Exhaust gas emissions, especially from coal power plants, must be made environmentally friendly. About 60% of the world's electricity currently depends on coal, this is because coal power plants can provide electricity at low prices. The exhaust gases that pollute and endanger the environment need to be treated in such a way that they no longer endanger the environment. Coal is a fuel that contains a high enough sulfur content, namely 0.5% (5 kg / ton of coal) so that when it is burned, it will cause SO2 and SO3 gas emissions or the SOx gas group. If these gases are emitted in the atmosphere, they will cause acid rain. The acid rain will damage agricultural land, forests due to imperfect photosynthesis, the death of marine life and corrosion of metal objects such as vehicles, buildings and so on, even human health in the form of respiratory problems such as asthma, chronic bronchitis to permanent lung damage.
For example, China says more than half of the country's cities experience acid rain and only a few have fresh air. One sixth of the major rivers are so polluted that their water is deemed unsuitable for agriculture. Pollution watchdogs say 16.4% of China's major rivers do not even meet agricultural irrigation standards. Water from big cities such as Shanghai, Tianjin and Guangzhou is said to be highly polluted. Only the tourist island area of Hainan and parts of the northern coast are considered truly healthy. Only 3.6% of the 471 cities monitored received the top ranking in terms of air cleanliness.
Efforts to minimize SOx gas emissions (including sulfur dioxide (SO2), sulfur monoxide (SO), and sulfur trioxide (SO3)) are carried out by treating flue gas desulphurisation or the term FGD (Flue Gas Desulphurisation). As of June 1973, there were 42 FGD units operating, 36 in Japan and 6 in the United States, with capacities ranging from 5 MW to 250 MW. Between 1999 and 2000, FGD units were in use in 27 countries, and there were 678 FGD units operating at a total power generation capacity of about 229 gigawatts. About 45% of the FGD capacity is in the US, 24% in Germany, 11% in Japan, and 20% in various other countries. Approximately 79% of the units, representing about 199 gigawatts of capacity, use wet limestone. About 18% (or 25 gigawatts) use spray-dry scrubbers or sorbent injection systems. The use of these FGDs has now been introduced to various places that use fossil fuels such as coal incinerators and waste incinerators.
Basically there are several types of FGD techniques but in general they can be divided into 2, namely the wet method, for example by absorption of wet lime or sea water solutions, dry methods such as activated carbon and semi-dry methods. The wet method is the method most widely used. The "gypsum lime method", which is one of the wet methods, has become a mainstream in the world as a large capacity exhaust gas treatment process especially for thermal power plants. The considerations for selecting the FGD technique include scale, cost, types of by-products, and their application. The lime-gypsum method complicates the gypsum recovery process and the wastewater treatment process, so it is not suitable for application in small boilers. Therefore, in small-scale plants, the "magnesium hydroxide method" which uses magnesium hydroxide, which is a cheap alkaline in addition to lime, is often used. The soda method was a wet method commonly used in pulp mills and small-scale equipment in the second half of the 1960s, but because caustic soda as an absorber is expensive and operating costs are high, a method using magnesium hydroxide, which is a cheaper absorber, has been adopted.
In the 1960s, Japan developed a lot of dry FGD techniques and since the 1980s wet FGD techniques have been widely used until now. Currently, Japan has all installed the FGD equipment, but it is certain that the need in China and Southeast Asia will increase in the future. Therefore, in recent years, most FGD equipment manufacturers have realized the technological developments being applied to overseas markets, and the development of simple desulphurisation devices suitable for developing countries that are easy to operate and low cost is underway. There is also a by-product from the FGD which has economic value, namely the gypsum FGD. In Indonesia, there is only one coal power plant that uses FGD with wet limestone and produces a gypsum FGD, namely at PLTU Tanjung Jati, Jepara, Central Java. Meanwhile, other coal power plants still use wet technique with sea water absorbtion.
The desulphurisation method with activated carbon is also simultaneous with denitration and functions to remove other components such as removing dioxins and removing heavy metal elements. The activated carbon adsorption method consists of an adsorption tower, a desorption tower, and an activated carbon circulation transfer device. When other components such as NOx are also adsorbed, a module consisting of a number of activated carbon cells forms an adsorption tower, and each component in the exhaust gas is removed as it passes through each module.
Activated carbon which has adsorbed SOx, etc. In the adsorption tower it is sent to the desorption tower, heated to 350 ° C or higher, and regenerated. The regenerated activated carbon is cooled to 150 ° C or lower in the cooling section, then the dust is removed by filtration and reused in the adsorption tower. Instead, the concentrated SO2 gas obtained is washed and purified, then oxidized or reduced, and finally recovered as sulfuric acid, elemental sulfur, or the like.
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