reduction of the temperature requ ired by the reaction (promotion of the kinetics thanks to the catalyst ) and an increase in the reaction rates (thanks to the formation of intermediate with lower energy). The increase in the speed of the reaction allows to have a more compact reactor. More over, when there are several possible pathways, the catalyst increases the selectivity (yield of a specific product). With thermal cracking there are a lot of products possible, thus there is no selectivity. Remember that with catalysis it is possible to improve the kinetics (reduce the energy barrier) , but the thermodynamics is the sam e as well the equilibrium conditions. It is a due unit process: - Reactor : it is in charge of the reaction - Regenerator : it is in charge of the continuous regenerat ion of the catalyst though a combustion reaction where the carbon is burned. The two processes must be carefully separated otherwise the hydrogen will react with the oxygen. The need a regenerator is because the reaction occurs so fast that there is an ine vitable layer of solid by - products (coke) that tends to accumulate on the surface of the catalyst so that the catalyst loses its activity (becomes useless). Since the coke is burned there is the formation of flue gas. It must go through an after -treatment unit in order to remove the solid particles, SOx (in the FGD ) and NOx (in the SCR ). Selective Catalytic Reduction (SCR): vD0D1 E vD0D*7\ vD06 E xD*6D1 Ammonia in injected in the flue gas and together with oxygen, nitrogen and water is formed. The feedstock is a complex mixture composed by: - The oil from atmospheric distillation . - The vacuum gas -oil . - The alkanes from deasphalter . - The liquid products from coking . The products of the process are: - Gas and LPG (C3 -C4) - Gasoline - Light Cycle Oil (LCO): it is in the range of diesel fuel , but it has poor cetan e number. It can be used as a blend for the diesel or can be up gradated into diesel. - Heavy Cycle Oil (HCO ): it is composed by polynuclear molecules with a lot of polyaromatic molecules. - Sour water that must be treated since it is containing of nitrogen and sulphur compounds and hydrocarbons. Its presence is due to the fact that water is a co -reactant. The mo st important feature of the cracking catalyst is the acidity which is property of releasing protons (H+). The proton is the result of a strong polarization of an OH bond due to the fact that oxygen is a strong electron attractor when it is bonded with meta llic atoms (metal oxide like ceramic materials ). The acidity promotes the rupture of carbon bonds in a reaction called C-C bond cleavage. Besides C -C bond cleavage a large number of other reactions occur: - Isomerization: reaction consisting in the rearrangement of the molecules in a different isomer (some number of atoms but differently organized ). - Protonation/deprotonation : reaction consisting in the movement of the proton (H+) from a molecule to another - Alkylation : it i s a condensation reaction between hydrocarbons. It is the opposite of the C -C bond cleavage. - Polymerization : small olefins tend to polymerize and form unsaturated rings (benzine) . - Cyclization : it is the closure of the ring, and it is thermodynamically favoured . - Condensation : it is the formation of solid carbon. From the Francis diagram, it is possible to notice that there is a thermodynamic driving force to pass from large molecule to small molecule . But these processes don’t lead always to methane (C1) since olefins have a slower temperature dependence (slope in the Francis diagram) on stability, so it happens that over a certain temperature , olefins tend to become more stable than the corresponding paraffinic . The graph represents the distribution of products (carbon number). With a thermal process the maximum is for D%6 since the olpehins D%6D*8 are not thermodynamically favourite to go to methane. Methane is still present, but it has been formed starting from longer molecules not small olefins . With a catalytic process the product distribution is completely changed: there is a minimum formation of D%5 and D%6 while the yield for the products good for gasoline is strongly improved. With catalytic cracking the maximum is around C4 because long chain hydrocarbon s are chopped well becoming C7 and C8 but then the process becomes less efficient due to the fact that is based on the formation of carbenium ion (tertiary carbon can be made on ly starting from molecul es longer than C6) and not only on the thermodynamic driving force The cracking mechanism relies on the formation of special intermediates that are called carbenium ions which are the result of the reaction of a hydrocarbon with a proton (they are ionic species) . Carbenium ions are only present on the surface of the catalyst (absorbed form). The carbenium ions are classified depending on the kind of carbon on which the plus charge is present. If the plus charge is in the last carbon atoms, it is called primary . If the plus charge is in a carbon also bounded with two carbon atoms, it is called secondary. … The probability of formation (and thus the speed of reaction) of these different carbeni um ions deals with stability, in particular the relative stability of carbenium ions decreases in the order: Tertiary > Secondary > Primary > Ethyl > Methyl This happens because the electronic anomaly can be distributed among more surrounding atoms and determines the selectivity of the reaction , the more stable species are the one produced. In other words , there is the faster formation of the more stable ion, because of the lower energy barrier . The fact that there is a reaction happening at higher reaction rate (speed) is what drives the selectivity (not the fact that there is only one route) . The most important reaction of carbenium ions in catalytic cracking is the scission of C -C bonds where a long molecule with the ion charge will tend to break in beta (in the C -C bond next to the plus charge) producing an ole fin and a smaller ionic species. This reaction is very fast (it is the faster) since we are starting from a tertiary carbon, and we are obtaining a tertiary carbon. Example: normal -C7 The C2 interacts with the proton donated by the catalyst, then the molecule rearranges such that H+ is shared between C2, C3 and C4 creating a cloud of carbon all secondary (all equivalent) interacting with the hydrogen. This cloud is the precursor of the branching (isomeric functionality of the catalyst) since the hydrogen will bond with the stabler con figurat ion which is the tertiary. In other words, the rule of the relative stability of ions has change the shape of the molecule. Finally, the bond in beta tends to break. This process involving the re-arrangement cannot be performed by molecules shorter that C7 due to a degree of freedom reason. It requires long chain. The relative stability of carbenium ions makes the branching (tertiary carbon) more favoured. Double bounds and branching are good conditions for improving the octane number of the gasoline. Therefore, with catalytic cracking it is possible both to increase the yield and the quality of the components of gasoline with respect to the therma l process . CATALYST (18.10) The catalysts required must have a very strong acidity , the hydrogen sitting on the surface of the catalyst is given by a strong polarization of an O -H bond since O belongs to a structure of oxid e in between Al and Si (the presence of acid sites is due to the fact that some Al ions occupy the place of Si ions). The zeolites are the catalyst used in FCC , they are 3D structures of Si 4+ (the prevailing part) and Al 3+ oxides, forming cages and cavities with regular size and distribution . Zeolite is a microporous material since the size of the cavities is comparable to the molecular one creating the size selectivity meaning that not all the molecules can enter and have access to acidic site (cages are more accessible to linear paraffins than branched ones or aromatics ). Due to the extremely high activity, zeolites are diluted with a n amorphous with larger pores and lower activity (silica -alumina). The matrix is only moderately active , but it contributes to crack large molecule (asphaltenic molecule ) into smaller molecule that can diffuse inside the zeolite cavities (staged cracking) , the polynuclear fragments of the cracked large molecules remains untouched since they cannot enter the zeolites’ sites (linear structure can enter the zeolite, benzene -like structures cannot). Micropores materials are characterized by an extremely high surface area (i.e. the surface of the pores per unit of weight of catalyst) thus the number of available active sites is huge but the accessibility of these active sites is limited by the very low diffusion inside the pour . The best way to fully use all the catalyst is to h ave particles that are not much bigger than the diffusion length . REACTOR The reaction occurs very fast (few seconds) essentially in the riser reactor (basically a tube or moving bed reactor ) where the mixing of the regenerated catalyst with the oil feed happens. The disengager is a separation unit (cyclones) where the deactivated catalyst s (covered by a coke layer) fall down, and the hydrocarbon s stream rises up with also the help of cyclones. There is also a steam injection (steam stripper) in the bottom counter currently flowing around the coke in the catalyst particles in order to release the “good” hydrocarbons that has remained glued around the catalyst (always in the disengager) . Finally , the deactivated catalyst is moved to the regenerator where it is burned in the fluidized bed . In the regenerator there is a gradient of density since the burned catalyst is lighter ad stay in the upper part. Note that the heat release in the combustion and the heat consumed in the cracking reaction (globally, the heat of reaction typically ranges between 900 and 1000 kJ/kg) must be balanced since there is not additional heat input from outside. D4 is the combustion efficiency of the regenerato r, that is the ration between heat adsorbed by the catalyst and heat produced by the combustion. D3��� L ���� D9��� D3���� D4 Where ���� is the weight of coke consumed in the regeneration over weight of catalyst and D9��� is the flow rate of the catalyst. If the process has very little production of coke (high quality feedstock) , then there is not enough coke to thermally balance the endotherm icity of the cracking reaction or, o n the other hand, if the heat released in to o high it is possible to start and unselective thermal cracking. Therefore , the feedstock must have the right quantity of nasty compounds. The feed is diluted with steam for better atomization and fed to the riser reactor together with regenerated catalyst. The mixture flows upward and cracking take place in few seconds . The spent catalyst is separated from the reaction mixture in a cyclone. Steam is added to the downcomer in order to strip adsorbed heavy hydrocarbons off the catalyst. It also creates a buffer between the reducing environment in the riser and the oxidizing environment in the regenerator. The catalyst is transported to a fluidi zed bed regenerator where coke is removed from the catalyst by combustion with air. Due to the limited life of commercially applied catalysts, approximately 30 days, up to 5% fresh catalyst is added every day. The flue gas is filter ed and then will go t hro ugh a cleaning process. The hydrocarbon mixture at the exit of the disengager go through a cooling stage (heat recovery) and then a distillation column where the components are separated based on the weight . In this stage is important to separate the LCO (in the range of diesel) which can blend with diesel after some upgrades. From the top, all the stuff lighter than the diesel are recovered : they pass through different condensation units and then the re is separation of the gasoline from the lighter compo unds . If the feedstock contains sulphur , it will be gassified though two possible paths : - If it is only present in the coke, it will be burned in the regenerator and it will become SO 2 (fully oxidized ). - If it is present also in gas phases in the disengager , it will become H2S (fully reduced) . But there is a way to reformulate the catalyst (adding to the amorphous matrix some solid sorbents) in order to trap the sulphur in the regenerator on the surface of the catalyst and the n release the sulphur in the disengager so that there is a higher formation of H2S since there is a better equipment for H2S removal then SO2 removal. HYDROTREATING (19.10) It is essential a clean -up process that uses hydrogen and the main goal is the removal of heteroatoms by the selective braking the C -S and C -N bonds . The removal of heteroatoms (S, O and N) and the purification process happens thanks to hydrogen. Also, the remov al of metallic atoms can be obtained using hydrogen as a purification agent. Mainly for this reason, r efinery is a huge consumer of hydrogen. D# EDJD*6\ D$ E ] D*6D5 D0D*7 D*6D1 The purification is very important not only in order to limit the emissions (environmental protection) but also to avoid the deactivation (poisoning) of the catalytic unit by sulphur absorption . The deactivation happens because H2 created by cracking reacts with N 2 to create NH 3 (ammonia) that is a very basic compound , so it deactivates the acid sites of the catalyst. One other effect of hydrotreating is the improvement of gasoline properties ( odour , colour , stability and corrosion) . But w hen the hydrogenating is too heavy , there is a decrease in the octane number of gasoline , because fully saturated molecules have a low octane number and unsaturated molecules have a high octane number (aromatics, for example) . It takes different name base on the final purpose of the process: - Hydrodesulfurization (HDS) : if the main goal is the elimination of sul ph ur, it will form SOx with negative environmental impact . - Hydrodenitrogenation (HDN) : if the main goal is the elimination of nitroge n, it will form NOx with negative environmental impact . - Hydrodeoxygenation (HDO) : if the main goal is the elimination of oxygen , it will decrease the calorific value of the product . - Hydrodemetallization (HDM) : if the main goal is the elimination of metals . THERMODYNAMICS The equilibrium constants are large and positive for all reactions considered under practical reaction conditions (350 -425°C, shaded area). All the hydrotreating reactions are exothermic with ∆H°1200°C) in which a carbonaceous material is transformed into carbon dioxide, water and heat under an oxygen -rich atmosphere. It is a highly exothermic process. Types of combustion: - Complete: all the fuel is oxidized, no intermediate products are formed - Incomplete : s ome elements are not fully oxidized - Stochiometric : c ombustion with the exact amount of ox ygen to obtain the complete oxidation of the products. GASIFICATION The thermochemical conversion process in which a carbonaceous material is transformed into product gas, mainly formed by CO, H2, CH4, CO2 and light hydrocarbons. This process takes place at mod erate -high temperatures (700 -1200°C) in an atmosphere with a small amount of ox ygen, less t han that required for stoichiometric oxidizing conditions. Gasification and pyrolysis reactions are mainly endothermic. The heat required for the process could be o btained directly (from the combustion reactions) or indirectly (from a separate combustion process). The oxygen needed for gasification might come from different gasifying agents: air (low gas HV) , steam, CO2, oxygen (high gas HV) or a combination of them. Benefits : - Increase the heating value of the fuel - Remove S and N compounds - Reduce C/H mass ratio in the fuel - Increase the energy density - The dependence on fossil fuels is reduced - The net contribution of CO2 is zero Applications : - Combustible for gas turbines or internal combustion engines - Production of hydrocarbons from methane and gasoline to wax - Production of syngas: H2 and CO - Production chemicals: hydrogen, methane, ammonia, met hanol... Gasification steps: - Drying Devolatiliza tion/pyrolysis : v olatile release, tar and char generation - Gas oxidation and reforming - Char oxidation: %:�; E 16 L %1 6 - Char gasification : %:�; E *61 L *6 E %1 %:�; E %1 6 L t%1 %:�; E t*6 L %* 8 Gasification reactions: - Oxidation reactions: they need oxygen and they are exothermic . - Char gasification reactions: they don’t need oxygen , they are prevalently endothermic - Water gas -shift reaction - Methanation reactions - Steam reforming reactions Gasification parameters: - Equivalence ratio (ER): the ratio between the air flow rate introduced into the gasifier and the stoichiometric air flow needed for the complete combustion of biomass . - Typical values are between 0.2 and 0.4 o Below 0.2: higher production of tars o Above 0.3: higher temperature, lower tars and lower HHV (higher oxidation and CO2 concentration) o Close to 0: pyrolysis reaction (high production of liquid) - LHV and HHV: these values depend on the gas composition . - Carbon and h ydrogen conversion : the ratio of carbon or hydrogen mass flow in the dry product gas to the mass flow rate of the relevant element in the dry and ash -free biomass . - Biomass conversion : how much of the biomass is converted into gas . - Gas yield (GY): the amount of product gas per unit of biomass on a dry ash -free basis . - Cold gas efficiency: the fraction of the energy in the fuel transformed into chemical energy in the product gas . REACTOR TECHNOLOGIES (in particular for gasification but there are not huge variation s for combustion or pyrolysis ) MOVING BED (UPDRAFT ) When the fuel is introduced into the reactor, it faces the produced gas which is at high temperature, thus the drying process happens. Since all the oxygen entering with air is consumed in the lower part, the pyrolysis and gasification reaction take place in a no -oxygen ambient. The combustion region is the one at the highest temperature, after the combustion region the temperature decreases due to the endothermicity of the following reactions. The %1 6 production in the combustion is used in the gasification process , thus there is a lower %1 6 concentration at the exit. Since t ar is the liquid fraction of the pyrolysis and the temperature in the pyrolysis region is low , cracking reaction is not signifi cantly so the tar content is high . High gas heating value because of the h igh tar content and h igh amount of hydrocarbons . The sensible heat of the gas is recovered for the pyrolysis and drying of the fuel , thus the reactor has high thermal efficiency. This type of reactor allows to removal easily the ash residue collected in the bottom part. Main characteristics : - Biomass with h igh ash content (up to 25% ) and h igh moisture content (up to 60% ) - Suitable for low volatile fuels (i.e: charcoal) - Tar producti on around 30 – 150 g/Nm3 - High CGE - Small units MOVING BED ( DOWNDRAFT GASIFIER) The air enters in the middle of the reactor and then it goes down with the biomass. The tar production is low thanks to the fact that all the liquids (including tard) produced in the pyrolysis cross the combustion region where the temperature is very high allowing the cracking reaction. The HV of the gas is lower than the one produced with an updraft gasifier because the gas produced during pyrolysis is burn during combustion. Also in this case the ashes are easily removable. Main characteristic : - Biomass is c rushed or pelletized relatively dry (less than 30% ) - Low ash content: less than 1% - Tar production around 0.015 – 3 g/Nm3 - Maximum mass fuel rate: 500 kg/h ENTRAINED FLOW GASIFIER It is mainly used for coal gasification which is introduced in the reactor in very small particles (125 –600 �I ) at the same time as the air (or oxygen or steam) . The operating temperature is high (around 1 200°C) as well as the operating pressure (20 ba r). The tar content is low due to the resident time in the combustion zone. FLUIDIZED BED GASFIER All the processes take place simultaneously in the reactor , there is no separation between zones . The d ifferent possible configurations are bubbling, circulating or dual : Main characteristics: - Uniform temperature distribution . - Good gas -solid mixing, and heat and mass transfer . - Wide vari ety of feedstock quality and size distribution. - Moderate temperatures: for biomass with high alkali con tent this could lead to agglomeration risks and it is necessary to reduce the temperature . - Low -medium tar content (cleaning methods are required). - High particle and ash content in the product gas. FLUID IZATION Process in which determined solids, generally with small particle size, behave as fluids when they are in turbulent movement in a gaseous flow. Ergun’s equation for minimum fluidization conditions . Minimum f luidization velocity is affected by temperature (at higher temperature the velocity is lower). Depending on the properties of this material the fluidization might be easier or more difficult. Typical materials in thermochemical fluidized beds (only sand -like particles) : - Silica sand (inert): the most used - Natural minerals : alumina, olivine, magnesite, dolomite... (catalytic properties) - Prepared catalysts: metal -based materials such as Ni -based catalysts GASIFICATION MODELS Sometimes it is difficult to get access to experimental facilities to get information about the gasification process. Experimental campaigns are expensive (huge amount of different and costly equipment) and with a high time consuming (plant preparation, experimental test, plant cleaning and data analysis). It is important to dev elop gasification models to predict the gasifier outputs due to the high cost associated with the experiments. THERMODYNAMIC EQUILIBRIUM MODEL Independent of the gasifier design , it is useful to study the influence of the fuel properties and operation par ameters. It g ives an estimation of the maximum achievable yield of the products. - The equilibrium constant: s imultaneous solution of the stoichiometric and equilibrium equations to give the product gas composition in the equilibrium. - Gibbs free energy: the equilibrium is reached when the Gibbs free energy of the system is minimized, and the mass balance is satisfied . Only the elemental composition of the biomass is required . - Kinetic: it provides the reaction time, the residence time and the reactor hydrod ynamics . The model depends on the type of reactor. - Neural network - CFD BIOFUELS Since transportation accounts for 30% of the %1 6 emission of the world, the main purpose of biofuels is to reduce this effect. Note that biofuels affected only the global %1 6 emission problem, not local pollution due to transportation. DEFINITION Fossil fuel energy is usually required to convert the biomass into biofuels. Besides economic aspects, the main objective of biofuel production is to maximize the renewable ener gy content of the fuel so as to minimize the environmental impact. All these indexes express how much energy is inside the fuel with respect to how much energy is consumed to produce it, thus they must be as high as possible. Net Energy Ratio: 0'4 L �� � ����� ������� ��� ����� ������ L H �������� I H �������� I Renewability: 4J L ��� ����� ������� ��� ������ ����� ������ L H �������� I H �������� I Cumulative Energy Demand : %'& L ������� ������ �� ��� ���� ���� ������ L H �������� I H �������� I Advanced biofuel : they are introduced with the “RED II” directive from the EU. For this biofuel the soil is specifically cultivated for the produc tion of the crop for biofuel conversion and the crop is not in competition with the food industry. In RED II, the overall EU target for Renewable Energy Sources consumption by 2030 has been raised to 32%. Member States must require fuel suppliers to supply a minimum of 14% of the energy consumed in road and rail transport by 2030 as renewable energy. BIOETHANOL PRODUCTION Bioethanol is produced through a biochemical conversion process with biomass characterized by high carbon content (C/N < 30, typical of young biomass) and high moisture content (>30%) due to the yeast . Ethanol is a complex molecule whose production from glucose is: %:*56 1:\ t%6*91* E t%1 6 .*8 ������� L t x
� z �� �� Basics of bioethanol are fermentation where sugars (molecule very similar to glucose) are converted into alcohol. The easiest way to make ethanol from biomass is to use sugar producing plants such as sugar cane , s uch sugars are directly fermentable. Considering glucose as the sta rting molecule, bioethanol is usually produced by sugarcane, sugar beet, but also from corn, potatoes, etc. The next easiest way is to use plants that produce starches (amidi) that can economically be transformed enzymatically into sugars and then fermente d (manioc roots ). With cellulose it is necessary to break the bond between the glucose through hydrolysis. The process depends on the feedstock: the closer is the primary molecule to glucose the easier is the process to produce ethanol. Therefore, s ugarca ne and sugar beet are easier to be converted than corn/potatoes and wood, e tc and moreover they required less energy in both cultivation and production of fuel . The first step is chopping then the harvested sugarcane is milled for the extraction of sucrose and bagasse (bagassa) . In this process, water must be added to the sugarcane. Sucrose is the primary source for bio -ethanol production while bagasse (possibly with low moisture content) is burned to cover the heat demand of t he production process making it self - sufficient (bagasse cannot be converted into ethanol) . Refining consists of chemical treatments and pastorization of the sucrose. The passages are sucrose to vinasse, which is then evaporated to separate the sugar from molasses. Sugar and molasses follow a different process for the production of bioethanol . Sugar is fermented into ethanol by the addition of yeasts within 4 to 12 hours. The produced ethanol is diluted in water with an ethanol concentration of about 10%. The yeast and water are separated by centrifuge and distillation respectively. After distillation, the water content is usually in the range of 5%. In automotive applications, the water content should be significantly lower for blending the gasoline and thi s can be done through rectification or water separation by chemicals . Gasoline and water stay separated (they don’t mix up) thus the fuel is not homogeneous and it is possible that only water (more heavy so it is on the bottom) enters the engine. In the case of engines working only with ethanol as fuel there is no problem with water content. Note that the less the water content the more is the efficiency of the engine and the less is the cost of transportation, but it requires more energy to separa te the two fluids. The efficiency of conversion from glucose to ethanol is very high (97,5%), from the energy point of view the wasted energy is mainly in the pre -processing, the post -processing and the cultivation. If the starting point is biomass reduces, it is necessary a pre -treatment with steam and acids (steam explosion) in order to obtain the glucose. All the additional steps required are very energy intensive. Drawbacks: - Bioethanol is produced by dedicated cultivation which is used for energ y purposes rather than food production. - The NER depends on the primary source (sugarcane, sugar beet , or corn) as well as solar radiation. It is usually above one but not higher than three. - The most energy -intensive step is the distillation of ethanol wher e all the ethanol must be evaporated to separate water, but it can be self -sufficient thanks to bagasse . - The bio -ethanol process has a very high usage of water . BIODIESEL PRODUCTION Biodiesel is produced through a physical -chemical conversion process. Oil -rich seed is crushed and filtered to give Straight Vegetable Oil (SVO), SVO is sometimes known as Pure Plant Oil (PPO). Common crops used are oil seed rape, sunflowers and soya beans. The pressing process is quite simple, it requires ambient temperatu re, so it can be performed close to the harvesting place. Warm pressing can also be carried out with extraction by organic solvent to increase the biomass yield to oil (economically effective but there is a threshold from the environmental point of view du e to the fact that the renewable energy content of the fuel goes to zero ). In principle, kitchen oil filtered and purified can be used as transportation fuel . It must be preheated before its utilization as the viscosity is higher than the diesel one. Once produced the oil, it must be converted into biodiesel using via transesterification : esters are derived from carboxylic acids . A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. Transesterification requires a catalyst based on sodium or potassium hydroxide. The main problem of transesterification is that methanol is required , and it is nowadays produced with fossil fuels (natural gas), thus more or less 10% of the energy content of the biodiesel is due to methanol . If the synthesis process from natural gas to methanol is considered (60% efficiency), thus the natural gas conte nt in the biodiesel is around 15% ( the strong limit for the renewability). Methanol and triglyceride react and produce a mixture of fatty esters (biodiesel) and glycerin (by - product). Conversion of soybean oil into biodiesel ( transesterification) is the p rocess which accounts for the highest primary energy, followed by the soybean crush and soy cultivation. Algae (next generation biofuel) are sunlight -driven cell factories that convert carbon dioxide to potential biofuels, foods, feeds and high -value bioactives . Main characteristics: - Commonly double their mass in 24 hours. - In s ome cases , more than half of that mass consists of lipids or triacylglycerides . - Recycling of nutrients . - Drying is energy -intensive and use solvent . - The most efficient microbial f uel synthesis techniques are tens of thousands of times slower than abiotic synthesis .