Aug-2012

Jade Ultra-low NOx Tail Gas Thermal Oxidiser Burner

The Jade burner from UOP Callidus is the first purpose-designed and built Ultra-Low NOx Tail Gas Thermal Oxidiser Burner that delivers ultra-low NOx emissions with no special physical or operational provisions.

Kurt Kraus, Stefano Bietto and Matt Martin
Honeywell Engineering Fellow

Article Summary

Incorporating the Jade burner into a thermal oxidiser delivers the lowest nitrous oxides (NOx) — in certain cases, essentially no NOx — and carbon monoxide (CO) while requiring no special operating practices.

UOP Callidus uses patent pending Balanced Staging technology to deliver the proper proportions of both waste gas and fuel (assist) gas to the proper locations in the flame zones of the combustion chamber. Application of Balanced Staging simultaneously optimises the counter-acting constraints of minimum NOx emissions, carbon monoxide emissions, operating temperature, assist gas consumption and residence time.

The term “thermal oxidiser” spans a broad range of equipment sometimes variously termed combustors, incinerators or thermal reactors. The purpose of the thermal oxidiser is to combust (oxidise) or destroy by heat various organic or inorganic compounds entering the thermal oxidiser from other processing units. Waste streams may be composed of combinations of gaseous, liquid and solid materials. These waste streams may contain high or low calorific content. The Jade burner’s Balanced Staging technology is best suited for waste gas streams that have relatively low calorific content such as tail gas from sulphur recovery units or CO gas from fluidised catalytic cracking (FCC) units.

Tail gases and similar low calorific content waste gas streams are usually comprised of large proportions of essentially inert constituents, principally molecular nitrogen and often varying amounts of carbon dioxide and water vapour. The stream of a low calorific value gas is regularly used to lower a burner’s flame temperature by mixing the waste gas stream with a portion of the fuel gas (staged gas) prior to combustion. Lowering the burner’s flame temperature and “inerting” the fuel stream retards the chemical mechanisms prevalent in the formation of thermal, prompt and fuel NOx thereby reducing the overall NOx emissions produced by the thermal oxidiser.

Balanced staging
The first and most apparent characteristic of the Jade burner is that it splits the waste gas stream into two portions delivering the gas both in an annular gap at the periphery of the burner tile and through the centre of the burner. In many thermal oxidiser applications, the waste gas is injected into the combustion chamber downstream of the burner. When the waste gas is injected downstream of the burner, the stream may have little effect on burner NOx production as the NOx has already been produced in the burner flame before mixing with the waste gas. Many other thermal oxidiser burners inject the waste gas in an annular gap surrounding the burner with or without staged fuel mixing with the waste stream before mixing with the combustion air and any remaining assist gas. Burners of this configuration can produce significantly lower NOx emissions when compared to burners without waste gas injection disposed such that it mixes with the assist gas.

By injecting a portion of the waste gas into the throat of the burner and premixing it with the combustion air, the Jade burner effectively reduces the partial pressure of oxygen in the resulting flame. Lower oxygen partial pressure serves to reduce the oxidation of nitrogen (NOx formation) in all of the principal NOx forming mechanisms (thermal, prompt and fuel). This proportioning of waste gas, both around an annular gap and through the burner centre, constitutes tail gas staging that is largely unique to the Jade. Also within the Jade burner, the assist fuel is staged to create primary and secondary flame zones. This considered combination of fuel staging and waste gas staging constitutes a portion of the Jade’s patent pending Balanced Staging approach.

But there is more to balanced staging
Many equipment suppliers have found out the hard way (in the field) that too much staging of fuel gas, and too much mixing of low calorific value waste gas and staged fuel gas in thermal oxidisers, can result in incomplete combustion of waste gas constituents and the assist gas. The result is elevated CO, unburned hydrocarbons (UBHC) and volatile organic hydrocarbon (VOC) emissions. The Jade remediates this issue using the patent pending feature of design-adjustable positioning of the staged fuel gas tips relative to the central air stream and the outer waste gas stream. The staged tips may be placed on a continuum from residing outside the burner, wholly within the oncoming externally staged waste gas stream, to inside the burner, residing wholly within the internal combustion air stream. This variable positioning is factory-set by UOP Callidus and is not required to be field-adjusted. Instead, the design is tuned to a customer’s specific requirements, minimizing the emissions of NOx without causing increased emissions of CO, UBHC and VOC.

In some cases, a high level of mixing of staged assist gas and waste gas is desired prior to the main flame mixing zone which contains the balance of combustion air, waste gas, and assist gas. In other cases, in order to control CO, UBHC and VOC emissions, a lower level of mixing of waste gas and staged fuel gas is desired – potentially to the point of requiring air and assist gas to be mixed prior to mixing the staged assist gas with the staged waste gas. In still other cases, it is preferable to mix a portion of the staged waste gas, staged assist gas and combustion air simultaneously prior to mixing with remaining waste gas, assist gas and combustion air in the flame zone. The Jade burner allows for great flexibility in the design process.

And still there is more
The portion of the waste gas that is directed into the throat of the burner into the combustion air stream is adjusted to either mix quickly and thoroughly with the combustion air or to mix more slowly. When rapid mixing is required, a patent pending lobed mixer waste gas injection nozzle is used. The lobed mixer injection nozzle has a round inlet fitted to the upstream pipe, and an undulating or lobed outlet. The cross-sectional flow area of the inlet and outlet are roughly equivalent; but, due to the lobes, the perimeter of the outlet can be much greater than the inlet. The result is that the waste gas comes into fluid contact with a much greater portion of the combustion air in a shorter distance, thereby inducing rapid mixing of waste gas and air.

Mixing of two fluid streams is often accomplished by inducing some pressure drop at or immediately downstream of the junction of the two streams. However, the allowable back pressure on waste gas streams is often very low. With the lobed mixer, effective mixing is achieved with little or no pressure loss on the system.