As syngas comes out of the gasifier at 1400K, it needs to be cooled down to around 600K in order to be put into catalyst in the next phase. The most efficient way to cool the gas down is to convert this excess thermal energy into electrical power to run the plant and sell back to the grid. By using a steam generator in conjunction with an electric turbine, we can easily convert this thermal energy into mechanical motion and then to electricity. Using water/steam as the working fluid, a boiler tube can be wrapped around the syngas pipe in a counter-flow1 arrangement and the surface area of the walls maximized, while minimizing the resistance to fluid flow, to create the most efficient heat exchanger. First capturing part of the thermal energy by conduction2 the metal tube then transfers the energy to the water through convection3. As the thermal energy continues to heat up the working fluid, enthalpy (h), the relative heat energy content of the fluid is increased. As more and more heat is transferred, the water in a saturated liquid state, then vaporizes remaining at a constant temperature until all the water is turned into steam (saturated vapor state), where the temperature can further increase creating a superheated vapor where it has the most enthalpy.
Because the density of a fluid decreases as it becomes a superheated vapor, the buoyant force acts on the fluid to move it to a cooler region due to a pressure difference. As the steam is accelerated through the pipe, it can be collided with a steam turbine and used to do work. This is where the mechanical energy of the working fluid is transferred into the rotation of a turbine. The steam turbine is a stack of fan blades having multiple layers to take advantage of all the different levels of fluid expansion. It spins just as a windmill would with blades angled so that the kinetic energy of the steam is transferred to rotational energy of the rotor.
Once the steam has passed through the turbine, it is still fairly hot and must be further cooled to reduce its pressure otherwise it would stop flowing from the hot to the cold side. This is usually done through the use of another heat exchanger just as had been done with the syngas but running the pipes through outside water or air. The alternative would be use the excess thermal energy for space heating. Because the plant will be built in Alaska, space heating will be an important aspect of the plant and the excess heat would be extremely valuable commodity and although a more expensive system would be required, it would be far more efficient.
The electrical generator converts the mechanical rotational energy of the spinning turbine to an electric current or voltage. Discovered in 1831 by Michael Faraday, electromagnetic induction is basis for how the generator works. Practically by accident, Faraday discovered that a changing magnetic field induced the flow of electron in a conductor and visa versa (current induced a magnetic field). This groundbreaking discovery showed that the fields of electricity and magnetism were in fact intertwined. This led to the invention of some of the most important advances in electric technology, namely advent of the motors and generators. Being the inverse of one another, motors and generators work on a relatively simple idea and any differences in models are simply individual company variations. The basic idea is to have a mechanical rotor spin a a coil of wire inside a magnetic field(see figure 1). As the wire spins past the positive pole of the magnet the conductors sees a changing magnetic field and therefore induces a current in one direction. It then passes the negative pole which induces the same amount of current but in the opposite direction. This causes a sinusoidal alternating current (AC) that can be transferred across any network of used to perform work. Thus mechanical rotational energy is converted to electrical energy. In our application, the rotor from the steam turbine is connected to such a generator and energy originally from the heat in the syngas is transferred into electrical energy.
Because the constant demand of the plant might not be met at all times by the output of the generator. The plant should be connected directly to the grid so that there is no dependence on the generator should any component fail. This will also enable excess electricity to be sold back to the grid. This too, would put the plant into the category of power plant as well as biofuel plant furthering the possible tax benefits of the business.
Electric Steam Generators
This area of research will mainly deal with different types of electric steam generators and the different companies that produce them.
Various types of Steam Generators (1)
There are 3 main types of steam generators. These are the vertical U-tube, horizontal, and Once through types. Most American, Japanese, and German power suppliers use the vertical U-tube configuration of steam generators.
Other differences between generators are of how steam is produced. Different things are heated in order to produce heat, and that is how generators are classified. (ex: Coal generators burn coal in order to heat water to produce steam. Nuclear plants use chemical reactions in order to produce heat.) The characteristic that differentiates between each generator is its ability to produce a certain amount of power. This is determined by the designer of the plant. Companies will then use specifications given by the designer to build the generator to fit in a plant.
The vertical steam generators generally have a feedwater ring supply header on the outer edge of the steam generator. The water is directed downward and flows along a wrapper sheet then is directed upwards to flow along the steam generator tubes where the water picks up heat, increasing in temperature until boiling occurs and the water is converted to steam. In the upper part of the steam generator is a moisture separator region which forces the steam-water mixture through channels which allow steam to pass, but not water. A vane arrangement in these steam generators will also force a swirling action that enhances the steam-water separation.
The water supplied to the steam generators must be very pure, free of particles, and chemicals. In the boiling environment of the steam generator these chemicals can concentrate resulting in undesired corrosion.
The horizontal steam generators have horizontal tubes as shown to the right. Reactor coolant flows through the tubes. Feedwater is supplied outside the tubes and is converted to steam that flows up into the header.
During refueling outages, the steam generator tubes are inspected for degradation and thinning using a non-destructive examination method known as eddy current testing. Probes as the one shown below are routed through the tubes and signals from the sensor are evaluated using a computer. The probes are used to find defects in the tubing material. If defects are found the tube may be plugged or have a sleeve installed to strengthen the tube.
(The probe is spinning as it goes through the tubbing)
(This may or may not be in issue as Dr. Catollica told us. They’re safety standards aren’t the same as the US, so the plant may or may not use this method.)
The function of the steam separators is to remove the water from the steam. This is done to ensure that the steam impinging on the turbine blades causes minimal erosion. The blades are large and steam laden with water can cause severe erosion and wear of the blading. So also can the presence of chemicals, e.g. silica, in the water that is converted to steam.
Usually the steam separator may be a separate component when horizontal steam generators are used. Examples of this are those used in the CANDU and VVER designs.
It should be noted that steam separation is accomplished internally to the boiling water reactor, as illustrated below.
This diagram illustrates the advanced General Electric Boiling Water Reactor design. Key areas to note relative to the steam separator discussion are:
(12) Fuel where the water is heated and converted to a steam water mixture
(4) Steam separators where initial steam separation occurs
(2) Steam dryer where additional steam separation occurs