Analysis of the high-temperature particulate collection problem (original) (raw)

Particulate agglomeration and separation at high temperatures and pressures are examined, with particular emphasis on the unique features of the direct-cycle application of fluidized-bed combustion. The basic long-range mechanisms of aerosol separation are examined, and the effects of high temperature and high pressure on usable collection techniques are assessed. Primary emphasis is placed on those avenues that are not currently attracting widespread research. The high-temperature, particulate-collection problem is surveyed, together with the peculiar requirements associated with operation of turbines with particulate-bearing gas streams SUMMARY The context of the present analysis is particulate separation and collection in the high-temperature high-pressure environment associated with direct-cycle application of fluidized-bed coal combustors. The problem of high-temperature collection has been actively studied for at least four decades without a wholly adequate solution. The 15-year effort of the Locomotive Development Committee of the Bituminous Coal Research Institute was directed toward developing a cyclonic device and represents the most comprehensive known investigation. In contrast to this work, most of the current research activity is committed to utilizing the granular bed concept. Recent surveys of this problem published by Stone and Webster for the Electric Power Research Institute and by the Midwest Research Institute for the U.S. Environmental Protection Agency have also generally favored filter and bed-type collectors. One of the central difficulties in evaluating collector concepts is the lack of an adequately precise and realistic turbine specification for allowable inlet concentration as a function of particulate diameter. The Aerospace Research Laboratory of Australia has concluded after 25 years of study that, for relatively low-tip-speed rotors, particulate smaller than 5 to 7 um diameter does not cause significant erosion, even in relatively high concentrations. However, domestic turbine manufacturers who are accustomed to high-rotor-speed turbines have concluded that the concentration is critical even for relatively small particles and have established concentration specifications which are only slightly above the ambient level of an urban environment. Thus, the current performance envelop for a collector system requires cleaning from ca. 40 000 mg/m 3 at the bed outlet down to ca. 1 mg/m 3 at the turbine outlet. I 2 There are two basic kinds of separation potentials: external field and streamline modification. None of the six known concepts employing external field separation appear to be usable in the envisioned environment of 1000*C and 10 atmospheres. Separation by streamline modification can be accomplished by either perimeter devices, which rely upon intense centrifugal velocity fields, or by distributed devices, which accomplish separation in two stages-first by precipitation onto an obstructing media and then deprecipitation of the media in a regeneration step external to the collector. Currently, the concept of distributed collection is receiving the primary attention of researchers. 4 PART I. ENG I NER I NG RELEVANCE AND A SURVEY Of PEUTIfUT RESEARCH A PRIMER ON AEROLS This section serves as a brief primer on aerosols and .s a glossary for terms used throughout the report. All micrometer dimasimis quoted will refer to particle diameter unless otherwise noted. The terms "high-twperature" and "high-pressure" are Intended to men approximately 1000'C and 10 stn (1 IFa), respectively; the phrase "hot-gas conditions" is usually employed as an abbreviation for a "hig-tsmerature, high-pressure sa 'roment." Symbole which refer to particulate propert ies are designated ! the ese of a tilda: e.g., # is used for particulate velocity. A relatively lomg-lived distribution of very mail solid or liquid particulate in a gaseous madma is called an "aerosol,"t and the field of study dealing with the comined motian of the particulate and medim is called the "Nechanics of Aerosols." The removal of the particulate from the =adim, termed "collection," is but one of the specialiaatians of this very diverse field of engineering and is based upon both the science of colloids (within the branch of physical chemistry) and fluid mechanics. A bibliography of the literature of aerosols is presented in Dppndimes 1t 2 and is taboo largely tron a recently pshished hbmook on the subject. e Aerosols are classified by the aniformity, origin, and size of the dispersed particulate. If the particulate is very nearly of P single sise, it is termed "momodisperse" as distinct from *Volydisperse" (a spectrum of sises). Aerosols can originate in tuo cmyst (1) by "dispersion," which is a purely mechanical process (sueh as grinding or pulverising) and tends to create large (>1 mm), irregularly shaped, polydisperse particulate chanically identical to its parent or (2) by "codemsation," which is a physico-cbemical process (such as combustion, coasmsation, or sublimatian) fomaing small particulate (tI om) which may be spherical, m disprse, and e. mically dif-ENGINEERING RILBYANCI The design of collection schemes for aerosols is an old and well-estab-Uised art. Aerosol collection has been widely applied for both material recovery and envirinmsntal protection purposes as the final step before discharge under virtually every kind of industrial process at low temperature (20*C ard less) and low pressure (atmespheric pressure 22). TWo new problems presently require satisfactory solution: fines removal and particulate removal in a bot-gas enirimnmt-and the possibility of both requirements have to be met simaltaneously. Since the current U S. ktvirinmatal Protection Agency (1P) NOW Source Ferforsance standards for particulate matter, 0.1 1W 106 Stu, do not distingpesh between rocks and W-om smoke, the fines removal requirement Ti treamlates to approximately 140 ms/m 3 (0.06 grainlft) at standard osmitles for the caob stion of coal at 132 excess air. APPENDIX 1. Bibliography-Periodicals Whose Scope Includes Aerosols (taken from Ref. 1