Encyclopedia of Renewable Energy. James G. Speight
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avoid the degradation or loss of precious carbohydrates and avoid the formation of by-products that can inhibit subsequent steps. However, pre-treatment often produces biological inhibitors, which affect fermentation. A large variety of pre-treatment processes have been developed. Common pre-treatments are steam-explosion, acid treatment, biological methods, and comminution. These methods can be used singly or in combination.
Steam explosion involves saturation of the pores of plant materials with steam followed by rapid decompression. The explosive expansion of steam reduces the plant material to separated fibres, increasing accessibility of polysaccharides to subsequent hydrolysis. Ammonia fibre explosion (AFEX) is similar to steam explosion except that liquid ammonia is used. It is effective on agricultural residues but has not been successful in pre-treating woody biomass. Biological pre-treatments employ microorganisms that produce lignin-degrading enzymes (ligninase). Comminution is an integral part of pre-treatment and uses a hammer mill to produce particle sizes that can pass through 3 mm screen openings.
The mechanisms by which pre-treatments improve the digestibility of lignocellulose are not well understood. Pre-treatment effectiveness has been correlated with removal of hemicellulose and lignin (lignin solubilisation is beneficial for subsequent hydrolysis but may also produce derivatives that inhibit enzyme activity). Some pre-treatments reduce the crystallinity of cellulose, which improves reactivity, but this does not appear to be the key for many successfully pre-treatments.
Hemicellulose is readily hydrolysed to pentoses (5 carbon sugars) but pentoses are difficult to ferment. The cellulose hydrolyses to hexoses (6 carbon sugars). Crystalline cellulose is difficult to hydrolyse but the resulting hexose derivatives are readily fermented. The three basic methods for hydrolysing structural polysaccharides to fermentable sugars (glucose, xylose, arabinose) are concentrated acid hydrolysis, dilute acid hydrolysis and enzymatic hydrolysis.
Acid treatment is the use of acid to hydrolyze cellulosic materials. Two acid processes hydrolyze both hemicellulose and cellulose with minimal pre-treatment beyond comminution of the lignocellulosic material to particles of approximately 1 mm in size. These are concentrated and dilute acid hydrolysis. Concentrated acid hydrolysis dissolves carbohydrates in woody biomass to form a homogeneous gelatine with acid where cellulose is extremely susceptible to hydrolysis. Typically, this is achieved with 70 to 90% v/v sulfuric acid (H2SO4) at room temperature, leaving lignin. Before fermentation, the solution of oligosaccharides is diluted to 4% v/v sulfuric acid and (i) heated (at the boiling point) for four hours or (ii) processed using an autoclave at 120°C (248°F) for one hour to yield monosaccharides. Following neutralisation with limestone, the sugar solution is fermented. The procedure takes 10 to 12 hours. Concentrated acid hydrolysis is attractive because it is relatively simple, and high sugar yields (approach 100% of theoretical hexose yields in pure samples and 90% in mixed samples which replicate municipal waste mixtures) are achieved. However, corrosion resistant equipment is necessary and acid recovery is expensive.
Dilute acid hydrolysis (1% acid w/w) greatly reduces the amount of acid required to hydrolyse lignocellulose. The process is accelerated at elevated temperatures: 100 to 160ºC (212 to 320°F) for hemicellulose and 180 to 220ºC (356 to 428°F) for cellulose. The high temperatures cause oligosaccharides released from the lignocellulose to decompose, greatly reducing yields of simple sugars to only 55 to 60% w/w of the theoretical yield. The decomposition products include toxins (acetic acid, furfural) which inhibit fermentation.
See also: Hydrolysis – Lignocellulosic Materials.
Acidity and Alkalinity
Acidity as applied to natural water and wastewater is the capacity of the water to neutralize hydroxyl ions (OH-). It is analogous to alkalinity, the capacity to neutralize the hydrogen ion (H+). Acidity is the quantitative expression of the capacity of the water to neutralize a strong base to a designated pH and an indicator of how corrosive water is.
Acidity can be caused by weak organic acids, such as acetic and tannic acids, and strong mineral acids including sulfuric and hydrochloric acids. However, the most common source of acidity in unpolluted water is carbon dioxide in the form of carbonic acid (H2CO3). On the other hand, The alkalinity of water refers to the capability of water to neutralize acid in which the water has a buffering capacity – a buffer is a solution to which an acid can be added without changing the concentration of available hydrogen (H+) ions (without changing the pH) appreciably.
A surface water body, such as a lake, the alkalinity in the water comes mostly from the rocks and land surrounding the lake. Precipitation falls in the watershed surrounding the lake and most of the water entering the lake comes from runoff over the landscape. If the landscape is in an area containing rocks such as limestone then the runoff picks up chemicals such as calcium carbonate (CaCO3), which raises the pH and alkalinity of the water. In areas where the geology contains large amounts of granite, for instance, lakes or ponds may have a lower alkalinity.
The acidity and/or the alkalinity of a surface water system can be influenced by the production of the products of the use of non-renewable fuels, such as those carbonaceous fuels that produce carbon dioxide during use or conversion to other forms of energy. However, switching from non-renewable (fossil fuels) to renewable fuels such as biomass is not always the answer. When biomass is directly used as a fuel or when biomass is used as a process feedstock the products can be sufficiently acidic, and it causes a change in the acidity or alkalinity of surface water systems.
See also: Acid Number.
Acid Number
The acid number (also known as the acidity, the acid value, and the neutralization number) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of the substance. The acid number (AN) or the total acid number (TAN) of crude oil is a measure of the amount of carboxylic acid groups and other acidic species (such as phenols and naphthol derivatives) in crude oil and indicates the potential corrosion during refining.
The acid number is used to quantify the amount of acid present, for example in a sample of biodiesel. It is the quantity of base, expressed in milligrams of potassium hydroxide, that is required to neutralize the acidic constituents in 1 g of sample.
In this equation, Veq is the amount of titrant (ml) consumed by the crude oil sample and 1 ml spiking solution at the equivalent point, beq is the amount of titrant (ml) consumed by 1 ml spiking solution at the equivalent point, and 56.1 is the molecular weight of potassium hydroxide.
The molarity concentration of titrant (N) is calculated as such:
In the equation, WKHP is the amount (g) of KHP in 50 ml of KHP standard solution, Veq is the amount of titrant (ml) consumed by 50 ml KHP standard solution at the equivalent point, and 204.23 is the molecular weight of KHP.
There are standard methods for determining the acid number, such as ASTM D974 and (for mineral oils, biodiesel), or specifically for biodiesel (ASTM D664). The acid number (mg KOH/g oil) for biodiesel should be lower than 0.50 mg KOH/g standard fuels.
See also: Acidity and Alkalinity, Neutralization Number.
Acidogenesis
Acidogenesis (sometimes referred to as fermentation) is the biological process that results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. In this process, volatile fatty acids are produced, along with (depending upon the feedstock) ammonia, carbon dioxide, and hydrogen sulfide, as well as other byproducts. Thus: