4.Gasification
Gasification is a process that converts carbonaceous materials, such as coal, petroleum, or biomass, into carbon monoxide and hydrogen by reacting the raw material at high temperatures with a controlled amount of oxygen and/or steam. The resulting gas mixture is called synthesis gas or syngas and is itself a fuel. Gasification is a very efficient method for extracting energy from many different types of organic materials, and also has applications as a clean waste disposal technique. The advantage of gasification is that using the syngas is more efficient than direct combustion of the original fuel, more of the energy contained in the fuel is extracted.
Syngas may be burned directly in internal combustion engines, used to produce methanol and hydrogen, or converted via the Fischer-Tropsch process into synthetic fuel. Gasification can also begin with materials that are not otherwise useful fuels, such as biomass or organic waste.
Most coals are suitable for this type of gasifier because of the high operating temperatures and because the coal particles are well separated from one another. The high temperatures and pressures also mean that a higher throughput can be achieved, however thermal efficiency is somewhat lower as the gas must be cooled before it can be cleaned with existing technology. The high temperatures also mean that tar and methane are not present in the product gas; however the oxygen requirement is higher than for the other types of gasifiers. All entrained flow gasifiers remove the major part of the ash as a slag as the operating temperature is well above the ash fusion temperature. A smaller fraction of the ash is produced either as a very fine dry fly ash or as a black colored fly ash slurry. Some fuels, in particular certain types of biomasses, can form slag that is corrosive for ceramic inner walls that serve to protect the gasifier outer wall. However some entrained bed type of gasifiers do not possess a ceramic inner wall but have an inner water or steam cooled wall covered with partially solidified slag. These types of gasifiers do not suffer from corrosive slags. Some fuels have ashes with very high ash fusion temperatures.
Alternatively, syngas may be converted efficiently to methane via the Sabatier reactionor diesel-like synthetic fuel via the Fischer-Tropsch process. Inorganic components of the input material, such as metals and minerals, are trapped in an inert and environmentally safe form as ash, which may have use as a fertilizer. Regardless of the final fuel form, gasification itself and subsequent processing neither emits nor traps greenhouse gasses such as carbon dioxide. Combustion of syngas or derived fuels does of course emit carbon dioxide. However, biomass gasification could play a significant role in a renewable energy economy, because biomass production removes CO2 from the atmosphere. While other biofuel technologies such as biogas and biodiesel are also carbon neutral, gasification runs on a wider variety of input materials, can be used to produce a wider variety of output fuels, and is an extremely efficient method of extracting energy from biomass. Biomass gasification is therefore one of the most technically and economically convincing energy possibilities for a carbon neutral economy. There is at present very little industrial scale biomass gasification being done.
Several waste gasification processes have been proposed, but few have yet been built and tested, and only a handful have been implemented as plants processing real waste, and always in combination with fossil fuels. One plant (in Chiba, Japan using the Thermoselect process) has been processing industrial waste since year 2000, but has not yet documented positive net energy production from the process.
5. Treatment of Radio-active Waste
How Is Low-Level Radioactive Waste Treated Prior to Disposal?
· Before low-level radioactive waste can be transported or placed in a disposal facility, it must be in an acceptable form
· Regulations require that the waste be solid and structurally stable so that it can be transported more safely and does not settle after being placed in a disposal facility. When the waste meets these requirements, the risk of human exposure to radiation is reduced.
· Low-level radioactive waste is generated in many forms. Some of it is solid, and some is liquid. Very little of it is structurally stable. Therefore, the waste must be treated to convert it to an acceptable form for disposal.
-Solid Low-Level Radioactive Waste
· Compaction involves compressing the waste to reduce its volume, much like a kitchen trash compactor (see Figure 1). Compaction is a relatively inexpensive and widely available option which is used by many low-level radioactive waste generators.
· Originally used to treat municipal solid waste, incineration can be used to reduce the volume of solid low-level radioactive waste. When any material is incinerated, the products are gases and ash. When radioactive material is incinerated, the gas and ash contain radioactive particles and must be treated. The gas is filtered to remove radioactive particles. The filters become contaminated and must be treated as radioactive waste. The ash is mixed with concrete or other material to prevent radioactive particles from blowing away.
· Usually both compaction and incineration are performed in conjunction with shredding. Shredding involves cutting solid low-level radioactive waste into smaller pieces. This allows for more efficient compaction and a more uniform burn for incineration.
-Liquid Low-Level Radioactive Waste
· Liquid low-level radioactive waste must be solidified for transportation and disposal. Usually, as much water as possible is removed from the liquid waste, and the remaining material is immobilized. Methods for removing water include evaporation and filtration. The remaining material is immobilized with solidifying agents such as cement or asphalt. The cement or asphalt is in a structurally stable form which can then be sent to a disposal facility.
-Short-Lived Low-Level Radioactive Waste
· Medical facilities produce both solid and liquid low-level radioactive waste, but some of their wastes have short half-lives. That is, they decay quite quickly. These wastes are stored in a container at the hospital until they decay. (The actual storage time depends on the half-life of the radioactive materials present.) After the wastes are analyzed for radioactivity to confirm that they have decayed, they can be disposed of as ordinary trash. This method of handling low-level waste is called storage for decay. It reduces the volume of waste to be sent to a low-level waste disposal facility.
(from http://ohioline.osu.edu/~rer/rerhtml/rer_40.html )
There are at least five methods of approaching the problem of radioactive wastes:
There are at least five methods of approaching the problem of radioactive wastes:
SORTING
Rigorous sorting prevents the mixing of waste that is lightly contaminated with waste that requires treatment and/or costly storage. Practiced by most operators today, it is the only means of avoiding the creation of secondary waste, waste that is a byproduct of the treatment of waste.
COMPACTION
Presses designed to reduce the volume of solid wastes are located at most major production sites. The risks are minimal, but, depending on the waste compacted, the wastes may disperse gaseous effluents and liquids that must be trapped and packaged.
DECONTAMINATION
Used since the creation of nuclear sites, decontamination consists of such treatments as coprecipitation of contaminated liquids, the sanding of metals to eliminate surface contamination, and the soaking of metallic wastes in a chemical bath. Secondary wastes are always created-the sludge from precipitation, contaminated sand, contaminated liquids . . . Today, decontamination is used to change the category of given wastes. Thus, one can begin with medium-activity solid wastes and end up with solid wastes said to be of low activity and a great quantity of effluents that are themselves treated to become sludge also described as of low activity or effluents of low activity that are then released into the environment.
REUSE
Called “recycling”, this method saves nuclear materials. However, the recovery of nuclear materials creates secondary wastes including radioactive effluents and sometimes consists of operations that clearly increase the risks for workers.
THERMAL TREATMENT
This method, which encompasses a wide variety of processes, is increasingly used:
--Melting metals
--Evaporation
--Incineration
Each time that wastes are treated by heat, whether to reduce the volume or for another reason, there is a risk that radionuclides and/or other toxic materials accompanying the radionuclides, will become volatile and escape into the environment. Filtration systems are never 100% effective. A loss of a minimal percentage of burned material can be dangerous. The release of one or two grams of uranium oxide, for example, represents the dispersion of a hundred million million (10E+14) particles of uranium, which can cause biological damage. Heat never destroys radionuclides, but it can help to distribute them.
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