Encyclopedia of Renewable Energy. James G. Speight
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and pollutants – such as the oxides of nitrogen and the oxides of sulfur as well as the oxides of carbon – should be removed as soon as possible after they appear from the combustor.
In areas where the biosphere is sensitive to acid rain, there has been ample evidence of the negative effects of acid rain on freshwater ecosystems. Elevated acidity in a lake or river is directly harmful to fish because it corrodes the organic gill material and attacks the calcium carbonate skeleton. In addition, the acidity dissolves metals that (such as aluminum from the sediments) that are toxic to flora and fauna (including humans). There is also evidence that acid rain is harmful to vegetation by a reverse effect insofar as the rain leaches nutrients (such as potassium) from the soil and allows the nutrients to exit the ecosystem by runoff.
Methods of mitigating acid rain formation include (i) reducing the sulfur content of fuels, (ii) using scrubbers such as flue gas desulfurization, (iii) lime injection multi-stage burning, (iv) fluidized bed combustion, or (iv) circulation dry scrubbing of the gas stream. To reduce the formation of nitrogen oxides, methods such as (i) changing air to fuel ratio, (ii) reducing the combustion temperature, which reduces the formation of the oxides of nitrogen that are formed when air is the combustion oxidant, and (iii) a selective catalytic reduction process in which injection of reactive chemicals such as ammonia (NH3) to react with the nitrogen oxides and convert them into nitrogen and oxygen, represented simply as:
See also: Biogas, Biomass, Waste, Wood.
Acid Rain – Formation
Acid rain is formed when sulfur dioxide (SO2) and the nitrogen oxides (NOx) react with water vapor and oxidants in the presence of sunlight to produce various acidic compounds, such as sulfuric acid and nitric acid.
Acid rain has a pH less than 5.0 and predominantly consists of sulfuric acid (H2SO4) and nitric acid (HNO3). As a point of reference, in the absence of anthropogenic pollution sources the average pH of rain is approximately 6.0 (slightly acidic; neutral pH = 7.0). In summary, the sulfur dioxide that is produced during a variety of processes will react with oxygen and water in the atmosphere to yield environmentally detrimental sulfuric acid. Similarly, nitrogen oxides will also react to produce nitric acid.
Precipitation in the form of rain, snow, ice, and fog causes approximately 50% of these atmospheric acids to fall to the ground as acid rain, while approximately 50% fall as dry particles (particulate matter, such as soot or aerosol particles) and gases. Winds can blow the particles and compounds hundreds of miles from their source before they are deposited, and they and their sulfate and nitrate derivatives contribute to atmospheric haze prior to eventual deposition as acid rain. The dry particles that land on surfaces are also washed off by rain, increasing the acidity of runoff.
The principle source of acid rain causing pollutants, sulfur dioxide and nitrogen oxides, are coal-fired power plants. Since natural gas emits virtually no sulfur dioxide, and up to 80% less nitrogen oxides than the combustion of coal, increased use of natural gas could provide for fewer acid rain–causing emissions.
Acid Rain – Mitigation
Acid rain reduction can be done either fuel switching or scrubbing. Fuel switching includes limiting the use of Sulphur-containing fuels such as coal or switching to low sulphur–containing coal or oil, switching to alternative energy sources such as using gas boilers instead of coal or oil boilers, nuclear power generation, using renewable energy sources such as wind, air, wave and geothermal energy.
Gas cleaning (gas scrubbing) includes use of electrostatic precipitators where positively charged sulphur particles are attracted by negatively charged plate or chemical means either wet scrubbing such as injecting water or chemical solution such as flue gas desulphurization (FGS) which has the sulfur dioxide removal rate between 80-95% or dry scrubbers such as lime injection multi stage burning (LIMB) or fluidized bed combustion (FBC or circulation dry scrubber) that react with sulphur in the absence of water medium.
To reduce nitrogen oxides (NOx), methods such as selective catalytic reduction process (SCR) which has the NOx reduction rate up to 80% where injection of reactive chemicals such as ammonia reacts with NOx and convert into nitrogen and oxygen, changing air to fuel ratio and changing the combustion temperature. In automobile NOx reduction, catalytic converters are used, e.g., three-way catalytic converters: (i) conversion of nitrogen oxides into nitrogen and oxygen, (ii) conversion of carbon monoxide into carbon dioxide, and (iii) conversion of hydrocarbon derivatives into carbon dioxide and water.
See also: Acid Rain, Gas Cleaning, Gas Processing, Gas Treating.
Acid Treating
Acid treating is one of the older treating processes in refining of liquids from renewable sources. This process found importance in the earlier days of crude oil refining and was preferentially used by oil industry for treating middle distillates like gasoline, kerosene, etc., and also for the refining of lubricating oils. Used oil refining was also carried out using this process. Primarily, acid treating processes were used to improve color, odor, removal/reduction of aromatic content and other undesirable constituents such as polar nitrogen containing compounds and/or some of the trace metals present in the products. Removal of these undesired constituents leads to considerable improvement in physical properties of the treated petroleum / petrochemical product.
Sulfuric acid treating is known to be one of the oldest and best suited treating agent for a large variety of petroleum-related applications. Use of sulfuric acid was widely accepted as an acid treating agent for upgrading petroleum products. Although petroleum refining industry and researchers have used other acids like hydrofluoric acid, hypochlorous acid for treating of the petroleum products, such as lubricating oil, best performance and economics of the operation could be achieved by using sulfuric acid treating only.
In the process, a product is thoroughly mixed with the acid in a mixer and this mixture is allowed to settle for some time for achieving complete phase separation of aqueous and organic layers. Treated product, i.e., upper organic layer is removed and water washed to give the finished product. If still better quality product is desired with further improvement in color and/or removal of polar constituents, this acid treated product layer may be further treated with clay then followed by water washing as per the process requirements. The bottom layer, i.e., acidic aqueous layer is taken out from the bottom of the settling tank for the recovery of extracted petroleum products and the recycle of the acid for further product treating or dilute acid; if it cannot be used further then this acid is treated with alkali to neutralize it and it is sent to disposal to water treatment plant.
As in the other treating processes in crude oil refining, acid treatment may also be used as a simple single-stage extraction process, or if required, as a complex multi-stage extraction process. The process requirements and treating scheme is designed based on the nature of the product to be treated and extent of product quality improvement requirement.
Pre-treating and post-treating of the products before and after acid treating also plays a significant role in product quality enhancement by acid treating. Whether it should be clay treatment or simple water washing after acid treating, the overall process requirement can be optimized easily depending upon quality requirements of the finished product. As discussed earlier, clay treatment after acid treating further reduces the polar constituents present in the product besides improving the color of the product. If desired product quality is achieved by acid treatment only than clay treatment step is not essential and simple water washing can give the desired product quality.
A number of process-related parameters play significant roles for the optimization of process requirements and these parameters depend upon the extent of refining