64 NOx CONTROL

Keeping NOx under control

To control NOx levels at both new-build and existing cement plants at a low cost with no

ammonia (NH3) slip, plant designers and operators need to understand the fundamentals

of controlling reburn, staging plus NH3 slip when applying selective non-catalytic reduction

(SNCR).

n by Tom Lowes, OPECS, UK and Joana Bretz, Cinar, Brazil

ICR JULY 2016 © COPYRIGHT TRADESHIP PUBLICATIONS LTD, 2016

The first online NOx measurement in Europe was made in 1971 at the

International Flame Research Foundation (IFRF) with a chemiluminescent analyser, made on-site by the staff from component parts to a design supplied by the US Environmental Protection Agency (EPA). This formed part of an EPA contract seeking to minimise NOx emissions from water boiler burners. This work gave rise to the AP series of trials that laid the foundations for industrial NOx measurements and their control.

Reported in the 1972 International and 1973 European Combustion Symposium by Lowes and Heap, it also showed a wider audience that for coal-fired burners the NOx emission was dictated by the volatile N2 and the O2 in the root of the flame when devolatilisation occurs. The work demonstrated that the normal range of NO emissions from water tube boiler burners could be dropped by 75 per cent. This reduction in NO emissions was achieved with ignition stabilisation on the burner by a swirl-induced internal reverse flow zone and the coal injected at a velocity to penetrate this zone.

A solution for the cement industry?The NOx reduction project was then extended to cement kilns in the 1990s via the Cemflame consortium which showed the route to low-NOx cement kiln burners was a flame front on or close to the burner plus a low O2 at this location, which would drop both fuel and thermal NOx.

While this was key in the development of low-NOx burners, these more often focussed on low primary air and momentum and not the flame front location. Hence it caused issues with output, SO3 cycles and build-up making them not so popular with the major cement producers. Generally, steps were taken to drop kiln NOx via

good process control – LINKman – and lower combinability temperatures, but after that using selective non-catalytic reduction (SNCR) to meet NOx emission requirements.

However, the current NOx limit for many plants of 500mg/Nm3 at 10 per cent O2 and the NOx limit of 200mg/Nm3 at 10 per cent O2 arriving in Europe (which will extend to the rest of the world in time), means that the cost of SNCR, problems of NH3 slip and use of the high-cost SCR and avoidance of NH3 emission issues, has increased focus on calciner design and operation. All efforts are directed toward optimising reburn and staging as well as maximising the efficiency of SNCR operation and avoiding NH3 slip.

This paper will focus on kinetics and combustion aerodynamics controlling these issues with practical examples of their application. Cases of calciner design that can potentially reach 200mg/Nm3 of NOx at 10 per cent O2 without SNCR, plus how to optimise the use of SNCR when needed, will be explored.

NO generation and controlIt is NO that is produced in the cement

kiln and calciner with NO2 being at a max of 50ppm. Hence NOx is used to cover all. However, in terms of emissions the limits are generally referred to as NOx and expressed in NO2mg/Nm3 at 10 per cent O2 dry basis. They are calculated by multiplying the NO in ppm by 2.05.

The rest of this paper will deal with NO, unless emission limits are referred to.

Figure 1 shows the reactions related to NO generation and control relationship.

This indicates that: • The fuel N is first evolved to HCN, then NH2, which subsequently reacts either with O2 to form NO or with NO itself to form N2.• The staging reaction involves delaying the mixing of O2 with the NH2 until it has had time to react with NO to form N2.• The reburn reaction involves forming CHi radicals that react with the NO to convert it back to NH2. Here again the NH2 needs time to convert to N2 in the absence of O2 to avoid reconverting to NO.Thermal NO is not dealt with in this

paper as it is associated with kiln NO which is generally at 75 per cent thermal NO. It is

+CHi

Fuel N HCN/NH2

+O 2

+NO+N2

NHi

NO ProductsThermal NOx at >1700˚C

Figure 1: fundamentals of NO generation and control

65NOx CONTROL

© COPYRIGHT TRADESHIP PUBLICATIONS LTD, 2016 JULY 2016 ICR

much more temperature sensitive that fuel NO having an activation energy of 90kcal/kmol compared to 5-10kcal/kmol for fuel NO.

ReburnThe reburn concept has been well known for many years. However, the idea of hot reburn was developed by Prof Mikko Hupa and communicated to an IFRF Cement Kiln Burner TOTem in Pisa in 2009. He explained the technique as “Combust the calciner fuels volatiles sub-stoichiometrically at 1300˚C for 0.15s and then quench to 1000˚C and allow a further 0.5s before allowing the tertiary air to mix and react with NH2 and HCN produced and

a good reburn will be achieved.”This concept was applied with the use

of MI-CFD by the authors et al, to Case 1 – a new plant that required SNCR to meet its limit at 500mg/Nm3 NOx at 10 per cent O2, as reported at the 2011 IEEE conference. The conventional burners below the tertiary air duct (TAD) were moved down to 0.15s residence time before the lower splash box and different angles and velocities simulated.

The volatiles concentration profile and NOx predictions calculated by MI-CFD simulations are shown in Figure 2. The first burner tried in practice was the B2, which gave an even better performance on reburn than predicted.

These results combined with the no- reburn situation and a Case 2 study, where the reburn zone was at 1.2s at 1200˚C, was estimated from MI-CFD to show an even better reburn (see Figure 3). All the data is for a bituminous coal with a 35 per cent volatile content and fuel N at 1.8 per cent.

Figure 3 also shows how well reburn and staging has taken place in the calciner. For a calciner with a 40/60 fuel split no reburn would give a slope of kiln back end NO against stack NOx at 10 per cent O2 of 0.4. Hence, in Case 1 with a slope of 0.11, the reburn is 73 per cent and in Case 2, a very high 92 per cent and in Case 1 before burner modification, 20 per cent.

The level at which the coal being injected well below the TAD and has allowed staged combustion of the fuel N to take place can be estimated by assuming that staging the volatile fuel N would convert completely to NO at the stack.

The coal had 35 per cent volatiles and 1.8 per cent N, which at the stack unstaged would be 581mg/Nm3 NOx at 10 per cent O2, giving 50 per cent and 70 per cent staging for Case 1 and Case 2, respectively.

Based on this the following rules for hot reburn were developed for a bituminous coal to fall below 500mg/Nm3 of NOx at 10 per cent O2 without SNCR:• 1300˚C for 0.15s, quench with 20 per cent meal, followed by at least 0.5s at sub-stoichiometric – <0.85 – combustion of the volatiles before the tertiary air is allowed to mix into the volatiles• 1200˚C for 1.2s controlled by the lower meal injection at sub-stoichiometric – <0.85 – combustion of the volatiles before the tertiary air is allowed to mix into the volatiles.This does not work as well for petcoke

due to its low volatiles releasing little fuel N and the char N – unlike coal – produces NO due to the fact that there is no pore surface and structure where NO exists and can reduce the char N when it goes to NH2 to covert to N and instead it produces NO.

On the other hand, for sub-bituminous coal and SRF there is strong data to support that they do not even need a reburn temperature of 1200˚C to give a good NO reduction, simply <1s at sub-stoichiometric combustion conditions before the tertiary air is allowed to mix with the volatiles.

The main reason appears to be that due to the high O2 attached to the C and H in these fuels they can more readily form HCCO which is twice as effective as CH2

Figure 2: coal volatiles concentration profile and NOx predictions

Figure 3: hot reburn data for cases 1 and 2

66 NOx CONTROL

ICR JULY 2016 © COPYRIGHT TRADESHIP PUBLICATIONS LTD, 2016

in achieving reburn. The key hot reburn reactions are shown in Table 1.

Air stagingThere are some manufacturers that produce tertiary air staging calciners where the fuels are injected below – with an option of also injecting some between the air levels – the lower TAD and having at 70 per cent sub-stoichiometry for the total fuel for 1s with its temperature controlled by the staged meal injection that 500mg/Nm3 at 10 per cent O2 can be met, providing a volatile coal or SRF is used.

Information indicates that using all the fuel below the bottom tertiary level produces the lowest NOx as it also helps reburn.

This is a little surprising considering that hot reburn focusses on sub-stoichiometric combustion of the volatiles and air staging works quite well with 70 per cent of the overall fuel.

The reason has to lie in how much actual volatiles come off under rapid heating and time. The work of the IFRF (see Figure 4) shows for a λ of 0.8 for the total fuel very effective staging can take place.

This leads to a low-NOx calciner concept, as shown in Figure 5, that combines hot reburn and air staging and where NOx should readily get below

500mg/Nm3 for all fuels and get close to 200 mg/Nm3 NOx for the more volatile coals and many forms of engineered SRF.

For plants with a hot meal molar ratio of >1.2, a hot reburn section at 1300˚C will break down the CaSO4 and produce build-up and hence silicon carbide as well as blasters and spaces for Cardox are recommended as part of the installation.

There is a myth that suggests that to reduce NOx emissions CO is essential. While operating the calciner in CO will undoubtedly reduce emissions, the CO levels needed to achieve this are >1000ppm and therefore outside normal emission limits for combustion-generated CO, plus the impact on the build-up and process operation are not acceptable as a way to drop the NOx emissions.

Figure 6 shows how a low stack NOx can be achieved by running a sub-bituminous coal in CO at the calciner and only resulting in 25 per cent reburn but virtually no fuel NO generated in the calciner. The reaction of CO to reduce the NO does not really exist as a major reducer of NO otherwise the reburn would be only 25 per cent effective.

SNCRIt is well established that the optimum temperature for SNCR with NH3 liquid is 950˚C and even the best arrangement/

Table 1: key reactions in hot reburn

Cradical

Reaction Rate (arb units)

Total rate (arb units)

Rank

C C+NO=CN+O =CO+N

11.6

2.6 6

CHCH+NO=HCN+O =H+NCO =N+HCO

251016

51 4

CH2

CH2+NO=H+HNCO =OH+HCN =H+HCNO

560140200

900 2

CH2 (S)CH2(S)+NO=H+HNCO

=OH+HCN =H+HCNO

143.65

22.6 5

CH3

CH3+NO=HCN+H2O =H2CN+OH

715.6 76.6

3

HCCO HCCO+NO=HCNO+CO 1860 1860 1

Figure 4: IFRF work on staged combustion Figure 5: generic calciner for 200-500mg/Nm3 via hot reburn and air staging for 95 per cent TSR

68 NOx CONTROL

ICR JULY 2016 © COPYRIGHT TRADESHIP PUBLICATIONS LTD, 2016

design of atomisers/injectors have lower efficiency of NOx removal at temperatures lower than this. What is not well known or applied in practice is the work by Heap et al at the 26th Combustion Symposium that showed quite clearly that in the presence of CO the optimum temperature drops to 850˚C for 1000ppm CO and 750˚C for 5000ppm CO, with an SNCR efficiency of only 50 per cent at 950˚C with 1000ppm CO.

This is due to the nature of the SNCR reactions as below:

NH3 + OH = NH2 + H2ONH2+NO=NNH+OHNNH+NO=N2+HNOHNO+M=H+NO+MH+O2=OH+OHence OH radicals are needed for the

NH3 to convert to NH2 to react with the NO and destroy it.

In the presence of CO, it preferentially takes the OH radical and the NH3 then either converts to NO or slips and gives NH3 emissions.

However, as the temperature drops the CO no longer takes preference over the OH radical formed from moisture in the combustion gases and in fact generates extra OH for the NH3 to use via the reactions below, by producing H in a radical pool. This then produces the OH radicals needed for the production of NH2 radicals for NO reduction (from Marc Cremer, REI).

CO+OH=CO2+H, H+O2=OH+O, O+H2O=2OH, NH3 + OH = NH2 etc.

This has been found to hold good in

Figure 6: impact of CO on reburn and staging to reduce NOx

Figure 8: case 4 – stratified NO, temperature and CO profiles

Figure 7: case 3 – 5500tpd calciner temperature profile and SNCR lance locations

69NOx CONTROL

© COPYRIGHT TRADESHIP PUBLICATIONS LTD, 2016 JULY 2016 ICR

practice on several installations. Figure 7, which represents Case 3, shows results from a calciner investigation in which 40 per cent less NH3 is used to get an 88 per cent reduction at 850˚C than an 80 per cent reduction at 950˚C, which 1000ppm CO at the calciner exit.

Generally, to get an optimum use of SNCR the CO, NO and temperature profiles in the upper regions of the calciner need to be known and then a ring of 4-8 injectors located before or after the calciner cyclone depending on this information. The best way to implement this is via a combination of plant measurement, process evaluation and MI-CFD modelling.

Figure 8 gives an idea of the issues plants face when trying to optimise SNCR operation to get a NOx/NH3 molar ratio (MR) of one and no NH3 slip. This shows some MI-CFD simulations of a gas riser with no fuel injection but stratified NOx, temperature and CO from the kiln and tyres on the feed shelf. Based on the Heap paper the SNCR injectors need to be at the 850˚C location.

However, their locations were much lower and a MR much greater than one (up to three) was needed to meet 200mg/Nm3 at 10 per cent O2. Moreover, it was being achieved with significant NH3 slip. SNCR penetration across a riser is also an issue and it is a false economy to minimise on the atomising air momentum. Sonic velocity is needed.

To evaluate the efficiency of this SNCR operation is not easy in a plant running under tight NOx regulations that do not allow much time above the limit. However, if the SNCR can be switched off for 20min

for a few times over a 12h period the baseline NOx emission can be found. Then using the fact that NH3+NO = N2+H2O +H and hence for 1kg of NH3 2.7kg of stack NOx should be removed it is possible to calculate the efficiency of the SNCR utilisation.

Figure 9 shows how this test when performed can be used to assess the SNCR efficiency, which in this case was 66 per cent.

Practical applicationsTo achieve the performance needed out of a new-build or an existing plant seeking to meet NOx limits at low cost with no NH3 slip, the fundamentals of controlling reburn, staging and NH3 slip need to be well understood and combined with a detailed plant process evaluation from good data and MI-CFD to arrive at an optimum solution.

While not dealt with in detail in

this paper, the CO issues produced when using solid recovered fuel with poor feed and quality control, as well as not using sufficient O2 to try limit the CO, will, and do, result in excessive costs of SNCR utilisation plus NH3 slip well above the 30mg/Nm3 at 10 per cent O2 limit. n

Tank June ad.indd 1 10/05/2016 09:59

Figure 9: case 4 – SNCR efficiency test“To achieve the performance needed... to meet NOx limits at low cost with no NH3 slip, the fundamentals of controlling reburn, staging and NH3 slip need to be well understood and combined with a detailed plant process evaluation from good data and MI-CFD to arrive at an optimum solution.”

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Building Bulletin is published monthly by:International Cement Review Old King’s Head Court, 15 High Street, Dorking, Surrey RH4 1AR, UK.Tel: +44 (0) 1306 740363/383 Fax: +44 (0) 1306 740660Email: info@CemNet.comwww.CemNet.comICR BUILDING BULLETIN n JANUARY 2016

1

The monthly newsletter covering the building materials sector

JANUARY 2016 n ISSUE 217

ContentsCRH reports on E8bn spend2015 saw 20 bolt-on acquisitions and investments 2

HeidelbergCement & Joule link upTackling carbon emissions 2Breedon scoops largest-ever contract GBP55m Scottish road project 2

Euro construction output up 0.5% Slovakia tops production chart 2US non-residential spend fallsNovember marks worst-performing month since April 2015 3‘Safeprecast” up and runningWebsite and app to tackle safety 3MPA: UK cement “under threat”Energy, environment and carbon costs take their toll 3

Hope announces depot plansHarnessing railway links at four new UK sites 3Saint-Gobain investmentsGysum joint venture in Morocco 4New UK plant for Celotex 4

Arkan welcomes new COOEnters next growth phase 4UK awaits volumetric ruling BSA speaks out on changes 4

Vulcan chairman retires Tom Hill takes over from Donald James 4

Revised gypsum application standard launchedTHE GYPSUM ASSOCIATION has released its revised application standard – GA-216-2016 Application and Finishing of Gypsum Panel Products. The new standard provides installation methods for most gypsum panels from the more common regular and type X wallboards to specialised performance boards such as glass mat water-resistant panels and abuse-resistant and impact-resistant panels. The new standard covers wallboard layout, selection of fastener types and dictates fastener spacing for many different types of gypsum panels. A new detail also illustrates and clarifies existing language when it comes to the proper placement of joints around window and door openings. Also new to this latest standard are specifications for the application of gypsum panels to flat wall insulating concrete forms (ICFs), a product that is gaining acceptance within the construction industry due to its combination of high thermal mass and insulation, which helps reduce heating and cooling costs.

UK construction sector rebounds for 2016UK CONSTRUCTION DEMAND “will remain high for many years,” according to Richard Threlfall, head of infrastructure, building and construction at KPMG. Following what he describes as a “ghastly” 2015, marred by legacy contracts signed cheaply in a bid to maintain volumes in the depth of the recession, 2016 is forecast to be a good year for the country’s construction industry thanks to strong demand on the back of steady growth.

“The government is committed to infrastructure programmes which will take decades to deliver. Still-improving economic sentiment will continue to drive commercial demand. Housing pressure remains acute and eventually some major supply-side intervention by government seems inevitable. 2016 is, therefore, the year for businesses in the construction sector to invest and build the capacity and capability to take advantage of this strong domestic market,” says Threlfall. “Recent months have seen output dip, and forecasts have been revised downwards. But it is clear that this is not reflective of underlying demand, which remains strong, particularly in commercial and civils. That demand is being suppressed by rising wages, which are causing clients to delay and re-scope schemes. Eventually recruitment into the industry will start to dampen that cost pressure and supply and demand will gradually move back towards equilibrium over the course of 2016.” Threlfall believes 2016 will see a consolidation trend within the UK construction industry with overseas buyers in particular taking a closer interest in the market.

Threlfall’s comments were echoed in the most recent data from the Markit/CIPS UK Construction Purchasing Managers’ Index (PMI), which rebounded in December to 57.8 from the seven-month low of 55.3 seen in the previous month. According to the index, commercial construction was the best performing sub-category of activity in December with improving economic conditions in the UK continuing to boost demand for commercial projects. Housing activity also increased at a robust rate that was much stronger than the 29-month low recorded in November 2015. Meanwhile, civil engineering activity declined marginally ending a seven-month period of sustained growth. Commenting on the results, Tim Moore, senior economist at Markit and author of the PMI, said, “Civil engineering remained the weakest performing area of construction in December. Nonetheless, civil engineering activity looks set to experience a near-term spike at the turn of 2016 from spending related to flood relief and additional capital budgets. In the immediate aftermath of the winter 2013/14 floods, UK civil engineering activity picked up at a survey-record pace.” Just over half of the survey panel (51 per cent) said they anticipated a rise in business activity over the course of 2016 while only seven per cent forecast a reduction. Respondents noted that greater client budgets, improving economic conditions and a strong pipeline of new projects had underpinned business confidence in December last year.

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