Coal Cleaning Technology

Coal Cleaning Technology

The National Energy Strategy (NES) and the Clean Air Act Amendments (CAA) of 1990 are examples of the nation’s commitment to a clean environment. The need to utilize our energy resources in an environmentally sound manner presents critical challenges and opportunities for coal preparation during the next decade, particularly regarding the control of SO2 emissions. The principal challenge is the development of cost-effective coal preparation approaches for sulfur removal beyond currently achievable levels. Opportunities are provided in the form of incentives associated with a novel system of SO2 allowances and a policy environment based on the proper functioning of familiar market forces.

An allowance, as defined in Section 402 of the Amendments, is “an authorization to emit one ton of sulfur dioxide.” To achieve the objective of the Clean Air Act Amendments, SO2 emissions will be eventually capped at 8.12 million tonnes annually by controlling the number of allowances issued nationally. By permitting allowances to be bought and sold once issued, the law encourages “overcompliance” as a source of profit for owners of excess allowances.

While many details concerning the administration of allowance trading are yet unwritten, it is reasonable to expect the worth of an allowance to be linked to the “buyer’s” cost of removing a tonne of SO2. That is, to achieve compliance (or to accommodate growth) a potential emitter can 1) bid for additional allowances (assuming their availability) and/or 2) reduce SO2 emissions. The cost of removing a tonne of SO2 depends on many variables, some of which are tractable (e.g., capital equipment, operating and maintenance, etc.), some of which are not (e.g., development of technology improvements/alternatives, economic effects of major domestic and world events, etc.), and many of which are site-specific (e.g., location, capacity, demand, furnace type, etc.). Cost estimates, while ranging from $275 to $1,650 per tonne of SO2, removed, provide a partial basis for gauging the potentlal of advanced coal cleaning as a precombustion compliance technology. A fairer basis would also account for the “value-added” to cleaned coal in terms of improved boiler efficiency and reduced waste. This paper presents the status of certain technologies being developed under the U. S. Department of Energy’s Coal Preparation Program as options for reducing energy-related emissions.

The Coal Preparation Program

DOE’s Coal Preparation Program is implemented and managed by the Pittsburgh Energy Technology Center (PETC), and consists of near-, mid-, and long-range, higher-risk ventures aimed at encouraging the commercial deployment of advanced coal preparation technologies. The mission of the program is to develop the scientific and engineering knowledge base that industry needs to produce economically competitive and environmentally acceptable clean coal products for introduction as market conditions warrant; and, in light of the foregoing discussion, market conditions are warranting such products now more than ever. The program has two objectives. The first is to assure the continued use of coal in traditional applications, i.e., large industrial and electric utility boilers, by dealing with the environmental concerns regarding local, regional, and global impacts of coal use. The second is to open new markets for coal use. Such new markets include: large industrial and electric utility boilers that now use oil; moderate-size boilers/furnaces (new and existing) for commercial, industrial, and cogeneration applications; and new concepts such as coal-fired diesels and turbines for both transportation and stationary power generation. Opening these markets to coal involves addressing not only environmental performance, but also serious technical and economic constraints associated with new end-uses. However, various advanced coal preparation technologies could produce the high-quality coal needed to penetrate these markets.

To accomplish its mission, the program is organized into four components, which, taken together, encompass a broad range of research in biological, chemical, and physical coal beneficiation:

Acid Rain Control Initiative/Compliance Technology (ARCI/CT) projects focus on developing advanced physical coal cleaning approaches in support of the first objective, and will be discussed more fully below;
High-Efficiency Preparation (HEP) projects will also support the first objective by developing low-cost approaches for the removal of more sulfur from coal with traditional technologies, while mitigating the processing and handling disadvantages that inhibit the marketability of coal fines a significant product of such technologies; Premium Fuels Applications (PFA) will support the second objective through the development of ultraclean coal products producing less than 0.26 micrograms of sulfur per Joule (0.6 lbm/MBtu) and less than 0.65 micrograms of ash per Joule (1.5 lbm/MBtu); and,
Technology Base Activities include fundamental studies and exploratory R&D that can contribute to scientific and engineering understanding within any of the above three components and are thus supportive of both main objectives, in the near-, mid- and long- term.

Advanced Physical Coal Cleaning

The Add Rain Control Initiative began in 1988 in anticipation of the potential passage of acid rain legislation. Advanced coal cleaning technologies being developed under the ARCI/CT component rely on extensive liberation of pyrite and ash-producing minerals by comminuting raw feed coal to fine, i.e., finer than 600 micrometers (28 mesh) and ultraflne, i.e., finer than 45 micrometers (325 mesh) particle sizes. At these particle sizes, deep pyritic sulfur removal is possible from moderate- and high-sulfur bituminous coals (the backbone of coal production in the Interior and Appalachian mining regions) using advanced physical cleaning methods, i.e., selective agglomeration, advanced froth flotation, and advanced cycloning. While the former two technologies exploit the differences in surface chemistry between inorganic and carbonaceous matter, the latter relies on the difference in density between these two constituent-types to effect their separation. It is hoped that all three advanced technologies can be demonstrated to meet the technical objectives of 85% pyrite rejection at 85% energy recovery and be regarded as commercially viable for deployment in add rain mitigation strategies by 1993. The developmental status of each is presented below.

Advanced Froth Flotation

Advanced froth flotation technology extends the principles of conventional froth flotation to particles of micron size. Development of the technology under ARCI/CT initiated with fundamental studies by the University of California, Berkeley (UCB) which Investigated the effect of surface control methods (e.g., storage modes, grinding environments, surface modifying reagents) on sulfur rejection and energy recovery. In September, 1988, ICF Kaiser Engineers, Inc. (ICF-KE) was awarded a contract to conduct the engineering development of advanced froth flotation technology. Along with ICF-KE, the project team Includes Babcock and Wilcox, Consolidation Coal Company, Eimco Corp., and Virginia Polytechnic Institute and State University (VPI). These organizations not only provide technical and engineering support, but also an Industrial perspective on how best to commercially apply advanced flotation.

The engineering development project is composed of 16 tasks as shown in Table 1. ICF-KE had completed the first seven tasks at the time this paper was prepared. Task 2 involved the preliminary conceptual design of a 20 t/h advanced froth flotation proof-of-concept (POC) plant. (The POC scale is intermediate to those of pilot plant and demonstration plant). The design and performance specifications for the plant were 1) at least 85% pyritic-sulfur rejection; 2) at least 85% energy recovery; 3) no more than 6% ash in the final clean coal product; and 4) a final clean coal and refuse moisture content of 35% or less. Pyritic-sulfur rejection and energy recovery are calculated based on the run-of-mine (ROM) coal quality.

The preliminary design incorporates several conventional precleaning steps including water-

coal-cleaning task structure

only cyclones, dense-medium cyclones, and froth flotation. The objective of precleaning is twofold: first, to reject as much mineral matter, including pyritic sulfur, as possible prior to fine grinding and subsequent advanced processing to minimize the size of these expensive steps; and second, to produce coarse- and intermediate-size clean coal streams that can be blended with the advanced froth flotation clean-coal product to aid in dewatering, transport, and storage.

A preliminary design schematic of the 20 t/h POC plant is shown in Figure 1. This preliminary design was carefully analyzed during Task 3 to identify areas in which insufficient operation and performance data existed to allow for a high degree of confidence in the detailed design and testing of the POC unit and the subsequent scaleup of the advanced flotation technology. This deficiency analysis indicated that further work needed to be performed in conventional precleaning, fine grinding of the precleaned coal, advanced flotation, and clean-coal product and refuse dewatering. A detailed plan (Task 4) was developed to address these deficiencies during bench-scale (Task 5) and component development (Task 6) testing. Results of Tasks 5 and 6 were evaluated during Task 7 to determine whether or not to proceed with POC design and operation as well as to make changes to the preliminary flowsheet.

The bench-scale tests were carried out on

coal-cleaning froth flotation plant

Pittsburgh No. 8 (Belmont County, Ohio), Upper Freeport (Indiana County, Pennsylvania), and Illinois No. 6 (Randolph County, Illinois) coals. Results showed that all three coals were amenable to precleanlng vis-a-vis the performance targets for ash, pyritic-sulfur rejection, and energy recovery. Precleaning tests were conducted on a semicontinuous and continuous basis at a nominal feed rate of 45 kg/h. As currently envisioned, the POC test unit will receive either a raw coal or a coal that has undergone various stages of precleaning. The ICF-KE POC advanced flotation process will be installed at the Ohio Coal Development Office’s Advanced Coal Cleaning Test Facility (ACCTF) in Beverly, Ohio. This facility will provide coal storage, handling, and crushing, as well as the equipment, such as hydrocyclones and conventional flotation cells, that will be used by ICF-KE to preclean the three test coals.

The laboratory and bench-scale testing confirmed that conventional precleaning unit operations can effectively reject pyrite from all three coals prior to fine grinding. Employing hydrocyclones (WOC) and/or dense-medium cyclones (DMC) and conventional froth flotation (CONV.FF) on plus 350 micrometers (48-mesh) and 350 x 75 micrometer (48 x 200 mesh) size fractions can accomplish high pyrite rejection at low energy losses. Precleaning was also shown to effectively reduce the amount of material that would need to be processed (including grinding to minus-200 mesh) in the advanced flotation (AOV.FF) unit operation. Further, the use of precleaning can produce coarse refuse and clean-coal-product streams that will help minimize dewatering and disposal requirements.

As part of Task 5, ICF-KE also conducted an evaluation of several advanced flotation processes. As a result, ICF-KE selected the microbubble column technology developed by VPI for POC testing and scaleup. A schematic diagram of the VPI column is shown in Figure 2.

The advantage of incorporating precleaning into an advanced flotation circuit is illustrated in Table 2. As can be seen, precleaning of the Upper Freeport coal rejected 57.5% of the pyritic sulfur and resulted in only a 4.5% loss of energy value.

The weight of feed to the advanced column was thereby reduced to 48% of the raw coal. The advanced circuit rejected an additional 24.2% of the raw coal pyrite with a concomitant energy loss of 5.3%. Overall, the advanced flotation cleaning circuit achieved an 81.8% pyritic-sulfur rejection and a 90.2% energy recovery based on the raw coal quality. Similar results were achieved with the Illinois No. 6 and Pittsburgh No. 8 coals.

Preliminary cost estimates completed by ICF-KE for a commercial advanced froth flotation plant cleaning 450 t/h of Illinois No. 6 coal show a processing cost (including precleaning, but not the cost of coal) of $27/t. The estimated annual cost for removing an equivalent tonne of SO2 by this process is $305.

ICF-KE will complete installation of the 20 t/h advanced flotation POC unit at the OCDO facility in early 1992. Testing of the POC circuit will begin in mid 1992 and will be completed in late 1992.

Selective Agglomeration

A contract for the engineering development of selective agglomeration technology was awarded to Southern Company Services, Inc. (SCS) in June 1989. Supporting SCS are Southern Electric International, Inc., Alberta Research Council (ARC), Praxis Engineers, Inc, and the Electric Power Research Institute (EPRI). Cofunding for the project is also being provided by EPRI. The project task structure is similar to that shown in Table 1.

Agglomeration is a process in which a liquid immiscible in water is used to “bridge” hydrophobic (oleophilic) coal particles while leaving the mineral matter particles dispersed in the aqueous medium. High-shear agitation is needed to displace the water on the surface of the coal with the agglomerating liquid, which is typically a heavy oil such as diesel or No. 6 fuel oil, or a light hydrocarbon like heptane or pentane (e.g., the Otisca T Process). During the high-shear step, the organic matter is separated from the pyritic sulfur and other mineral-matter components. Hence, it is often referred to as the “cleaning” step.

Southern Company Services investigated several approaches to selective agglomeration. Many of the

coal-cleaning flotation circuit

laboratory and bench-scale studies were conducted at ARC’s facilities in Devon, Alberta, Canada. An initial task was to narrow the list of viable agglomerating liquids. Based on batch experiments and technical, economic, safety, and health factors, two liquids were selected, heptane and diesel oil. Further testing of these liquids established ranges for key operating conditions such as oil-to-coal ratio, mixer speed, and residence time in the high-shear mixer for each of the test coals. Like ICF-KE, SCS will test Pittsburgh and Upper Freeport seam coals, but will use a precleaned Kentucky No. 9 (Webster County, Kentucky) coal in place of the Illinois No. 6. which is characteristically similar. Representative analyses for the test coals are shown in Table 3 below.

coal-cleaning-selective-agglomeration

A critical step in the agglomeration process involves the recovery of the high-shear agglomerates. For a diesel system, low dosages of oil (about 1%-2% by wt.) are used during high-shear, resulting in the formation of fins (less than 100 micrometer) “microagglomerates.” The low dosage of oil is necessitated by two factors: 1) the relatively high price of heavy oil and 2) the difficulty and expense of recovering the oil if it were to be used at higher concentrations. Because of the fine size of the microagglomerates, traditional methods of recovery, such as screens, are ineffective. Instead, flotation is used to recover the microagglomerates. For the heptane process, the light oil is added in the range of 20% to 30% by weight (depending on the coal), and screens are used to collect the agglomerates from the high-shear mixer. The high oil-to-coal ratios require that the heptane be recovered and recycled back to the process. Vendor tests with samples of heptane agglomerates indicated that fluidized-bed and rotary-tray driers could be used to recover the light hydrocarbon.

The results of 5 kg/hr continuous bench-scale high-shear agglomeration tests are shown in Table 4. While direct comparison is confounded by topsize differences, on the whole, little difference in energy recovery and pyritic sulfur rejection can be discerned between the heavy (diesel) oil tests and the light (heptane) oil tests.

Southern Company completed a technical and economic feasibility study of three agglomeration process alternatives. Two of the processes utilize low levels (approximately 1%) of diesel in high shear, but differ in the method used for product size enlargement. In the first option, size enlargement is typically accomplished through low-shear agitation with a viscous petroleum binder such as asphalt. However, pelletization technology was also considered as a second option because of its potential to require less binder and produce a more handleable product. The third alternative involved high-shear agglomeration with heptane and the addition of heptane and asphalt during low shear. Two variants utilizing diesel and heptane agglomeration with no product size enlargement were also investigated.

The three process alternatives were evaluated against eight selection criteria. These included: 1) technical feasibility; 2) process performance; 3) product handleabllity and storage; 4) product acceptability; 5) process economics; 6) reliability; 7) environmental factors; and 8) operational and ancillary considerations. The feasibility of scaling up all three process options (including the two not involving final product enlargement) in the near term was judged good. In addition, based on the bench-scale test results as illustrated in Table 4, all were expected to achieve equivalent performance in terms of energy recovery and ash and pyritic-sulfur rejection.

As part of the evaluation, a commercial scale 200 t/h agglomeration plant conceptual design was generated for each of the three alternatives. A preliminary economic assessment for each plant design based on the Upper Freeport coal showed cleaning costs ranging from $22 to $25 per tonne of clean-coal product. On the basis of $-per-tonne- of-SO2-removed, the costs ranged from $460 to $990.

Based on technical and economic evaluation, and on the results of the laboratory, bench-scale, and vendor tests, the diesel pelletization process has

coal-cleaning-performance

been recommended by SCS for POC testing. From SCS’s perspective, this alternative represents the most technically mature agglomeration process option, having the best economics and promise for commercial application in the mid-1990s.

The low-moisture pelletized product should be handleable using conventional utility equipment. The effects of oil price escalation, one of the major risk factors associated with agglomeration, are minimized by the low amount of oil (1%) and asphalt (1%) used in the recommended process. In addition, the low oil and asphalt concentrations reduce the potential for volatile organic emissions, odor problems, and contaminated runoff from product storage piles.

The diesel-based 20 t/h POC system will be installed at a coal research facility in Wilsonville, Alabama. Much of the coal handling, conveying, crushing, grinding, high-shear agglomeration, and dewatering circuitry for the POC unit has been installed. Installation and shakedown of the entire POC test module will be completed in early 1992, with operation conducted throughout the remainder of that year.

Advanced Cycloning

The significant density difference between the organic and mineral constituents of coal is the fundamental separating principle for most conventional coal cleaning processes. In particular, a medium of specific gravity intermediate to the two constituent types is used in dense-medium washers and cyclones to separate the lighter organic material (float/overflow) from the heavier mineral matter (sink/underflow). The dense-medium is generally a suspension of fine, high-density particles (e.g., magnetite or sand); the density of the medium varies directly with solids concentration. The separating performance and downstream recovery of the dense-medium suspension requires that the difference in size between dense-medium particles (i.e., magnetite) and coal particles be significant. This size difference diminishes with conventional dense-media as the quest for maximum liberation of pyrite from coal is pursued through particle size reduction. Advanced cycloning seeks to extend the density-based separating principle to fine and ultrafine coal through the use of alternative media such as heavy liquids, and the application of centrifugal force to accelerate separation by gravity. It should be noted that a suspension made of micronized magnetite, a process patented by researchers at PETC, was dropped from consideration as part of this evaluation when Custom Coals International, a joint venture between Genesis Research Co. of Carefree, Arizona and Duquesne Light Co. of Pittsburgh, Pennsylvania announced a plan to pursue the commercialization of an advanced cycloning process based on such a medium.

Previous work in advanced cycloning was conducted by Process Technology, Inc. (PTI) under DOE auspices in cooperation with EPRI. Its work showed that Illinois No. 6 coal with a topsize of 75 micrometers (200 mesh) can be cyclonically separated at 45 kg/h using an organic liquid, methylene chloride, to achieve 83.2% pyrite rejection at 87.7% energy recovery. Coal Technology Corporation (CTC) was awarded a contract in September 1990 for engineering development of an advanced cyclone process based on the best available medium that could achieve the grade/recovery goals of ARCI/CT. CTC’s project team consists of ICF-KE, PTI, and Intermagnetics General Corporation. The project task structure is shown in Table 5, and work is proceeding in three phases: literature review and laboratory-scale studies, bench-scale performance testing, and POC-scale plant design and operation.

coal-cleaning cyclone process

Nearly 50 candidate media were considered for evaluation. Evaluation proceeded in three steps: initial screening based on environmental, economic, and engineering considerations; final scoring based on industry surveys and laboratory-scale test results; and selection based on performance testing. The criteria used to screen and score candidate media were the following:

  • ability to achieve/adjust specific gravity
  • physical and chemical properties
  • health, safety, and environmental implicatlons
  • cost and availability
  • anticipated separation performance
  • anticipated handleability of clean coal and refuse
  • anticipated adverse cleaning plant and boiler impacts
  • anticipated ease of medium recovery and regeneration

Laboratory-scale tests were designed to provide a basis for scoring under the “anticipated” criteria. The screening/scoring process produced three finalists for further evaluation during performance tests at 450 kg/h with commercially available separating devices including a small-diameter/high-pressure cyclone, a disk stack centrifuge, and a dynawhirlpool-like device.

The finalists, i.e., methylene-chloride/perchloroethylene mixture, calcium nitrate, and magnetically enhanced ligno-sulfonic acid each represents a class of heavy media: organic liquids, aqueous solutions, and colloidal suspensions. Each class presents its own set of challenges for medium recovery and reuse ranging from simple mechanical dewatering to multistage distillation. The efficiency and economics of medium recovery schemes are crucial to selecting the medium upon which preliminary conceptual plant design is based, since these factors affect several other evaluation criteria (i.e., handleability, separation performance, medium availability, end-user impacts, as well as environmental, safety, and health).

As in the other ARCI/CT process development projects, the preliminary conceptual design will provide the basis for selecting equipment to be assembled into a POC-scale test circuit. Given the straightforward scaleability of cyclonic processes, the test circuit, while only 450 kg/h, is considered the POC equivalent to the 2-3 t/h test circuits called for in advanced froth flotation and selective agglomeration. The test circuit is scheduled for construction at CTC’s industrial site in Bristol, Virginia, in the spring of 1992 with operation to take place throughout the summer and fall. Test results will be instrumental in finalizing the conceptual design of a 3 t/h advanced cycloning demonstration plant. This conceptual design is a major project deliverable.

Summary

The national commitment to reducing SO2 emissions is clear in policy and law. Moreover, the Clean Air Act Amendments of 1990 feature an allowance system that encourages competition among technologies on the basis of cost-per-tonne of SO2, removed. Three precombustion technologies are being developed under DOE’s National Coal Preparation Program for potential deployment in SO2, mitigation strategies by 1993. The lowest preliminary cost estimates for removing an equivalent tonne of SO2 from moderate- to high- sulfur bituminous coals by these technologies are $305 for a commercial-scale advanced froth flotation process, and $460 for a selective agglomeration process using heavy oil. (Preliminary estimates have not yet been made for advanced cycloning). These preliminary figures suggest that the precombustion approach to sulfur removal is competitive with current post-combustion technologies that are variously estimated to cost between $275 to $1,650 per tonne of SO2, removed. Products from precombustion approaches, however, have additional value associated with reduced mineral matter content, e.g., improved combustion efficiency and reduced waste disposal and transportation costs.

coal-cleaning microbubble column

a status of advanced coal cleaning as a compliance technology