Gasification and Treatment of Radio active wastes

Waste Treatment- Gasification and Radio-active waste treatment



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

· Most of the low-level radioactive waste generated by nuclear power plants, industry, hospitals, and research institutions is in dry, solid form such as cardboard, paper, plastic, cloth, and glass. To reduce the amount of space required to store the low-level waste, three processes are used to reduce its volume: compaction, incineration, and shredding.
· 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 is generated primarily by nuclear power plants during purification of cooling water. Lubrication oil and sludges from filters are other examples of liquid low-level 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:

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.

(from http://www.francenuc.org/en_chn/trt_waste_e.htm )

Waste Treatment

Waste treatment

What is waste treatment?

Waste treatment refers to the activities required to ensure that waste has the least practicable impact on the environment. In many countries various forms of waste treatment are required by law.


Some examples of waste treatments are:


1.Disposal of wastes
Disposal of waste includes land filling, composting, and incineration. Disposal of waste here is defined as getting rid of waste, especially non-biodegradable waste. Because of this broader sense of word, gasification is, by right, a method of waste disposal.



-Composting
Composting is the aerobic decomposition of biodegradable organic matter. It was originally used for creating garden-ready soil. Nonetheless, it is now an important way to get rid of solid waste that is biodegradable in nature. However, the potential of this method has not been realised, with only 8% of the more than 60% of materials being treated this way in the US. People often feel that composting is complicated. However, “passive composting”, that is, leaving the material to rot [pun intended] serves as well as active composting [making sure that the condition is close to perfect for the aerobic bacteria by adding extra chemicals and/or ingredients]

In our opinion, composting is important as it recycles or downrecycles or "downcycles" organic household and yard waste and manures into an extremely useful humus-like, soil end-product called compost. Some examples are fruits, vegetables and yard clippings.
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Composting is widely believed to speed up the natural process of decomposition appreciably as a result of the raised temperatures that often accompany it. (The elevated heat results from exothermic processes, and the heat in turn reduces the generational time of microorganisms and thereby speeds the energy and nutrient exchanges taking place)





-Incineration
Incineration involves burning materials and it is therefore classified as “Thermal Treatment”. There is a debate over the environmental and health risks incinerators pose to the environment as in some countries, there is no material separation, which means that hazardous, recyclable materials as well as bulky materials are thrown into the incinerator. This might cause poisonous fumes, especially associated with burning of plastics, to be emitted and even non-toxic gases, such as carbon dioxide, are responsible for the formation of acid rain, as well as the greenhouse effect [Carbon Dioxide is one of the Greenhouse Gases]. Incineration reduces the mass of material by 95-96%, so it is often used in conjunction with land filling as it reduces the volume needed for land filling.


Incineration has particularly strong benefits for the treatment of certain waste types in niche areas such as clinical wastes and certain hazardous wastes where pathogens and toxins can be destroyed by high temperatures. Examples include chemical multi-product plants with diverse toxic or very toxic wastewater streams, which cannot be routed to a conventional wastewater treatment plant. However, due to the high cost needed to carry out incineration, it is not advisable to use this method as a treatment for waste.



-Landfill
Land filling is the method of burying your waste underground. This is the oldest and most popular method of waste disposal by fair, being widely used in many countries like Indonesia and the Philippines, because it is relatively inexpensive, unlike incineration. Landfills are also used to dispose of radioactive materials, often rendering the land unusable, because of the possible effects of radioactivity on humans as well as the environment.
Even if it’s not radioactive waste, land filling can have potentially harmful effects of the environment, like the contamination of groundwater, the attraction of pests like rats, and the production of gas. This might cause buildings built on the landfills to experience liquefaction during earthquakes.


We feel that the method of landfillingmay not be very feasible as huge areas of land are needed. For countries like Singapore, being a small country as it is, not much land can be used for landfilling. Also, in the process of landfilling, fatal accidents may occur. (Eg:scavengers buried under waste piles) ,infrastructure damage. (Eg: damage to access roads by heavy vehicles) and lastly, pollution of local environment. (Eg: such as contamination of groundwater and/or aquifers by leakage and residual soil contamination during landfill usage, as well as after landfill closure.




-Fun Fact:
U.S. landfills consist of 40% to 50% paper waste, 20% to 30% construction debris, and 1.2% disposable diapers
Windrow composting makes use of the same principal as composting [ie: breaking down biodegradable waste to form compost as fertilizer for the plants], except that it is on an industrial scale and it is similar to active composting [ie. People have to calculate how much carbon-rich materials and nitrogen rich materials there are, moisture etc.etc. ] There are machines that turn the materials for windrow composting for better oxygen levels and moisture content for the aerobic bacteria to act upon the waste.







2. Reverse Osmosis-The treatment of water
NEWater is reclaimed water which has gone through treatment in NEWater Plants which consists of four processes namely Micro Filtration, Reverse Osmosis, Water Conditioning and Ultraviolet Disinfection, before turning into pure water. It can be used for potable and non-potable purposes, mainly for wafer fabrication and air condition cooling towers.
NEWater was first implemented in the United States. It has been in place for more than 20 years.



In Singapore:
Singapore’s water problem- WHY WAS NEWATER INVENTED?

· There are many answers to this question. One answer is that Singapore’s daily water consumption keeps increasing daily. As Singapore’s population increases and the economy grows, more water is needed to keep factories running and people living. More and more people are immigrating to Singapore, and more and more businesses are being set up. Also, as water is so easily accessible – just turn on the tap and water will flow – more and more people are taking it for granted. They use water excessively without a second thought. All this contributes to the steadily growing water consumption.



· Another answer is that our previous sources of water are diminishing. This is because one big source of water for Singapore is water from Johor, Malaysia. We have 2 treaties with them, one expiring in 2011 and the other, in 2061. As 2011 draws near, Malaysia and Singapore are still not able to agree on terms for another water treaty. Malaysia is negotiating for higher prices, while Singapore wants everything to remain the same. If we could not come to an agreement by 2011, Singapore would be doomed.



· The Singapore government also realized that depending on other countries for water put us in a very vulnerable position that we did not want to be in. If Malaysia suddenly decided to cut off our supply of water, what would we do?



· This led to the conclusion that other sources of water needed to be created. After a long search, the government came up with NEWater to serve the purpose of the being the fourth national tap to fulfill the growing water consumption.

-Process of NEwater:
1. First water goes through Micro filtration. Micro filtration consists of many tiny fibers, which act like filters. Each fiber can filter particles up to 0.02 microns (the unit used to measure micro-organisms) in size. During Micro filtration, dirty water passes through the fibers. Solids, most bacteria and many harmful organisms are trapped in the tubes. Micro filtration also ensures a good stream of steady flowing water (water particles are able to pass through) to prevent blockage and clogging up.

2. Next, the water undergoes Reverse Osmosis. Reverse Osmosis applies pressure such that the water would flow from the concentrated side, to the less concentrated side through a semi-permeable membrane. This process removes the dirty contents of water, like nitrate, chloride (the nasty tasting water in the swimming pool is full of them), sulphate etc.

3. It also disinfects the water, and removes some of the pesticides in the water from the Micro filtration process. The Reverse Osmosis membrane has a pore size of 0.0001 microns (*gasp*) and through this process; about 95% of the dissolved solids in the water are removed. The Reverse Osmosis process also removes viruses.
4. Then, as a safety precaution back up if the Reverse Osmosis surprisingly fails, the Ultra violet Rays in the next step, Ultra Violet disinfection, will wipe out any harmful particles in one zap.

5. Finally, the fully cleansed water is mixed with water from the reservoir to give it a “water taste”, as it is too clean such as it does not have any taste at all. The NEWater is now ready to be drunk.

Sources: http://schools.moe.edu.sg/rgps/html/home.htm

-Disadvantages of NEwater:

It is a costly process. Some countries would rather use the method of landfilling over the process of reverse osmosis.

3. Gasification