November 21, 2005

Burn Flame Burn 

Ok, it's time to go back to some hardcore technolgy here; enough of blogosphere wars.

The topic now is combustion technology: I attended a lecture by a serious combustion expert, Adam Williams of Leeds University.

Combustion (flame, but not always) is a process very often involved in extracting energy from fuels, be they fossil or otherwise. In order to maximize the amount of usable energy that can be extracted from fuels and at the same time reduce polluting emissions, it is necessary to understand the combustion phenomenon, so that it can be influenced in the desired way.

No big deal, you may think. Wrong.

Volatile substances burn in gas phase, through a series of radicalic chain reactions in which carbon combines with oxygen to form carbon dioxide and hydrogen forms water. If sulphur is present, it will form sulfur dioxide; metals will generally be converted in oxides and phosphorous will form phopshates. A small amount of air or organic nitrogen will also react with oxygen to form nitrogen oxides, and some of the carbon will fail to burn and end up as soot, a mixture of various polynuclear aromatic hydrocarbons and other substances (among them, the famous fullerenes).

Soot (especially the nanometric particles), sulphur and nitrogen oxides are polluting agents and it is desiderable to minimize their formation; soot also represent wasted carbon and thus a lower efficiency of combustion. There are mathematical model to simulate combustion, and these must take into account reaction nergy and kinetics, flow properties and mass and energy transfer.

In power stations, coal is pulverized and mixed with air before being injected in the combustion chamber, and when coal particle are exposed to the high temperatures in there something quite convenient occurs: the particles melt partially, assuming a sufficiently spherical shape that makes calculations relatively easy. The burning coal particles are transported upward by the flow of hot gases, and when they have burnt conpletely, the denser ashes fall at the bottom of the chamber (there are also other systems to separate fine ashes from the flue gases). Coal combustion is a mature technology, and the best coal-powered power stations have something like 50% efficiency. Biomass is seen by many as a viable fuel to reduce CO2 emissions, and indeed it does so; biomass is already burnt alone or with coal in a number of power stations around the world. The amount of biomass potentially available is huge, but it is also very dispersed.

However, biomass particles have an irregular, elongated shape that makes modelling of their combustion much more difficult, and there is a concrete risk that in conventional combustors a significant fraction of the biomass fuel (say, 1%) may remain unburnt. Considering that a power station can use several hundred thousand tons of fuel per year, even 1% becomes a sizable amount - and all this not to mention other problems like the high humidity content of biomass.

The carbon credits trade created by the Kyoto protocol also has a perverse effect: british power stations burn biomass (presumably wood) imported from Malaysia and olive oil production waste (crushed and pressed olives, a material used as fuel since time immemorial) from Greece, where the strict environmental regulations make its disposal exceedingly expensive. While this system can be economically viable and Kyoto-wise, it's hard to see how all this long-distance shipping of biomass can be truly beneficial to the environment.

Professor Williams suggested the use of very intensive cultivation techniques and/or genetically modified plants in order to boost the production of biomass without having to subtract too much land from the food crops usage, and I think this proposal makes sense: if we want to take the biomass way, we have to do it properly.

Another fuel-saving technological advance is the flameless combustion of natural gas in the reheating furnaces used for still processing. In conventional burners, methane burns in air within a narrow flamefront, where temperature is very high, something like 1500 C: this process produces an irregular temperature profile in the furnace, noise and a higher amount of nitrogen oxides. Instead, when methane is put in contact with air above 850 C (and the composition of the mixture is kept within precise limits by recirculating exhaust gases), a flameless (almost; sometimes green light is emitted) chemical reaction occurs over the entire volume of the chamber producing a much flatter temperature profile, no noise, very little NOx and a sensible fuel saving. The problem of this technolgy is that flameless combustion conditions are unstable; however, methane burners built on this principle are already available.

Combustion processes are here to stay, and it's only in our own interest to make them better.

Update: Quick and unrelated. Dan Darling at WoC has a post dealing with the labels to use in this conflict; I myself posted the second chapter of my short story.


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