The human population on Earth is growing rapidly. Current projections maintain that by 2050 there will be 9 billion humans, and 10 billion humans by 2100. The rapid growth leads to rapid reduction of areas for landfills. A good solution is to recycle waste into alternative fuels. The product of waste recycling is syngas. It can be used in generation of electricity, as well as raw material in many chemical and petrochemical processes. However, syngas contains hydrogen sulfide, which is a toxic and corrosive sulfur compound, and must be removed from syngas for environmental and safety reasons.
One method of HS removal is used in Japan, in waste recycling facilities. Japan is a good example, since it is there where waste recycling is especially efficient.
The Claus process is used for removal of sulfur from crude syngas in large refineries. This method is currently standard for such tasks. It is not, by far, the most efficient process, however. This is due to high costs and complications when operating relatively small plants.
Liquid redox process was first used in gasification in Japan in 2001. The method proved successful and was gradually adopted for other processes. The first European redox project was launched. The US and China are in the late stages of developing coal gasification project with the use of this reduction-oxidation process. The use of this technology is limited by the total amount of sulfur in crude syngas. If it is less than 40 ton/day, redox process is viable.
The first plants were not too efficient, extracting about 100 kg of sulfur per day. When technology was proven, this number rose to 4 ton/day.
Chemically, the liquid redox process is a case of Claus reaction, which can be divided into five stages. The first stages – hydrogen sulfide absorption, then its ionization, then oxidation of sulfide, then oxygen absorption and oxidation of iron. The presence of iron in this process is due to two reasons. First, it is a donor and acceptor for electrons. Second, iron accelerates the reaction. In practice, the functions can be performed by other metals, but iron is relatively cheap and non toxic to the environment.
Today a rather flexible process has been developed, which allows processing of gas stream containing small amounts of hydrogen sulfide. The main advantages of such systems are:
- ability to process aerobic and anaerobic gas streams;
- hydrogen sulfide yield of 99.9% or more;
- harmless byproducts.
The process allows production of high quality sulfur:
- more plastic particle structure;
- quick absorption by soil and degradation;
- ability to mix with water, general hydrophilic properties.
The redox technology is a good source of active sulfur for agriculture. Sulfur is used for soil acidity correction, plant nutrition and as fungicide.