OCR Text |
Show 3 At this point I will depart slightly from my written abstract and begin to introduce some of the ideas relating to Nitrogen Oxide emissions. In general we talk about the Oxides of Nitrogen emanating from the combustion process as NOx and as such we discuss limiting the total amount NOx being emitted. To keep this in perspective Transportation accounts for 51.3% of the total emissions whilst 26.4% comes from burning Natural gas or Oil at stationary sites, including all power generation. There are three oxides of Nitrogen, Nitric Oxide NO, Nitrogen Dioxide N02 and Nitrous Oxide N20. The latter is extremely stable in the environment and is generally not considered to be a pollutant. Nitric Oxide on the other hand is considered highly toxic and is highly reactive. Nitric Oxide and Nitrogen Dioxide also have some unfortunate characteristics, when mixed together with sundry hydrocarbons in the presence of sunlight, they become a reactive component of the dangerous soup of compounds in industrial smog. The chemistry for these reactions is beyond the scope of this lecture however ref 4 provides a good description. The other point to note is that NOx usually has a very short life in the atmosphere often with a half life of five days or so depending on local conditions. The compounds are washed out of the atmosphere as nitrates and as nitric acid however their effects can be much longer lasting. When we consider technologies for the reduction of NOx we must therefore be cognisant of the possibility that we could drive the reaction equilibrium in the direction of the more noxious compounds and would therefore see no real environmental benefit. The reaction equilibrium are affected by the composition of the combusting gasses the temperature of the flame and the temperature of the furnace. Here we arrive at a potential conflict which will require one trade off as to the importance of the various pollutants in the combustion gasses. It is possible to generate very low NOx levels by allowing the flame to generate reducing gasses such as Carbon Monoxide. With sufficient concentrations it is possible to almost suppress the formation of NOx. This procedure for controlling NOx is obviously unacceptable because of the emission of an equally nasty pollutant with a longer half life. In some instances however this type of control would be acceptable, where the waste gas was treated with an after burner, for instance. There is an economic consideration concerned with the wasted fuel which is carried through the process and which does not release its heat. Carbon Dioxide may end up as the public enemy number one which would require that waste heat be returned to the process in some form. One convenient method has been to add some of the waste heat to the incoming air supply for the burners. The efficiency improvements mean that less fuel is required for the process and therefore less Carbon emitted into the environment. A win win situation on the face of things. Well almost, in a rather circuitous route we get back to the real NOx problem. The reaction kinetics predict an ever increasing volume of NOx will emanate from an ever higher temperature process and the net effect of adding heat to the combustion air supply of the burner is to increase the flame temperature. |