World Cement - North America 2018

North America 2018

28 \

World Cement

excess oxygen decreases in the flue gas, NO

formation is

reduced; however, this also increases CO emissions (Figure 2;

operating point 2).

The addition of combustion air in stages to reduce

primary NO

is a well-established technology (power plants,

low-NO

calciners, etc). However, the confinement of a

rotary kiln makes it difficult to add hot burnout air in the

later stages of combustion after the main flame. An oxygen

injection at the kiln feed end offers a low cost opportunity

for

reduction through combustion staging. As shown

in Figure 2, reducing the amount of excess O

to the kiln

leads to reduced

emissions, but may also increase the

formation of CO.

The arrangement of a single large burner firing into

the narrow kiln pipe of a cement rotary kiln and the

counter-current flow of material can lead to significant

stratifications at the feed end of the kiln. Flue gas travelling

at the bottom of the kiln may have much higher CO

concentrations than the flue gas in the upper part of the

kiln pipe. This is exacerbated by the use of solid fuels,

specifically coarse RDFs or alternate fuels (AFs). Although

the residence time in the flame is quite large, the trajectory

of these large fuel pieces brings them to the bottom half of

the kiln and they may be trapped in the clinker bed where

they burn much more slowly. Low oxygen availability near

the clinker bed, combined with limited cross-mixing in the

long and narrow kiln, results in high CO concentrations in

this region. If this CO stratification is not mixed with oxygen

at sufficiently high temperatures for reaction (>1000 °C or

1800 °F), then high CO emissions from the kiln can be

the consequence. CO emissions from cement kilns have

been traditionally high in the range of thousands of ppm,

especially with high use of AFs.

Recently, the permitting process has focused more on

CO, in addition to NO

, and producers are increasingly

boxed in by NO

, CO, and in case of SNCR and SCR

end-of-pipe solutions, by ammonia slip in the flue

gas. Plants may be required to reduce CO or will see

higher CO when they try to further reduce NO

with

combustion control methods. The combined emission

requirements present a difficult choice: as retrofit NO

control technologies cannot address high CO or aggressive

combustion, air staging leads to higher CO emissions.

Increasing excess oxygen has a detrimental effect on NO

emissions and efficiency.

Oxygen injection at the correct locations can reduce

CO from the kiln process and meet emission requirements.

Praxair offers oxygen lancing at the kiln feed end. The

key to this technology is to break up CO stratification

and oxidise the CO in the high temperature region of the

kiln. Two lancing methods are used: the first comprising

a lance delivering highly pure, non-preheated oxygen

to the flue gas; the second, utilising the OPTILANCE hot

oxygen technology. Cold oxygen lances rely on optimal

positioning on the ductwork and a specially designed

nozzle to facilitate mixing between the cold O

jet and the

hot flue gases. Once mixed, CO in the flue gas contacting

the O

is reacted to form CO

. Further CO reductions are

possible using Praxair’s OPTILANCE technology, which

enhances mixing and reactivity above and beyond that

of standard cold O

lances. The high-momentum hot

oxygen jet effectively mixes with the CO laden flue gas,

provides additional oxygen for combustion, and maintains

high-temperature reactive conditions when installed in the

kiln feed shelf or in the calciner at temperatures sufficiently

high for CO burnout.

Tests that compare cold oxygen injection to OPTILANCE

hot oxygen technology show the high momentum of the

lance improves CO reduction, as higher jet momentum

drives improved mixing of flue gas with the highly reactive

oxygen. One constraint to combustion staging in a cement

kiln is the potential impact of reducing kiln atmosphere on

the clinker quality, specifically the clinker colour and sulfur

retention. Quality parameters vary based on feed material,

operating conditions, and kiln equipment, so testing is

recommended to identify how much the secondary air in

the kiln can be reduced. Typical kiln excess oxygen levels

are 2 − 4% and there is opportunity to optimise the kiln

combustion process with respect to emissions and clinker

quality. Process control improvements may be required

to allow a consistent operation at lower excess oxygen

levels. A targeted oxygen injection over the clinker bed at

the kiln discharge can mitigate colour changes resulting

from low excess oxygen operation. For a successful

project implementation, a detailed analysis of the specific

operating conditions and the kiln design is required.

Injecting oxygen in the cement kiln feed end allows staging

combustion in the entire rotary kiln, which reduces NO

from the combustion.

Case study

The OPTILANCE technology was successfully

implemented at a cement plant with a preheater kiln to

reduce CO emissions from the kiln in response to new

Figure 1. OPTILANCE

oxygen injection − CO and NO

emission reduction for cement kilns.

Figure 2. Typical relationship of excess oxygen on CO

and NO

formation.