Editing Combustion (section)

==Combustion management==
Efficient [[Industrial furnace|process heating]] requires recovery of the largest possible part of a fuel's [[heat of combustion]] into the material being processed.<ref>{{cite journal | title = Calculating the heat of combustion for natural gas | journal = Industrial Heating | page = 28 | date = September 2012 | url = http://www.industrialheating.com/articles/90561-calculating-the-heat-of-combustion-for-natural-gas | access-date = 5 July 2013 | archive-date = 10 July 2013 | archive-url = https://web.archive.org/web/20130710085624/http://www.industrialheating.com/articles/90561-calculating-the-heat-of-combustion-for-natural-gas | url-status = dead }}</ref><ref name="HeatCalc">[http://www.industrialheating.com/HeatCalc] HeatCalc</ref> There are many avenues of loss in the operation of a heating process. Typically, the dominant loss is [[sensible heat]] leaving with the [[Exhaust gas|offgas]] (i.e., the [[flue gas]]). The temperature and quantity of offgas indicates its heat content ([[enthalpy]]), so keeping its quantity low minimizes heat loss.

In a perfect furnace, the combustion air flow would be matched to the fuel flow to give each fuel molecule the exact amount of oxygen needed to cause complete combustion. However, in the real world, combustion does not proceed in a perfect manner. Unburned fuel (usually {{chem|CO}} and {{chem|H|2}}) discharged from the system represents a heating value loss (as well as a safety hazard). Since combustibles are undesirable in the offgas, while the presence of unreacted oxygen there presents minimal safety and environmental concerns, the first principle of combustion management is to provide more oxygen than is theoretically needed to ensure that all the fuel burns. For methane ({{chem|CH|4}}) combustion, for example, slightly more than two molecules of oxygen are required.

The second principle of combustion management, however, is to not use too much oxygen. The correct amount of oxygen requires three types of measurement: first, active control of air and fuel flow; second, offgas oxygen measurement; and third, measurement of offgas combustibles. For each heating process, there exists an optimum condition of minimal offgas heat loss with acceptable levels of combustibles concentration. Minimizing excess oxygen pays an additional benefit: for a given offgas temperature, the [[NOx]] level is lowest when excess oxygen is kept lowest.<ref name="NOx formation" />

Adherence to these two principles is furthered by making material and heat balances on the combustion process.<ref>{{cite journal | title = Making a material balance | journal = Industrial Heating | page = 20 | date = November 2012 | url =  http://www.industrialheating.com/articles/90764-making-a-material-balance | access-date = 5 July 2013}}</ref><ref name="MatBalCalc">[http://www.industrialheating.com/MatBalCalc] MatBalCalc</ref><ref>{{cite journal | title = Making a heat balance | journal = Industrial Heating | page = 22 | date = December 2012 | url =  http://www.industrialheating.com/articles/90812-making-a-heat-balance | access-date = 5 July 2013}}</ref><ref name="HeatBalCalc">[http://www.industrialheating.com/HeatBalCalc] HeatBalCalc</ref> The [[material balance]] directly relates the [[air/fuel ratio]] to the percentage of {{chem|O|2}} in the combustion gas. The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion.<ref>{{cite journal | title = Available combustion heat | journal = Industrial Heating | page = 22 | date = April 2013 | url =  http://www.industrialheating.com/articles/91014-available-combustion-heat | access-date = 5 July 2013}}</ref><ref name="AvailHeatCalc">[http://www.industrialheating.com/availheatcalc] AvailHeatCalc</ref> Additional material and heat balances can be made to quantify the thermal advantage from preheating the combustion air,<ref>{{cite journal | title = Making a system balance (Part 2) | journal = Industrial Heating | page = 24 | date = March 2012 | url =  http://www.industrialheating.com/articles/90954-making-a-system-balance-part-2 | access-date = 5 July 2013}}</ref><ref name="SysBalCalc2">[http://www.industrialheating.com/SysBalCalc2] SysBalCalc2</ref> or enriching it in oxygen.<ref>{{cite journal | title = Making a system balance (Part 1) | journal = Industrial Heating | page = 22 | date = February 2012 | url =  http://www.industrialheating.com/articles/90897-making-a-system-balance-part-1 | access-date = 5 July 2013}}</ref><ref name="SysBalCalc">[http://www.industrialheating.com/sysbalcalc] SysBalCalc</ref>