Posts Tagged ‘Gas turbine’

A gas turbine as a higher level of art

August 4, 2015

It all started with the wheel of course. Mechanical engineering became art – or was it the other way around?

13th century stone chariot wheel - Konark

13th century stone chariot wheel – Konark

The “art” of mechanical engineering is not just about beauty of form, it is about a beauty of both form and substance. It is a higher level of art because it must not only be aesthetically pleasing to the eye – look good – it must also function as it was meant to.  Da Vinci’s helicopter sketches are interesting and even look good but they don’t represent anything which ever worked or can work. Lovely things which do nothing or do things badly may be some kind of art, but they are not examples of the “art” of engineering. Even sketches of humble pulleys or levers or gears which actually work are – for me – a higher level of art in that they have both the form and the substance.

gt blades

GT Blades (image Siemens)

The requirement of substance – that some artefact functions as it should – constrains the degrees of freedom available for aesthetic expression or satisfaction. It is easier to design pleasing shapes which don’t also have to work. When things that work have a form which is also aesthetically pleasing, or inspiring or challenging we have the art in engineering. And in the world of mechanical engineering, it is turbines in general (wind , water, steam and gas), and modern heavy duty gas turbines in particular, which represent, for me, an awe-inspiring and almost frightening beauty in the sublime combination of form and function. From windmills to jet engines there is art in the engineering. State-of-the-art gas turbines are art epitomised.

GT Compressor blading

GT Compressor blading (image Stork)

Artefacts of engineering don’t have to be beautiful. Not all engineering is art. Gas turbines don’t have to be pleasing to the eye – but they are. Mathematics and physics and chemistry are combined to satisfy the substance and the resulting form – perhaps not entirely intentionally – becomes beautiful. The beauty lies not only in the shape and profile of the compressor and turbine blades (which are in themselves almost mesmerising in their irregular regularity), but in the sheer cleverness of the whole engine. The concept of how a gas turbine functions is itself beautifully ingenious. It is far more “intelligent design” than the ineffective (99% failure rate), hit and miss of evolution (just a trial of random mutational errors), which requires billions of years and innumerable failures. A compressor is an unnatural animal. The “natural” order of fluid flow from higher to lower pressure is subverted. To conceive of the use of an unnatural machine, the compressor, to create a high pressure stream of air, to burn a suitable fuel and raise the temperature of the gas such that it can be expanded in a turbine which not only provides the power to drive the compressor but produces surplus power, is genius. And then to do all that and drive an electrical generator as well , while the blades are rotating at 3000 or 3600 revolutions per minute, when they are at temperatures where even the best steels have the strength of soft butter, is more than awesome. Fine, powerful stuff.

GE 9HA

The GE 9HA “Harriet” gas turbine

An infinite number of monkeys pounding away at keyboards for an infinite amount of time would surely reproduce the works of Shakespeare.

But they couldn’t produce a gas turbine.

Advertisements

Composition of exhaust gases from humans and from fossil fuels

April 13, 2013

It occurred to me when carrying out some combustion calculations that what humans breathe out is pretty close to the flue gas from a gas-fired, gas turbine combined cycle plant.

In a gas turbine combustion chamber, fuel is burned typically at an excess air level of about 200% (the amount of oxygen available in the combustion air compared to that which is needed for complete oxidation of the fuel). This means that about one third of the oxygen available is used and converted to carbon dioxide and water while about 2/3ds just passes through (i.e of the 21% oxygen in air, about 6-7% is “consumed” and about 14 -15% passes through unused). In coal-fired plants the excess air levels are usually only about 25% which leads to about 15 -16% of the incoming 21% oxygen being consumed with about 5% passing through. The amount of oxygen actually consumed depends on the fuel composition and the oxygen demands of the elements which are oxidised during the combustion process. Carbon, hydrogen and sulphur (giving CO2, H2O and SO2) are the main oxygen consumers. All the other constituents of air pass through – heated up of course – but otherwise unchanged. Minute quantities of the fuel- nitrogen and the nitrogen in the incoming air can – depending upon the combustion temperature – be “fixed” to create the nitrogen oxides – nitrous oxide (N2O) and nitrogen dioxide (N2O). The higher the combustion temperature the greater the “fixing”. Too low a combustion temperature – for example with very wet fuels and bio-mass – can give “incomplete combustion” with some carbon monoxide (CO) and even some dioxins and hydrocarbons with a particularly poor combustion process. Internal combustion petrol engines essentially run at stoichiometric conditions (zero excess air) and there is no oxygen in the exhaust. However combustion is never quite complete and around 1% carbon monoxide is usually present (which is why suicide by exhaust fumes becomes possible). Diesel engines on the other hand have 10% oxygen in the exhaust when idling and this reduces to 1 or 2% when fully loaded.

All fuels essentially contain carbon and hydrogen as the main energy releasing elements when oxidised. Most industrial combustion processes happen fast and speed of combustion – which is desirable for complete combustion – has to be tempered by the need to keep temperatures at levels which can be handled by the materials used. The human use of the same elements of carbon and hydrogen for the release of energy however is by a relatively slow oxidation processes. Not all the water produced leaves the human body with our expelled breath since some part of it leaves in liquid form with urine. But from the composition of the waste gas we breathe out it seems that the carbon/hydrogen ratios in our food intake cannot be so very different to the natural gas burned in gas turbines (and not very surprising considering that plant-life is the ultimate source of both).

exhaust gas compositions

 

Since human exhaust gases emit the same concentration of carbon dioxide as gas turbine, combined cycle power plant perhaps we should penalise every human as well?

 


%d bloggers like this: