IE010315A

Chapter 3
Thermal Combined Cycle RDF

What is a chapter on Combined Cycle and Thermal Power Plant Combustion of RDF doing in a book on wind energy? There being some need for diversion from the seriousness of the business at hand, it was decided to give the reader a moment or two of something a little different. Besides, a purpose does present itself in the matter of opening a discussion on mass, the mass of air. You see, wind power and biomass energy share something in common that is often taken for granted by most popular treatments of these technical concepts - the fact that air has substance to it, a substance called mass that is necessary for energy production and is much greater than is normally supposed.

The subject of producing power by making use of combined cycle technology in the combustion of RDF (the acronym means "Refuse-Derived-Fuel") is something that may be taken in somewhat of a light vein. At the time this is being written, it is probably an energy-production procedure that is not done in any power plants anywhere, the combustion of RDF being still subject to further standardization. Those who consider the generation of energy to be a highly thought-intensive, even academic, process, evoking the best of what educated minds can produce, would be interested to know that not all share this view. A point was once made by the highly authoritative owner of a major shipyard construction and refitting facility on the Great Lakes who had some contact with electrical power generation technology to the effect that the solid waste detritus of the nation, trucked into landfills in some abundance, is the best fuel for energy production and that the entire technology of power generation never rises above the level attained by that of incineration. These views were made known despite the clear options available for this shipyard's steel fabrication capability being used for wind turbine tower fabrication.

Not much can be said in argument. RDF has become a popular source of energy generation in many parts of the world and continues to develop as more power plants are enabled to burn this fuel. It is, after all, considered to be part of what is normally called "biomass". In fact, the industry of power generation with steam has always lumped RDF in with other forms of biomass fuel when conducting studies on power plant design, in particular, involving such refinements as gasification and combined cycle. It may only be a matter of time before these ideas see implementation and the light of day. The below diagram copied in from a freely available report is an example of how such power plant gasification processes are designed to work:

The above figure was excerpted from a report "FERCO...at a Glance" an updated version dated 1/27/97 published for free distribution by Future Energy Resources Corporation, 950 East Paces Ferry Road NE, Suite 810, Atlanta, GA 30326, Tel: (404) 842-9355, Fax: (404) 814-0549.

Much can be said in favor of technologies such as this. The world, it seems, has been given a taste of combined cycle in the case of that for natural gas combustion. It goes well with the concept of cogeneration, both technologies bringing the engineering studies subject of Thermodynamics and its prognostications about Entropy and the Carnot Cycle efficiency into a more up-to-date frame of reference with a certain acceptability. If thermally-produced power done in a combined cycle mode can be made to go from an efficiency of about 33% to one of about 67%, then technology has truly achieved something, a doubling of units of energy produced from the same resources and disposables. An unspoken reason among those more well-known, for example, why nuclear has slipped in popularity may very well be its characteristic limited ability to run at these higher rates of output favored now by the public mood. The view ahead for other nuclear forms of energy isn't good, either. Fusion energy, the on-again, off-again, doorway open to a virtually unlimited energy future has all the appearance of something of low, if not prohibitively low, efficiency. Everyone always knew something like this was true but now it is beginning to be said that these efficiency considerations do make a difference to those deciding among energy alternatives.

So combined cycle looks pretty good and anything it touches turns to gold, even ignoble RDF. The gasifier equipment necessary as a first step for solid fuels in being combusted for combined cycle operation, for example, itself has seen development. At one time, the process was done with an ordinary steel vessel that was blown with air and the product gas delivered straight to the turbines. But since air largely consists of nitrogen, the gas produced could only come to about one fifth of the energy content of natural gas per cubic foot. This requires modifications to the turbines and so other, more complex systems have been created to provide a gas of a higher fuel value and overall more acceptability. Some systems are blown with steam instead of air and the hot steam breaks down into hydrogen and oxygen in a process called "reformation" that creates a product gas that contains little nitrogen. There is need to process the product gas to remove other unwelcome substances in it and so more complexities arise. It is thought that all would be well and good if once a practical design could be found that could be relied on to make the process achieve a high degree of workability. Indeed, such is now claimed to be the case with proven technology by at least one company and it is only necessary to look up the material available in the literature and electronic media to find out more.

Other advantages present themselves. The process can be tuned to obtain some "methanation", or creation of methane from other constituents in the product gas, and this makes it more suitable for fuel cells, which have always been thought of primarily in terms of their use with natural gas as a fuel. Using the process to create fuel for running reciprocating engines (diesels, etc.) has also been tried with success. Many people remember the cover picture on a "Mother Earth News" magazine issue of years back showing a pickup truck that was converted to include a gasifier vessel on its truckbed to run down the highway powered by biogas produced with the help of stopping now and then to stoke the unit with pieces of wood.

But something always happens when extrapolating an energy technology to the big time. The world has developed a need for enormous amounts of energy being made available for our homes and businesses. Finding other sources for this energy takes much investment and time to bring them into the same pro ballpark that furnishes the bulk of this commodity that we have come to enjoy for such a long time. The road to this level of capability is littered with the remains and struggling bits and pieces of technologies that have not made this transition. It is instructive to understand these biomass technologies in this light.

Combined cycle is a means of bypassing the notorious difficulty of containing working fluids such as steam or hot gases while they are at both high temperatures and high pressures at the same time. This is solved by limiting the existence of these circumstances to the smallest possible volumes of space such as from gas turbine burner tips to the first few blade sets of the spinning turbine. Turbine blades handle the temperatures and high velocities quite reliably and the pressures are limited to surface areas of not great significance. The higher the temperatures involved at the start of the energy conversion process, the higher the efficiency that results; it's that simple. Once the hot gases have been tamed by the turbine and reduced in their energy, then they can be directed into a boiler to transfer heat to boiler tubes and convert water to steam to begin the second phase.

But something seems to be missing. The hot gases can contain mechanical energy in either of two forms, pressure or velocity, or so says the well-known equation formulated by Daniel Bernoulli back in the 1700s. In particular, the velocity of the gas is of interest. We know from wind generator theory that the energy potential of the wind machines is a function of the cube of the velocity of the wind, irrespective of its pressure (always at or near atmospheric) or temperature (other than for a slight density effect). Air, or any gas, has a characteristic called mass. To speed up the airflow entails the storing of energy in propelling this mass, something not at all to be considered lightly.

Now consider the following carefully. In creating a product gas that can be used as a fuel, the gasifier vessels and piping need contain nothing beyond low to moderate pressures. Standards dictate some additional capability to support such containment but the process involving this incomplete combustion normally does not require much. Suppose it were decided to add more pressure containment capability, step up the combustion to be complete, and discharge the fully combusted gas at high energy in the same manner as an air-fed, solid fuel rocket booster. Just direct the hot gas, as it burns, out of the chamber at atmospheric pressure but high velocity.

Turning for a moment to the wind energy side of things, wind developers crave winds that blow at speeds of on the order of 20 to 30 miles per hour. These numbers are then cubed to provide a relationship to the energy that can be produced by the blades of a wind machine. So what results in the Lanchester-Betz formulations are something like energy factors represented by numbers such as 8000 to 27000. But now suppose the windspeed were to be increased to 600 miles per hour, or close to the speed of sound. Cubing this results in energy factors, for the same quantity of air passing through the blades, of like 216 million. Guess what. That's the sort of energy available in the gas leaving the gasifier vessel, again, at ordinary atmospheric pressure.

We are getting a little ahead of the story, however, and must take a moment to reflect on this. Gas turbines represent a highly developed technology and, if anything is to accede contested ground, it is the fuel and gases being provided to them for operation. What we are discussing above, after all, is a rather crude method of exhausting the products of combustion of indeterminate solid fuel, not immediately up the stacks to the outside but first through the blades of a gas turbine. Something as unfortunate as the inability of gas turbine technology to accept this sort of treatment may be the controlling reason why this approach can not be put to use in simplifying the biomass combined cycle process for the immediate use of this technology in such a conjectured arrangement. It is fervently hoped this makes sense to the reader. Feet firmly planted on the side of combined cycle, the energy industry may wish to continue to extract further gains from this direction of energy development. Again, the purpose of drawing all this out is to talk about not the potential of biomass to supercede its own limitations but about something much more mundane, that is, the importance in the production of energy of the characteristic of air called mass.

For it is the mass of flowing substances that confuses and perplexes the ordinary person into disbelieving physical processes that perform contrary to what is expected. For example, the flow of a liquid or gas through a nozzle has always been characterized by misconception. If the walls of a flow regime such as a pipe narrow down in a constricting manner, it seems they should see an increase in pressure, not the decrease which actually results. The explanation is quite simple. The walls just nudge the flow medium into a narrower path and then Newton's Law takes over. Force in the form of pressure is required to propel the flowing substance into moving at the faster velocity required to pass through this smaller aperture and this force and pressure is not transmitted through to the walls of the pipe as we normally expect it would be. In fact, the pressure is used up in accelerating the flow and the walls see less pressure as a consequence, hence the unexpected result, and the practical application of this so-called venturi effect in providing for use what are called "jet pumps". Again, it's all due to the mass of fluids, even those of such low density as air, something rarely given the consideration that it merits.

Here are some interesting thoughts about the mass of air. The air in an empty box one cubic foot in size would weigh one and a quarter ounces and require extra postage to mail (beyond the tare weight) except that its bouyancy in the great ocean of air we call the atmosphere renders it weightless. However, when we go to a larger box, that is, a box in the shape of a cube that is about 30 feet on each side, this larger volume of air has a mass of one ton. Further yet, an even larger block of air that rests over one square mile of land with a height dimension of 1000 feet has a mass under most normal conditions of one million tons. It is no wonder that aircraft that now reach takeoff weights of 350 tons can support themselves on nothing but cushions of air and fly.

So those puffy clouds to be seen on lazy summer days actually represent masses of air of tonnage no less than that normally associated with great ships of the sea, hardly to be believed by most anyone looking at them. Not to be disconcerting to the reader who views with sympathy the development of renewable technologies, but all the drama that goes on with respect to how many hundreds or even thousands of tons of carbon dioxide and other gases released by energy combustion processes by one power plant or in one local area seems to be a little overwrought when this fact is fully understood, i.e. the weight of ordinary air all by itself is so many orders of magnitude greater as to render such quantities by comparison of little consequence. Many reasons apply for going forward with all deliberate speed in creating a renewables-powered economy but, in the light of understanding the simple fact of the large value of the mass of air itself, the reduction of the venting of greenhouse gases into the atmosphere, if the numbers as to the extent of this practice are no greater than those normally published, does not hold a high place among them.

Other comparisons can be made. The weight of coal carried by each coal car of a railroad train is about 100 tons. So a trainload of 100 coal cars contains about 10,000 tons of coal. Going further, it takes no fewer than 100 of such trains to carry the amount of coal whose weight is about the same as the one million ton mass of just one square mile of air with only an elevation of 1000 feet, which is not much of the atmosphere by any standards of earth studies. It's one of those hard-to-believe-but-true facts and can be considered a good thing from the standpoint of the possible use of such large masses of this substance called air as it moves in the process known as wind for energy generation but a not quite so good thing if one wants to believe that coal is a villain about to turn the earth into another planet Venus with its carbon dioxide atmosphere and surface temperatures of 870° F.

Whatever. Air has mass. So we have "biomass" energy and "wind mass", so to speak, energy.