1. Background

Industrial [GLOSS]combustion[/GLOSS] is introduced in Combustion File (CF) 32.  Industrial [GLOSS]fuel[/GLOSS]s are introduced in CF62.

The potential energy released by combustion is often seen to reside in the fuel, mainly as carbon or hydrogen. In fact, as far as typical industrial combustion is concerned, the energy results from the oxidation of these components. Thus the oxidant is as important as the fuel. In this Handbook the generic term for such an oxidizing medium is “comburent”.

[GLOSS]Comburent[/GLOSS]s are the gaseous mixtures, which contain the oxygen used in typical industrial combustion processes. As stated, the oxygen reacts with the carbon and hydrogen, which are the most important “fuel” components found in industrials fuels, thereby releasing heat for utilisation in the industrial processes.

The present Combustion File introduces industrial comburents.

2. Overview of Comburents

Comburents range from:

·          Regular atmospheric [GLOSS]air[/GLOSS] containing approximately 21% v/v O₂; up to

·          virtually pure oxygen as may be used regularly within steel making processes; down to

·          turbine or diesel exhaust gases, which still contain significant concentrations of oxygen; and finally to

·          oxygen enriched combustion products to produce a synthetic “air” with low N₂ content, which may be of interest for concentrating CO₂ in the flue gas.

The oxygen concentration range is listed in Table 1 below.

Table 1. Industrial comburents listed in order of oxygen concentration

3. Advantages of the different Industrial Comburents

Regular Atmospheric Air – A^o

Air – typically referred to as “Combustion Air” – is freely available in virtually unlimited amounts, taken directly from the Earth’s atmosphere.

The residue or [GLOSS]ballast[/GLOSS] is almost entirely molecular nitrogen, which effectively controls the rate of mixing and reaction of the oxygen with the fuels and absorbs a significant proportion of the heat released. This prevents the occurrence of extreme flame temperatures but can give rise to excessive heat losses if effective heat recovery is not practiced.

Preheated Combustion Air – A^p

Preheated Air Combustion can prevent excessive heat loss and thus enhance energy utilisation efficiency and reduce CO2 emissions in the industrial heating process.

This can be achieved either by the use of [GLOSS]recuperator[/GLOSS]s or [GLOSS]regenerator[/GLOSS]s, through which waste heat from the process exhaust gases is transferred to the combustion air.  The combustion products leave the heat recovery system at a temperature typically between 200 and 400°C.

The use of such techniques also enhances flame temperature and radiative heat transfer. Theoretical flame temperatures in excess of 2000°C can be obtained with modern high efficiency regenerators – see CF171, CF172 and related Combustion Files.

Enhanced flame temperature will also broaden flammability limits and thus improve flame stability.  But, the higher flame temperatures achieved can also increase NOx (oxides of nitrogen) emissions – see CF66 and CF40.

However, in most modern combustion system, the driver for increasing the combustion air preheat temperature is primarily the recovery of heat from the waste gas and the increase of the global thermal (and thus fuel) efficiency.  The risk of excessive NOx emissions resulting from a very high peak flame temperature is nowadays avoided by the use of a suitable burner design (see CF171).

Oxygen Enriched Air – A⁺

In many industrial locations, oxygen is available on site from production facilities or from bulk storage facilities serviced from external production sites.

Such oxygen can be used to enrich regular air, with potentially beneficial effects. For example, oxygen enriched combustion air decreases the nitrogen ballast exhausted with the combustion products, and thus improves the global thermal efficiency.

As with preheated air combustion, oxygen enriched combustion decreases fuel consumption, can improve flame stability, but can also give rise to higher NOx emissions if used with traditional burners. Modern implementations of oxygen enrichment often show a reduction of specific NOx emissions.

Variation in oxygen enrichment can be used to achieve variable flame design – e.g. flame lengthening and shortening – as used in the cement industry and historically used in process heaters in the iron and steel industry.

Exhaust gas volumes are reduced with the positive effect that flue gas system size can be reduced. However the volume of flue gas recirculating in the furnace is also reduced which can affect temperature uniformity.

“Pure” Oxygen – OXY^o and OXY^–

In former times the “regular” oxygen available from an onsite production facility, for example on integrated iron and steel works, was produced through a cryogenic separation process – OXY^o – a very pure oxygen.

Present day cryogenic oxygen plant production capacities range typically from 80 to 5000 tons per day (4700 to 300,000 m_o³/h). Such oxygen is also frequently used as a source of oxygen for liquid (bulk) oxygen supply for truck delivery.

In recent years however, the industrial gas producers have put a considerable amount of work into the development of smaller and cheaper separation plants with consequently cheaper oxygen for tonnage requirements between 4 and 150 tons per day.  Adsorption systems, which produce a relatively high concentration of oxygen – OXY^– is representative of such plant.

The oxygen concentrations are as indicated in Table 1.

Further descriptive details on the processes for producing oxygen, their relative merits and disadvantages, and the relative production costs are given in an associated Combustion File (CF234).

Exhaust Gases – EG^o – EG^–

Exhaust gases from diesel engines or gas turbines can vary significant in quality; however they contain significant quantities of residual oxygen and are available at temperatures up to 600°C or more. Together these two characteristics can result in valuable exhaust gas based comburents.

Combustion systems employing exhaust gas comburents require special design considerations similar to those employing lean fuel gases such as [GLOSS]Blast Furnace Gas[/GLOSS]. Designers need to take due account of the low oxygen partial pressure.

With increases in efficiency of gas turbines, oxygen levels are decreasing which exacerbates the design problems.  In any event the utilisation of exhaust gases as comburents is regularly employed in industrial and power generation systems.

Sources

Jacques Dugué, Private Communication, Data on oxygen products, 2001

John Smart, Private Communication, Data on turbine exhaust gases, 2001