There are different ways to study the effect of substrate pretreatment on AD (see Figure 2.1), from laboratory-scale experiments to trials at full-scale biogas plants. A lot of information can be obtained from lab-scale experiments but to prove that a pretreatment method is effective under real conditions,it must be tested at full-scale biogas plants. This is mainly because the equipment used for pretreatment at large scale is not the same as the equipment used at lab scale. Another factor is that reported methane yields may be theoretical values obtained from chemical analysis or batch tests, and methane yields under real conditions could be different because factors like altered pH and accumulation of toxic compounds are not taken into account with these methods. These methods determine how much the lignocellulose has broken down on a chemical level. These values can then be used to calculate theoretical methane yields. However, greater lignocellulose breakdown does not necessarily translate into greater biogas production because substances that inhibit methane production can also be produced during pretreatment.
BMP test, a batch test or cumulative biomethane production test. This method gives information about the amount of biogas produced and its production rate. However, this method can be interpreted differently, depending on the duration of the batch test A pretreatment method can increase the rate of anaerobic digestion (pretreatment b) or can increase the methane yield (pretreatment c). Both effects will improve the running of a biogas plant. (t1: pretreatment b doubles the methane yield; t2: none of the pretreatment methods increase methane yield; t3: pretreatment c increases the methane yield by 25% but pretreatment b has no effect).
Batch AD does not always correlate with continuous AD, because during continuous AD, microorganisms have more time to adapt to new substrates or inhibitors and inhibitors have more time to accumulate from bacteriostatic to toxic levels. For more information about the long-term effects of pretreatment on AD, laboratory-scale and pilot-scale continuous AD (e.g. VDI 2006) can be carried out.
Mechanical pretreatment is carried out by mills and either makes the pieces of substrate smaller or squeezes them to break open the cellular structure, increasing the specific surface area of the biomass. This gives greater possibility for enzymatic attack, which is particularly important for lignocellulosic substrates. Particle size reduction not only increases the rate of enzymatic degradation, it can also reduce viscosity in digester (thus making mixing easier) and can reduce the problems of floating layers. All particle size reduction is helpful, but a particle size of 1 to 2 mm has been recommended for effective hydrolysis of lignocellulose .
The presence of heat and water disrupts the hydrogen bonds that hold together crystalline cellulose and the lignocellulose complexes, causing the biomass to swell .
At very high temperatures, certain dark-coloured xylose and lignin breakdown products are formed. These compounds include heterocyclic and phenolic com-pounds (such as furfural). Although it is known that these compounds are toxic to yeasts, it is not entirely clear if they are toxic to all AD microorganisms or if they are simply very difficult to degrade anaerobically. There is evidence to suggest that they inhibit AD microorganisms
Solid biomass, as shown in Figure 4.1, is an important fuel and can be burned directly to generate heat and power. It can also be gasified to obtain synthesis gas or syngas (i.e., H2 & CO). However, solid biomass has the following disadvantages: (1) a high moisture content; (2) a low calorific value when compared with solid fossil fuel, such as coal; (3) biomass is inherently hygroscopic; (4) the bulk density of biomass is low, so its volume is large; and (5) the specific properties of different biomass samples can vary significantly, due to the dif-ferent sources used to obtain it. As a result, the utilization efficiency of biomass is low, and its grinding, storage, and transportation are difficult. For these reasons, biomass is frequently blended with coal for co-firing, rather than used alone in power plants.A number of biomass pretreatment methods have been developed to address these disadvantages, with torrefaction being one promising method for solid fuel production that has received a great deal of attention. Torrefaction is a thermal pretreatment process that has been shown to improve the properties of biomass.
Dry torrefaction means that biomass is pretreated in the gas phase and can be classified into nonoxidative and oxidative torrefaction. Wet torrefaction means that biomass is upgraded in the liquid or steam phase, and can be categorized into dilute acid treatment and steam explosion.
The biomass feeding section; The reactor section, where biomass is converted into torrefied material and a combustible gas. The cooling section; the torrefaction product is highly flammable, necessitating the need for a cooling system. The combustor section; the produced gases & vapours are burned with an excess of oxygen in the combustor, and the heat generated is used to heat the process.
The hot flue gas from the combustion is forced along the wall of the reactor to indirectly heat the biomass.
There are many inclusions in the thick-walled fibers of C. japonica, and after undergoing nonoxidative torrefaction the number of these is significantly reduced, as clearly seen by the cell structures. This change in the microstructure improves the grindability of the torrefied biomass, which leads to an increase in the weight percentages of fine particles at the same grinding conditions, and also reduces the amount of energy that needs to be consumed to grind the biomass.
During the alkaline pretreatment, the lignocellulose undergoes two reactionsd solvation and saponification dwhich cause the structure of the lignocellulose to swell, decreasing the degree of polymerization, thus making the lignocellulose components more accessible to enzymatic and microbial degradation. It also has been found that alkaline solutions can be used in the solubilization, redistribution, and condensation of lignin, which also leads to the modification of the crystalline cellulose.
During alkali pretreatment, saponification of ester bonds takes place, which results in the swelling of wood and enhances the enzyme penetration into the cell wall fine structure.
Native samples exhibited a rigid, highly compact, and nonporous structure, while the pretreated samples showed an increase in porosity and greater surface area. This is due to the removal of lignin and hemicelluloses, which in turn destroyed the cellulose-hemicellulose-lignin network, leading to the disruption of the hydrogen bond between the cellulose and becoming more susceptible for enzymatic hydrolysis. The loose structure as well as an increase in surface area of the alkali pretreated sugarcane bagasse allows hydrolytic enzymes to penetrate, adsorb, and hydrolyze the lignocellulosic materials more easily, thus increasing the hydrolysis efficiency.
FTIR spectra of lignocellulosic materials were influenced by three main polymersdcellulose, hemicelluloses and lignin. FTIR spectra of native and alkali pretreated sugarcane tops showed difference in the absorption spectra (Figure 5.3). The carbonyl band at 1735/cm was weakened on pretreated sugarcane bagasse indicating removal of hemicellulose. The peaks corresponding to aromatic ring stretch at 1590/cm also were weakened indicating delignification.
Salma A. Iqbal et al conducted to investigate the production ability of biogas as an alternative energy from KW with co-digestion of cow manure (CM) through anaerobic digestion (AD). Three alkali (NaOH) doses 1.0%, 1.5% and 2.0% on wet matter basis of kitchen waste were applied to improve biodegradability and biogas production. The highest degradation rate was 6.8 ml/gm which was obtained from 1.5% NaOH and also observed that biogas production was almost doubled from treated KW than untreated KW.
The general advantages of biological pretreatment over chemical or thermal pretreatment is that biological pretreatment can take place at low temperature without using chemicals. One disadvantage is that it can be slower than non-biological methods.
Anaerobic microbial pretreatment, also known as pre-acidification, two-stage digestion or dark fermentation, is a simple kind of pretreatment technology in which the first steps of AD (hydrolysis and acid production) are separated from methane production as shown in figure 6.1. While the pH during methane production must be between 6.5 and 8, the pH value of the first digester (the preacidification step) should lie between 4 and 6, which inhibits methane production and causes volatile fatty acids to accumulate. Microbiological pretreatment can speed up the degradation rate of substrates in AD. In general, cellulosedegrading, hemicellulose-degrading and starch-degrading enzymes work best between pH 4 and 6 at temperatures from 30 to 50 C, so the pre-acidification step increases the degradation rate by creating an optimal environment for these enzymes. Another positive effect of this pretreatment method is on the methane concentration in the biogas. In addition to H2 and volatile fatty acids, CO2 is formed during the pre-acidification step. CO2 can be present in three forms: at higher pH values it is present in the form of the carbonate ion CO32, at neutral pH asHCO3 and in acidic environments as CO2. Due to the low pH, most of the carbonate is in the form of CO2, which is volatile and is released into the hydrolysis gas produced from the pre-acidification step. less CO2 in the gas phase of the methanogenesis step, and therefore a higher CH4 concentration is obtained. Another advantage of two-stage digestion is that the microorganisms of the first stage are less sensitive to many chemicals (such as phenols, ammonia, etc) than the microorganisms of the second stage, and many inhibiting chemicals can be broken down in the first stage.
The effect of white-rot fungus on the Yard trimmings was studied by Jia Zhao et al,Effects of moisture content (MC), at 45%, 60%, and 75%, on the degradation of holocellulose and lignin in the fungal pretreatment step and on methane production in the digestion step were studied with comparison to the control group (autoclaved without inoculation) and raw yard trimmings. The results of this work shown in figure 6.2.
Rudianto Amirta et al study oyster mushroom, Pleurotus ostreatus was cultivated on mixed sawdust of Shorea wood (major species; Shorea leprosula), and then the residual wood obtained from mushroom production (figure 6.3) was subjected to methane fermentation by mixing with cow dung in two steps utilization process.
During 44 days, fruiting bodies were harvested and collected four times, and lignin and holocellulose in the Shorea wood decreased by 24.7% and 15.8%, respectively
Biological activity expressed as yields of the fruiting body was the maximum at the first flush, and decreased gradually. In methane fermentation, addition of pretreated wood waste increased production of biogas by 2-3 times higher than those without the bio-treated wood.
Pretreatment of biomass
TARUN B PATEL
ME ENERGY ENGINEERING
GUIDED BY – Dr K V Modi
PRE-TREATMENT OF FEEDSTOCK
ENHANCED BIOFUEL PRODUCTION
• Methods used to assess pre-treatment
• Mechanical pre-treatment
• Thermal pre-treatment
• Chemical pre-treatment
• Biological pre-treatment
• substrates for biogas production
• Ligno cellulosic substrates very slow to break down
• Breaking down this lignocellulose complex is the key to
• pretreatment technologies
• AD faster, reduction of the retention time, increase
biogas yield, make use of new and/or locally available
substrates, prevent processing problems such as high
• substrate is heated (typically 125 to 190 C) under pressure
• held at that temperature for up to one hour
• pressure cookers, autoclaves or microwave heaters, Reactor.
• Dry substrates need additional water before thermal treatment.
• Carried with chemicals or in combination with mechanical
• increased biogas yields of 20 to 30% for energy crops
• effective up to a certain temperature
• biomass is heated in an inert or
nitrogen atmosphere at
temperatures of 200-3000C
• mild pyrolysis
• improve the properties of raw
• Main product is the solid, torrefied
• industrial furnaces, such as boilers,
gasifiers, and blast furnaces
• hemicellulose is removed
and the cell wall in
biomass is destroyed
• scanning electron
microscope (SEM) images
of Cryptomeria japonica
• at 300C for 1 h
• inclusions in the thick-
• Improves the grindability
• reduces the amount of
• used to achieve the destruction of the organic
• acids, alkalis or oxidants
• AD generally requires an adjustment of the pH by
• positive effect on substrates rich in lignin
• ALKALI PRETREATMENT
• utilize lower temperatures and pressures
• carried out at ambient conditions
• but pretreatment time is in hours or days rather than minutes or
• Alkali addition causes swelling of lignocelluloses and partial
• carried out with different alkalis
• Lime or sodium hydroxide (NaOH)
• Economically unattractive due to the high costs of alkalis
• useful for acidic and lignin rich substrates
PHYSICOCHEMICAL CHARACTERIZATION OF ALKALI PRETREATED BIOMASS
• NaOH pretreated sugarcane bagasse
• Scanning electron micrographs
• With alkaline pretreatment, there is 82%
reduction in lignin content
FTIR spectrum of native and alkali pretreated sugarcane tops
• carbonyl band at 1735/cm indicating removal of hemicellulose
• aromatic ring stretch at 1590/cm indicating delignification.
• Biological pretreatment includes
• both anaerobic and aerobic methods,
• Fungal treatment
• addition of specific enzymes
• biological pretreatment can take place at low
temperature without using chemicals
• slower than non-biological methods
ANAEROBIC MICROBIAL PRETREATMENT
pH must be between 6.5 and 8
pH value lie between 4 and 6
temperatures from 30 to 50 C
• FUNGAL PRETREATMENT
• associated with the use of fungal species
• produce enzymes capable of biodegradation of
substrates like lignin, hemicelluloses and polyphenols.
• White and soft-rot fungi were found capable of
biodegradation of lignocellulosic material
• white rot fungus was most effective in pretreatment
• not been carried out at large scale.
Effect of white-rot fungus on the Yard trimmings
Effect of oyster mushroom fungal on Shorea wood
 Parameswaran Binod, Ashok Pandey,Pretreatment of Biomass: Processes
and technologies, 1st Edition,Elsevier, 2014.
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