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Listen to part of a lecture in an environmental engineering class.
Professor: As you know, we have been discussing fossil fuels and the issues that the world is facing due to our finite supply of fossil fuels, like oil. Would anyone like to provide a quick summary of how oil is formed?
Student 1: Well, dead plants and other organisms, they get buried under the earth, under the sea, and they get pushed deep underground by geological forces. Then they’re heated and pressurized and you get oil.
Professor: Yes, and of course this process takes millions of years, today let’s consider the pros and cons of some alternative ways of making oil. Some of them use recycled waste products to create oil. That’s right, recycled garbage! The early efforts to create oil from waste resulted in the low quality oil that was not a serious alternative to natural crude oil, but there’s a newer process I want to start with. It’s called "thermal depolymerization." With this process of thermal depolymerization, it only takes hours to create oil, and it’s a pretty decent quality. It uses water pressure and heat to convert organic material into a variety of useful products including crude oil, which can be refined into gasoline and other oil-derived products, and it doesn’t produce any polluting emissions.
Student 2: Um...l think I remember from the article you assigned that thermal depolymerization can use both organic and inorganic waste as its source, right?
Professor: Yes, and that’s another big improvement from earlier attempts. Waste products, almost any kind of waste, both organic and inorganic, can be used a source. For example, old tires, plastic bottles, even old household appliances. You chop up everything into tiny pieces, dump the stuff into a vat and then add water. Water is an important ingredient because it reduces the amount of heat needed, which increases the efficiency of the process. Remember, if the process isn’t efficient, you’ll end up using nearly as much energy to produce the oil as you’ll get from the oil itself and it won’t be a viable process.
Student 1: Yeah, that wouldn’t make much sense.
Professor: So after the waste and water are added to the vat, the mixture is ground into a pulp then the mixture is heated. Now, after that, and this is another feature that previous processes didn’t have, there are two reactor stages. In the first reactor stage, the mixture is cooked with heat and pressure for about an hour to break apart the molecules that the waste material is composed of. Then the excess water and minerals are removed. In the second reactor stage, the mixture is heated to 260 degrees Celsius and pressurized to 42 kilograms per square centimeter and in 20 minutes the process replicates what it takes nature thousands or even millions of years to do. There are several more steps, which we aren’t going to discuss shortly, but one point I do want to make relates to the efficiency of the process. Currently, the claim is that only 15% of the energy obtained is used to power the process and 85% of the energy is usable for other purposes. Not bad, huh?
Student 2: But what about global warming and the greenhouse effect? I mean it would be great if we could recycle garbage into a useful product, but that useful product is oil, and as long as we continue to burn oil, we’ll continue to pollute the atmosphere with greenhouse gases containing carbon, So is this process really a win for the environment in the long run, or does it cause environmental damage?
Professor: Well, I should mention that research on the thermal depolymerization process has received funding from environmental groups, but you raise a good point. Proponents of the process claim that we could eventually find enough sources of waste containing carbon to produce oil, so that we could eliminate the need for the traditional sources of oil completely, and therefore the only carbon that we’d use would already be above ground, thus making it a so-called "carbon neutral process." But—and this is a very big—but it might be overly optimistic, and perhaps naive, to assume that oil created by the thermal depolymerization process will completely replace traditional oil. Not only that, if the price of oil were to go down over time, then the demand for oil might increase, and we’d actually end up using even more oil than we do now.