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Are you familiar with the adage, “Things are not always as they seem”? It’s probably come into play in your life at one time or another, like when you opted to buy the cheapest model of something, only to find out that its life span was two weeks before falling apart. Not such a bargain after all. Well, it’s kind of that way with low sulfur coal and its application in electric power production. All coals contain some sulfur, their content ranging from trace amounts to as high as 8%. This sulfur ends up as a byproduct of the combustion process, meaning it is released into the atmosphere when coal is burned. There it combines with moisture in the air to form sulfuric acid. If you will remember from last week’s blog, this is the stuff that forms acid rain, able to dissolve marble statues, corrode metal, and disrupt eco systems. In the process of generating electricity for homes and businesses, many utility power plants of the past burned coal with high sulfur content. This was the case through the middle of the 20th Century. This coal was brought into power plants by trains and river barges from nearby coal mines. In some cases power plants were actually built next to the mines, thereby eliminating shipping cost. It was effective and cheap. Then, in 1963, the Clean Air Act was signed into law, its purpose to improve, strengthen, and accelerate programs for the prevention of air pollution. By 1970 the Act had empowered the federal government to set and enforce national air quality standards for sources of air pollution, like coal burning power plants. Under the Clean Air Act, government was able to mandate to utilities that they reduce sulfur emissions or face court injunctions to shut them down. Caught between a rock and a hard place, utilities learned to comply, switching over to lower sulfur coals. But the story doesn’t end here. That lower sulfur created a whole host of new problems, for the power plant and their consumers. To begin with, low sulfur coals are scarce in areas of the country where electricity is needed most, like the densely populated eastern half of the country. It has to come from mines in the western states like Wyoming, and for a power plant located in Chicago, for example, this can get costly. A lot more costly than simply getting the coal, high sulfur content coal, that is, from nearby mines in southern Illinois. The result is higher transportation costs, and this cost is passed on to consumers. Another problem with low sulfur coals is that they tend to release less heat energy than higher sulfur coals when they are burned. That means that you have to burn more of it to generate the same amount of power. As a result, utilities ended up having to buy more coal, another cost that was passed on to the consumer. Yet another issue with the switch from high sulfur to low sulfur coals involved the reconfiguration of power plants that was made necessary. You see, when power plant boilers are designed, they have a particular type of coal in mind, and that originally was high sulfur coal. In addition, many power plants have been required to install equipment to scrub sulfur from the gases produced when the coal is burned. This scrubbing equipment is expensive to purchase, install, and operate. Pollution control equipment like this consumes power, but it does not facilitate the process of generating electricity. In addition to these costs, the switch to low sulfur coal causes many other problems that can raise the cost of operations and make the power plant less reliable. For example, some low sulfur coals have properties that tend to make ash stick to the surfaces inside of boilers, often leading to boilers overheating and springing leaks. If these leaks are bad enough, the boiler has to be shut down for cleaning and repair, and when this happens the electrical generating unit has to be taken off the utility grid. The net result is less power being available to meet consumer demand. We can thank the Clean Air Act for effectively reducing the amount of airborne pollutants, but we must acknowledge the cost to do so. Electric utilities are for-profit corporations, not charities, and someone has to pay for the increased coal consumption, higher transportation costs, equipment additions, and operating problems that are a result of the usage of low sulfur coal. That someone is the consumer. _____________________________________________
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Posts Tagged ‘power plant’
Nuclear Power, Is It The Answer?
Sunday, April 18th, 2010|
In weeks past we’ve explored wind energy and the possibility of it overtaking fossil fuel burning plants as our main source of power. This week we’ll discuss the next most viable option to do the job, that of nuclear power. Nuclear power, unlike fossil fuel plants, doesn’t combust fuel and therefore doesn’t contribute to air pollution. But unlike wind turbines, their electrical output is reliable, that is to say, we know, save for a major breakdown, that they will put out X-amount of power every day, regardless of weather conditions. As a matter of fact, according to the Nuclear Energy Institute, the 103 nuclear power plants in operation in the United States today are the most reliable and efficient producers of energy to our electric power grid. They account for about 20% of the power generated and produce a total capacity of 96.245 gigawatts, meaning, a whopping 96.245 billion watts. Nuclear energy is clean, reliable, and produces loads of power, so why not initiate a program to begin immediate replacement of our dirty fossil fueled plants? It’s time to take a closer look. Needless to say, large scale replacement of fossil fueled power plants with nuclear power plants would be a huge undertaking. You’ll remember from my previous blog postings that the US Department of Energy reports that 71.2% of our power is currently being produced by burning fossil fuels. All power plants, and especially nuclear power plants, are extremely expensive to build. Let’s look at an example. In 2007, Florida Power & Light informed the Florida Public Service Commission that the cost to build a new nuclear plant in south Florida would be approximately $8,000 per kilowatt-hour. How does this large sum affect the consumer in terms of real dollars? Well, let’s say you want to build a 3,000 megawatt (3,000 million watt) nuclear plant. This is enough capacity to provide power for about 2 million people in the US. When all is said and done, you’ll end up having to pay out $24 billion before you can start generating electricity. That looks like a lot of cash outlay for one plant, but what does it mean to each individual? If we do the math, a nuclear plant that is capable of supplying 2 million people with electricity will result in a cost of approximately $12,000 per person. Considering that, will investors, taxpayers, and consumers be willing to cover the losses that accrue when all existing fossil plants are closed and nuclear plants are erected to replace them? As with wind powered energy, cost is an enormous factor when considering the viability of nuclear power plants, but there is something way more profound to consider. Nuclear power plants produce radioactive waste. This waste remains radioactive, and therefore highly poisonous to the environment, for millions of years. That’s right, millions, not hundreds, not thousands, millions of years. The Nuclear Energy Information Service states that for each nuclear reactor that exists, 50 to 60 tons of high level radioactive waste is produced every year. So you’ve got all this waste as a byproduct of nuclear energy production, and, of course, there’s a lot of controversy surrounding its safe disposal. Not only does it lay around for millions of years, the costs of dealing with it are staggering. The US Department of Energy estimated in 2008 that it will cost around $96 billion to construct the Yucca Mountain nuclear waste repository in Nevada, which is basically a huge underground garbage dump for nuclear waste. And this amount of money will only keep it in operation for about 150 years. What happens after that? And if we build more nuclear plants in addition to those that currently exist, what will then be the cost of disposing of their waste? No one knows for sure, but they know it’s a mighty large sum, and certainly much too large for the ailing American economy to absorb. Now, Dr. Seuss, the guy that wrote The Cat in the Hat and other wonders, was an actual person, and he had a lot to say about things that didn’t involve gnarly looking creatures that go “BUMP!” in the night: “Sometimes the questions are complicated and the answers are simple.” Well, that’s sort of the case here. There are a lot of seemingly simple answers being posed to address our energy and environmental problems, but when you start asking pointed questions to delve deeply into the feasibility of those answers, things can get extremely complicated. We have seen through our present blog series that these answers inevitably lead to more questions and a multiplicity of other problems, and so far we haven’t seen an easy fix to our energy issues. But are we just making an issue where none exists? Are we making a mountain out of a mole hill? Next week we’ll explore a few more options that are being considered as alternative energy sources. Perhaps there is an easy answer to our power dilemma. _____________________________________________ |
Thermodynamics in Mechanical Engineering, Part IV, Stoichiometry
Sunday, December 27th, 2009|
Last week we talked about an area of thermodynamics that concerns refrigeration cycles as presented through the example of an air conditioner. This week, we’ll learn about stoichiometry, which is concerned with the math behind chemical reactions, like those that take place during the burning of fuels. During the combustion process, heat energy is released from a fuel when the combustible elements in the fuel combine with oxygen. This is known as oxidation, or in common everyday language as burning. The most important thing to remember about oxidation is that it obeys the first law of thermodynamics. That is, mass cannot be created or destroyed. In a chemical reaction like combustion, particles of fuel and air are rearranged in space and then combine to form different substances. However, despite the rearranging, the mass that goes into the reaction must equal the mass that comes out. This conservation of mass is the basis of stoichiometry. For example, if pure carbon (represented by the chemical symbol “C”) is burned in pure oxygen (O2), you can represent the combustion process as: C + O2 → CO2 This is chemistry shorthand for representing how carbon and oxygen combine during burning to form carbon dioxide (CO2). The elements to the left of the arrow are known as “reactants” and the elements to the right are known as “products.” In stoichiometry, the mass of the reactants must equal the mass of the products. But, how do we quantify the mass of reactants and products? Now this is where it gets a little weird. To make use of our chemistry shorthand above, we have to consider something called moles. No, these aren’t the little furry creatures that tunnel under your lawn and eat your tulip bulbs. In stoichiometry a mole is considered to be 6.02×1023 molecules of a substance. That is 602,000,000,000,000,000,000,000 molecules! Okay, so we have one heck of a lot of molecules in a mole. So, what does that have to do with figuring out how much mass we are dealing with in the combustion process? Well, in order to make moles work for us, we have to take into consideration the differing molecular weights of substances. Molecular weight is the number of grams (g) of mass that are contained within one mole of a substance, like the element carbon in our example above. To help make stoichiometry more workable, scientists created a table that provides the molecular weight of all known chemical elements. This table is known as the Periodic Table of Elements, or the “Periodic Table” for short. Now going back to our example above, if we know from the Periodic Table that carbon has a molecular weight of 12 g per mole and oxygen has a molecular weight of 16 g per mole, then how many grams of carbon dioxide do we get by burning carbon in pure oxygen? The combustion process can be represented by this equation: C + O2 → CO2 (12 g/mole) × (1mole of carbon) + (16 g/mole) × (2 moles of oxygen) = 44 g of carbon dioxide This is a fairly straightforward example of how stoichiometry works. In reality, things can get far more complicated. In a power plant for example, fuels like coal contain substances in addition to carbon, such as hydrogen and sulfur, and they, too, must be factored into the stoichiometric accounting system. To further complicate things, fuels are usually burned in air, rather than pure oxygen. Air, too, contains substances other than oxygen, including nitrogen, argon, and molecules of water. These other substances’ presence in fuel and air make the combustion process more challenging to account for, because they all get mixed together, and they can combine into all sorts of other substances. Despite these complicating factors, the first law of thermodynamics must be obeyed, so the balancing act is still the same: mass of the reactants must equal the mass of the products. Once mechanical engineers use stoichiometry to figure out what’s going in and coming out of the combustion process, they can then use the data provided by chemical analysis of the fuel to calculate the heat energy that is released. They can also calculate the air required for proper combustion. This helps them to design things capable of delivering enough fuel and air to meet the heat input requirements for a diversity of power cycles, from the engine in your car to the coal fired power plant supplying electricity for your home. Next week, we’ll talk about psychrometric analysis. No, this has nothing to do with psychiatry. Psychrometrics involves the analysis of gas and vapor mixtures like air and water. _________________________________________________________________ |
Thermodynamics In Mechanical Engineering, Part II, Power Cycles
Sunday, December 13th, 2009|
Last time we talked about some general concepts in an area of mechanical engineering known as thermodynamics. In this week’s article we’ll narrow our focus a bit to look at a part of thermodynamics that deals with power cycles. One mammoth example of a power cycle can be found in a coal-fired power plant. You can’t help but notice these plants with their massive buildings, mountains of coal, and tall smoke stacks. They’ve been getting a lot of negative press lately and are a central focus of the debate on global warming, but most people have no idea what’s going on inside of them. Let’s take a peek.
Figure 1 – A Coal-Fired Power Plant A power plant has one basic function, to convert the chemical energy in coal into the electrical energy that we use in our modern lives, and it’s a power cycle that is at the heart of this conversion process. The most basic power cycle in this instance would include a boiler, steam turbine, condenser, and a pump (see Figure 2 below).
Figure 2 – A Basic Power Cycle When the coal is burned in the power plant furnace, its chemical energy is turned into heat energy. This heat energy and the boiler are enclosed by the furnace so the boiler can more efficiently absorb the heat energy to make steam. A pipe carries the steam from the boiler to a steam turbine. Nozzles in the steam turbine convert the heat energy of the steam into kinetic energy, making the steam pick up speed as it leaves the nozzles. The fast moving steam transfers its kinetic energy to the turbine blades, causing the turbine to spin, much like a windmill (see Figure 3 below).
Figure 3 – The Inner Workings of a Steam Turbine The spinning turbine is connected by a shaft to a generator. The turbine works to spin the generator and thus produces electricity. After the energy in the steam is used by the turbine, it goes to the condenser, whose job it is to convert the steam back into water. To accomplish this, the condenser uses cold water, say from a nearby lake or river, to cool the steam down until it converts from a gas back to a liquid, that is, water. This is why power plants are normally found adjacent to a body of water. After things are cooled down, the pump gets to work, pushing the condensed water back into the boiler where it is once again turned into steam. This power cycle keeps repeating itself as long as there is coal being burned in the furnace, the plant equipment is functioning properly, and electrical energy flows out of the power plant. Thermodynamics sets up an energy accounting system that enables mechanical engineers to design and analyze power cycles to make sure they are safe, reliable, efficient, and economical. When all is said and done, a properly designed power cycle transfers as much heat energy as possible from the burning coal on one end of the cycle to meet the requirements for electrical power on the other end of the cycle. As was mentioned in last week’s blog, nothing is 100% efficient. Next time we’ll learn about being cool. No, I’m not going to talk about the latest cell phone gadget or who’s connected on Facebook. We’ll be covering refrigeration cycles. _________________________________________________________________ |
