Syngas, Hydrogen and Liquid Fuels
Fischer-Tropsch Process
(2n+1)H2 + nCO --> CnH2n+2 +nH20 By the use of catalysts, target product n=10-20 length alkanes Side unwanted reaction is methanation.
F-T Tubular Slurry Reactor Adv/Dis
FOR LTFT ADV- High heat capacity therefore good temp control Catalyst can be recovered and replaced DIS- Uses more catalysts than fixed bed Generates fine particles which cause abrasion
Syngas -Composed of.. -Made from..
Fuel gas mixture ideally consisting of H and CO. Often CO2 & methane as well. Produced from natural gas, biomass, coal, any hydrocarbon feedstock by gasification
Fluidized Bed Gasification -main feature -Types of these gasifiers
Fuel is fluidized by the gasification agent -Bubbling Fluidized Bed Gasifier For <25MWth Tolerates various sizes of biomass -Circulating Fluidized Bed Gasifier Large scale (10-100MWth) Easier to scale up
Fixed Bed Gasification -Main feature -Types of these gasifiers
Fuel is in contact with the gasification agent -Updraft (counter-current) Usually for small scale (1-10MWth) Used for heat generation as opposed to power -Downdraft (co-current) Fuel from top, air from middle, product gas comes out bottom. For very small scale. (100kWth-2MWth)
Biogas -Composed of.. -Made from..
Gas mixture composing of mainly CH4 (plus small amounts of CO, H2 and H2S). Produced from breakdown of organic materials through bio-processes such as anaerobic digestion. Can be combusted to convert it into usable form of energy
Gasification Def Adv
Process that converts coal, petroleum or biomass into CO & H2 using a gasification agent (steam, O2 or air) Occurs in two stages: -Initial devolatilization -Subsequent gasification reactions Has high efficiency
CO Methanation Process
Process which generates methane from a mixture of gases such as biogas/syngas, exothermic reaction. Required a catalyst such as Ni, Fe etc. as it is optimal at low T CO+ 3H2 --> CH4 + H20
Fast Pyrolysis
Produces a pyrolsis oil in a VERY short time (few seconds) at temps of 450-650C. Requires finely ground biomass so the v high heating rates can be achieved Then rapid cooling occurs
Contamination Control
To reduce the number of microbes in the feed sugar solution --> resulting in the level of end bacterial products being too low to affect the ethanol yield
Carbon Formation and Prevention
Totally unwanted -breaks down catalysts -causes over-heating of the tubes Use of a potash doped catalyst reduces probability of carbon formation
Air as a gasification agent
Used for partial combustion to supply heat for gasification Product gas is composed of: H2, CO, CO2, CH4, N2 and tars The large amount of N2 in product gas means large amount of heat lost
Oxygen as a gasification agent
Used for partial combustion to supply heat for gasification Similar product gas as when air is used, however N2 content much lower therefore a lot less heat loss. More expensy than air
Lignocellulosic Biomass to Ethanol: Route 1
Enzymatic Hydrolysis & Fermentation pretreatment: Removes lignin so cellulose and hemicellulose are free! hydrolysis: cellulose & hemicellulose are depolymerised to glucose so can be fermented to ethanol
Primary Reformer -To maximise heat transfer
Primarily a heat exchanger which is needed to heat up gas Need to minimise the gas film at the tube wall in order to maximise efficiency of heat transfer
Chemical Composition of Wood
-Cellulose (40%) multiple units of glucose tied together -Hemicelluloses (30%) less organised, from lower molecular weight polysaccharides -Lignin (27%) High MW polymer, functions like glue
Upgrading of pyrolysis liquid- Physical methods
-Polar solvents used to reduce viscosity -Hot-gas filtration which reduces ash/alkali content. This increases burning rate + decreases ignition delay (for combustion in engine)
Reactors used for anaerobic digestion
-Sequential Batch Reactor and Operation -CSTR -Continuous Anaerobic Plug Flow reactor
Upgrading of pyrolysis liquid- Chemical methods
-Volatilization then High temperature cracking which reduces bio-oils oxygen content & improve its stability. Occurs using zeolites as catalysts -To decrease viscosity and aging rate: -acetalization or esterification -Hydrotreating breaks down heavy oil into lighter hydrocarbons and removes oxygen
Important factors in Anaerobic Digestion
-Volumetric Organic Loading Rate (amount of COD loaded) -Mass (microbial mass) yield (a measure of cell growth in a system) -Hydraulic Retention Time (the time the waste remains in the reactor) -Solid Retention Time (the time the waste remains in the reactor to achieve a given degree of stabilization) HIGH SRT/HRT ratio key factor
When Carbon formation occurs in reformer:
-steam to carbon ratio too low -catalyst not active enough -tube walls are too hot
Undesirable properties of pyrolysis liquid
-v high viscosity -unstable, will deteriorate with time (visc will increase, volatility decreases
Pyrolysis- Circulating Fluidised Bed System
Available commercially, liquid yield 60-70% from woody biomass. Has a char combustion unit to provide heat
Water Gas Shift Reaction
CO + H20 --> CO2 + H2 Produces Hydrogen in a slightly exothermic reaction
CO2 Methanation Reaction
CO2+4H2 --> CH4 +2H20 exothermic reaction, ideal as it uses CO2 and produces an energy source Requires a catalyst also as it is exothermic and therefore optimal at low T
Continuous Fermentation -Types
Can either be CSTR / plug flow reactor / cascade continuous (same volume tanks linked in series) Higher fermentation efficiency & productivity, reduced downtime for cleaning
Slurry Reactors
Can use liquid/gas/solid Usually consist of a solid catalyst suspended in liquid, which a gas is bubbled through
Hydrogen -Made from...
Clean energy carrier as has no carbon it it. Produced by steam reforming reaction Very low density so difficult to capture a reasonable amount
Ethanol Fermentation
Conversion of sugars (fructose, glucose & sucrose) to ethanol & CO2 as a side product. Yeasts are used for this conversion in the absence of oxygen, so technically an anaerobic fermentation process. Zymase is an enzyme found in yeast which catalyses the fermentation.
Lignocellulosic Biomass to Ethanol: Route 3
Direct acid hydrolysis for Fermentation Acid hydrolysis occurs uses conc H2SO4 or HCl
Biomass Pyrolysis
Direct thermal decomposition in absence of oxygen --> vapours, non-condensable gases, char
Bacteria Required for Anaerobic Digestion
Group 1: Fermentative Bacteria Responsible for hydrolysis & acidogenesis. This breaks down the organic wastes to amino acids, sugars, fatty acids Group 2: Hydrogen-Producing Acetogenic Bacteria Metabolize ethanol, fatty chains to produce acetate H2 and CO2 Group 3: Homoacetogenic Bacteria Converts H2 and CO2 to acetate Group 4 & 5: Methanogenic Bacteria Convert acetate and H2 respectively to methane
Steam as a gasification agent
H20 is used for water shift of gases from decomposed wood Steam + char --> H2 + CO2 + CH4 etc. Needs external heat input
Pre-treatment of the starch feedstock -Why must this happen
Has to occur to separate the fermentable sugars Starch --> Sugars occurs by hydrolysis
Material which could be used for Anaerobic Digestion What pre-treatment these require
Linocellulosic wastes (low degradability) such as -crop residues -logging residues These require pre-treatment by heat to separate the lignin, cellulose and hemicellulose. Organic fraction of Municipal Solid waste (high degradability) Manure
Natural Gas
Mainly composed of CH4. It is a fossil fuel
Optimal Temps for Anaerobic Digestion
Mesophiles (35-40degreesC) group of MO that are most active at these mild temps Thermophiles (55C) heat-loving!
LTFT
Occurs at 220-250C, products in liquid phase Uses Fixed Bed Reactor and Slurry Reactor
HTFT
Occurs at 320-350C. Reactants and products in gaseous phase, then condensed in separate heat exchanger. Uses circulating fluidised bed reactor
Batch Fermentation ADV & DIS
Rapid batch fermentation is easy to control, can handle high conc of ethanol concs DIS in that cleaning takes a while, incurs higher $$
Why pre-treatment must occur
Reducing particle size Increasing solubility size Enhancing biodegradability (thermal, mechanical (grinding))
Steam Reforming Reaction
Reverse of CO methanation CH4 + H20 --> CO + 3H2 Endothermic reaction therefore occurs at high T
Anaerobic Digestion
Series of biological processes which break down biodegradable materials in absence of O2, produces mainly CH4
Pyrolysis- Bubbling Fluidised Bed System
Silica bed, only been used as a pilot unit but generates high liquid yield of 70-75% from wood. Heat transfer rate is limited by the rate of heat transfer through a biomass particle
New Developments- Biomass Direct Liquefaction
Similar to pyrolysis but biomass is in solvent of bio-oil w/ catalyst, and hydrogen is used for the removal of oxygen in liquid product. Problematic as v high pressure, still has too high O2 content to be used as a vehicle fuel.
Lignocellulosic Biomass to Ethanol: Route 2
Simultaneous Saccharification & Fermentation pretreatment: same as route 1. uses either a mixture of saccharification MO and fermentation MO or GMO that can do both
SNG
Synthesis Natural Gas. A fuel gas produced from syngas (therefore could be from natural gas, biomass, coal etc) by methanation CO+H2->CH4 + H2O
Fast pyrolysis reactor cost
only accounts for approx 20% of capital cost, as has to have -high heating rate -rapid cooling of vapours to obtain a liquid product
