March 30, 2006 

 

Christopher Recchia 

Executive Director 

Ozone Transport Commission 

Hall of the States, Suite 638 

444 North Capitol St. 

Washington, DC 20001 

Re: Comments on Draft NOx Control Measure Summary for Lime Kilns 

Dear Mr. Recchia: 

The National Lime Association (NLA) appreciates the opportunity to comment on OTC’s 

characterization of NOx control technologies for lime kilns. NLA is the trade association for 

commercial manufacturers of high calcium quicklime, dolomitic quicklime, and hydrated lime, 

collectively referred to as “lime.” Lime provides cost-effective solutions to many of society’s 

manufacturing and environmental needs. 

The commercial lime manufacturing process involves combustion of fossil fuels to calcine 

limestone in kilns. Kiln emissions include NOx and SO2. Lime kilns are also very different than 

cement kilns, which employ kiln designs not applicable to lime production and use a variety of 

different feedstocks (other than limestone) and fuels in their process. 

Emissions from lime kilns are more difficult and costly to control than those from many other 

sources. OTC’s control measure summary states that NOx control technologies applicable to 

cement kilns are “readily available and can be used” on lime kilns. However, as described below, 

this is not the case. Neither selective non-catalytic reduction, nor selective catalytic reduction, is 

a feasible NOx control technology for commercial lime kilns. 

NOx Emissions from Lime Kilns & OTC’s Proposed Control Measures 

OTC estimates that commercial lime kilns emitted 4,379 tons of NOx in 2002 in the OTC region, 

which equates to less than 350 tons, on average, per kiln. Total emissions from the lime sector 

are about 1 percent the emissions from electric generating units (446,692 tons) and less than 1/5th

the emissions from cement kilns (31,822 tons). 

OTC’s draft NOx control measure summary for lime kilns (attached) identifies two candidate 

control measures: 

• Selective non-catalytic reduction (SNCR) and combustion modifications; and 

• Selective catalytic reduction (SCR). 
Page 2 

The summary states that SNCR “is readily available and can be used across the variety of 

vertical1 and rotary [lime] kilns used in stand-alone operations and in conjunction with pulp mills 

and chemical processing.” The document is silent on the feasibility of using SCR for the lime 

industry. 

Turning to costs, OTC estimates that SNCR can reduce NOx emissions by 60 percent at a cost of 

$1,017 per ton of NOx, and that SCR can reduce NOx emissions by 70-90 percent at a cost of 

$1,500 to $2,000 per ton of NOx. The basis for OTC’s identification of SNCR and SCR for lime 

kilns is EPA’s AirControlNET database ( “AirControlNET 4.1 Control Measures Documentation 

Report,” pages III-377 to III-384, entitled “Lime Kilns, Selective Non-Catalytic Reduction 

(SNCR) Urea Based, Selective Non-Catalytic Reduction (SNCR) Ammonia Based, and Selective 

Catalytic Reduction (SCR)“).2

 

Despite the titles of the referenced pages, EPA’s lime kiln control technology descriptions are 

actually cement kiln applications, based on EPA’s 1994 Alternative Control Technologies 

document for cement kilns.3 The discussion of costs makes it clear that the evaluation of the 

technologies are based solely on cement kiln applications. 

EPA and State Evaluations of SNCR and SCR for Lime Kilns 

All U.S. regulatory authorities that have evaluated SNCR and SCR have reached the conclusion 

that both technologies are infeasible for lime kilns: 

In the 1998 NOx OTAG Transport Rulemaking, EPA examined the appropriateness of 

imposing NOx control technologies on the lime industry, including urea-based SNCR. 

EPA recommended to the OTAG states that lime plants not be targeted for NOx 

reductions (63 Fed. Reg. 56,401). This contrasts with the cement industry, for which EPA 

recommended that 30% NOx reductions should be sought because control technologies 

were available that fell within EPA’s cost-effectiveness range (Id. at 57,416). 

Since 1998, BACT analyses of NOx controls have been prepared for 11 new commercial 

lime kilns that have been thoroughly reviewed by U.S. EPA, state and local agencies, and 

the public. They all reached the conclusion that add-on controls for NOx for new lime 

kilns are not technically or economically possible. Instead, permits for these new kilns 

define BACT as efficient combustion practices, minimization of fuel consumption, and 

excess air for the combustion process, or some similar control techniques (see EPA 

Clearinghouse). 

VISTAS is in the process of updating its control technology summaries for states to use to 

develop regional haze control requirements. Based on recent responses to requests for 

comments from BART sources, VISTAS expects to delete both SCR and SNCR from its 

 1

 There are no operating vertical kilns at commercial lime plants in the OTC region. 

2

 E-mail from OTC contractor, Ed Sabo (MACTEC) to Eric Malès, NLA dated February 28, 2006. 

 3 USEPA, Office of Air Quality Planning and Standards. Alternative Control Technologies Document – NOx 

Emissions from Cement Manufacturing. EPA-453/R-94-004 [hereinafter referred to as Cement NOx ACT Document] 
Page 3 

compendium of presumptive control technologies for lime kilns.4 

NOx controls would be even more costly for existing commercial lime kilns in the OTC region 

than for the new kilns subject to BACT analyses since 1998. There are two reasons why costs 

would be greater: control costs do not scale down in direct proportion to the production rate and 

there would be additional costs for retrofitting NOx controls on existing kilns, which are not 

included in the BACT analyses for new kilns. 

The average capacity of the 11 new BACT kilns is about 1,100 tons of lime per day (tpd), nearly 

three times greater than the 375 tpd capacity of the average lime kiln on the OTC list. Because 

NOx control costs would be spread over fewer tons of lime for the OTC kilns, the costeffectiveness of NOx controls for them would be less “reasonable” than for the 11 kilns subject to 

BACT review. 

EPA and the states have generally recognized a 0.6 power (i.e., exponent) factor to be appropriate 

when scaling NOx control equipment costs.5 Applying this factor to the average size of a new 

lime kiln vs. the average OTC lime kiln, suggests that costs for OTC kilns on a dollar per ton 

basis would be more than 50 percent higher than for the 11 new BACT kilns mentioned above. 

As discussed later in this document, SNCR costs evaluated in two recent BACT analyses for lime 

kilns were between $8,000 and $25,000/ton NOx removed – well above OTC’s cost estimate of 

$1,017 listed above. Applying a 0.6 power factor increases this discrepancy even further. 

NOx Control Technologies for Cement Kilns are Not Transferable to Lime Kilns 

Lime manufacturing -- including the size of the production units, kiln technology, raw materials, 

and process conditions -- is significantly different than cement manufacturing: 

1) Lime kilns generally produce smaller quantities of material than cement kilns. As discussed 

above, the size difference alone makes NOx controls significantly more expensive per ton of 

product. 

2) There are significant process differences between cement and lime manufacturing. For 

example, SNCR in cement kilns usually relies on the precalciner tower of the kiln. Lime kilns 

do not employ precalciners. 

There are also important differences between cement and lime in the size of raw feed. 

Particle size of feed to a cement kiln is measured in microns (e.g., 80% less than 75 microns--

about 0.003 inches); limestone fed to a lime kiln is measured in inches (e.g., 1 inch to 21⁄2 

inches). As discussed below, this difference in feed particle size makes injection of reactants 

infeasible in lime kiln preheaters. 

Set forth below is a discussion of the feasibility of the NOx control technologies identified by 

OTC. 

 4

 VISTAS’ draft control measures summary sent to BART sources for review is dated Nov. 2005. Before the 

revisions are made, however, a second round of review between states and non-BART sources must be completed. 5

 For example, Cement NOx ACT Document, pg. 6-24. 
Page 4 

Selective Non-Catalytic Reduction (SNCR)

This technology has been determined to be technically or economically infeasible in all BACT 

determinations for lime kilns. As noted above, SNCR in cement kilns usually relies on the 

precalciner tower of the kiln. Lime kilns do not employ precalciners. 

A common theme in the RACT/BACT analyses is that SNCR is infeasible in lime kilns because it 

requires a high but very specific temperature range (between 1,600 and 2,100oF) to be effective. 

At lower temperatures, the NOx reduction reaction is incomplete, and at higher temperatures NOx 

emissions can actually be increased. There are three possible locations where this temperature 

profile could be achieved in lime kilns: in the preheater, after the air pollution control device (in 

most cases a baghouse), or within the kiln. 

Regarding the first option, none of the commercial lime kilns in OTC’s inventory have 

preheaters; all are straight rotary kilns. For lime kilns, feed particle size makes injection of 

reactants infeasible in preheaters, because large stone would either damage spray nozzles, or the 

sprays would impinge on the stone, wetting the stone but not entraining the reagent in the gas 

stream. 

Regarding the second option, flue gas exhaust temperatures from lime kilns baghouses and wet 

scrubbers (generally below 450oF and 212oF, respectively) are substantially below the SNCR 

operating range. Consequently, the exhaust gases would need to be reheated. Recent BACT 

analyses estimate that the cost of reheating alone -- excluding reagent and capital costs -- would 

result in an average cost/ton of NOx removed greater than $8,000.6 Reheating exhaust gases 

would also result in additional pollutants being formed from the combustion products from the 

fuel used to reheat the exhaust gases. 

Turning to the third option, several RACT/BACT evaluations demonstrate the infeasibility of 

injecting the reagent within the kiln. The Portland Cement Association (PCA) comments state 

that there is currently no method of injecting reagent into a rotating kiln.7

 Furthermore, injection 

of the reagent within the lime kiln would contaminate the product. For straight rotary kilns, such 

as those in the OTC region, temperatures in the reagent injection region would result in increased

NOx formation. 

Another concern raised in BACT analyses of SNCR is the formation of unreacted ammonia or 

urea that will react with sulfur oxides in the flue gas in the presence of water to form ammonium 

bisulfite (NH4HSO4), a sticky compound that can cause corrosion, fouling, and blockages 

downstream of the injection point. This would create serious problems with the preheater, 

 6

 In the PSD permit analysis for 625 tpd kiln at U.S. Lime & Mineral’s Arkansas Plant, exhaust gas reheat is 

estimated to be $8,322/ton NOx removed, at $6.45/MMBtu natural gas. In Pennsylvania, SNCR was estimated to 

cost $18,000 - $25,000/ton NOx removed for the 2 new Graymont kilns (1,050 and 1,200 tpd), at $10.00/MMBtu 

natural gas, including capital and reagent costs. 7

 “Evaluation of Suitability of Selective Catalytic Reduction and Selective Non-Catalytic Reduction for Use in 

Portland Cement Industry.” Robert J. Schreiber et al, March 2006. Also see PCA’s Jan. 24, 2006 comments to 

TCEQ, which state that SNCR is not currently available for long wet kilns and that the logistics of injection in the 

middle of a long rotating kiln have not been developed. (“Initial Comments of PCA on Draft Final Report 

‘Assessment of NOx Emissions Reduction Strategies for Cement Kilns-Ellis County’”) 
Page 5 

ductwork, baghouses, and fans; reduce kiln draft; and cause excessive outages. Furthermore, 

ammonia slip from such systems would increase formation of secondary fine particulate in the 

atmosphere. 

Finally, ammonia absorption into the lime kiln dust collected in the baghouse would seriously 

impact, if not eliminate, the ability to sell this byproduct. Not only maintaining but also 

increasing the sale of these byproducts is a key industry strategy in meeting our CO2 intensity 

reductions under DOE’s Climate Vision Program. 

For all of the above reasons, SNCR is not a feasible NOx control strategy for the lime industry. 

Selective Catalytic Reduction (SCR)

Selective catalytic reduction (SCR) is an exhaust gas treatment process in which ammonia (NH3) 

is injected into the exhaust gas upstream of a catalyst bed. On the catalyst surface, NH3 and nitric 

oxide (NO) or nitrogen dioxide (NO2) react to form nitrogen and water. OTC estimates that 70-

90% reduction of NOx is achievable with this technology for lime kilns, at a cost of $1,500 to 

$2,000/ton NOx removed. 

Even for cement kiln applications, this technology is unproven--see PCA’s comments, which 

discuss significant shortcomings in the one application of SCR at the Solnhofen cement plant in 

Germany.8 At that plant, SCR is not consistently achieving 80% removal (70-90% is presented in 

OTC’s control document). The maximum removal rate achieved is 70%. Furthermore, in 2004 

the actual removal rate was less than 55%, 28% of the time. As Tom Carter of PCA explained at 

OTC’s meeting in Washington D.C. in February, the SCR installation at Solnhofen is very large 

and would be extremely difficult to retrofit on existing kilns. The costs of the system were 

partially funded by the German government. 

Particulate fouling and masking of the catalyst is a significant problem for SCR, particularly with 

calcium-based particulates from kilns. Comments by PCA discuss this problem for cement 

kilns.9

 Irregularly-shaped particulates from kilns are more likely to create fouling and plugging 

problems than spherical particulates from utility boilers. At the Solnhofen cement plant, even 

with a complicated catalyst cleaning system, a 24-hour shutdown of the SCR unit is required for 

manual cleaning every 7-14 days.10 This problem would be even more significant for lime kilns, 

which use 100% limestone (calcium carbonate) feed and therefore have a greater concentration of 

calcium in flue gas particulates. 

To achieve the 80% removal rate, an SCR would need to be operated at a temperature between 

700 and 750°F.11 To avoid fouling the catalyst bed with the PM in the exhaust stream, an SCR 

unit would need to be located downstream of the particulate matter control device. However, due 

to the low exhaust gas temperature exiting air pollution control devices at lime plants (in most 

cases, a baghouse), a heat exchanger system would be required to reheat the exhaust stream to the 

 8

 Ibid. 

9

 Ibid. 

10 Ibid. 

11 Air Pollution Control Cost Manual, Sixth Edition, January 2002, Section 4.1, Chapter 1, Selective Catalytic 

Reduction, NOx Controls, EPA/452/B-02-001, page 2-10. 
Page 6 

desired reaction temperature. The cost of exhaust gas reheat would be prohibitive, as with the 

SNCR systems discussed above. 

Efficient operation of an SCR process requires fairly constant exhaust temperatures (usually 

±200°F).12 Fluctuation in exhaust gas temperatures reduces removal efficiency. If the 

temperature is too low, ammonia slip occurs. Ammonia slip is caused by low reaction rates and 

results in both higher NOx emissions and appreciable ammonia emissions. If the temperature is 

too high, oxidation of the NH3 to NO can occur. Other emissions potentially generated by SCR 

include increased PM emissions (from ammonia salts in a detached plume) and increased SO3

emissions (from oxidation of SO2 on the catalyst). 

Although SCR is being used in the utility industry, there are significant differences between the 

exhaust streams generated by the two industries that account for the difference in the application 

of the technology. A utility boiler’s exhaust gas stream does not vary over time. That is, the gas 

stream characteristics do not change greatly, whereas, the lime kiln exhaust gas stream 

temperature has a high degree of fluctuation. The temperature variability of the exhaust gas 

stream makes the technology technically infeasible. 

For all of the above reasons, SCR is not a feasible NOx control strategy for the lime industry. 

Conclusion 

OTC’s control measure summary should be revised to reflect the infeasibility of SNCR and SCR 

controls for lime kilns. This finding is consistent with all other authorities that have evaluated the 

potential application of NOx controls on lime kilns. 

If you have any questions regarding these comments, please do not hesitate to contact me at 

(703) 243-5488. 

Sincerely, 

 Arline M. Seeger 

Executive Director 

cc: Terry Black, PADEP (OTC category lead for lime kiln control measures) 

Ali Mirzakhalili, DNREC (Chair, OTC Stationary/Area Source Committee) 

Ed Sabo, MACTEC (OTC consultant) 

Attachment: OTC Control Measure Summary for Lime Kilns (Review Draft), Jan. 13, 2006 

 12 Id, page 2-11. 
Summary of OTC Candidate Control Measures REVIEW DRAFT Date: January 13, 2006 

CONTROL MEASURE SUMMARY FOR 

Lime Kilns 

Category Lead: Terry Black

Control Measure Summary: 

Emissions (tons/year) in Ozone 

Transport Region 

2002 existing measure: NSR; PSD; State RACT. 

Emission Reductions: 

Control Cost: 

Timing of Implementation: 

Implementation Area: OTR 

NOx 

 Uncontrolled: 

2002 Reduction: 

2002 Base: 

4,649 

 0

 4,649

Candidate measure 1: Selective Non-catalytic Reductions 

(SNCR) and Combustion Modifications 

Measure ID: SNCR 

Emission Reductions: 60 percent from 2002 base 

Control Cost: $ 1017 per ton 

Timing of Implementation: 2009 

Implementation Area: OTR

 NOx 

2009 Base 

including growth: 

2009 Reduction: 

2009 Remaining: 

5,228 

- 3,136

 2,091 

Candidate measure 3: Selective Catalytic Reduction (SCR) 

Measure ID: SCR 

Emission Reductions: 70-90 percent from 2002 base 

Control Cost: $ 1,500 to 2,000 per ton 

Timing of Implementation: 2009 

Implementation Area: OTR

 NOx 

2009 Base 

including growth: 

2009 Reduction: 

2009 Remaining: 

 5,228 

-4,706 

 * 522 

* 90% reduction

Policy Recommendation of State/Workgroup Lead: Consider the option of recommending that a program 

be developed that requires combustion modifications and SNCR for this category of sources. 

Brief Rationale for Recommended Strategy: This technology is readily available and can be used across the 

variety of vertical and rotary kilns used in stand-alone operations and in conjunction with pulp mills and 

chemical processing.