Thermochemical conversion, characterized by higher temperature, is best suited for low-moisture feedstock and is generally less selective for products. Thermochemical conversion consists of three stages: incineration, pyrolysis and gasification. The incineration is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification are refined thermal treatment methods as alternatives to incineration. They are the transformation of the waste into product gas which can be stored as an energy carrier for later combustion (i.e. in a boiler or a gas engine).
Biochemical Conversion
The bio-chemical conversion process, which includes anaerobic digestion and fermentation, are preferred for wastes having high percentage of organic biodegradable matter and high moisture content. Anaerobic digestion is a reliable technology for the treatment of wet, organic waste. Organic waste from various sources is composted in a highly controlled and oxygen-free environment; circumstances resulting in the production of biogas which can be used to produce both electricity and heat. Anaerobic digestion also leaves behind a dry residue called digestate which can be used as a soil conditioner.
Alcohol fermentation is the transformation of organic biomass to ethanol by using specialized micro-organisms to carry out biochemical reactions.
GEM (Green Energy Machine)
Even though those methods work well, scientists and energy companies are trying to come up with better methods and machines to create energy through trash. They might be researching on how to maximize the amount of trash used in the production and also to maximize the efficiency. They would also try to leave zero carbon foot print.
A company called IST Energy, has been doing this. They have developed a shipping container size machine that turns your building’s trash into energy. The machine is called the Green Energy Machine (GEM). It is currently the best method used to produce electricity through trash. It can use anything to produce energy except for metal and glass as they have no energy content. It can convert upto 95% of the waste into energy and can process 3 tons of trash every day. The remaining 5% come out as ash. The machine can produce about 120 kW of electricity and double of that through 3 tons of trash.
The machine then removes any moisture and creates pellets. The pellets are then put into a gasifier and a synthetic gas or producer gas is generated and mainly consists of hydrogen and carbon monoxide. That gas is stored and used for making electricity and gas. Since the GEM runs on its own energy output, the only carbon emissions produced by it are coming from burning the synthetic gas that it produces.
Efficiency of conventional methods compared to modern method
When comparing the conventional methods to the modern methods such as the GEM, there are differences. The conventional methods such as incineration are not very efficient and they release a lot of pollutants. The efficiency of incineration depends on the trash that goes in and sometimes another fuel has to be added. In contrast, the GEM utilizes 95% of its trash to produce the electricity and the remaining is harmless ash. But one advantage of incineration over the GEM is that it can burn plastics and metals but some people argue that it is a disadvantage as less metal and plastic are recycled and burning them can produce harmful gases.
How does it affect the environment, society and economy?
Note: The issues discussed below do not concern the GEM but rather the conventional techniques such as incinerators, etc.
Environment
Emissions from incinerators can include heavy metals, dioxins and other harmful substances which may be present in the waste gases, water or ash. The combustion of plastics, like polyvinyl chloride (PVC) gives rise to these highly toxic pollutants.
Toxics are created at various stages of such thermal technologies and not only at the end of the process. These can be created during the process in such places such as the stack pipes, the residues in ash, scrubber water and filters, and in fact even in air plumes emitted. There are no safe ways of avoiding their production or destroying them, and at best they can be trapped at extreme cost in sophisticated filters or in the ash. The ultimate releases of these pollutants are unavoidable, and if trapped in ash or filters, these become hazardous wastes themselves. The pollutants which are created, even if trapped, reside in filters and ash, which need special landfills for disposal.
The production of electricity through trash does have its positive impacts on the environment. Since all the trash are being reused, they do not end polluting any water bodies or filling up landfills. Producing electricity through trash also saves energy for us and so our energy demands decrease and less fossil fuel are burnt. Since less fossil fuel is burnt, less greenhouse gases are being produced and the total effect of global warming is reduced.
Society
The waste-to-energy concept affects the society in a big way. There are positive and negative effects of this concept. One of the positive impacts is that as we start to develop better methods of using trash to produce electricity, more plants will be built to make more electricity through trash. This will increase employment opportunities. The people who live in the cities that have those plants will receive some satisfaction from donating their trash to these plants as they know they are playing an active role in saving the environment.
The negative effects of this concept concern the health of people. Those who work at these plants or those who live nearby can be affected by breathing in the air. People will also be affected by consuming any water, locally grown food or wildlife that has been contaminated by the waste. The amount of toxins inside each organism builds up the food chain and maximum amount of toxins will be transferred to humans as we are on top.
Economy
All over the developed world, almost half the investment is put in control systems to reduce toxic emissions such as mercury, cadmium, lead, dioxins and other harmful substances. For example a 2000 MT per day incinerator can cost upwards of $500 million in Europe and half of that goes into emission control. Another problem arises in the case of developing countries because the average calorific value garbage in such countries is about 800 cal/kg. For combustion technologies to succeed they would need about 2000 to 3000 cal/kg or other auxiliary fuels have to be added. This makes the process more uneconomical and polluting than it already is.
Most of the size and expense of the incinerator is dedicated to the pollution control equipment. Thus, it can be realized that the cost of the pollution control system over-rides the cost of the incinerator by a huge margin.
Experts say that to have an economically practical operation, incinerators have to burn at least 1000 tons of garbage a day. The cost to build such a facility is approximately $100 million. The operation costs to maintain the equipment especially the pollution control are too high.
Conclusion
The concept of energy through trash is very innovative and ground breaking. It has many advantages mainly to the environment but there are also disadvantages. The conventional methods produce harmful gases and substances and that is why it is not wise to use them anymore. I personally think that the best method is the GEM because it has the least negative impact on the environment and society despite its limitations.
Bibliography
LaMonica, Martin. "Turn Trash into Energy in Your Office Parking Lot | Green Tech - CNET News." Technology News - CNET News. CNET News. Web. 17 Sept. 2011. <http://news.cnet.com/8301-11128_3-10140500-54.html>.
"Negative Impacts of Incineration-based Waste-to-Energy Technology." Alternative Energy News. Web. 17 Sept. 2011. <http://www.alternative-energy-news.info/negative-impacts-waste-to-energy/>.
"Waste as a Renewable Energy Source." Alternative Energy News. Web. 17 Sept. 2011. <http://www.alternative-energy-news.info/waste-renewable-energy-source/>.
"Municipal Waste and Landfills - An Overview of Municipal Waste and Landfills."Geography Home Page - Geography at About.com. Web. 21 Sept. 2011. <http://geography.about.com/od/globalproblemsandissues/a/municipalwastelandfills.htm>.