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THERMODYNAMIC SIMULATION OF BIOGAS COMPOSITION INAGRICULTURAL WASTE GASIFICATIONMUHAMMAD SYAFIQ BIN AZIZA Thesis submitted in fulfillmentof the requirement for the award of the degree ofBachelor of Chemical Engineering (Gas Technology)Faculty of Chemical & Natural Resources EngineeringUniversiti Malaysia PahangJULY 2013
THERMODYNAMIC SIMULATION OF BIOGAS COMPOSITION INAGRICULTURAL WASTE GASIFICATIONABSTRACTBeing one of the world’s largest producers of palm oil, Malaysia has an abundantsupply of oil-palm fronds due to the large plantation areas in the country. Considering thelarge and consistent supply, oil-palm fronds could be a promising source of biomassenergy through gasification. In this work, the study is conducted by thermodynamicsimulation to determine the biogas composition of oil palm fronds (OPF). Computersimulation using MATLAB is performed to predict composition of the synthesis gas. Apreliminary gasification of oil palm fronds is conducted to understand the process and toimprove the study in the future. It is proven that oil palm fronds are feasible forgasification and have a good potential as a renewable energy source. Biomassgasification is a thermochemical process of converting biomass into the producer gas orsynthesis gas which can be subsequently used for heat, power and liquid fuels productionthrough various synthesis processes. The major objective of this study was to identify thebiogas composition from oil palm fronds (OPF) by thermodynamic simulation usingthermodynamic equilibrium model as suit to the Gibbs equations and formulations. Fewassumptions were made as to ease the analysis of data. Results obtained were tabulatedand graphs were sketched. Different temperature with range of 650 C to 1200 C is themanipulate variable. The composition of combustible gases was decreasing as thetemperature increase. Also as the moisture content is increase, the composition of COdecrease, while other gases were slightly increases and decreases.
SIMULASI TERMODINAMIK UNTUK KOMPOSISI BIOGAS DALAMPENGEGASAN SISA BUANGAN PERTANIANABSTRAKSebagai salah satu pengeluar minyak sawit terbesar di dunia, Malaysiamempunyai banyak bekalan pelepah kelapa sawit kerana kawasan ladang yang besar dinegara ini. Disebabkan itu, ianya boleh menjadi sumber yang menjanjikan tenaga biomasmelalui pengegasan. Dalam kertas kerja ini, kajian dijalankan menggunakan simulasitermodinamik bagi menentukan komposisi biogas pelepah kelapa sawit (OPF). Simulasikomputer menggunakan MATLAB dilakukan untuk meramalkan komposisi gas terhasil.Kajian ini membuktikan bahawa pelepah kelapa sawit sesuai untuk pengegasan danmempunyai potensi yang baik sebagai sumber tenaga yang boleh diperbaharui.Pengegasan biomas adalah satu proses termokimia, menukar biomas kepada gas sintesisyang boleh kemudiannya digunakan untuk tenaga haba, kuasa dan pengeluaran bahan apimelalui pelbagai proses sintesis. Objektif utama kajian ini adalah untuk mengenal pastikomposisi biogas dari pelepah kelapa sawit dengan simulasi termodinamik menggunakanmodel keseimbangan termodinamik yang bersesuaian dengan rumusan dan persamaanGibbs. Beberapa andaian telah dibuat untuk memudahkan analisis data. Graf telahdilakarkan berpandukan data yang telah diperolehi. Suhu yang berbeza antara 650 Chingga 1200 C adalah pemboleh ubah yang dimanipulasikan. Komposisi gas mudahterbakar telah berkurangan dengan peningkatan suhu. Juga dengan meningkatnyakandungan kelembapan, komposisi gas CO menurun, manakala gas lain sedikitbertambah dan berkurangan.
TABLE OF CONTENTCHAPTER1TITLELIST OF TABLESiLIST OF FIGURESiiNOMENCLATUREiiiINTRODUCTION1.1General Overview11.2 Previous Developments and Motivation31.3 Research Background41.4 Research Objective71.5Research Project71.5.1 Introduction71.5.1.1 Stage 171.5.1.2 Stage 281.5.1.3 Stage 381.5.2 Research Scope2PAGE8Literature Review2.1Introduction102.2Biomass Gasification10
2.32.434Gasifier Types and Processes122.3.1 Downdraft Gasifier162.3.2 Updraft Gasifier162.3.3 Fluidised Bed Gasifier17Computational Approach Using MATLAB Coding182.4.1 Thermodynamic Equilibrium Modelling182.4.2 Model Formulation192.4.3 Equations Governing21RESEARCH METHODOLOGY3.1Introduction273.2Process Work Flow283.3Software293.3Experimental Methodology293.4Assumption for Thermodynamic Equilibrium Model30RESULT AND DISCUSSION4.1Introduction314.2Effect Of Temperature On Biogas Composition324.2.133Relation For Temperature and EquivalenceRatio4.2.2Effect Of Temperature At Fixed Equivalence34Ratio4.3Effect Of Moisture Content to Biogas Composition36
5CONCLUSION AND REFERRENCES39APENDIX A42APENDIX B47
LIST OF TABLESTable No.TitlePage1.1Biomass stock in Malaysia (Abdulmuin et al., 2001)22.1Coefficients of specific heat capacity for various gases262.2Enthalpy of formation and coefficient26B.1Data for Figure 4.147B.2Data for Figure 4.248B.3Data for Figure 4.349B.4Data for Figure 4.450B.5Data for Figure 4.551B.6Data for Figure 4.652i
LIST OF FIGURESFigure No.TitlePage1.1Ash content in biomass samples (S. Balamohan, 2008)51.2Energy content in biomass samples (S. Balamohan, 2008)52.1Updraft gasifier132.2Downdraft gasifier142.3Bubling bed fluidized142.4Circulating fluidized bed154.1The effect of temperature on concentration biogas324.2The effect of temperature on number of mole biogas334.3Effect of temperature on equivalent ratio344.4Effect of temperature on biogas (vol %) at fixed35equivalent ratio4.5Effect of temperature on biogas (no. of moles) at fixed35equivalence ratio4.6Effect of moisture content on biogasii36
NOMENCLATUREa–gCoefficient for gibbs free energy empirical relationC, H, O, N, SCarbon, hydrogen, oxygen, nitrogen and sulfurfraction in biomass (dry basis)C*, H*, O*Carbon, hydrogen and oxygen fraction in biomass (wetbasis)c1 – c4Coefficient for specific heat capacity GTGibbs free energy (KJ/kmol)) gf,T,iChange in Gibbs free energy for individual gas withtemperatureHHVHigher heating value (MJ/kg)HbioHeat of formation (kJ/kmol)H fEnthalpy of formation (KJ/kmol)K1Equilibrium constant for water-gas shift reactionK2Equilibrium constant for C 2H2 CH4LHVLower heating value (MJ/kg)MbioMolecular weight of the biomassmMoisture content in biomass (% dry basis)mwNumber of moles of water vapor (dry basis)niNumber of moles of species intotTotal number of gaseous moles in the reactorPxPartial pressure of species of I inside the reactoriii
PiProducts of complete combustion of biomass (kmol)for species iwStoichiometric coefficients of water vaporx, y, zNormalized coefficient of atomic hydrogen, oxygenand nitrogen for biomass moleculex1 – x2Number of moles of H2, CO, CO2, H2O, CH4respectivelyxgNumber of moles of oxygen for gasificationiv
CHAPTER 1INTRODUCTION1.1General OverviewThe high prices of fossil fuels and environmental concerns have driven manyinterests for the field of biomass gasification. A large number of organizations andcountries around the world nowadays are investing development dollars in an attempt toget place in this critical market. As we all know, biomass is an indigenous,environmental friendly, often cheap and the most important it is renewable fuel. Besidesthat, so many agricultural wastes that either been destroyed or left to rot, although thewastes have so many other benefits as important as for biogas production. Handbook ofBiomass Downdraft Gasifier Engine Systems mentioned that the benefits gain frombiomass for energy use include low cost by-product of agriculture, low ash and sulfurcontent, and also decreasing the level of CO2 in the atmosphere.1
Palm oil industry waste provides the biggest potential for biomass energyutilization in Malaysia. It is because they are abundant, easily available and moreoverthere is a need to be disposed of cost effectively. Today, 3.88 million hectares of land inMalaysia is under oil palm cultivation. These palm oil plantations yield a large amountof harvestable biomass that is about 50 to 70 tons per hectare per year. 10% of this totalresults in the finished products as palm oil and palm kernel oil. While the remaining90% are Empty Fruit Bunch (EFB), trunks, kernels, and palm oil mill effluent that willbe discarded as waste or left to settle in waste ponds, as well as Oil Palm Fronds (OPF)that are left to rot for soil nutrition (S. Prasertsan et. al., 2006)Table 1.1 Biomass stock in Malaysia (Abdulmuin et al., 2001)YearFelled Trunks (mil. tones, dwb)Felled Fronds (mil. tones, 10.210.010.611.12.52.42.72.92.72.62.82.9The present research focuses on OPF, as they are easier to collect, store and toprepare as feedstock. Table 1.1 shows the projected availability of the field residue oftrunks and fronds in Malaysia (Abdulmuin et al., 2001).2
Gasification as the matter of fact already had been used for almost 200 years ago,in various types and forms. It is essentially an oxygen limited thermochemicalconversion of carbonaceous material to a useable gaseous fuel. The wide range ofsources including energy crops, agricultural waste, and municipal solid waste, createeasy accessibility to low cost fuel sources for gasification almost anywhere in the world.Small scale (20-250 kWth) downdraft gasifiers are a valuable source of energy forpopulated rural areas. These units are cheap, easy to use, require minimal maintenanceand support. It also can drastically improve the lives of rural communities that don’thave electricity or gas for cooking. This potential for future growth gives rise to the needfor an accurate model for predicting the heating value and composition of the synthesisgas produced from many different biomass sources.1.2Previous Developments and MotivationSeveral models have been proposed in literature to predict downdraft gasifiersynthesis gas composition and heating value from the ultimate and proximate analysis ofthe fuel source. Zainal et al., (2001) had proposed using a thermodynamic equilibriummodel for such predictions and concluded reasonable agreement with experimental data.From the other journal, Sharma (2008) compared a kinetic model to a thermodynamicmodel. That study concluded that the kinetic model is limited because it uses rateconstants based on the difference from the equilibrium value, so for gas composition theequilibrium model is a reasonable alternative to the complexity of the kinetic model. The3
equilibrium model computes the gas composition, for complete thermochemicalconversion, at a specified temperature. In any small scale commercial downdraftgasifier, there will not be a single reaction temperature or complete thermochemicalconversion. The equilibrium model compared well against experimental data in (Zainalet al., 2001) but for an actual downdraft gasifier the inhomogenous reaction zone causesfluctuating reaction temperatures and conversion efficiencies. These factors lead toinaccuracies in the prediction of composition and heating value of expected synthesisgas. An accurate model of gas composition and heating value is critical for the optimalselection of biomass sources in various parts of the world.1.3Research BackgroundThe research of biomass has been conducted a decade ago till now, in variousways, samples and methods. The research will be more focus on the composition ofbiomass. To be more specific, Oil Palm Frond (OPF) had been chosen as the subject.Plus, thermodynamic equilibrium by using Gibbs formulations is necessarily applied.Recently a number of studies were made by final year students of UniversitiTeknologi Petronas [(Kimin, 2008), (S. Balamohan, 2008)], on the feasibility of OPF asa feedstock for gasification and preparation of the feedstock. Their scope for chemicalproperty was studied by Ultimate Analysis, Calorific Test, and Proximate Analysis. Theresults they obtained show that OPF has a higher quality in terms of lower ash content,higher calorific value and acceptable moisture content, compared to other biomass4
resources as shown in Figures 1.1 and 1.2. Figure 1.1 shows the comparison of ashcontent of OPF with other biomass resources. While Figure 1.2 shows the comparison ofenergy content of OPF with other biomass resources. From the analysis also Balamohanfound out that the OPF has average density of 712.8kg.m-3, which is more suitable ascompared to other feedstock materials in terms of bulk handling (S. Balamohan., 2008).Figure 1.1 Ash content in biomass samples (S. Balamohan, 2008)Energy Content KJ/KgFigure 1.2 Energy content in biomass samples (S. Balamohan, 2008)5
A few another studies have been conducted to determine synthesis gascomposition and heating value of synthesis gas using thermodynamic equilibriummodeling on limited biomass types. Watkinson et al., (1991) have developed athermodynamic equilibrium model and compared their result with various types ofgasifiers used for coal. Their research found the best prediction for entrained bed gasifierbut a lower degree of accuracy while predicting synthesis gas composition fromfluidized bed and moving bed gasifiers. Meanwhile, Jarungthammachote and Dutta(2007) and Melger et al. (2007) predicted synthesis gas composition from variousbiomass types using thermodynamic equilibrium modeling at a fixed equivalence ratio.Their researches predicted gasification temperature through an iterative process and thesynthesis gas composition at a given equivalence ratio. It is stated that the compositionof temperature is varies with the temperature for the gasification. Thus, this research wasto identify how far can this be true.6
1.4 Research Objectives To identify the composition of biogas in agricultural wastes (oil palm fronds) bythermodynamic simulation. To study the agricultural wastes gasification process.1.5 Research Project1.5.1IntroductionIn order to manage this research successfully, 3 stages had been planned earlier. Thestages are as below:-1.5.1.1 Stage 1Firstly, collects any data and journals that related to the research in order tounderstand the core and concept of the research. With the data and journalscollected, introduction for the proposed research have been made where it willclearly stated the objectives that want to be achieve accompanied with literaturereview that will relate the finding and the entire journals together. After that, themethodology will be plan. For simulation, the software will be identified and thehow to use the software will be learned further. Furthermore, the problem statementalso will be identified.7
1.5.1.2 Stage 2During the second stage, the basic concepts are already mastered and will focusmore on the experiment methodology. Also, in order to do the simulation, softwarechosen is MATLAB that fit the best for the models and simulation. In this stage, itwill be more on preparing models and equations for the simulation so the simulationcan be run smoothly so that successful result can be obtained. Other than that, all thevariable and parameter will be identified so that the objectives will be more cleared.1.5.1.3 Stage 3With all the variables and parameters have identified, the model will beconstructed using MATLAB coding software while referring to the governingequations, which are mostly related to Gibbs. Then the simulation will be runrepeatedly as to achieve the objectives of this research. The results are taken and willbe analyzed.1.5.2Research ScopeIn this proposed project, a thermodynamic simulation of biogas composition inagricultural waste gasification will be conducted. For the specific sample of theagricultural waste, Oil Palm Fronds (OPF) is chosen. This is because Malaysia has avery large amount of OPF produced as referred to Table 1 (Abdulmuin et al., 2001).This simulation will be run by using MATLAB. The model chosen are8
thermodynamic equilibrium. It is because this technique is easier and also thesimplest compare to other technique such as kinetics-free, steady-state, semitransient and transient. Plus more, this technique give more accurate result of gascomposition value (Reed, 1985). Some assumptions are made or retrieved fromjournals. Moreover, Gibbs equations and formulations are necessarily needed as forthe calculation in order to run a complete simulation and achieving the objectives ofthis research.9
CHAPTER 2LITERATURE REVIEW2.1IntroductionThis literature is based on the objectives and scopes of this research. As for theresearch is focusing on the biogas composition in agricultural waste, that is OPF.Besides that, the thermodynamic simulation and the software used, that is MATLABalso discussed. This includes the modelling and governing equations. A reports will bemade at the end of the experiment and the results of MATLAB analysis, backed up bypublished experimental data, where there are applicable will be presented.2.2Biomass GasificationModern agriculture is an extremely energy intensive process. However the highagricultural productivities and subsequently the growth of green revolution have beenmade possible only by large amount of energy inputs, especially those from fossil fuels(Leach, 1976). Rajvanshi (1978) mentioned that there has been a trend towards use of10
alternative energy due to recent price rise and scarcity of these fuels. As example ofalternative energy sources are solar, wind, geothermal and other renewable energy.However these energy resources have not been able to provide an economically viablesolution for agricultural applications (Dutta and Dutt, 1981).Solar Energy Research Institute (SERI) mentioned that one biomass energybased system, which has been proven reliable and had been extensively used fortransportation and on farm systems during World War II is wood or biomassgasification.Biomass gasification on the other hand means incomplete combustion of biomassthat resulting in production of combustible gases. The gases are consisting of Carbonmonoxide (CO), Hydrogen (H2) and traces of Methane (CH4). This mixture is calledproducer gas. Producer gas can be used to run internal combustion engines. It also canbe used as substitute for furnace oil in direct heat applications. The most important is itcan be used to produce methanol in an economically viable way. Reed et al. (1982)mentioned that methanol is an extremely attractive chemical which is useful both as fuelfor heat engines as well as chemical feedstock for industries. Since any biomass materialcan undergo gasification, this process is much more attractive than ethanol productionwhere only selected biomass materials can produce the fuel.11
2.3Gasifier Types And ProcessesWarnecke (2000) had classified the gasifiers into four different categories whichare based on the fluid and solid movement inside the reactor. The types of gasifiers areas follow:-i. Quasi non-moving or self-moving feedstockii. Mechanically-moved feedstocka. Downdraft gasifierb. Updraft gasifierc. Cross-draft gasifieriii. Fluidically-moved feedstocka. Bubbling bed (BB) gasifierb. Circulating fluidized bed (CFB) gasifierc. Entrained-bed gasifieriv. Special reactorsa. Spouted bed gasifierb. Cyclone gasifierAmong of all the gasifiers stated, it is proven that downdraft (Figure 2.2), updraft(Figure 2.1), bubling bed fluidized (Figure 2.3) and circulating fluidized bed (Figure 2.4)gasifiers are the most well known and widely used (Knoef, 2000). However, bubling bedand circulating fluidized bed both are conclude as one section, that is as fluidized12
gasifier due to only slight different between them. Figure 2.1 to 2.4 show schematics ofvarious gasifiers that are commonly used in the commercial market.BiomassSynthesis gasoutGasificationagentGrateAshFigure 2.1 Updraft gasifier13
BiomassAir / Gasification agentSynthesis gas outAshFigure 2.2 Downdraft gasifierSynthesis gas outFluidizing mediumDistrubutorFigure 2.3 Bubling bed fluidized14
assumptions were made as to ease the analysis of data. Results obtained were tabulated and graphs were sketched. Different temperature with range of 650 C to 1200 C is the manipulate variable. The composition of combustible gases was decreasing as the . kuasa dan pengeluaran bahan api melalui pelbagai proses sintesis. Objektif utama kajian .
