Environmental Concerns
The increased utilization of fossil fuels and subsequent industrialization in most part of the world has led to a remarkable increase in the atmospheric sulfur compounds concentrations. Pollution released by the use of petroleum-based fuels contributes immensely to the deterioration of air quality despite regulatory and technological advances in place. SOx, NOx, and particulate matter are constantly emitted to the environment which affects public health, ecosystem and general wellbeing of the people living mostly in urban areas. Sulfur dioxide which is the immediate sulfur compound found in the lower atmosphere after combustion of fuels has a major role to play in the formation of acid rain, smog formation, and particulate aerosols. Each of these formations affects the healthy living of animals, plants, soils, water, and the general ecosystem.
The four-day event of “London smog” that took place in December 1952 claimed the life of over 4000 people is a noticeable example of the dangers of sulfur compounds in the atmosphere. A similar tragedy happened in London in 1957 with a recorded casualty of over 700 lives. The lower the sulfur content in fuel the better its quality and marketability (Song, 2003). The purpose of this chapter is to provide cheap and reliable nanomaterials that will help in reducing the amount of sulfur released from hydrocarbon-based fuels into the environment.
There are pieces of evidence of substantial the increase of sulfur compounds over the past few decades in our immediate environment (Georgii, 1970). These compounds are the major contaminants found in hydrocarbon fuels (Shahadat et al, 2013) and they are equally reactive. Sulfur compounds released mainly from the burning of fossil fuels results in breathing impairment, respiratory illnesses, alterations in lung defenses, and aggravation of the existing cardiovascular disease. These compounds equally have grievous consequences for asthmatic patients.
Many countries have laws for regulating the emission of sulfur from fuels e.g. In the US, USEPA enacted a law known as ultra-low sulfur diesel (ULSD) since 2006 for maintaining the sulfur content in diesel at 15ppm from the previous 500ppm law (low sulfur diesel) (USEPA, 2014). Diesel engines since then were produced with devices that minimize the emissions of sulfur to the environment. This had the immediate health benefit to the population and the environment. The expected results from the enactment of the law were that: 8300 premature deaths, 23100 cases of bronchitis, and 360,000 asthma attacks are expected to be prevented annually. In addition, 7100 hospital visits, 2400 emergency visits of asthmatic patients and 1.5 million days of productivity loss will be avoided (USEPA, 2006).
USEPA has a tier 3 motor vehicle and fuel standard approach to reduce the sulfur content in gasoline. It is an upgraded version of the tier 2 approach and it is one of the most effective air quality control measures in place (USEPA, 2014a). This rule is expected to start by 2017 and it will consider both the vehicle and the fuel as an integrated system; the expected sulfur level will be 10ppm on an annual average. Japan, Europe, Canada, and South Korea have already achieved the tier 3 limits of sulfur in gasoline (Dasgupta et al, 2013). Lower sulfur gasoline will aid in the development of better technologies that will further improve fuel economy, reduce greenhouse gas effect and improve public health (USEPA, 2014b). It is projected that by 2030, tier 3 will annually prevent: close to 2000 premature death, over 2000 hospital visits and asthma-related cases, and almost 1.5 million days of productivity loss (USEPA, 2014a). Therefore, it is required to reduce the sulfur content in the fuel itself during the refining process to protect the environment.
The removal of sulfur from fuels involves three different approaches: decomposing the sulfur compounds, removal of the compounds without decomposition and finally separating the compounds followed by decomposition (Babich et al, 2003). The conventional method used by refineries for the removal of sulfur from the fuel is hydrodesulphurization (HDS). It is efficient in the removal of most aliphatic sulfur compounds from fuels e.g. thiols. However, it is not efficient in the removal of aromatic and refractory sulfur compounds e.g. dibenzothiophene which poses a more serious danger to the environment. In addition, it requires high temperature, pressure and high dosage of the catalyst before achieving the desired objective (Song, 2003).
Other methods of desulfurization in use to curtail the problems of hydrodesulphurization include: oxidative desulfurization where all the refractory sulfur compounds are oxidized to less harmful polar derivatives that can be isolated easily at room temperature and pressure, ionic liquids desulfurization (Prashant et al, 2010), and bio-desulfurization (Soleimani et al, 2007) such methods are considered viable alternatives for the desulfurization. Catalyst is combined with most of the processes to speed up the rate of desulfurization. Adsorbent materials are currently used in desulfurization for the selective removal of sulfur from petroleum products to trace limits. It has advantages over other methods of desulfurization because it is cheap and does not have scaling abilities. It has ease of application, possesses the regenerative ability and has the potential to remove the sulfur compounds to ultra-deep limits (Kostas et al, 2014). The major drawback of this process is the absence of efficient adsorbent materials that will reduce the sulfur content of fuel to minimal concentrations and be easily recycled to minimize environmental concerns of its disposal.