compound, such as a fatty acid, or in a mixture of compounds.
In a typical procedure, a known amount of sample dissolved in an organic solvent (often an alcohol, such as isopropanol) and titrated with a solution of potassium hydroxide (KOH) of known concentration using an indicator (such as phenolphthalein) to monitor the color change and thereby determine the neutralization point as a colour indicator (see ASTM D664. Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration).
As an example, the acid number is used to quantify the acidity of a substance such as biodiesel. As oil-fats become rancid, the triglyceride derivatives are converted into fatty acid derivatives and glycerol, thereby causing an increase in acid number. A similar observation is that of biodiesel aging through analogous oxidation processes and when subjected to prolonged high temperatures (ester thermolysis) or through exposure to acids or bases (acid/base ester hydrolysis).
See also: Acidity and Alkalinity, Acid Number, Neutralization Number.
Activated Sludge Process
Activated sludge consists of sludge particles, teeming with living organisms, produced in either raw or settled waste-water by the growth of organisms (which include bacteria) in aeration tanks where dissolved oxygen is present.
In general, sludge is defined on the basis of the solids content (Figure A-1) but in order to develop a quantitative definition of sludge which will also help with correlating waste forms with disposal options, a simple and measurable characteristic of all three waste forms needs to be identified.
Figure A-1 Simplified distinction between solid waste, sludge, and liquid waste.
Activated sludge processes treat waste streams that contain 1% or less of suspended solids. In this process, flocculated biological growths are continuously circulated in contact with organic wastewater in the presence of oxygen. The process is widely used for industrial wastes and is even more common in municipal treatment plants.
In the process, air or pure oxygen is bubbled through industrial wastewater combined with organisms to develop a biological floc, which reduces the organic content of the wastewater.
The combination of industrial wastewater and biological mass is commonly known as mixed liquor. Once the industrial wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant liquid is run off to undergo further treatment before discharge. Part of the settled material (sludge) is returned to the head of the aeration system to re-seed the new sewage (or industrial wastewater) entering the tank. Excess sludge which eventually accumulates beyond what is returned is the waste activated sludge which is removed from the treatment process to keep the ratio of biomass to food supplied (wastewater) in balance. The waste activated sludge is stored away from the main treatment process in storage tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.
See also: Chemical Waste, Sludge.
Additives – Catalysts in Combustion Systems
Additives may catalyze or otherwise affect combustion processes. For example, salt has long been known to be of some assistance in removing soot deposits from chimneys and the carbonaceous feedstock treated with a more complex mixture of metal oxides has been reported to be activated in combustion systems but it is apparently not resolved whether the catalytic effect is with regard to carbon-oxygen reactions or whether it is more indirect since the effect resembles (to a degree) the catalysis of feedstock-steam systems in which alkali salts serve to catalyze the carbon-steam reaction to produce synthesis gas (carbon monoxide/hydrogen mixtures). This mixture may then, in turn, react to produce hydrocarbons and oxygenated materials in the presence of the multitude of trace metals that can (and do) occur in biomass feedstocks.
In addition to causing objectionable stack emissions, ash and volatile inorganic material generated by thermal alteration of mineral matter in the feedstock will adversely affect heat transfer processes by fouling heat-absorbing and radiating surfaces and will also influence the performance of the combustion system by causing corrosion, and operating procedures must therefore provide for effective countering of all these hazards.
Corrosion is mainly caused by oxides of sulfur, but in certain parts of a combustion system, specifically on furnace wall tubes with metal temperature of 290 to 425°C (550 to 800°F) and superheater or reheater tubes with temperatures in the range 600 to 700°C (1,110 to 1,300°F), corrosion can be induced by tube deposits that destroy protective surface oxide coatings.
Corrosion damage that is usually ascribed to sulfur is actually caused by sulfuric acid, which is generated from organic and inorganic sulfur-bearing compounds:
Oxidation of sulfur dioxide to sulfur trioxide occurs mostly in flames where (transient) atomic oxygen species are thought to be prevalent by interactions of hydrogen atoms with oxygen:
As well as by interactions of carbon monoxide with oxygen:
The process can be catalyzed by the ferric oxides which form on boiler tube surfaces and show excellent catalytic activity for sulfur dioxide oxidation at approximately 600°C (1110°F), i.e., at temperatures which occur in the superheater section of a boiler.
The presence of water has a marked effect on combustion (by participating in various combustion reactions) and there is evidence for the existence of active centers for chain reactions involved in the further combustion of carbon monoxide and hydrogen (which would be reaction intermediates in the combustion process).
The endothermic steam-carbon reaction is primarily responsible for cooling effects in furnaces and the presence of moisture is believed to cause heat generation at the surface of the bed and in the combustion by virtue of the (endothermic) formation of carbon monoxide and hydrogen in the bed which then burn at the surface. On the other hand, the presence of water vapor appears to assist in the formation of carbon dioxide:
Moisture appears to play a more integral role in the combustion of hydrogen-deficient carbonaceous fuels (such as coal) than has been generally recognized. The carbon-steam reaction to produce carbon monoxide and hydrogen (which are then oxidized to the final products) is an important stage in the combustion sequence as is the carbon monoxide shift reaction to yield carbon dioxide and hydrogen:
Since the whole system involves reaction (and equilibria) between the fuel (i.e., carbon), water, carbon monoxide, hydrogen, and carbon dioxide, the rapid rates of the reactions render it difficult (if not impossible) to determine precisely which of the reactions are the major rate-controlling reactions. In addition, the heterogeneous nature of the system adds a further complication.
