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:
Whereas the production of volatile fatty acids (VFAs) is increased when the process pH is in excess of 5, the production of ethanol (C2H5OH) is characterized by a pH lower than 5 with reaction process coming to a halt at a pH less than 4.
In a balanced bacterial process approximately 50% of the monomers (glucose, xylose, amino acids) and long-chain fatty acids (LCFA) are broken down to acetic acid (CH3COOH). Twenty percent is converted to carbon dioxide (CO2) and hydrogen (H2), while the remaining 30% is broken down into short-chain volatile fatty acids (VFAs). Fatty acids are monocarboxylic acids that are found in fats and have fewer than six carbon atoms whereas long-chain fatty acids. If there is an imbalance in the digester process, the relative level of volatile fatty acids will increase with the risk of accumulation, since the bacteria that degrade the volatile fatty acids have a slow growth rate and cause an imbalance between the various phases of the process. A steady degradation of the volatile chain fatty acids is therefore crucial and often a limiting factor for the biogas process.
Hydrolysis of simple fats results in 1 mol glycerol and 3 mol long-chain fatty acids and, therefore, high proportions of fat in the digester feedstock will result in large amounts of long-chain fatty acids, while large amounts of protein, which contain nitrogen in amino groups (-NH2), will produce large amounts of ammonium/ammonia (NH4+/NH3). In both cases this can lead to inhibition of the subsequent decomposition phase, particularly if the composition of the biomass feedstock varies.
See also: Acetogenesis, Acidogenesis, Acidogenic Digestate, Anaerobic Digestion, Methanogenesis.
Acidogenic Digestate
Anaerobic digestion produces two main products: (i) digestate and (ii) biogas.
The digestate is the material remaining after the anaerobic digestion of a biodegradable feedstock which is then used as a source of renewable energy through the participation of a variety of chemical reactions (Table A-4).
Acidogenic digestate is fibrous and comprises structural plant material which includes lignin and cellulose. It is the acidogenic digestate that possesses the high moisture retention properties and the raw digestate
