Precise Combustion ControlGerald Lefebvre and Eric Hansen, EcoSource Technologies, Ronald Baker,Robert Peters and Edul Chikhliwala, EcoChem Analytics, USA, discuss howperformance CEMS enable precise cement kiln combustion control.

Reprinted from WORLD CEMENT April 2004

Introduction

In cement manufacture operations, ContinuousEmission Monitoring Systems (CEMS)1 have primarilybeen used for regulatory reporting purposes - eitherfor compliance with air quality regulations, or foremissions inventory purposes. Measurements fromthese CEMS have been used only occasionally for kilncombustion process control. Historically, tight combus-tion control was not feasible because of unreliableoxygen and other emission measurements, and havingverified data is critical to very tight control. An addi-tional obstacle was the cost, especially when monitor-ing several compounds (for example O2, CO, CO2, SO2

and NOx). Performance CEMS contribute towards reduced

emissions, improved kiln thermal efficiency, increasedproductivity, and better clinker quality by bringinggreater kiln stability. The production rate can also beincreased with the same gas flow by optimising thefuel to air ratio at all times. In the past, due to lack ofreliability, plant personnel were reluctant to invest inperformance CEMS although they were aware of thepotential benefits. An ideal candidate for this applica-tion is the Hot-Wet multicomponent CEMS technology. This technology has evolved over threegenerations and two decades of experience. It hasperformed reliably under harsh and rigorous condi-tions encountered in waste-to-energy facilities andcoal-fired utilities. The Hot-Wet multicomponentinfrared gas analyser technology addresses past con-cerns regarding reliability by offering a robust, accu-rate and cost-effective technique.

What is a performance CEMS?

A performance CEMS delivers high quality validatedemission data for precise control of combustion. This isachieved by using a combination of optimised compo-nents (sampling system, multicomponent gas analyserand combustion control software) that monitor andcontrol the combustion in a cement kiln. This CEMS hasdual application - meeting regulatory requirementsand providing inputs that can be used for process con-trol (Figure 1). Some of the benefits of a reliable andaccurate performance CEMS for the cement kiln appli-cation include:

� Better combustion control with an aim towardsreducing the excess oxygen leading to higher kilnproduction without an increase in emissions.Hansen2 provides an example of a cement kilnwhere a 1% reduction in average excess oxygen canresult in a 5% increase in production throughput(Figure 2).

� Performance CEMS are equipped with combustioncontrol enhancement software that minimisesexcess air while maintaining compliance and con-trol of fuel, feed and the ID fan to ensure maximumproduction (Figure 3). Better combustion controlachieved through a performance CEMS results inemissions minimisation. The air-fuel ratio can alsobe controlled in relationship to multiple emissions(NOx, SO2, CO).

� Performance CEMS can assist in fulfilling emissionstandards. This is particularly applicable in manycountries like the USA where regulations such asthe US Environmental Protection Agency’s PortlandCement Maximum Achievable Control Technology(MACT) and long standing regulations of 40 CFRPart 60 have to be met.

Reliable and validated measurements

In order to obtain a reliable measurement the limita-tions of an historical CEMS must be addressed. CEMSused for cement kiln monitoring consisted of an arrayof discrete analysers utilising a cold-dry sample condi-tioning system (a system of gas coolers remove thewater moisture from the sample stream and provideclean dry flue gas to the analysers). These systems wereinherently troublesome leading to maintenance night-mares for plant operating personnel. Taking note ofthe difficulties encountered by plant personnel, theHot-Wet multicomponent gas analyser technology was

Figure 1. Application of a CEMS for a cement kiln (adapted fromJahnke1).

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developed where appropriate systems are matchedand operating variables are optimised to maximise reli-ability. The advantages of this type of gas analyser overa collection of discrete analysers using the traditionalcold-dry sampling approach include:

� In the cold-dry approach, sample gas coolers typi-cally have condensates and encounter loss of reac-tive and water-soluble gases like SO2, NO2, HCl andNH3. The Hot-Wet approach eliminates gas coolersand maintenance of the sampling system is oftensignificantly reduced.

� A multicomponent IR gas analyser enables directmeasurement of relevant gases (NOx, CO, SO2, CO2,HCl) in one compact instrument (Figure 4). In addi-tion, a zirconium oxide sensor is integrated withinthe analyser to provide an O2 measurement. Thusthe cement plant gets all the necessary emissiondata from a single analyser with one power supply,motherboard, temperature-controlled oven and I/Ointerface.

� Since the same analyser is measuring all gases, real-time correction for quantifiable interfering com-pounds using computerised algorithms is possible.This results in accurate data with high reliability.

� Due to reduced parts count, the maintenance effortfor the analyser is reduced. A single source lamp,pump, flow meter, detector, power supply and opti-cal bench replace the redundant componentsfound in each analyser.

� Through the use of common components, the Hot-Wet multicomponent performance CEMS has a sig-nificantly lower purchase cost.

Low-maintenance sampling system

In order for a performance CEMS to function asdesired, it must be configured to deal with the harshenvironment and complex chemistry of alkalis, chlo-rides and sulfates encountered in the cement manu-facture process. The application engineering aspectmay be one of the most important parts and it is insampling systems where that is most critical.

The primary key is to keep the system as simple aspossible while dealing with four primary concerns: par-ticulates, corrosion, condensates and reactive gases.This is accomplished in the Hot-Wet (HW) sampling sys-tems by using specially designed components. The sys-tem maintains a high temperature throughout thesampling and analysis process (for measurements ofreactive compounds such as SO2, HCl or NH3 the entiresample system will be heated up to 450ºF or 230°C). Inthe HW system only four parts are in contact with theflue gas, as defined below.

Probe assemblyThe probe assembly provides specific functions neededfor reliable sampling of flue gases while closely main-taining temperatures at elevated levels. Those func-tions are probe-back purge with instrument air,calibration gas injection, and failsafe inert gas protec-tion of the system from corrosion if loss of tempera-ture control should occur.

Heat traced sample lineThe heat traced sample umbilical will maintain thegases at an appropriate temperature all the way fromthe probe assembly to the analyser enclosure. The sam-ple umbilical bundle will contain one or more tubes forthe sample gas and may contain additional tubes forinstrument air and calibration gas. It may also haveconductors and control wires for other functions needed at the probe assembly. PFA-Teflon is typicallythe material used for the sample tube.

Heated sample pumpA key in reactive gas sampling is minimisation of the res-idence time in the sampling system and that is achievedby using a custom-engineered high capacity heatedpump. A three-layered Teflon diaphragm is used.

Analyser sample cellThe analyser sample cell may be of straight or foldedpath construction depending on the gases to be mea-sured and range of analysis. An integral zirconium oxideoxygen sensor is also included. The entire sample cellalong with the oxygen sensor is temperature controlledand a single sample exhaust is vented to the atmosphere.

Accurate measurements from a multicom-

ponent gas analyser

In order to understand the operation of the multi-component analyser, here is a summary of the impor-tant aspects of its functioning:

Reprinted from WORLD CEMENT April 2004

Figure 2. Kiln production as a function of excess oxygen2.

Figure 3. Advanced kiln control strategy 3.

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Reprinted from WORLD CEMENT April 2004

Analysis techniquesThe multicomponent IR gas analyser can measure dif-ferent gases using techniques like gas filter correlationor the single beam dual wavelength1,4. Sophisticatedmathematical algorithms are implemented whichallow the computation of multiple gas concentrationswhile properly taking into account associated interfer-ence effects during each measurement cycle (which isusually completed within 10 sec).

ConstructionThe gas analyser is specifically designed for continuousservice and the optical bench, sample cell and controlcomponents are all integrated into one convenient,and easily serviced compact enclosure (Figure 4). Theanalyser uses a folded path sample cell that is achievedby a specialised mirror configuration. In particular, theintegrated design allows the photometer source anddetector to be housed in the same temperature con-trolled housing which significantly improves stability. Asolid-state detector is located on its own subassemblybut in the temperature controlled housing.

Programmable logic controller (PLC)A PLC controls the entire CEMS (sampling system andmulti-component analyser). A commonly used indus-try standard PLC is deployed. The PLC can be re-pro-grammed in the field using a touchscreen display.Using the PLC, the set points (e.g. sampling line tem-perature) and other crucial parameters (e.g. time val-ues associated with a calibration sequence) can bedefined.

Combustion control software

Borrowing from standard CEMS practice, data valida-tion and quality assurance is done on a continuousbasis. Measurements are crosschecked with other fluegas constituents indicative of excess air levels (forexample, the O2 measurement that is used in controllogic would be constantly validated against simultane-ous CO, SO2 and NOx values to verify the credibility ofthe measurement). With this validated data, controlstrategies can be aggressively pursued. Algorithms formodelling the heat and mass balance in a cement kiln(Figure 5) along with process control loops implement-ing fuel and oxygen trim are incorporated in the soft-ware. Other potential enhancements includemodel-based predictive controls and strategies thatprioritise emissions control3.

Conclusion

The benefits of cement kiln combustion control arenumerous. Hot-Wet sampling eliminates the problemsassociated with condensates and significantly increasesthe reliability of the CEMS. Coupled with a multicompo-nent analyser it is now possible to obtain verifiableemissions data that is continuously crosschecked. Hot-Wet multicomponent CEMS technology is an ideal can-didate for a performance CEMS. It is a cost-effective andlow-maintenance approach for obtaining high-availability emissions data. By virtue of the robustdesign and accurate data, the Hot-Wet multicomponen-

t CEMS applied to the cement kiln industry can resultin minimising emissions, improved thermal kiln effi-ciency, increased productivity and better clinker quali-ty through greater kiln stability.____________________�

References1. JAHNKE, J.A., Continuous Emission Monitoring, John Wiley &

Sons, New York, 2000.2. HANSEN, E., The Use of CO and Other Trace Gases for Process

Control, IEEE Transaction on Industry Applications, Vol. 1A-22,No. 2, March/April 1986.

3. HANSEN, E., Changing Process Priorities when FiringAlternative Fuels, paper presented at the 45th IEEE-IAS/PCACement Industry Technical Conference, Dallas, USA, May 2002.

4. DILLEHAY, D., Computerized Control and Signal Processing forInfrared Analyzers In Proceedings - Speciality Conference onContinuous Emission Monitoring - Present and FutureApplications,Pittsburgh: Air and Waste ManagementAssociation, 1990, pp. 215-226.

Figure 4. CEMS enclosure and typical dimensions of a multicom-ponent gas analyser.

Figure 5. Process control software incorporating heat and massbalance of a cement kiln with fuel and oxygen trim loops.

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