National Lime Association v. Environmental Protection Agency and Douglas M. Costle, Administrator of Environmental Protection Agency

The National Lime Association (NLA), representing ninety percent of this country’s commercial producers of lime and lime hydrate (the industry), challenges the new source performance standards (NSPS) for lime manufacturing plants issued by the Environmental Protection Agency (EPA, Administrator or Agency) under § 111 of the Clean Air Act (the Act), 42 U.S.C. § 7411 (Supp. I 1977). The standards limit the mass of particulate that may be emitted in the exhaust gas from all lime-hydrating and from certain lime-manufacturing facilities and limit the permitted visibility of exhaust gas emissions from some facilities manufacturing lime. We find inadequate support in the administrative record for the standards promulgated and therefore remand to the Administrator.

I. RELEVANT PARTICULARS OF THE LIMESTONE INDUSTRY

A. The Industry

In sheer size and weight of production, the limestone industry ranks among the largest in this country. Limestone production in the United States ranks second only to sand and gravel in commodity tonnage and exceeds petroleum, coal and iron ore in volume produced. Limestone deposits can be found beneath an estimated fifteen to twenty percent of the surface of the United States and occur in every state. Total national production approximates twenty-two million tons annually and derives from plants in over forty states. 1

The recent development of two important industrial uses for lime 2 has ensured the continuing growth of production 3 despite a decline in agricultural use. 4 The industry is capital-intensive with declining employment, but because so many other industrial processes depend on the use of lime, any decline in production would have “a large multiplier effect on U.S. employment.” 5

B. The Production of Lime From Limestone

The process by which commercially valuable lime is produced is relatively simple. Limestone is quarried, crushed, sized and fed into a kiln where it is subjected to high temperatures (1100°C/2000°F). By a proc *423 ess known as “calcination,” the heating (“burning”) of limestone produces quicklime, a soft, porous, highly reactive material commonly used in industry. As might be expected, the process generates a substantial quantity of dust, or in the language of the Agency, particulate matter, sufficiently lightweight to be carried off in the hot exhaust gas and emitted from the kiln. The particulate matter thus released is composed of partially burned limestone, raw limestone feed, deadburned lime 6 and quicklime. Typically, the process also releases sulfur dioxide (S02). 7

Almost ninety percent of total United States lime production is processed in rotary kilns. 8 Uncontrolled emissions from rotary kilns have been reported to run from 150 to 200 pounds per ton of lime produced, roughly five percent of the feed poundage and nine percent of the produce. 9 A typical lime plant 10 producing 500 tons per day from a rotary kiln, conforming to typical state pollution-control standards, 11 emits about 150 megagrams (165 tons) of particulate matter per year. Rotary kilns produce a greater volume of particulate emissions than the formerly widely used vertical kilns but they are also the only kilns which can retain product quality while burning coal, a fuel on which the industry has become increasingly dependent. 12

*424 C. The Production of Hydrated Lime

A comparatively small amount (ten percent) of all lime produced is further processed into hydrated or slaked lime. This is done by adding water to lime and introducing the mixture into an agitated hydrator. An exothermic reaction occurs and a fluffy, dry, white powder, known as hydrated lime, is the result. 13 Particulate matter is carried off in the steamy exhaust emitted from the hydration process.

D. Emissions Control in the Production of Lime

Rotary kilns here and abroad have employed several different methods of emissions controls including the fabric filter baghouse, the electrostatic precipitator (ESP), the high energy scrubber, and the gravel bed filter. 14 One survey showed that of eighty-five domestic rotary kilns, twenty-four percent used a baghouse, thirty-one percent used a high energy scrubber and eight percent used an ESP. 15 However, use of the baghouse method is increasing because this method requires less energy and does not itself create additional problems of pollution control. 16

EPA has identified baghouses, ESPs and scrubbers as “best systems” of emissions control for rotary lime kilns. 17 Baghouses

The operation of baghouses and electrostatic precipitators was briefly explained in our initial review of EPA’s performance standards for portland cement plants, Portland Cement Association v. Ruckelshaus, 486 F.2d 375, 390-91 (D.C.Cir. 1973), cert. denied, 417 U.S. 921, 94 S.Ct. 2628, 41 L.Ed.2d 226 (1974) [hereinafter cited as Portland Cement I]. The baghouse method employs fabric filters (“bags”), situated within an enclosed area (a “house”), to remove particulate from the kiln exhaust gas which is channeled through the house.

As the exhaust gas passes through, a dust cake forms on the filters. The cake itself improves filtration efficiency, but from time to time the filters must be cleaned. This is done by forcing a reverse gas flow through the fabric, thus releasing the cake for disposal.

EPA acknowledges that fabric filter effectiveness is primarily a function of kiln exhaust particle size distribution, fabric type, fabric age and maintenance history. 18

Electrostatic Precipitators

Under this method, “dust particles are charged [by discharge electrodes] and pass through an electrical field [collector plates] of the opposite charge, thus causing the dust to be precipitated out of the exhaust gas . . . .” Portland Cement I, 486 F.2d at 390. Two basic criteria must be met before an ESP can be utilized: (1) the suspended particle must be able to accept an electric charge; and (2) the particle must then pass through an electric field of sufficient strength to ensure removal of the particulate from the gas stream at the desired efficiency.

Precipitability is a function of the chemical composition of the dust particles, and will vary with the different kinds of material that make up the kiln exhaust dust (limestone, quicklime, fly ash, calcium sulfate, etc.). 19 Assuming precipitability, the two main factors influencing the efficiency of a precipitator are the gas velocity and treatment time. The ESP method experiences a relatively low collection efficiency on sub-micron particles.

*425 Although most particles collected by an ESP fall by gravity into waiting hoppers, programmed rapping of the electrodes is also required to keep the collector plates and discharge electrodes clean. As with the baghouse method, the dust collected is dry and may be disposed of in a variety of ways. A high level of maintenance skill is needed to keep an ESP in operation at design conditions.

Scrubbers

Scrubbers operate on the principle that wet particles are easier to control than dry. High pressure (or high energy) scrubbers of the type EPA considers capable of meeting the promulgated standards are those which because of their design increase the likelihood of contact between particle and water.

The most common high pressure drop scrubber used for controlling emissions from rotary lime kilns is the venturi scrubber. This scrubber operates by accelerating the velocity of the exhaust gas through a narrow venturi-shaped throat, where it is then brought into contact at great force with a spray of water. The particles thus dampened coalesce to form a slurry that can then be collected by a comparatively simple water-gas separation device. The separated gas is then released into the atmosphere.

The efficiency of particulate removal is a direct function of energy input, measured by pressure drop across the venturi throat. 20 Gas-water contact in the venturi scrubber is so thorough that even submicron particles are removed. Although low pressure drop scrubbers use less energy than high pressure drop scrubbers, even a low efficiency scrubber requires more energy than either the baghouse or the ESP. The slurry which is the by-product of scrubber use is deposited in ponds, where the collected particulate settles out from the scrubbing water. The “clean” scrubbing water is then reused. Under present law settling ponds must be located so that they do not receive excessive rainwater run-off, causing overflow into local navigable waters.

E. Emissions Control in the Production of Hydrated Lime

Hydration emissions have been shown to be most effectively controlled by wet scrubbers and they are the only system of emission reduction considered by EPA for lime hydrators. 21

The most common type of scrubber used on lime hydrators is the wetted fan type with centrifugal separation. In this scrubber water is sprayed into the center of a draft fan where it is forced to mix with the exhaust gas. More water is sprayed just after the fan into the duct carrying this gas-water mixture. The dust laden slurry water is then removed from the cleaned gas stream by centrifugal separation and the “scrubbed” gas is then vented to the atmosphere.

Slurry water is returned immediately to the hydrator for reuse; the hydration process requires the addition of water and the captured dust seems to contribute to, rather than interfere with, the production of hydrate. Recycling the slurry water eliminates the settling ponds and waste sludge disposal problems usually associated with particulate scrubbers.

II. PROCEDURAL HISTORY

Section 111 of the Clean Air Act, formerly 42 U.S.C. § 1857c-6 (1976) (repealed 1977), now 42 U.S.C. § 7411 (Supp. I 1977), authorizes the Administrator to limit the air pollutants that can lawfully be emitted from newly constructed 22 or modified 23 *426 plants. This the Administrator can do by promulgating new . source performance standards requiring new or modified plants to meet standards which can be met through application of the best system of emission reduction (considering costs) which has been “adequately demonstrated.” The purpose is to assure that new or modified plants will not create significant new air pollution problems. 24

On May 3, 1977, EPA added lime manufacturing plants to the list of sources that “may contribute significantly to air pollution which causes or contributes to the endangerment of public health or welfare” pursuant to section 111(b) of the Clean Air Act, 42 U.S.C. § 1857c-6(b)(l)(A) (1976) (repealed 1977). 25 42 Fed.Reg. 22510 (1977). At the same time, EPA proposed NSPS for lime plants. 42 Fed.Reg. 22506 (1977). The information underlying both actions was contained in the SSEIS. 26

Although lime plants were determined to be sources* of nitrogen oxides, carbon monoxide and sulfur dioxide as well as particulates, standards of performance were proposed and ultimately promulgated only with respect to particulate matter. 27 Furthermore, of the various types of kilns that may be used in the calcination of limestone, only rotary kilns are regulated by the standards. 28

*427 The kiln standards limit emissions 29 to 0.15 kilogram of particulate matter per megagram of limestone feed (0.3 pound per ton) and ten percent “opacity.” 30 The owner or operator of an affected facility is required by the regulations to monitor continuously the opacity of emissions. Where the scrubber method 31 is used for control, both the opacity standard 32 and the opacity monitoring requirement are waived, and the pressure drop and liquid supply pressure of the scrubber must be monitored instead. 33

The standard proposed and promulgated for lime hydrators limits emissions to 0.075 kilogram of particulate matter per mega-gram of lime feed (0.15 pound per ton). No opacity standard was set. 34 The hydrator standard requires that the electric current and the liquid supply pressure of the scrubbers 35 used to control emissions be monitored continuously.

The standards promulgated for particulate emissions are considerably stricter than the average applicable state regulations already in effect. Plants conforming to the NSPS here would — in the case of rotary kilns — be required to emit less than one-third the particulate permitted under average state regulations and — in the case of hydrators — less than one-sixth the particulate permitted by these regulations. See SSEIS 4-15.

Evidently, EPA had engaged in a dialogue with the NLA concerning the anticipated NSPS for at least a year before the standards were proposed. 36 After publication of the proposed standards on May 3, 1977, 37 EPA received additional written comments both from the NLA and from others and on June 16, 1977 held a public meeting to “provide[] an opportunity for oral presentations and comments on the standards.” 38

*428 Final responses to some of the comments received were issued in a final support statement document in October 1977. 39 The final notice of rulemaking was published March 7, 1978. 40 Except for two minor changes the final standards did not differ from those proposed ten months earlier. 41 A petition for review was timely filed in this court, the exclusive court of review of new source performance standards. 42

III. PREVIOUS REVIEW UNDER SECTION 111

As amended in 1977, section 111 of the Clean Air Act requires the Administrator to prescribe standards of performance for new statutory sources that reflect

the degree of emission limitation and the percentage reduction achievable through the application of the best technological system of continuous emission reduction which (taking into consideration the cost of achieving such emission reduction, any nonair quality health and environmental impact and energy requirements), the Administrator determines has been adequately demonstrated. .

42 U.S.C. § 7411(a) (Supp. I 1977). 43 As the court of exclusive review for NSPS, we *429 have examined section 111 standards on several prior occasions. Portland Cement Association v. Ruckelshaus, 486 F.2d 375 (D.C.Cir.1973), cert. denied, 417 U.S. 921, 94 S.Ct. 2628, 41 L.Ed.2d 226 (1974) (Portland Cement I) (Portland cement plants); Essex Chemical Corp. v. Ruckelshaus, 486 F.2d 427 (D.C.Cir.1973), cert. denied, 416 U.S. 969, 94 S.Ct. 1991, 40 L.Ed.2d 558 (1974) [hereinafter cited as Essex Chemical] (sulfuric acid plants and coal-fired steam generators); National Asphalt Paving Association v. Train, 539 F.2d 775 (D.C.Cir.1976) (asphalt concrete plants) [hereinafter cited as Nat’l Asphalt]; Portland Cement Association v. Train, 513 F.2d 506 (D.C.Cir.), cert. denied, 423 U.S. 1025, 96 S.Ct. 469, 46 L.Ed.2d 399 (1975) [hereinafter cited as Portland Cement II].

These decisions, viewed independently, have established a rigorous standard of review under section 111. We have not deviated from the approach applied to the first NSPS to reach this court. In that case, Portland Cement I, we acknowledged that

[w]hile we remain diffident in approaching problems of this technical complexity, . the necessity to review agency decisions, if it is to be more than a meaningless exercise, requires enough steeping in technical matters to determine whether the agency “has exercised a reasoned discretion.” . . . We cannot substitute our judgment for that of the agency, but it is our duty to consider whether “the decision was based on a consideration of the relevant factors and whether there has been a clear error of judgment.” . . . Ultimately, we believe, *430 that the cause of a clean environment is best served by reasoned decision-making.

486 F.2d at 402 (citations omitted).

In Essex Chemical we reiterated this concept of the court’s role in examining the basis for section 111 standards:

The judgment of the Administrator is to be weighted against his statutory function and limitations, the record searched to determine if indeed his decisions and reasons therefor are themselves reasoned, and at that point our function terminates. Our expertise is not in setting standards for emission control but in determining if the standards as set are the result of reasoned decisionmaking. Yet even this limited function requires that we foray into the technical world to the extent necessary to ascertain if the Administrator’s decision is reasoned. While we must bow to the acknowledged expertise of the Administrator in matters technical we should not automatically succumb thereto, overwhelmed as it were by the utter “scientificity” of the expedition.

486 F.2d at 434. The search for reasoned decisionmaking in a world of technical expertise must continue if judicial review is to have any meaning in the statutory scheme.

Section 111 requires that the emissions control system considered able to meet the standard be “adequately demonstrated” and the standard itself “achievable.” 42 U.S.C. § 7411(a) (Supp. I 1977). We have in the past remanded section 111 standards for the “seeming refusal of the agency to respond to what seem to be legitimate problems with the methodology of the [ ] tests,” Portland Cement I, 486 F.2d at 392; and the limited relevance and reliability of the tests relied upon in support of the standard. Id. at 396, 401. In Essex Chemical as well as Portland Cement I we expressed concern that the standards set might not have been achievable in periods of abnormal operation, e. g., during the “startup, shutdown and [equipment] malfunction” periods that occur in plant operation; and we remanded for further consideration of this issue. Portland Cement I at 398-99; Essex Chemical, 486 F.2d at 433. We have also questioned the significance of tests conducted for purposes of standard development under conditions different from those specified by the regulations for enforcement. Essex Chemical at 436. In analogous review proceedings under other sections of the Clean Air Act and under the Federal Water Pollution Control Act, 33 U.S.C. § 1251 et seq. (1976), this court and other courts have evinced a similarly rigorous approach. 44

However, we think it serves little purpose to elaborate on the standard of review as applied before we explain how, under the general approach required by statute and our earlier decisions, we have evaluated petitioner’s and respondents’ contentions.

The issue presented here is primarily one of the adequacy of EPA’s test data on which the industry standards are based. NLA disagrees with EPA’s conclusion that the standards are achievable under the “best technological system of continuous emission reduction which . . . the

Administrator determines has been adequately demonstrated.” Specifically, NLA claims that the test data underlying the development of the standards do not support the Administrator’s conclusion that the promulgated emission levels are in fact “achievable” on a continuous basis. Promulgation of standards based upon inadequate proof of achievability would defy the Administrative Procedure Act’s mandate against action that is “arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law.” 5 U.S.C. § 706 (1976). 45

*431 IV. ASSESSMENT OF THE OBJECTIONS RAISED BY THE INDUSTRY

Our review has led us to conclude that the record does not support the “achievability” of the promulgated standards for the industry as a whole. 46 This conclusion is a cumulative one, resulting from our assessment of the many points raised by the industry at the administrative level and in this court; 47 no one point made is so cogent that remand would necessarily have followed on that basis alone. 48 In the *432 analysis that follows, common threads will be discerned in our discussions of individual points. Chief among these common threads is a concern that the Agency consider the representativeness for the industry as a whole of the tested plants on which it relies, at least where its central argument is that the standard is achievable because it has *433 been achieved (at the tested plants). The Agency’s failure to consider the representativeness — along various relevant parameters — of the data relied upon is the primary reason for our remand. The locus of administrative burdens of going forward or of persuasion may shift in the course of a rulemaking proceeding, 49 but we think an initial burden of promulgating and explaining a non-arbitrary, non-capricious rule rests with the Agency and we think that by failing to explain how the standard proposed is achievable under the range of relevant conditions which may affect the emissions to be regulated, the Agency has not satisfied this initial burden.

Bearing this initial burden will involve first, identifying and verifying as relevant or irrelevant specific variable conditions that may contribute substantially to the amount of emissions, or otherwise affect the efficiency of the emissions control systems considered. And second, where test results are relied upon, it should involve the selection or use of test results in a manner which provides some assurance of the achievability of the standard for the industry as a whole, given the range of variable factors found relevant to the standards’ achievability.

EPA itself acknowledged in this case that “standards of performance . . . must . meet these conditions for all variations of operating conditions being considered anywhere in the country.” SSEIS 2-6 (emphasis supplied). As set forth in the standards support statement, EPA’s guidelines require data to be assessed with consideration of the “representativeness” of the source tested, including the “feedstock, operation, size and age” of the source. SSEIS at 2-7. Furthermore, the record strongly suggests other factors that may affect the particulate emissions from lime plants. Yet at no point does EPA evaluate the relevance or irrelevance of such factors to regulable emissions; nor does the Agency explain how such factors might have been taken into account in choosing test plant sites or in analyzing the data from the sites it chose.

The critical question presented here is whether the regulated industry, through its trade association, should have borne the entire burden of demonstrating the unreliability for the industry as a whole of the conclusions drawn by the EPA. In this connection we are candidly troubled by the industry’s failure to respond, at a crucial juncture in the standards development process, to the Agency’s invitation to submit data supporting a fundamental industry objection to the achievability of the standard. 50 We would have expected the industry to have been eager to supply supporting data for its position, assuming the “cost” of obtaining such data were less than the “cost” of compliance with a standard that was argued to be unachievable on any reliably repetitive basis for the industry as a whole. 51 We cannot help but wonder if the industry’s failure to supply such data means that the data available or obtained would not be favorable to the industry’s position. Nevertheless we remand because we think, on balance, EPA must affirmatively show that its standard reflects consideration of the range of relevant variables that may affect emissions in different plants.

The showing we require does not mean that EPA must perform repeated *434 tests on every plant operating within its regulatory jurisdiction. 52 It does, however, mean that due consideration must be given to the possible impact on emissions of recognized variations in operations and some rationale offered for the achievability of the promulgated standard given the tests conducted and the relevant variables identified. To facilitate public comment, 53 we think this rationale should have appeared in the Agency’s initial standards support statement.

We must remand to the Agency for a more adequate explanation or, if necessary, for supplementary data to justify the standard in terms of the “representativeness” of the sources tested. The specific doubts generated by our review of the record in light of the lime industry’s attack on the standard are more fully explained below. 54

*435 A. The Particulate Emission Standards

1. Rotary Kilns

EPA tested emissions at six plants 55 before it proposed its mass emission standard for rotary lime kilns. These six plants were selected for testing on the basis of visits to thirty-nine plants, during which the visibility of emissions was observed and information obtained on the emissions control systems employed. 56 The thirty-nine plants were themselves selected because they had been identified as effectively controlled after a review of the literature and contact with industry representatives. SSEIS A-l. The results of the tests of one plant (Plant A) which could not meet the proposed standard were excluded from consideration because the plant was thought not to represent best technology. 57 From what we can gather from the record, three plants were able to meet the standard consistently. 58

Our doubts about the representativeness of the data relied upon are grouped under three subheadings below: Variations in Quantity of Particulate Generated in the Kiln ; Variations in Controllability of Particulate Generated; and Explanation of Discarded Data from Plants A and F. Under the subheading Variations in Quantity of Particulate Generated in the Kiln, we discuss the possible impact on the standard’s achievability of composite dust levels generated by the tested plants and two factors (feedstock variations and gas velocity) that may contribute to composite dust levels. Under the subheading Variations in Controllability of Particulate Generated, we discuss two factors — apart from sheer quantity of dust — that may affect emissions control: coal usage and particulate size. Finally under the subheading Explanation of Discarded Data from Plants A and F, we discuss the EPA’s handling of the results of two tested plants that were unable to meet the standards proposed.

a. Variations in Quantity of Particulate Generated in the Kiln

That the quantity of dust produced in the kilns would affect the controllability of emissions and the achievability of the stan *436 dards does not seem an unreasonable expectation. The Agency, however, appears to have taken conflicting positions on the reasonableness of this expectation and perhaps as a consequence has devoted inadequate attention to several variables which EPA’s own documents and the industry suggest may affect the volume of dust produced in different kilns.

(1) Feedstock Variations

For example, the record suggests that the size and chemical composition of the limestone feedstock used will affect the amount of dust produced.

The MRI Report, prepared for EPA as a prelude to proposal of the particulate emission standards and an important background document considered in developing the proposed standards, 59 stated that “[d]usting in the kiln with the resulting generation of particulate emissions is reportedly a function of the limestone raw material, the rate of rotation of the kiln, and the velocity of the gases in the kiln.” R. 8, 2 (emphasis supplied). This suggests to us that some analysis should have been performed or tests conducted which took into account significant variations in limestone feed, or other variables relevant to dust generation.

The same theme was struck by NLA’s comments on the proposed standard: “No consideration has been given by EPA to variations produced in stone size or preparation, or to the physical characteristics of the stone feed and lime produced, with the resultant variations in the quantity of flue dust to be handled.” R. 103, 10.

The EPA did note in its SSEIS that “[r]otary kilns can handle a range of stone feed sizes between Vi inch and 2Vi¡ inches,” SSEIS 3-6, and that larger feed size generally results in lower dusting in the kiln. See SSEIS 3-14. 60 The Agency also acknowledged that the grade and composition of limestone varies widely across the country. SSEIS 3-1. 61 However, no data on stone size are included by the EPA in the summary data on plants tested (SSEIS App. C) and little information concerning the chemical composition of the feed used at the tested plants is provided. The feedstock at two of the plants tested is characterized as “high calcium lime” (Plants E and F, SSEIS C-6-C-7, and at two other plants as “dolomitic limestone” 62 (Plant C, SSEIS G-3) or “dolomitic stone” (Plant D, SSEIS C — 4). The feedstock at one plant (Plant B) is not described at all. We are, more importantly, left in the dark about which kinds of limestone can be expected to produce the greatest volume of emission dust and what, if any, processing adjustments can be expected of producers using particular kinds of feed in order to achieve the standard proposed. 63 For all we know, *437 the six plants tested could be using kinds and sizes of feed which are representative of only a small segment of the industry spectrum. 64 If that were true the plants may not be “representative” and the regulation might not be “achievable” by the industry as a whole.

(2) Gas Velocity and Operation Levels

According to the MRI Report, quoted above, dust generation is in part a function of gas velocity in the kiln. Gas velocity appears in turn to depend on several factors, including the percentage of capacity at which the kiln is operating. The MRI Report stated that kiln gas velocity has “the most [apparent] effect [on dust generation] when the kiln is operated close to 100 percent of design capacity,” and noted that in one plant studied an increase in production — from 100% to 135% of design capacity — resulted in double the rate of emissions where a reduction from 100% to 75% resulted in only an eight percent reduction. R. 8, 2-3. 65 This seems to mean that at levels close to or exceeding capacity, gas velocity and consequently dust generation increases at a faster rate than at lower levels of production. Thus the level of capacity at which the plant was operating at the time of sampling and the gas velocity would appear relevant to the representativeness of the test data.

Both in this court and at the administrative level the industry has addressed the possible atypicality of the production level of some of the test plants, which it alleges were not tested at full capacity 66 and in doing so, it has echoed a concern expressed by this court in an earlier case. 67

Data on the production level and air flow rate (velocity) at the tested plants were included in the support document filed in this case. 68 These data indicate that the two baghouse-controlled kilns relied upon (Plants B and E) were operating at 111% and 91% of rated capacity, respectively. Plant A, also baghouse-controlled, but whose test results were rejected as unsatisfactory, operated at 92% capacity during the tests. One of the two ESP-controlled kilns (Plant C) was operating at 97% of design capacity and the other (Plant D) was tested at capacity, slightly over capacity, and 86% capacity, but achieved the standard consistently only when operated at less than capacity. The scrubber-controlled kiln, whose test results were discounted in the formulation of the standards, was operated at 95% of design capacity during the tests. Stack effluent flow rate (gas velocity) for the tested plants ranged from a high of *438 180,000 ACFM for Plant C to a low of 48,100 ACFM for Plant E. 69

Having stated that much, however, the Agency did not explain how the range of test results fully takes account of any significant differences in operating conditions in the industry. The support document is totally devoid of analysis of the relevance or irrelevance of operating level or gas velocity to the achievability of the standard, notwithstanding assertions in the EPA’s own contracted-for report 70 that gas velocity bears upon dust generation rates.

(3) Dust Levels at the Tested Plants

The SSEIS asserts, without explaining how the conclusion was reached, that Kilns A, B and E each generated dust at a rate of twenty-two to twenty-five percent (pounds of dust collected per pound of lime produced), 71 higher rates than the average rate of dust generation at the eleven plants for which data were submitted by the NLA. The NLA data, however, indicate a much greater range in dust generation levels than that suggested by the EPA’s test plant figures. R. 103, 13, App. 72 (figures ranging from low of six percent to high of thirty-five percent of lime produced). 72

As laypersons it seems entirely logical to us to suppose that dust generation levels would directly affect emissions controllability, viz., the higher the dust generation, the more difficult the achievability of the standard by the technological control device.

But the exact relationship between volume of dust generated and. the efficiency of the emissions control systems is never clearly stated or explained by the Agency. Instead, the Agency sends us several mixed signals.

On the one hand, the Agency suggests both directly and indirectly that more dust means a more difficult control problem. The direct suggestion is made in the Agency’s rationale for the standard, which states that the two baghouse-controlled test kilns generated “higher [dust levels] than the industry reported average and therefore represent difficult control situations ” SSEIS 8-17 (emphasis supplied). The indirect suggestion is made by the standard itself, which permits higher levels of emissions when larger quantities of feed are being burned, a circumstance under which the production of more dust would be expected.

On the other hand, the Agency asserts that the amount of dust generated is irrelevant to the efficiency of at least one control method and therefore to the achievability of the standard. In correspondence with the NLA antedating the standard’s proposal, EPA stated, 73 “It is generally accepted that outlet dust concentrations from baghouses vary only slightly with changing inlet dust concentrations.” R. 71, 2, App. 57. 74 This statement finds some support in the MRI Report which notes at one point:

*439 The general opinion among the manufacturers of emissions control equipment was that all four types of control systems would be equally tolerant of process upsets leading to short-term heavy dust loadings. In fact, as the dust loadings increase, within a certain limit, the emissions removal efficiency of some of the systems will reportedly increase.

R. 8, 10 (emphasis supplied). However, the MRI Report does not indicate whether long term heavy dust loading or extremely heavy short term dust loading would impair the efficiency of the control system; nor does the report indicate what manufacturers consider to be a heavy or short term dust loading; nor does it indicate on what basis the. manufacturers’ opinion is predicated. 75

Our examination of the record thus yields a conflict: while in one breath EPA appears to acknowledge the relevance of dust generation levels to the proposed standard, in another breath the relevance is denied. In our view, the conflict is not adequately explained, nor is the industry-wide achievability of the standard adequately justified, in light of the acknowledged possibility that heavy dusting creates a more difficult control problem. From what appears in the record, both variations in dust volume produced and its contributing factors received inadequate attention from the Agency in the development and explanation of this standard. 76

b. Variations in Controllability of Particulate Generated

The record points to other variables which were also given short shrift in the stated rationale: the use of coal to fuel the kiln (as it relates to controllability of emissions); and variations in size of emitted particles. The record strongly supports the relevance of coal usage to the efficiency of at least the ESP control method and it also suggests a relationship between particle size and the efficiency of both the ESP and the baghouse control method. Nothing indicates how — if at all — variations in these factors were considered in proposing an “achievable” standard.

(1) Coal Usage

It is clear that the trend in the industry is not only toward coal, but toward high sulfur coal, as other energy sources become scarcer. EPA estimates that by 1986, fifty percent of the lime plant new capacity will have high sulfur coal as the only fuel available. SSEIS 3-5. One-half of all coal used will be between one and four percent sulfur content; the average, as high as three percent. SSEIS 6-6. Moreover, conversion to coal is expected to be a major “modification” that will bring old plants into the regulatory orbit under section 111. SSEIS 5-2-5-3, 8-23. Finally, Congress was especially concerned in passing the 1977 Clean Air Act Amendments that the increased use *440 of coal enter into the Agency’s regulatory approach. 77

However, the impact of high sulfur coal usage on the controllability of particulate emissions under any of the three “best” emissions control systems was not clearly or closely examined by EPA in the development of this standard.

With respect to the ESP system, for example, EPA acknowledged that “precipitability [or efficiency of the ESP method] is a function of the chemical composition of the dust particles and will vary with the different kinds of material that make up the kiln exhaust dust (limestone, quicklime, flyash, calcium sulfate, etc.).” SSEIS 4-6. However, neither of the two ESP plants burned coal, the burning of which will affect the chemical composition of the dust and hence the “precipitability” of emissions. 78

The support document acknowledged:

The tests that were performed on the ESP-controlled kilns are not indicative of normal operation since the current trend in the lime manufacturing industry is toward the use of coal as fuel and the kilns that were tested were fired by oil and natural gas. It is expected that this use of coal would produce a more difficult cont