versus Gas Cogeneration

We intentionally skipped over the gas (typically natural gas = methane) thermal power plant comparison since it is very similar to the oil thermal power plants, and also because it has been replaced in most cases of new construction by the gas cogeneration process. Gas cogeneration uses a two-stage process to extract energy from the methane fuel. The first stage is a stationary gas turbine, and the second stage is a heat recovery system that makes steam and then recovers energy from the steam using either the gas turbine or a separate steam turbine.

How Cogeneration Works

A gas cogeneration (also called combined cycle) system can be large (200MW or more) or small (as small as a few hundred kW), depending on the design (of course) and the need. The basic denominator is the size of the gas turbine. The smaller turbines are typically utility and aviation turbines that have been modified for stationary service. They are set up in a fixed location, and are coupled to a generator to produce electricity. What happens downstream (on the electrical side) is the province of the electrical engineers. What happens downstream on the exhaust gas side is heat recovery from the hot exhaust gases. This typically is done with a heat exchanger that has a low pressure drop, because the gas turbine’s performance is very sensitive to a buildup of back pressure.

There is also another add-on to improve efficiency of the overall system. The gas turbine is not a very efficient combustion device; it needs an excess of oxygen in the exhaust gas to keep from making smoke (carbon particulates in the exhaust gas). This means that the exhaust gas has somewhere around 10 percent oxygen left in it. The tweak here is to add more fuel (methane) after the turbine and burn the oxygen to create more heat. This optimises the heat recovery for the entire system. Aviation turbines can also be designed to do this – called an “afterburner”, but the afterburner is designed to provide more thrust, and is inefficient in terms of combustion (you have maybe seen the films of military aircraft using afterburners for takeoff, often in the early days with large amounts of black smoke trailing behind – smoke means fuel was not completely burned). Caution: the video below is VERY LOUD!

The steam that is generated in the gas cogeneration system can be utilised in three different ways, either separately or jointly:

• it can be re-injected into the gas turbine to produce more electricity through the generator,
• it can be fed to a separate steam turbine that also drives a generator – maybe on the same shaft as the generator coupled to the gas turbine
• the steam can be used elsewhere for motive or heat, such as in a process plant

All of these methods create an additional benefit for the cogeneration unit.

The result is that the gas cogeneration system is the most efficient of the stationary thermal power generation systems. It is also generally less expensive, since the facilities are relatively easy to package, and also because the gas combustion process creates little pollution, aside from carbon dioxide (CO2) and potentially nitrogen oxides (proper controls of flame temperature and addition of ammonia can eliminate almost all of the nitrogen oxide emissions).

It is also possible to burn light gasoline (naphtha) or kerosene or even fuel oil in a stationary gas turbine, but some of the advantages disappear in this case (more CO2 per volume exhaust gas, potential sulphur oxide and nitrogen oxide emissions – still essentially controllable). Also, the flexibility of the system is reduced – you cannot change the load as quickly to adjust to changes in electrical requirements – load-following – but this is not a problem for a base-load unit.

An aside here: it is also possible to build a similar small unit using a automotive piston engine and a thermal recovery system on the exhaust system. These units are generally less than 100 kW in capacity.

GEOCOGEN in Comparison

The gas cogeneration process is probably the most environmentally friendly process amongst the combustion processes, but it is still a combustion process. A hydrocarbon (methane) is burned, and there are two problems here. One is that the supply of methane in large quantities (supplied by pipeline or as liquified natural gas – LNG – at a cryogenic temperature) is limited, and the other is that it still produces carbon dioxide (CO2). As we noted previously (many times) GEOCOGEN does not produce carbon dioxide because it does not burn anything. Also, the price of energy from a GEOCOGEN Power Plant is significantly lower. According to the US Dept of Energy estimates for new facilities coming into service by 2016 (http://www.eia.doe.gov/oiaf/aeo/electricity_generation.html) cogeneration is the least expensive electricity producter at about USD 0,08/kWh total cost. Compare this to GEOCOGEN at less than USD 0,04/kWh (a conservative GEOCOGEN estimate) and you will see what we mean!

Download the table here

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Power Plant Comparisons:

GEOCOGEN versus Oil-Fired Thermal

The basic principals of an oil-fired thermal power plant are quite similar to a coal-fired thermal plant, but simpler – heat water until it boils, put the steam through a turbine and generate electricity from the rotation of the steam turbine’s shaft.

Since you have been reading our website http://geocogen.net, you already know what makes a GEOCOGEN Power Plant work. What do you know about an oil-fired power plant? Here is some information.

Oil-Fired Power Plant

The heat to “boil the water” comes from burning oil, usually heavy oil that is called “residual oil” or “heavy fuel oil” or “number 6 fuel oil”. There are various kinds of heavy fuel oil, and they are usually segregated by the sulphur content. The reason for this is that heavy fuel oil can have very high sulphur content – as much as 4 to 6% by weight or even more – and as low as almost zero. The high sulphur fuel oil is usually the product of the Middle Eastern countries – but this is a question of the composition of the crude oil or petroleum as it comes out of the ground.

What is Heavy Fuel Oil?

The way heavy fuel oil is produced is relatively simple, for the most part. Think about when you make coffee by boiling water with the ground coffee inside. When you’re finished, there are the coffee grounds and maybe some tar still left in the bottom of the pot. It’s similar with crude oil – when you boil it (in a refinery, not on the kitchen stove!), you get light colorless product called naphtha (usually used in gasoline or petrochemicals), then clear or lightly coloured kerosene, then somewhat darker but still transparent distillate oil that can be used as a component of Diesel fuel or as home heating oil (also called “No. 2 fuel oil”). What’s left that didn’t boil is called residue or atmospheric residue. Depending on the conditions of the distillation and the quality of the crude oil that was the starting point, this residue is typical of heavy fuel oil. It’s qualities (density, viscosity, sulphur content, etc.) depend on the crude oil it comes from, and how hard it was boiled. In any case, it is almost a heavy as water, may or may not be liquid at room temperature, and generally has sulphur content between 0,1% and 5% by weight. It generally will contain polynuclear aromatic compounds, or PNAs. If you remember your chemistry at all, PNA’s are made up of benzene rings that are linked together. PNAs are generally toxic and cancer-forming on long-term skin contact. They also have a very low ratio of hydrogen to carbon, and usually contain contaminants like sulphur, nitrogen, and heavy metals.

The advantages that an oil-fired power plant has over a coal-fired power plant are that the fuel is much easier to handle (kept heated, it remains in a liquid form), and there are almost no waste disposal problems for oil-fired power plants, except for the sulphur, nitrogen, and minor flyash in the flue gas. The sulphur problems have been legislated away in many areas by requiring that oil-fired power plants use low sulphur heating oil, typically 0,3 weight percent or lower, or use treatment systems to remove the sulphur from the flue gas. Flyash can be removed by particulate removal systems. All of this is not free, but it is typically much less expensive than the systems required for a coal-fired power plant. On the other hand, the heavy fuel oil is considerably more expensive than coal, with the relative prices being driven by the availability and prices of alternative fuels.

As in coal-fired power plants, the effort to obtain the maximum energetic value from the oil burned means that there is a distinct advantage to cool the exhaust stream from the steam turbines to as low a temperature as is feasible. The lower the exhaust temperature is, the greater the pressure differential is across the turbine, and this is what determines the power that can be extracted from the turbine. To get to the desired low temperature/pressure (actually a vacuum when the temperature is below 100°C – 212°F) a lot of low-level energy will be released to the atmosphere via a cooling tower or to a lake, a river, or the ocean as water that is warmer when it goes out than when it came in.

The gas from the firebox of the boiler – the exhaust gas or flue gas – is also cooled to the extent economically reasonable to recover as much energy here as possible. This is where the point made above enters the picture – there are certain design criteria that are required to be able to remove sulphur and nitrogen oxides from the flue gas. There are additional requirements for removing particulate material – dust – and also in modern boilers to remove or substantially reduce the carbon dioxide in the flue gas. All this equipment that is necessary for controlling the quality of the combustion gasses becomes a large economic burden in the design and construction of new oil-fired power plants.

Additional social costs that are not always easy to evaluate are the devastation of the landscape made in oil fields in previous years (and still are present in some areas today) as well as the pollution produced even from a modern clean oil refinery and the leftover toxic oil wastes that still litter many parts of the world.

source: Wikipedia.org http://upload.wikimedia.org/wikipedia/commons/thumb/f/f1/Signal_Hill_California_1923.jpg/1000px-Signal_Hill_California_1923.jpg

GEOCOGEN Power Plant

How does a GEOCOGEN Power Plant stack up against an oil-fired power plant? The most obvious comparison are the social costs involved in the production of heavy fuel oil, and the emission of sulphur, nitrogen and carbon oxides and heavy metals into the air. Since the GEOCOGEN process does not burn any fuel, there are essentially no sulphur or nitrogen oxides, no volatile organic compounds, no carbon dioxide (CO2), no dust, no ash, etc., and the social costs are nil.

As in the comparison with a coal-fired power plant, what does remain is a way to eliminate or convert the low level steam energy. The advantage of the GEOCOGEN Power Plant is that, since our energy is essentially free, we don’t have to pay that much attention to efficiency in the heat train. We will put our excess low level heat back into the shaft and it will get recycled. In theory, that should lengthen the usable life of the stone heat harvesting area, but that probably will not make a measureable difference in the cycle length of the stored stone heat.

We also will not be drilling oil fields anywhere to provide our fuel, there will be no refinery to distill the crude oil, and there will be no extensive transportation system for supplying the fuel or removing the ash and the fly ash from the site. Our site will tend to look like a park, if the underground option is selected – all you probably will see from outside the area is a nicely landscaped park or forest.

You can tally up the pluses and minuses – tell us what you think? If you click here, you will come back to this same article, but with a form for comments below it. There you can write quite a bit (at the moment the comment length is limited to about 300 characters)

Oh, for a modern oil-fired power plant with CCS (“carbon capture and storage” or “carbon capture and sequestration”), the capital cost is greater than for a GEOCOGEN Power Plant, and the operating costs are also higher, and will get worse as more plants are built and with a limited amount of oil available. Finally, there is no guarantee that the carbon will not return to circulation!

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- Top of the page
- Go to the beginning of the GEOCOGEN Project thread

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Power Plant Comparisons:

GEOCOGEN versus Coal-Fired Thermal

The basic principals of the two systems are similar – heat water until it boils, put the steam through a turbine and generate electricity from the rotation of the steam turbine’s shaft.

If you have been reading our website http://geocogen.net, you already know what makes a GEOCOGEN Power Plant work. What do you know about a coal-fired power plant? Here is some information.

Coal-Fired Power Plant

The heat to “boil the water” comes from burning coal. There are various kinds of coal, in general ranging from anthracite to lignite.

Anthracite coal is a hard coal that produces a clean blue flame when it burns – and it has low levels of contaminants compared to other coal types. It generally contains over 90% carbon. It burns cleanly, with little if any soot, and is not dusty (it has a hard shiny appearance). It is denser than other coal types, and has a low moisture content – around 15%. These qualities make it popular but also make it expensive. Also, there is not so much of it in the World – probably less than 1% of the total known coal resources. It is too expensive today to be utilised for anything except small “exotic” operations that can afford the more expensive fuel (2-3 times bituminous coal – see below). The exotic operations are things like coal heaters in a household, or older automatic feeders, where the coal is first crushed.

Bituminous coal (called “Steinkohle” in German – translated “stone coal”) is the “next best” grade of coal; it has a lower carbon content – about 65-75% – and the rest is water and contaminants, including sulphur. It can be shiny or dull in appearance. Bituminous coal is used today for electrical generation and – after a drying process – to make steel from iron ore. There are large deposits around the world, including Pennsylvania in the USA, originally in western Europe, although these are becoming exhausted, in Russia and Australia. The sulphur and (to a lesser extent) nitrogen content of bituminous coal cause problems when it is burned, creating sulphur and nitrogen oxides which are both toxic and acidic. There are ways to control these contaminants (see below).

The lowest grade of coal is called lignite or soft coal or brown coal. It is formed from the mild compaction of peat bogs around the world, and is present in substantial quantities in central Europe, Russia, the USA, Australia, and South Africa; it is commercially mined in all these countries. Lignite has a high amount of contaminant materials and not completely converted bog components, and is less dense than the other types of coal. Commercially, it tends to be used only for power generation close to the mines. Since lignite is a “new” sedimentary product, it is often found close to the surface. This means that it is easier to mine using strip mining or open-pit mining. These practices result in a devastated environment, with thousand of square meters of soil being removed and vast cavities are left in the area where the lignite is mined.

The Results of Strip Mining in Germany. Source: Wikipedia. “This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Germany license. Attribution: Bundesarchiv, B 145 Bild-F088901-0001 / Thurn, Joachim F. / CC-BY-SA”

In South Africa, lignite is used as a feedstock for the SASOL lignite-to-hydrocarbons plants, which produces a large variety of specialty chemicals in addition to gasoline and distillate type motor fuels. When burned, lignite produces a large amount of carbon dioxide (CO2) as well as sulphur and nitrogen oxides, and dust. It can also produce volatile hydrocarbons if the combustion process is not complete. These contaminants can be controlled industrially by many of the same control systems used for the combustion of bituminous coal.

Generally, in a coal-fired power plant, the coal is broken down into very small pieces – pulverised – and is fed into the firebox of the boiler almost as a liquid. The quality of bituminous coal allows the production of steam at very high temperatures – limited primarily by the metallurgy of the tubes in the boiler. The reason that higher temperature and pressure are desirable is that they help to increase the efficiency of the steam turbines.

In the effort to obtain the maximum energetic value from the coal burned, there is a distinct advantage to cool the exhaust stream from the steam turbines to as low a temperature as is feasible. The lower the exhaust temperature is, the greater the pressure differential is across the turbine, and this is what determines the power that can be extracted from the turbine. To get to the desired low temperature/pressure (actually a vacuum when the temperature is below 100°C – 212°F) a lot of low-level energy will be released to the atmosphere via a cooling tower or to a lake, a river, or the ocean as water that is warmer than what came in.

The gas from the firebox of the boiler – the exhaust gas or flue gas – is also cooled to the extent economically reasonable to recover as much energy here as possible. This is where the point made above enters the picture – there are certain design criteria that are required to be able to remove sulphur and nitrogen oxides from the flue gas. There are additional requirements for removing particulate material – dust – and also in modern boilers to remove or substantially reduce the carbon dioxide in the flue gas. All this equipment that is necessary for controlling the quality of the combustion gasses becomes a large economic burden in the design and construction of new coal-fired power plants.

Additional social costs that are difficult to evaluate are the vast empty and infertile areas that often are created from strip-mining bituminous coal and lignite, and the need for controlling the dust produced in handling the raw fuel coal and the solid waste are additional problems – both social and economic – that make coal-fired power plants more expensive.

GEOCOGEN Power Plant

How does a GEOCOGEN Power Plant stack up against a coal-fired power plant? The most obvious comparison is the flue gas situation. Since the GEOCOGEN process does not burn any fuel, there is essentially no sulphur or nitrogen oxides, no volatile organic compounds, no carbon dioxide (CO2), no dust, no ash, etc.

What does remain, at least in theory, is a way to eliminate or convert the low level steam energy. The advantage of the GEOCOGEN Power Plant is that, since our energy is essentially free, we don’t have to pay that much attention to efficiency in the heat train. We will put our excess low level heat back into the shaft and it will get recycled. In theory, that should lengthen the usable life of the stone heat harvesting area, but that probably will not make a measureable difference in the cycle length of the stored stone heat.

We also will not be strip-mining anywhere to provide our fuel, and there will be no extensive transportation system for supplying the fuel or removing the ash and the fly ash from the site. Our site will tend to look like a park, if the underground option is selected – all you probably will see from outside the area is a nicely landscaped park or forest.

You can tally up the pluses and minuses – tell us what you think? If you click here, you will get back to this same article, but with a form for comments below it. There you can write quite a bit (at the moment the comment length is limited to about 300 characters)

Oh, for a modern coal-fired power plant with CCS (“carbon capture and storage” or “carbon capture and sequestration”), the capital cost is greater than for a GEOCOGEN Power Plant, and the operating costs are also higher, and will get worse as there are more plants built and a limited amount of coal available. And there is no guarantee that the carbon does not return to circulation!

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.

- Top of the page
- Go to the beginning of the GEOCOGEN Project thread

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