IGNITION AND IGNITIBILITY HORACE C. PORTER 1833 Chestnut Street, Philadelphia, Penna.
Courtesy, Phzladelphia Euenino Bulletin
FIREI N GRAINAND FEEDWAREHOUSE, WHICH MAY H.4VE BEEXCAUSEDB Y SPONTANEOUS IGNITIOS
A
SERIES of determinations has been made (Table I) of the relative ignitibilities of a number of solid materials under certain chosen conditions. That chemical structure, as well as physical, in a combustible solid should influence its reactivity toward oxygen and the acceleration rate of this reaction with temperature is forecast by the known effects of chemical structure on reactivity in general. This work shows, by an experimental method which minimizes certain disturbing factors, that ease of ignition in solids is closely related to the presence of particular types of chemical structure. The open and long-chain structure of paraffin wax is much more easily ignitible than the closed-ring structure of naphthalene. Similar in import is the finding that wood, peat, lignite, and class B bituminous coals (all of which contain either humic bodies or lignin with structures comprising openchain groupings, 1) are of greater ignitibility than semibituminous coal, anthracite, and the cokes, whose structures are more largely made u p of multiple rings. They range themselves roughly in the order of their humic or lignin-pluscellulose content. Cellulose itself, as in loose cotton or paper pulp, is very high in ignitibility, conforming to its known chemical structure, but the influence of physical state may outweigh that of the chemical. As between two sugars, a dextrose with an aldehyde end group was shown to be more easily ignitible than the disaccharose, sucrose, without an aldehyde group. The data show also that the ether or bridged linkage of oxygen in the molecular structure tends to promote ease of ignition, as in phenolformaldehyde resins and the lignin of peat and wood. Among liquids, ethyl ether and carbon disulfide are of a high order of ignitibility (7).
If carried out within certain limits of temperature, moderate charring and partial decomposition by heat (low-temperature carbonization) increase ignitibility, as in wood charcoal, peat char, and the low-temperature caked product of bituminous coal (under 450" C.). This phenomenon is probably due to the setting up of anhydride-type linkages or other groupings sensitive to oxygen attack a t weak structural points. Peat char, in hydrogenation under pressure, has been found ( I O ) to retain good reactivity in comparison with the raw peat. Carbon black, deposited from natural-gas flames a t moderate temperature and containing some volatile matter, is of moderately low ignitibility, but not so low as that of anthracite or high-temperature coke, possibly because of a smaller ring structure with some attached side groups. I n coke making in by-product ovens and gas retorts, the bulk of the charge is heated to 950" C. or higher, which gives a highly condensed multiring structure with probably no side-chain or interconnecting groups of a type which would lend ease of ignition, such as are present in the intermediate chars and semicokes. Ignitibility in cokes, therefore, depends on the temperature a t which they are manufactured. Oil-coke formations, as in cracking-still operations, are the more easily burned away, the lower the temperature a t which they have been formed. In general, the ignitibility of coals increases with the increase of oxygen in the dry, ash-free material (that is, with decreasing rank), but the differences are small within the group of class A bituminous and semibituminous coals. Although not included in the series of tests here reported, i t has been found that iron sulfides, probably containing persulfides, are of a high degree of ignitibility when formed on t h e
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interior surfaces of oil tanks or gas purifiers; their chemical structure may involve bridged sulfurs.
gines) of various chemical types offer a means of relating chemical structure to the ignitibility characteristic.
Nature of Ignition
Test Method
Newall (6) used a method for determining rates of propagaIgnition (of solids, as well as of gases and liquids) is best tion of combustion in solid fuels t h a t had some points of simidefined as the initiation of self-supporting and self-propagab larity to the ignitibility method used here, but he did not deing combustion, with accompanying glow or flame. I n the termine relative ignitibilities. Moore and Sinnatt ( 5 ) and Case of SolidsJignition is assured when the rate of heat producMoore (4) used for comparing solid fuels in their combustion tion by oxidation of a given surface area begins to exceed the characteristics a method in which a small sample of 200rate of loss of heat to the surroundings. The ensuing autogemesh coal, laid on a hearth in a furnace, was heated gradually nous temperature rise then brings the material to the glow through its glow point and beyond in a current of oxygen. or flame point (ignition) a t a rate dependent upon its coeffiThe differences between this method and the one used here cient of oxidation-acceleration with temperature, its exposed are obvious. surface per unit weight, and the rate of oxygen supply or air renewal upon that surface. Our method was chosen with a view toward minimizing disIgnition of gases and of liquids involves direct attack of turbing factors, such as preliminary decomposition and oxidation oxygen on molecular or near-molecular subdivisions, each surrounded with air. Various studies indicated that reactions TABLE I . RELATIVE IGNITIBILITIES OF SOLID MATERIALS(IN OXYGENAND in gases may be propagated by active particles of ON 60-80 MESHMATERIAL UNLESS OTHERWISE NOTED) some sort (possibly intermediate oxidation prodDecreasing Amp. Apgox., % Order of Required of urning ucts) and that the diffusion of these particles Ignitiin in, 2 bility Material IgniterMin. may be as important a factor as heat transfer. 1 Cellulose nitrate in airbjc 2.85 0 combustion in air, containing 77 per cent 37 Special tests: Dextrose deposited on asbestosbm 1.95 .. .. .. nitrogen by weight, the importance of air 2.15 Sucrose, he osited on asbestoabsn movement and a blowing effect becomes evident 1.72 Pittsburgh {jtuminous coal, under 80 meshk ... Pittsburgh bituminous coal, 80 mesh, tested in from consideration that a t least 11.5 pounds of 1.80 1 atm. Oz ... gaseous products of combustion for each pound Higher amperage indicates higher temperature, although not in direct linear relation. A t e s t on the melting of pure K S O s indicated t h a t 2.00 amperes gives about 325-335O C. on of solid burned must be moved away from the surface of sample. surfaces before combustion can proceed. b Sample not sized between 60 and 80 mesh. c Plasticized b y evaporating t o dryness a solution of nitrated cotton in acetone and breakFor testing “autogenous ignition temperaing u p into small pieces (over 80 mesh). d Burning was so rapid as t o fuse t h e igniter wire. tures” of liquids, the laboratory of the National e Ashless filter paper, twisted strips. Board of Fire Underwriters developed an emf Commercial Bakelite containing filler. B Sample of “Textolite” containing a fibrous filler. pirical method which has been adopted as h Air-dry a t room temperature. contains about 40% volatile. i A hard, dense, nonfibrous for&, prepared from wet paper pulp.by compressing in screw standard by the American Society for Testing press, dipintegrating t h e cake, boiling in water, a n d a ain ressing. This was repeated Materials and the American Petroleum Instiseveral times. T h e pulp waa then dried, broken u p a n d size$ between 60 and 80 mesh. i Fine commercial gas black was wet, compact$ as much a s practicable, and dried; tute ( 7 ) . The conditions used in that test (as this was repeated several times. Crumbles easily. k One sample from Warden mine, Allegheny G o . , Penna., of 35-36% volatile matter, and in the test on solids to be described) are mainanother from Belmont Co., Ohio (No. 8 seam), both giving same test for ignitibility. T h e tained constant but do not duplicate those special testa a t bottom of table, on -80-mesh size, show t h a t the finer size gives a considerably higher degree of ignitibility, b u t t h a t the excess pressure of oxygen (above atmospheric) met in practical utilization of the materials; makes only a small difference. I Commercial lov-temperature coke, made b y t h e “Carbocite” or Wisner process, in the the test is for comparative purposes only. The early trial plant at Canton Ohio Underwriters’ Laboratory (7) has reported relam Commercial low-tem ekature’coke made b y International Coal Carbonization or “KSG” process in trial plant at &e, Brunswick, N. J. tive ignition temperatures of a large number n These tests were made under t h e special condition of depositing on asbestos strips a thin coating of sugar from water solution. and drying for 5 or 6 days over sulfuric acid. It of liquids, some of which are referred to later. was found difficult t o ignite them when laid in a loose train a s powder, owing t o t h e quick Such values for liquids and the known pera n d easy melting b y low heats i n t h e igniter. formance (as i n i n t e r n a l c o m b u s t i o n e n 0
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of the sample during the period prior to ignition and the effect of the supplying of extraneous heat in relatively large amount. With this objective, the method applies a small igniter to the surface of the material, in one spot only, and raises its temperature until self-propagating combustion is initiated. This is done in oxygen, under moderate pressure in a closed bomb, t o accelerate the reaction and make the test more sensitive. Ignition temperatures mere not determined, but only the required current in the electrically heated igniter wire t o indicate relative ignitibilities. Table I gives the results of the tests. The sample was approximately air-dry, crushed, and sized between 60 and 80 mesh; 0.4 gram was laid in a train, 42 mm. long and 2-3 mm. deep, on asbestos in a nickel boat without compacting and was supported in the center of a calorimetric bomb of 400-ml. capacity containing a little water. Oxygen was admitted, without displacing air, up to 50 pounds gage pressure (the gage had been checked previously by measurement of volume of gases released from the bomb). The atmosphere in the bomb was calculated to be 76-78 per cent oxygen. The igniter was No. 36 platinum wire, 21-22 mm. long, looped so as to allow 5-7 mm. of contact with the surface of the sample. I t was heated by 110 volts of direct current at the source, passing through a rheostat in series and a lamp bank in parallel (three 100-watt carbon lamps). The current was left on for 10 seconds in the igniter; ignition was detected by the development of heat in the bomb (not immersed in water). Any fluctuation of voltage caused an error by variation of the te perature in the igniter with equal current strength. Good contact of igniter with sample was essential. The oxygen present was equal to 4.4-4.5 times the weight of sample, or 120-180 per cent of the theoretical requirement, so that in those few cases (of the slower burning materials) where observations were made, complete combustion was accomplished when sufficient time was allowed after ignition. The precision attainable by this method was indicated by an average variation in duplicate runs of about 0.03 ampere from the mean. Relative rapidity of burning was noted, merely as an approximation, by observing the extent of the combustion in the train of sample after one minute following ignition, plus a second minute after releasing the excess pressure of oxygen (column 3, Table I).
Ignition Temperatures Temperatures of the igniter corresponding to the different current strengths were not determined; an indication a t one point was obtained by determining that the current required to melt pure potassium nitrate (melting point 335" C.) in the 50-pound oxygen atmosphere was 2.00 to 2.04 amperes. The temperature of the igniter was probably somewhat higher because of the latent heat of fusion and other cooling effects. Pittsburgh seam coal, in graded 60-80 mesh size, required 1.93-1.97 amperes for ignition. Therefore its ignition temperature under these conditions would seem to be about 300310" C., if latent heat effects were not greatly different from those of the melting salt. This coal, in a mixture of finer sizes (passing 80 mesh), required only 1.72 amperes for ignition, which may be approximately equivalent to 260-280" C. RIoore and Sinnatt (4, 6 ) on a 200-mesh-size bituminous coal (with 38 per cent volatile content) in an oxygen current in a furnace held at 225" C. found a glow point of 240-250" C. The use of finer sized coal and of extraneous heat supply no doubt had a part in giving a lower ignition point than that indicated here for Pittsburgh coal. For Pittsburgh seam coal, of graded 14-28 mesh size, heated in a rapid air current during a preliminary period of about 50 minutes, Jones and Scott (a) found an ignition temperature of 327" C., for wood 270-290", and for paper "packing" 228". (The conditions of the author's method, avoiding preliminary alterations, would tend t o give lower values.)
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greatly as to ease of ignition by their physical state. illthough cotton, paper, and the common woods, for example (and synthetic resins of the phenolic type), are among the most easily ignited materials when in light fibrous form, in thin sections, or in a finely comminuted state, their ignitibility is much less when they are in massive or compacted form. Relative behavior in ignition, however, when in comparable physical state serves to indicate effects of chemical structure; some of these typical effects have heen mentioned in the introductory paragraphs. A test method for relative ignitibility of solids which allows free play to the unassisted heating of the unaltered material by its own oxidation, so as to reach the point of self-propagating active combustion, is the type of method best suited to characterize the material in this respect. Although the ignitibility of cokes becomes less, the higher the temperature used in their manufacture, it is possible to improve the ignitibility characteristic of high-temperature coke by having even a small part of the product made a t a relatively low temperature (below 650" or 700" C.). Relative burning rates of some of the materials tested (column 3, Table I) are instructive. The lightly charred Pittsburgh bituminous coal (item 19) burned completely in 2 minutes; the raw coal (item 24) burned only 60 to 80 per cent. The low-temperature cokes (items 2.5 and 26) burned more rapidly than the Pittsburgh coal; lignite and the Illinois bituminous burned more rapidly than the Pittsburgh. An activated carbon (item 30), probably made from coke or anthracite by regulated heat treatment in steam, burned rapidly (over 95 per cent in 2 minutes); the raw hard anthracite or the oven coke burned only 30 to 50 per cent. Soda-treated anthracite (item 31) burned much more rapidly than the untreated coal. Carbon black, although not easy to ignite, burned rapidly (over 90 per cent in 2 minutes). PROPOSED MECHANISM OF IGNITION I N SOLIDS.The indications of various links between ignitibility and molecularstructure types such as oxygen linkages, open or closed chains, long chains, etc., lead to the supposition that, for oxygen attack upon the molecule, these particular types of structure offer vulnerable points a t which rupture possibly first occurs, accompanied or followed by immediate union of the nascent opened bonds with oxygen. Macromolecular structure or intermolecular arrangement, as in the thread molecule of fibril cellulose, may also play an important part in facilitating oxygen attack.
Literature Cited (1) Fuchs, W., "Die Chemie der Kohle", pp. 24 ff., 472, Berlin, Julius Springer, 1931. (2) Jones, G. W., and Scott, G. S., U. S. Bur. Mines, Rept. Investigations 3468 (1939). (3) Lewis, B., and Elbe, G. von, J.Applied Phys., 10, 344-59 (1939). (4) Moore, B., Fuel, 10,205 (1931). (5) Moore, B., and Sinnatt, F. S., Ibid., 4,194 (1925); 5,377 (1926). (6) Newall, H. E., Ibid., 14,34 (1935); 17,260, 292 (1938). (7) Nuckolls, A. H., "Underwriters' Laboratory Method for the Classification of the Hazards of Liquids", Natl. Board Fire Underwriters Lab., Chicago, 1929; Am. SOC.Testing Materials, Standards on Petroleum Products D-286-30; Am. Petroleum Inst. Standard 522-30. (8) Price, D. J., and Brown, H . H., "Dust Explosions", pp. 40-1, Boston Natl. Fire Protection Assoc., 1923. (9) Taylor, G. B., Porter, H. C., and White, E. C., U. S. Bur. Mines, Rept. Investigations 2306 (1922). (IO) Warren, T. E., Bowles, K. W., and Gilmore, R. E., Fuel, 19, 72 (1940).
Conclusions These data are not to be taken as indicating relative fire hazards since the materials, as used in the arts, are influenced
PamsmNTmn as part of the Symposium on the Combustion of Solid Fuels before the Division of Gas a n d Fuel Chemistry at the 98th Meeting of the American Chemical Society, Boston, Mass.
