Along with the dissemination of design information, the Council also evaluated the effect of oxidation at various mills employing black liquor oxidation. Better than 90 per cent reduction of H2S was shown to be effected on all tested furnaces which did not exhibit excessive losses from the firing zone. The benefits of black liquor oxidation with respect to ambient sulfide levels were shown in the results of a 2-year testing program conducted by a member mill with the cooperation of the National Council. The curves shown in Figure 2 indicate the rate of decrease of sulfides at various distances from the mill when half and all the black liquor were oxidized. Although uncontrolled foam was a serious problem in attempting weak black liquor oxidation, some laboratory mass transfer devices which were designed to minimize foam were evaluated. This research was described in Technical Bulletin No. 165. Fundamental absorption rates were measured at low turbulence in thin film devices which were successful in controlling foam. These, however, were found to be impractical since too much surface area was required for effective oxidation. In a recent study, the foaming tendency of weak black liquor has been put to some use in a proposed foam fractionation process to increase soap yields. 3. Absorption and oxidation of sulfur compounds To determine whether any alternatives to burning odorous gases for their ultimate destruction might be feasible, methods for absorption and chemical oxidation came under investigation. Chemical streams inherent in the kraft process, such as chlorine bleach effluents and dilute caustic solutions, were obvious possibilities, since this chemical is effective in absorbing H2S and methyl mercaptan. Laboratory and field studies have shown that the absorption rate is dependent on the pH of the scrubbing solution, a stream with a pH of less than 11.0 being no more effective than plain water. With the use of streams of pH near 14.0, the absorption rate can increase 20- to 30-fold. The effectiveness of an existing scrubber is increased accordingly. The use of caustic in scrubbing mercaptans is of limited odor control value due to the formation of odorous disulfides in the presence of oxygen. These reaction products will be stripped out of the exhausted scrubbing liquor unless handled in a closed system. The use of chlorine in acid and alkaline solutions has been studied and found to be effective in the oxidation of H2S, methyl mercaptan, and dimethyl sulfide. 10 FIG. 2. EFFECTS OF BLACK LIQUOR OXIDATION ON REDUCIBLE SULFUR GASES In acid media, reaction is slow, but a stable, innocuous end product results. Reaction of reduced S compounds with chlorine at alkaline pH levels was shown to be more rapid. However, methyl disulfide, an insoluble, odorous compound is formed by partial oxidation and stripped out of solution. Field evaluation of existing odor control scrubbers is currently under way, and the findings will be summarized in a technical bulletin to be distributed shortly. Fundamental studies on other chemical oxidants and oxidation catalysts have been initiated and will be extended as benefits are shown. 4. Particulate emission control Previously, reference was made to the work of Dr. Ingraham in preparation of the material for Technical Bulletin No. 12. It suffices to say that this comprehensive review illustrates that the air pollution problem associated with fuel burning and related to particulate discharge from such a cause, is usually amenable to reasonably precise definition and solution. This is in contrast to the knowledge that has been available on reducing particulate emissions from recovery furnaces beyond that achieved in conventional control devices. The Council's West Coast Research Center has been active in this area since initiation of the air pollution program at Oregon State. Interest in this area stemmed from the need of local mills to comply with existing ambient air quality requirements for particulate matter. After cooperation with two mills on pilot studies of secondary scrubbers behind the primary recovery units, an evaluation of field installations was initiated. Seven such units now exist or are under construction, handling flows of from 60,000 to 450,000 cfm. The objectives of this survey are set forth below: (1) To determine effectiveness on particulate discharge control. (3) To compile corrosion problems involved and determine suitability of promising materials of construction. The effectiveness of particulate control is being measured by stack sampling, and in most cases can be correlated with ambient-air survey data collected prior to scrubber installation. Energy and water requirements are related both to the economics of operation as well as efficiency. Due to high corrosion rates in early scrubber installations, the suitability of more resistant materials now in use, such as tile and fiberglass, as materials of construction are being documented. Stack survey and ambient air data confirm that these devices are capable of reducing salt cake fallout in the mill vicinity to a minimum, well within existing ambient air quality requirements. They are also capable of meeting the most rigorous source emission regulations now in force. These devices are not, however, effective in reducing gaseous sulfur emissions unless the pH of the scrubbing solution is above 11. Pilot and one full-scale field investigation has shown that 80 per cent of H2S in relatively high concentration are removed with caustic use amounting to approximately 125 lb/ton pulp produced. 5. Meteorology and ambient air sampling techniques A significant part of the Council's effort in air pollution work has been related to means of defining the conditions that can, or do, exist in the ambient air surrounding mill environs. Those areas of principal interest where work has been done and technical bulletins have been issued are listed below: (1) Meteorology. (2) H2S tile study. (3) Techniques for high-volume sampling. (4) Methods for conducting ambient air surveys. The application of techniques for collecting and tabulating meteorological data set forth have constituted the basis for several sampling programs, either now in existence or since terminated. There is a need for continuous monitoring of H2S in some areas and work was undertaken to evaluate the simple lead acetate tile technique for this use. The research showed that uncontrollable environmental factors limit the effectiveness of the method as a quantitative analytical tool. A recent bulletin describes sampling and analytical techniques for measuring suspended particulates in the atmosphere. This permits interested mills to collect information and compare local conditions with those in the 250-station, national air-sampling network operated by the Public Health Service. It may also define a mill's contribution to local situations. Councils staff has integrated the most desirable meteorological and ambient air sampling techniques for routine ambient air surveys into bulletin form. is in the process of being updated at the present time. 6. Cooperative mill service activities This Service activities in air pollution by the regional engineers cover a wide spectrum and are increasing. Typical of these are those listed below: (1) Collection, compilation, and dissemination of information. (3) Training of mill personnel in sampling and analytical techniques. (4) Conference with members on control measures as well as data interpretation. 7. Staff technical activities Previous factors of the report have described those programs and activities which have as their main purpose generation of new knowledge in the fields of emission measurement and control technology, and pollutional effects of our atmospheric emissions. There are several additional technical areas calling for increased activity on the part of our staff. These involve (a) the continuing inventory of new emission control measures, their costs, effectiveness, and application; and (b) the continuing review of existing and proposed control regulations as an aid to member mills involved in cooperative efforts to develop equitable control codes at the state and local levels. (a) Continuing review of emission control measures Traditionally, the National Council has played an important role as a clearing house of stream-improvement technical information, combining compilations of current practice with technical analysis to identify important process parameters, and to indicate which practices are worthy of extension. To an increasing degree, we are being called on to play this same role in the air pollution areas as more mills adopt control measures or begin active evaluations of alternative control systems. In this respect, mention should be made of a number of such studies performed in the areas of black liquor oxidation, eletrostatic precipitation ratings, lime-kiln, venturi-scrubber performance, total sulfur emission, and the impact of particulate emissions on ambient air quality as measured by fallout rates and suspended particulate levels. We are now completing an industry-wide enumeration of control practices employed at kraft mills and are preparing reviews of heavy black liquor oxidation practice and secondary scrubbing of recovery furnace stack gases. Some preview data is presented in Figure 3. Black liquor oxidation_ Smelt tank demisting___ Non-condensible oxidation_. High efficiency fuel burning stack devices_-_ (b) Continuing review of control agency regulations We have been continually active since 1956 in providing information on proposed air pollution control legislation, particularly that involving the setting of standards of emission or of ambient air quality. Such legislation has prompted increased activity on the Council's part in evaluating existing and proposed regulations. In such states as Pennsylvania and South Carolina this has recently taken the form of: (1) Determining what levels and forms of emission control will be required to achieve compliance with proposed regulations. These frequently include limitations on sulfur concentration in exist gases as well as total emission weight per unit of material processed. This inventory information is of great value in answering questions. (2) Indicating under what conditions certain forms of regulation are more pertinent than others (i.e., limits on emission vs. changes in ambient air quality). (3) Indicating the extent and type of monitoring efforts that will be required to demonstrate compliance with various regulations. We expect to see a steady growth in the Council's efforts in these two directions which provide both a balance to our total program and a necessary link between our own investigative efforts and the mills' continuing progress in air pollution control. C. IDENTIFICATION OF RESEARCH UNITS 1. Odor evaluation and non-sulfur odor sources The most significant need is in the area of evaluation of source and control. While the sulfur compounds have been given credit for the typical kraft odor, little has been done in attempting to chemically characterize other compounds present in small concentrations, which are contributing factors, by the use of chromatographic isolation and infra-red spectrophotometry. Chemical characterization of odor may have merit over subjective testing by humans, whose response is variable. It is, however, becoming obvious that at least some subjective testing in connection with chemical characterization may be desirable and a necessary adjunct to study programs where various control techniques are being evaluated. 2. Role of direct contact evaporation and black liquor oxidation in odor emission and control Considerable emphasis has been placed on the role of the contact evaporator as a source of H2S emission. With present black liquor oxidation practices this component can be substantially reduced or virtually eliminated at this source. This has reduced but not eliminated the odor problem, and at least three facets of black liquor exidation are deserving of further attention. These include the following: (1) Refined chemical characterization of gaseous effluent components present with and without black liquor oxidation. (2) Relationship of degree of black liquor oxidation and sulfur emission or odor emission. (3) Relationship of high-degree black liquor oxidation, odor threshold, and refined chemical characterization. The first of these is under study at the present time. It has been demonstrated that the rate of H2S evolution is inversely proportional to the degree of black liquor oxidation. There is no information to indicate whether organic sulfur or other odorous compound evolution follws a similar pattern. Since the organic sulfurs may constitute the bulk of the sulfur loss at this source, this aspect is deserving of attention. There is considerable speculation concerning the merits of indirect evaporation in controlling odor emission. The issue is by no means resolved and at least one major manufacturer is piloting a unique air-to-air heat exchanger for this purpose. Without exception the merits of indirect evaporation are based on the premise that odor emission from a furnace is, or can be, relatively nonexistent. This, to our knowledge, has not been demonstrated by subjective odor tests nor have sulfur losses from the furnace per se been adequately documented using the more refined analytical techniques now available, such as chromatography. The actual role of the recovery furnace in sulfur and odor emissions under normal load should be established in order that indirect evaporation can be properly evaluated as a pollution control device. 3. Absorption and oxidation of odor compounds Beyond the conversion of some sulfur compounds to more stable forms or major process changes, such as elimination of contact evaporation, the ultimate control of odor rests almost entirely on oxidation of malodorous compound to ones of lower odor threshold or preferably having no unpleasant odor. A multitude of odor sources exist which may ultimately require treatment. These include those listed below: (1) Recovery furnace stacks of kraft and semi-chemical pulp mills. (2) Digester relief and blow gas. (3) Evaporator non-condensables. (4) Black liquor oxidation tower off-gas. (5) Condensate scrubbing off-gas. (6) Washer room vents. (7) Lime kiln stack. This is a wide array of point-source emissions, varying both in intensity and volume. All, however, are presently under scrutiny to varying degrees by regulatory agencies, and five mills are now required to report loads from two or more of these sources on a routine basis. (a) Chemical absorption and oxidation The present status of knowledge and work in progress in the area of chemical oxidation or absorption with particular reference to chlorine and caustic was described earlier in the report. Their limited effectiveness suggests the need for further evaluation of other oxidants such as iron, ozone, and permanganate as well as oxidation catalysts such as oxides of nitrogen. The need for development of a highly effective oxidant is increasing as the older decentralized kraft mills initiate pollution abatement at widely dispersed points. This need is not, however, confined to older mills, since it is impossible to devise a highly refined control scheme, short of thermal oxidation in the furnance or auxiliary equipment, which does not encompass chemical oxidation. (b) Thermal oxidation Thermal oxidation in theory is the most positive means of reducing the threshold odor level of sulfur compounds as well as many other organics to a minimum. Cursory laboratory observations and sulfur measurements on lime kiln exhausts where non-condensables are being burned confirm this theory. Its use as an odor control measure on recovery stack gas may seem premature. Its effectiveness cannot be overlooked, however, and subjects such as minimum temperature requirements for oxidation of odorous compounds as well as compilation and evaluation of heat recovery schemes should be a portion of the research program. 4. Summarized odor research needs Three areas of study have been set forth as probably being the most productive in getting at solutions to the odor problem. These include: (1) definition of full capability and limitations of black liquor oxidation; (2) evaluation of the effects of major process changes, such as elimination of the stack evaporator; and (3) evaluation of chemical and thermal oxidants. While these cover a wide spectrum, all are of major significance. 5. Particulate emission Data tend to point to the fact that in most cases adequate means are available at present for gross particulate matter control from power boilers, as well as the bulk of the recovery operations. There exist localized situations where questions are arising concerning the role of particulate matter as nuclei in fog formation. The most significant lack of knowledge in particulate removal lies in the means to reduce discharge from stacks behind venturi recovery units to a level commensurate with that obtainable where precipitators are employed. The question of relationship of particulate matter fallout and corrosion, particularly paint damage, arises on occasion. No controlled experimental data are available relating such factors as amount of salt cake, humidity, and time of exposure on corrosion rate of metal surfaces or paint. Constructive work could therefore be done in this area. 6. Control of SO2 emissions Finally, the increased attention being given to sulfur dioxide emissions and the sulfur content of fossil fuels suggests that we analyze our position in this matter. We are conscious of the implications, and future developments may require more emphasis on researching means of controlling sulfur dioxide. ATTACHMENT 2 INDEX OF NATIONAL COUNCIL FOR STREAM IMPROVEMENT RELEASES ATMOSPHERIC POLLUTION-TECHNICAL BULLETINS Number 1. Sampling and Analysis of Air-Borne gas- Number 4. Application of Instrumentation to Pulp Mill Number 5. Methods for the Determination of Sulfur September 24, 1957. November 20, 1957. March 24, 1958. March 31, 1958. September 29, 1958. April 13, 1959. |