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Week 1 Week 2 ● Units, Conversions and Key Quantities ● Material Balances ● Energy Balances Week 3 ● Thermodynamic Models for Bioenergy Process Design ● Chemical and Biochemical Reactions Thermodynamics and Kinetics Week 4 ● Organic and Bioenergy Chemicals ○ organic molecules ■ Alkanes Complex alkanes are named according to IUPAC system the higher the degree of branching, the lower the boiling point ■ Cycloalkanes - general molecular formula, CnH2n - in water, while soluble in most organic solvents - have higher boiling points and densities than the corresponding alkanes - Low reactivity, except for small cycloalkanes, particularly cyclopropane ■ alkenes and alkynes - unsaturated hydrocarbons - general molecular formula, CnH2n - compared to alkanes, alkenes have higher reactivity - undergo addition and polymerization reaction (eg polyethylene) - unsaturated hydrocarbons - general molecular formula, CnH2n-2 - one characteristic carbon–carbon triple bond ■ Alcohols (-OH) - three major types of reaction - dehydration, for the formation of ethers and alkenes; - oxidation, for the formation of aldehydes, ketones, or acids; - and esterification, for the formation of esters ■ Aldehydes & ketones - Aldehydes (R–CHO) and Ketones (R–C(=O)–R) ■ carboxylic acids - compounds with carboxyl functional groups ■ Aromatics - Benzene is a six-membered cyclic hydrocarbon with the molecular formula C6H6. ■ Heterocyclics - Compounds with heteroatoms, atoms other than carbon, in their cyclic structures - primarily include N, O, and S atoms ○ Carbohydrates - Consist of C, H, and O atoms with empirical molecular formula Cn(H2O)n - Major components of food and building units for lignocellulosic biomass ○ Lipids - A class of hydrophobic compounds, including animal fats, vegetable oils - Both animal fats and vegetable oils are triglycerides with various fatty acid compositions - Cross-linked to cellulose fibrils - hydrolyzed by dilute acid, alkaline, or hot water to release its monomeric sugars - highly branched, hydrophilic, and amorphous - hemicellulose, which has a high quantity of xylose or arabinose, is referred to as xyloglucans or arabinoglucans, respectively - Arabinoxylans (AG) are mainly found in the primary cell walls of the grass family (Poaceae) ● Pectin in the Primary Cell Wall - a mixture of polysaccharides mostly in the primary cell walls and middle lamella ■ Secondary Cell Wall - constitutes the major structural component of wood that holds trees upright against the force of gravity - generally thicker than the primary cell wall and stacked in multiple layers - most dominant polysaccharides in the secondary cell walls of common biomass feedstocks include: •Cellulose: 41 –51 % •Hemicelluloses:29 –37 % •Lignin: 14 –28 % ● Microbial Metabolism ○ Microbes ○ Complexity of microbial metabolism ○ Cell growth and biofuel synthesis ○ Metabolism ○ Metabolic energy carriers ○ Carbon metabolisms ○ Biofuel synthesis microbial pathways ● Microbial Growth 1:Lag phase 2:Exponential growth phase 3:Declining growth phase 4:Stationary phase 5:Death phase Product 1 is growth associated Product 2 is likely non-growth associated. Lag phase -Cell numbersremain unchanged for a certain time after the initial inoculation -Cells are metabolically active and may grow in size -Lag phase may be reduced by: •adding an actively growing inoculum •Increasing the inoculation ratio (vol of microbes per volume of liquid) •Improving the culture medium –e.g. by adding nutrients -Cells start to divide at a constant rate -Metabolic activity and chemicalcomposition of cells can be assumedin a pseudo steady-state -growth rate -initial time (t0) and concentration (X0) -lag phase may also be incorporated Declining growth phase -microbial biomass growth rate becomes nutrient limited -Waste byproducts may also cause metabolic stress and reduced growth Stationary phase -cell population remains stable as key nutrients have been exhausted and toxic products become inhibitory -Non-growth associated biofuel products can still be produced such as lipids and higher alcohols -Cell growth may be balance by cell death Death phas -cell lysis occurs and population declines. ● Monod equation for microbial growth ● Elementary balances ○ A general aerobic growth elemental balance unknown coefficients the respiratory quotient (RQ) is measured (rate of CO2 produced per O2 Consumed) ● Product and Biomass Yields ● Always a trade off between biofuel and biomass production ● Biofuels Opportunities and Challenges ○ Using genetic modification it is possible to engineer microbe’s metabolic pathways to produce biofuels, however there are several challenges: 1.Genetic instabilities (mutations) may deactivate biofuel pathways 2.As pathways become more complex optimisation of the yield is difficult due to enzyme overexpression and metabolite imbalances ● Additionally, in general the challenges are •Low production yields and titres (concentrations) •Expensive biological synthesis processes (rich medium and aerobic fermentation) •Poor mixing leading to suboptimal conditions (unfavourable pH or oxygen levels for example) that reduce biofuel production for scaled-up processes in particular. Week 5 ● Starch Based Feedstocks ○ Corn Corn –Growing Degree Days For corn are Tbase= 10 °C and Tceiling= 30 °C If the daily maximum temperature is higher than the ceiling temperature, the ceiling temperature is used as the daily maximum. Corn Cultivation -Selection of Seed -Seedbed Preparation -Drainage -Planting Corn harvesting and storage -harvesting dates vary in different locations depending on the regional climates -Threshing -Harvested corn should be dried to a moisture level of about 12% for long-term storage ● Oil-based feedstocks Soybean Rapeseed-Canola Growth -can be grown in tropical, subtropical, and temperate climate zones -Soybean plant development can be divided into vegetative stages (VE, VC, V1–V5) and reproductive stages (R1–R8) -Soybean is propagated from seed -Planting into cool and wet soil may delay germination and result in disease infestation -In the early growth stages, soybean plants cannot compete with weeds -weeds need to be controlled until the crop develops a canopy -Glyphosate-tolerant cultivars have been developed Growth -Rapeseed and canola are same species but rather cultivars containing different fatty acid profiles in the seeds. •Rapeseed is commonly referred to as industrial rapeseed with high erucic acid and glucosinolates •Canola is primarily used for human consumption with very low erucic acid and glucosinolates concentrations in the seeds. -Rapeseed plant development can be divided into vegetative stages (seedling, rosette, bud) and reproductive stages (flower and ripening). -Rapeseed has little tolerance to heat and drought. -Rapeseed seedlings can tolerate temperatures as low as −4 °C with a minimum 3 °C of soil temperature for germination. Harvesting and Storage -Soybean is directly harvested using a combine harvester -must be stored with a moisture content of less than 13% -Soybean seeds may become moldy if the moisture content is high and the bin temperature is warm Harvesting and Storage -The seeding rate can be calculated according to seed weight while accounting for germination rate, and for stand losses during emergence and establishment. -Like soybean, rapeseed is not very competitive with weeds in the early growth stages, but it becomes more competitive after the late-rosette and bolting stages. -Rapeseed must be harvested at the appropriate time, either by direct combining or by swathing and combining. -Rapeseed is usually stored at a moisture content of less than 8% in bins Nutrient and Water Use -Soybean is a leguminous crop that can biologically fix nitrogen from the atmosphere for plant growth -soybeans require other macro-and micronutrients such as phosphorus (P), potassium (K), etc. •Most soils can provide sufficient micronutrients, while soils should be tested for macronutrients to determine nutrient deficiency •A starter fertilizer of 10–20 kg N/ha at planting is beneficial for good early growth -Water requirements for producing 2.4–3.0 tonnes/ha of soybean seed vary between 50 and 65 cm/season depending on climate and the growing period -To reduce the incidence of disease, soybean should not be planted in the same field every year. •Soybean planted after a previous soybean season usually Nutrient and Water Use -Macronutrients such as N, P, K, and S must be supplied from fertilizers. -Unlike legume crops, rapeseed requires N fertilizer (5.0–7.5 kg N to produce 100 kg of seed). -Sulfur is important to rapeseed, with about 1 kg S needed to produce 100 kg seed. -Rapeseed requires 125–152 mm of baseline water use. •After reaching the baseline water use, it produces 7.6 kg of seed with each additional millimeter of water used •using the average baseline water use (139 mm), rapeseed seed yield and water use may be predicted by the following equation, where W is the total water used by the crop (mm). -To reduce soil-borne diseases such as Sclerotiniastem rot and blackleg, rapeseed should not be grown after a previous rapeseed crop. shows an increased incidence of root rot fungi, such as Phytophthora and Rhizoctonia Yield and Oil Content of Major Oilseed Feedstocks -typically contain about 21% oil and 40% protein (on a dry wt. basis). -After the oil is extracted, the remaining soybean meal can be used as a valuable protein supplement in animal feed or as a source of soy lecithin. contain 40% oil and 23% protein (dry wt. basis). The average seed yield for rapeseed is 1.98 metric tons/ha in the USA. ● Lignocellulose Based Feedstocks ● Algae-Based Feedstocks Week 6 ● Pretreatment of Lignocellulosic Feedstocks ○ Goals and Impact ofPretreatment •remove hemicellulose and/or lignin •modify the lignin structure to reduce negative impacts on enzymes •break the matrix structure of the feedstock to reduce particle size, thereby increasing the surface area for the enzymes to access cellulose •destroy the crystalline structure of cellulose •pre hydrolyze cellulose to reduce the degree of cellulose polymerisation. ○ Physical - Used to disrupt cellulose crystallinity, reduce particle size & increase feedstock surface area - In general, physical pretreatment does not affect the chemical composition of the feedstock - No cell walls are degraded, nor do significant structural changes occur to the cellulose, hemicellulose, or lignin components - Physical pretreatment alone is usually insufficient to achieve satisfactory enzymatic digestibility ○ Thermochemical Pretreatment - chemical reagent(s) at elevated temperatures to reduce the resistance of the biomass to breaking down - Can cause •removal of hemicellulose by dissolution or hydrolysis; •removal of lignin (delignification) by breaking the structures (depolymerization and dissolution); •recrystallisation of cellulose; and/or •pre hydrolysis of cellulose (reduction of cellulose polymerization) ■ Thermochemical –acid - Sulfuric acid is the most widely used because of its low price and high efficiency - Most hemicellulose is hydrolysed and dissolved from the cell wall into a liquid hydrolysate comprising oligomeric or monomeric hemicellulosic sugars - Cellulose is partially depolymerised and may even be hydrolysed to glucose during acid pretreatment - no significant delignification during acid pretreatment - Undesirable side effect of acid pretreatment is heat-and/or acid-induced sugar degradation. - the sugar degradation products (e.g. furfural,levulinic acid and formic acid) are toxic to fermentation organisms and referred to as fermentation inhibitors. . ○ Pentose Fermentation - Saccharomyces cerevisiaeis unable to utilise pentoses - other microorganisms can utilise pentoses (e.g. arabinose and xylose) and thus can produce ethanol - ■ Microbes ○ Byproducts Of Fermentation - fermentation the amount of acetaldehyde is limited and yeast use a different pathway to regenerate NAD+ - Yeast cultivation at pH 7 or above leads to acetic acid and glycerol production - microbial contamination can result in byproducts–other microbes compete to utilise sugar and nutrients thereby limiting ethanol production - Some microbes also use enzymes to degrade ethanol ○ Microbial Contamination Byproducts - Lactic acid bacteria (Lactobacillus Sp.) can survive high ethanol concentrations, low pH and low O2. which Causes lactic and acetic acid production and inhibits yeasts - Acetobacter can oxidise ethanol to acetic acid - Clostridium Sp Can result in formation of butyric acid - Wild yeast Dekkera Bruxellensis Utilises ethanol as a substrate for acetic acid production when other carbon substrates are limited ○ Industrial Fuel Grade Ethanol Production ■ Seed Culture Propagation - Frozen or freeze-dried stock culture reactivated in an enriched medium in a test-tube - Then propagated in 100-250 ml of enriched medium - The propagated in 1-4 L of saccharified (sweetened) product derived from feedstock - Then amplified 1:10-15 steps - Final yeast inoculum 5-10 % of the medium required for the fermenter ■ Industrial Ethanol Fermentation - Both batch and continuous processes used but batch generally used - Batch is easier to control with less chance of contamination, the steps include: 1.Fermenter disinfection 2.Substrate/nutrient loading and yeast culture inoculation 3.Mass production of active yeast culture •Requires oxygen for energy generation and growth 4.Ethanol production (active fermentation) •Begins after oxygen is exhausted •Rapid ethanol, CO2and heat generation –lasts about 12 hour •Later substrate becomes limited and high ethanol conc. inhibits the yeast and by products such as glycerol can form 5.Harvesting of fermentation broth at end of predetermined period •Normally takes about 72 h for complete ethanol fermentation from glucose with final ethanol conc. in the broth around 10-15 vol% ■ Ethanol Recovery - Critical for the industry as it determines theproduct quality & quantity - The ethanol-water mix from fermentation isabout 10-15 vol % ethanol - Typically concentrated by distillation butazeotrope means concentration beyond84 mol % (93 vol%) not possible - A further dehydration step required post-distillation - In distillation or rectification column mixture separated by volatility (boiling temperature) –ethanol overhead product ~ 90 vol % - Molecular sieves typically used for final dehydration stage –wet ethanol vaporised and based through bed containing sieves –they capture water (diameter 0.28 nm) but let ethanol (0.32 nm) pass Week 7 ● Fundamentals of Anaerobic Digestion ○ anaerobic digestion - a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen - In an anaerobic digester, organic matter is decomposed by diverse microbial communities through a series of metabolic stages, and produce the final gaseous product: biogas. ■ Hydrolysis - Complex organic compounds are brokendown into simple soluble molecules by theaction of extracellular enzymes excretedby hydrolytic microorganisms - Proteins → amino acids - Polysaccharides → sugars - Lipids → long-chain fatty acids (LCFAs) and glycerine - Also small amount of acetic acid, H2, and CO2generated - Hydrolysis is can be a rate-limiting step in the overall AD process for feedstocks containing high lipids, and/or a significant amount of particulate matter and plant materials ■ Acidogenesis - Takes simple soluble molecules (sugars, amino acids, peptides, and LCFAs) - fermented into: - short-chain or volatile fatty acids (SCFAs or VFAs) and a small amount of alcohol, especially ethanol - CO2, H2, NH4+and S- - Acetic acid - Nitrogen is most commonly supplemented as: - Urea - Aqueous ammonia - Ammonium chloride - Phosphorus is most commonly supplemented as - Phosphoric acid - Phosphate salt. - Toxic materials and inhibition - Toxic substances may be present in either the influent or the byproducts of the metabolic activities of the microorganisms - Influent examples: Heavy metals, halogenated compounds, cyanide and phenol - Microbial metabolism byproducts examples: Ammonia, sulfide, and long-chain fatty acids - Heavy metal toxicity follows the following order: Ni > Cu > Pb> Cr > Zn - Total solids content - The volumetric methane production rate during AD often increases with the TS content until a threshold TS content of 15–20% - Volumetric organic loading rate - Commonly mass of organic matter fed per unit reactor volume per unit time (m/V/t) - For highly dilute substrates such as municipal & industrial wastewaters (with TS content <1–2%) - OLR is expressed in terms of COD(e.g. units: kgCOD·m-3·d-1) - For substrates with high TS contents(e.g., energy crops, sewage sludge, food wastes, OFMSW) - OLR is expressed in terms of volatile solids (VS) (e.g. units: kgVS·m-3·d-1) - A higher OLR means that more substrate can be digested per unit reactor volume - Hydraulic Retention Time and Solids Retention Time - HRT indicates the time which the substrate remains in the bioreactor in contact with the microbes. - Shorter HRTs for readily degradable substrates - Longer HRTs required for harder to degrade substrates - SRT is a measure of the time which the microbes stay in the bioreactor. - Longer SRT helps maintain stability sand provides better ability to withstand and recover from toxicity - Ideally SRT > HRT which can be achieved with biomass immobilisation - Start-up - Start-up is the initial commissioning period - The anaerobic digester is brought to the point at which normal performance with a continuous feed of substrate is achieved. - Start up is a key consideration due to the slow growth of anaerobic microbes, particularly methanogens as they’re vulnerable to environmental changes and the rate-limiting hydrolysis step for high solids feedstocks. - Start up times may be substantial: 1-6 months depending on conditions - The start-up time could be reduced considerably if the exact microbial culture for the substrate is used as a seed. ● Anaerobic Digesters:Classification and Design Considerations - Digester the key of the AD system where biochemical reactions take place - Bioreactor used as a term for a reactor where biochemical reactions are taking place whereas digester is specific to reactor in which AD is taking place. - However digester and bioreactor sometimes used interchangeably - Key aspect of anaerobic digester design is the biomass retention capacity as anaerobes such as methanogens grow slowly. - Must maintain long solid retention time (SRT) irrespective of hydraulic retention time (HRT) to prevent washout of slow growing anaerobes - SRT may be decoupled from HRT by biomass immobilisation in attached growth systems, granulation and floc formation, biomass recycling and biomass retention ○ Classifications of AD systems - Decoupling SRT and HRT can be challenging in the high solids substrate systems encountered in bioenergy applications - Substrate often digested in completely mixed systems called continuous stirrer tank reactors (CSTR), for which SRT = HRT - Based on these considerations AD systems can be broadly classified as suspended growth systems or attached growth system - Can also be classified for example by whether: - continuous or batch - Low rate or high rate microbial activities - Dry (solid-state) or wet (slurry) - Stages (one-stage, two-stage etc) - Temperature such as mesophilic and thermophilic ○ Continuous Stirred Tank Reactors (CSTR) - Majority of reactors are suspended growth systems –microbes are in suspension in the reactor - CSTRs are the most common reactor configuration in AD - Contents completely mixed by continuous or intermittent stirring - Concentration of constituents throughout the reactor and the digestive (effluent) almost the same - Typically operated at HRT of 20-50 days - Since inflows are diluted rapidly by mixing they are not sensitive to shock loadings or toxicity - Can be designed by considering a material balance either on substrate or biomass ■ Design Consideration - Feed cycle –substrate feed to reactor - React cycle –substrate metabolised to microbial biomass. Some biogas feed back into reactor to provide mixing - Settle phase –Biomass is allowed to settle - Decant period –the clear supernatant is decanted - As sequential, a single vessel can be used as reactor and settler. - Maintains high biomass levels due to bioflocculation and biogranulation ○ Attached growth systems ■ Anaerobic Filter ■ Expanded and Fluidised Bed Reactors ○ Solid-State Anaerobic Digestion (SSAD) Systems ■ Batch and continuous ● Biogas Upgrading and Utilisation ○ Biogas value chain ○ Biogas cleaning and upgrading - Typical biogas composition is shown in table - Biogas will also contain water and may also contain NH3 siloxanes - Moisture can be removed by passing gas through pipes underground (for cooling) or by refrigerated cooling - Biogas cleaning –removal of H2O, H2S, NH3 (present at less than 5%) –does not target CO2-sufficient for direct burning - Biogas upgrading –removal of CO2to generate biomethane with CH4content of over 95 %. Used if to be used as transport fuel or injected into natural gas grid ■ Physical Methods - Water scrubbing –water absorbs acid gases (CO2and H2S) as trickles down packed column - Pressure swing adsorption –solid adsorbent with high affinities to bond impurities such as activated carbon, zeolites or molecular sieves - Adsorb at high pressure, regenerate at low - Membrane separation –based on difference in gas permeabilities –material selected to have a high permeability for impurities and low for CH4 - Cryogenic separation –conducted at p= 40 atm and Tas low as -100 °C –impurities liquified or solidified ■ Chemical methods - Cleaning -metal sulphide formation: biogas passed through permeable bed containing hydrate ferric oxide: Regeneration of ferric oxide –add water too: - Acid gas removal (H2S, CO2) with alkali chemicals in scrubber - Acid gas removal with amines: - Commonly alkanolamines such as monoethanolamine(MEA) - The absorption reaction is driven in forward direction at high pressure: - At low pressure and high temperature the solvent is regenerated (reverse reaction) ■ Biological Methods - Sulfide Can be biologically oxidised under aerobic conditions: - Thiobacillusis the best known and utilises reduced inorganic sulfur and CO2. ○ Biogas utilisation - In developing countries –lighting and cooking - In developed countries –generate electricity and heat via combined heat an power unit (CHP) - CHP are modified natural gas engine that generates power - 35-40 % of energy in biogas converted into electricity–remainder released as heat and recovery of this heat is important - After upgrading to biomethane it can be used as a transport fuel or injected into the natural gas grid. ○ Biogas Utilisation - In developing countries –lighting and cooking - In developed countries –generate electricity and heat via combined heat an power unit (CHP) - CHP are modified natural gas engine that generates power - The equivalence ratio, λ, is the ratio of actual oxygen supplied vs. that required stoichiometrically: - Air-biomass equivalence ratio is used to define combustion regimes: - λ = 1, Stoichiometric air for complete combustion - λ < 1, Biomass rich regime – insufficient air for complete combustion - Λ > 1, Biomass lean regime – excess air for complete combustion - The term e expressed in percentage is used to determine the excess air in combustion ○ Nitrogen and carbon monoxide in combustion - Stoichiometric balance equation previously presented was simplified – often other compounds such as CO and NOx are formed - To account for this and unreacted oxygen (excess O2) ● Pre-combustion Processing and Biomass Furnaces ○ Biomass composition and properties - Lignocellulosic biomass is composed: - C: 43-51 wt%, H: 4-6 wt%, O: 34-48 wt% - Small amounts of N, S and Cl: depend on biomass type - Inorganic elements such as Ca and Mg which increase the ash melting temperature and K and P which have opposite effect - Volatile matter: 70-86 wt% - Ash content: 0.5-35 wt% - Ash melting has a negative impact and lowers the allowable temperature of combustion process - Sintering of ash reduces air flow to base fuel layers in combustion chamber - Some grasses have ash softening temperatures as low as 800 °C due to high K (34 % K2O in ash) vs oak which is 1360 °C (9.9 K2O in ash) - Elements such as Cl, S, K and Na in biomass play major roles in corrosion - Nitrogen in biomass leads to NOx with NH3 and HCN – ideally all converted to NOx - Sulfur is not significant in woody biomass but can be in grasses ○ Biomass pre-combustion processing - Most common are size reduction and drying - Size reduction includes chunking, chipping and grinding - Preprocessing depends on the type and quality of biomass - Sometimes pre-processed into pellets to minimise variability and increase energy density - Feedstock size is critical for feeding - Biomass drying is often required to improve combustion efficiency - Also storage of wet biomass often results in microbial degradation ○ Biomass furnaces ■ Fixed-Bed Furnaces ● Travelling grate furnace - Biomass fed onto a conveyor grate and transported through the combustion chamber with minimal disturbance of biomass bed - Uniform combustion conditions for wood chips and pellets - Minimal dust production as bed not mixed and easy maintenance - But requires long residence time and high excess air ● Vibrating grate furnaces - Inclined grate placed on springs - Biomass transport on grate via gravity and vibrating drive - Well suited for biomass with slagging and sintering tendencies e.g straw or waste wood - Fly ash emissions can be high due to bed vibrations ■ Fluidised-Bed Furnaces - Do not feature a grate - Hot inert material in reactor maintained in suspended fluidised state using air - Biomass enter and begins burning as it mixes with hot inert material - Fluidisation results in good mixing, uniform temperature, lower excess air and higher combustion efficiency - Suitable for medium to large scale (>20 MWth) - Fly ash and dust entrainment more significant that for fixed bed furnaces & therefore particulate removal cost higher - Emissions from combustion consist of gases, particulates and residual ash - 1. Species produced due to poor mixing: Smoke, PAH, CO & volatile organic compounds (VOCs) - 2. Pollutants formed in the reaction: NO, NO2, N2O, SOx, H2S, HCl, etc - 3. Stable species emitted: Potassium salts and other inorganic aerosols ● Gasification ○ Gasification of lignocellulosic biomass into Synthesis gas - Gasification is the thermal conversion of carbonaceous material under limited oxygen at high temperature (700-1400 °C) - Products are “producer gas” and char - Old technology – in 19th century was used to produce town gas from coal before natural gas was tapped - In the 20th century used to generate synthesis gas or syngas – primarily a mixture of CO and H2 - Syngas can be used to produce artificial petrol and diesel and is a key precursor to chemicals such as methanol and ammonia - Gasification of biomass mainly used for power generation but growing interest in fuel and chemical production ○ Fundamentals of gasification The key difference the three thermochemical conversion processes is the amount of oxygen supplied: Gasifying/oxidising agent: O2, air, steam or steam + (O2 or air) When steam is used steam to biomass (S/B) and steam to carbon (S/C) ratios expressed: ○ Gasification Process & Reaction ■ Process I. Drying: T < 150 °C rapid heating causes evaporation and drying of biomass II. Devolatilization or pyrolysis: (200 <T/°C< 400) removal of volatile compounds – rapid mass loss and increase in porosity III. Char reactions: C-rich char reacts with gas such as O2, H2O, CO2 and H2 from gasifying agent or from products of prior steps IV. Gas phase reactions: Gas phase CO2, H2, CO, CH4 and H2O react with each other as well as gasifying agents ■ Reaction - Char Reaction stage (III): - Gas Phase Reaction stage (IV) - Final products of the producer gas include: CO, H2, CO2, CH4, H2O, N2 (when air used), C2-C6 hydrocarbons and a small amounts of other byproducts - The final composition of producer gas dependent of gasifying agent (ER and S/C), temperature, pressure, feedstock type and gasifier design ○ Types of gasifiers ■ Fluidised bed gasifiers - High throughput than fixed bed, high mass and heat transfer and can be operated at higher pressure ○ Applications of biomass gasification - Heat and power production (internal combustion energy, steam turbine or combination) - When power generation combined with heat recovery referred to as combined heat and power (CHP) or cogeneration - Fuels and chemical synthesis - Clean syngas can be used in Fischer-Tropsch (FT) process to generate liquid transport fuels: - Methanol can also be produced using catalysts and converted to petrol ● Pyrolysis ○ Pyrolysis of lignocellulosic biomass - Pyrolysis is a thermochemical process that occurs in the absence of oxygen at temperatures between 350 and 600 °C. - Used for the production of charcoal or liquid products (also known as tar or bio-oil) - About 220 million metric tons of biomass are used annually for global charcoal production - Brazil produced 6.8 million tonnes of charcoal (2012), making it by far the largest charcoal producer in the world - Nigeria (4.1 million tonne/year) - India (2.8 million tonne/year) - China (1.7 million tonne/year) - USA (0.8 million tonne/year) ○ Slow pyrolysis - Slow pyrolysis (also known as carbonization) is a process in which relatively large biomass particles (chips, logs) are heated in the near absence of air/oxygen for the production of charcoal - Carbonization reactors, classified into three types: kilns, retorts, and converters - Kilns are used in traditional charcoal making, solely to produce charcoal - Retorts are reactors with the ability to pyrolyze pile-wood or wood logs over 30 cm long and over 18 cm in diameter - Converters produce charcoal from the carbonization of chips or pelletized wood of Brazilian kiln - Despite the growing interest in charcoal production, many of the technologies currently available do not recover the energy contained in the volatiles. ○ Fast pyrolysis - Fast pyrolysis is a process in which very small biomass particles (less than 2 mm diameter) are rapidly heated to between 450 and 600 °C in the absence of air/oxygen to produce high yields of bio-oil (60–75 wt%) ○ Pyrolysis reaction - When a biomass particle is exposed to a hot environment (over 200 °C), pyrolysis reactions take place as the temperature front progresses toward the core of the particle - Most of the vapors will diffuse in the axial direction while the heat is transferred mostly in the radial direction - At slow heating rates (10 °C/min) - Hemicellulose and the amorphous fraction of cellulose typically break down at temperatures between 200 and 350 °C - Crystalline cellulose degrades between 300 and 400 °C - Lignin pyrolysis happens over broader range, between 200 and 600 ° C ○ Production of bio-char and bio-oils - Charcoal is a combustible solid (with a high heating value, around 30 MJ/kg) - Can be burned to generate energy in most systems currently burning coal - Low-ash-containing charcoal can be used as feedstock for activated carbon production - Pyrolysis oil, also known as bio-oil, is a black or dark red-brown liquid, derived from plant material via fast pyrolysis - Bio-oil is miscible with polar solvents (methanol, acetone), but it is almost totally immiscible with petroleum-derived fuels - Density of approximately 1,200 kg/m3, and a heating value of 16–19 MJ/kg (~55% of the heating value of diesel on volume basis and 45% on a mass basis) Week 9 ● Biorefineries for Sugar, Starch and Lignocellulose ○ Sugar based biorefineries for ethanol production - Other feedstocks include - Microalgal oil – a promising future feedstock - Animals fats are favourable for small producers because of their low price – but they have limited availability - Waste cooking oils – used vegetable oils or vegetable oil/animal fat blends. - Contain impurities like water and food particles and high fatty acid from degradation during cooking. Tend to form undesirable soaps which cause emulsification ○ Triglycerides – chemistry and their molar mass - Oils and fats are both composed of triglycerides and free fatty acids - Triglycerides are chemically are fatty acid esters of glycerol - The fuel properties of biodiesel is depending on types and quantities of fatty acid present in the feedstock ● Molar Mass of Triglycerides ○ Biodiesel Production via Transesterification ○ Free fatty acids and soaps ○ Biodiesel quality ○ Lipid-based biorefinery coproducts and glycerol utilisation ● Techno-Economic Assessment Week 10 ● Life-Cycle Assessment ● ARENA Bioenergy LCAs Guidelines ● Bioenergy with Carbon Capture and Storage Week 11 ● Government Policy and Standards ● Australian Emissions and Bioenergy Policy ● Climate and Renewable Energy Policy ● The Sustainability of Bioenergy Week 12 ● Revision