He also gives the yield of gas as about 240 cu. ft. per hundred pounds of wood or roughly 8,000 cu. ft. per cord. The fuel value of the gas from 100 Kg. of wood is computed by Klar to be equivalent to 2.59 Kg. of coal with 5,000 "available" calories per gr., or the gas from one cord (3,300 pounds of dry wood) would be equivalent to 85 lbs. of coal of 9,000 "available" B.tu. per pound. Klason2 shows the gas from small scale experiments to contain more methane and less hydrogen than is given by Klar and the figures in Table 13 also show more methane, but Klar's figures may be correct for commercial samples of wood gas.
Bergstrom and Wesslen 8 give the following data on the gas obtained from air dry softwoods in Swedish ovens heated by flues within the oven:
Heavy hydrocarbons Carbon monoxide ..
They also give the yield of this gas as 18 per cent by weight on the dry wood distilled or about 4,600 cu. ft. per cord. It seems likely that the figure for methane in this table is too high, since it would require nearly all the methoxyl in the original wood in order to give this percentage of methane in the gas, and it has already been shown that other products of distillation beside the gas contain high proportions of methoxyl.
The 1.5 per cent "vapors" shown as part of the gas constituents are made up of all the volatile constituents of the two other groups, pyroligneous acid and tar, the most volatile constituents naturally being present in the higher proportion. This point will be discussed further under refining methods.
The pyroligneous acid is a very complex mixture, only a few of whose constituents have been identified. It contains from 80 to 90 per cent of water, of which part is the moisture in the wood distilled and part (about 20 per cent of the weight of the dry wood) is formed, by the decomposition of the wood. Following is a list4 of the products which have been reported, not including those found mainly in the*tar and occurring only in small quantities in the pyroligneous acid:
Pyromucic acid Methyl alcohol Allyl alcohol Acetaldehyde Furfural Methylfurfural Acetone Pyroxanthen Methyl formate Methyl propyl keton.e
Formic acid Acetic acid Propionic acid Butyric acid Valeric acid Caproic acid Crotonic acid Angelic acid Methylamine Isoamyl alcohol o-Methyl ¿9-keto-penta-methylene
* Muspratt, Vol. Il, p. 1869, aB given by Klar, loc. «(., p, 51» and Fraps, Am. Chm. Jour. »5, a6-sa.
Methyl ethyl ketone Ethyl propyl ketone Dimethyl acetal Methylal Valero lactonc Methyl acetate Pyrocatechin Ammonia Isobutyl alcohol Keto-pentamethylene Methyl pyridine
To these should be added formaldehyde as shown by Klason (see page 56).
Acetic and formic acids, methyl and allyl alcohols, acetone and methyi acetate are the most important and occur in greatest quantities, being the only ones separated or recognized in the refining process except for the "alkaline constituents" separated by acid treatment in refining the alcohol. Except for some of these important constituents there are no quantitative data on the composition of pyro-ligneous acid.
Pyroligneous acid also contains from 7 to 12 per cent of soluble tar but this will be discussed under the composition of the settled tar, since it is probably made up mostly of constituents the same as those occurring in settled tar. Certain heavy oils are also contained in solution in the pyroligneous acid6 but they too are made up of constituents occurring mostly in the tar. Pyrocatechin in the list above is one of these and should not be mentioned in preference to many other phenolic bodies which also occur in both the tar and the pyroligneous acid. After the pyroligneous acid has been distilled from the non-volatile settled tar and neutralized with lime other non-volatile compounds are produced as shown by the insoluble "sludge" and by the soluble material which remains as an impurity in the acetate of lime. The composition of these non-volatile products in the neutralized pyroligneous acid is unknown and the compounds from which they may have been formed can only be guessed at. Di-acetyl has been identified in the products of resinous wood distillation 0 but very likely it is a product of the lignocellulose and occurs also in hardwood distillates. This is a compound which would distil with the pyroligneous acid and on neutralization would polymerize to an insoluble product, xylochinon, which would contaminate the acetate. The organic imparities in the acetate are probably farmed from similar complex di-ketones or aldehydes which polymerize on neutralization.
# The tar is a very complex product of which only a few constituents have been identified. It consists of all the substances previously mentioned as constituents of the pyroligneous acid together with "light oils," "heavy oils" and pitch. The light oils (with boiling points below 140° C.) have been studied by Fraps 7 and a series of acids and ketones have been identified. A complete list of identified substances is given below:
.Hawley, Chem. & Mat, Eng. g$, 197 (ipai). Aschan, Zeit. fOr. angew Chem. J907, p. 1811. Am. Chem. Jour. 35, 36-53,
Valeric aldehyde Acetone
Methyl ethyl ketone Methyl propyl ketone Methyl butyl ketone Adipic ketone Nitriles
Di-methyl furane Unsaturated compounds m-xylene
Methyl acetate Methyl propionate Methyl n-butyrate Methyl n-valerate Esters of unsaturated acids Sylvane
Di-ethyl ketone Tri-methyl furane Toluene
Hydrochloric acid addition products
Few quantitative data are given and no indication of what proportion of the oils is made up of unidentified constituents. Probably there is a large proportion of less reactive constituents, such as hydrocarbons, which have not been identified.
The heavy oijs (heavier than water with boiling points above 200 0 C.) have been rather thoroughly studied so far as the alkali soluble constituents are concerned. The following list is taken from Beilstein, all the phenols being included which are there mentioned as having been found in wood tar;
Phenol Di-methyl ether of homopyrocatechin
0-, m- and p-cresol Coerulignol
Besides most of this list ethyl guaiacol is mentioned by Fraps.8 No quantitative data on single compounds are available, but from the boiling points and gravities a rough grouping of the constituents can be made. (1) Phenol and the cresols occur only in very small quantities because the alkali soluble constituents of wood tar begin to boil very close to 200° C., thus excluding phenol and ortho cresol while the gravity of the fractions in the vicinity of 200° is higher than the other cresols, showing that they can occur only in small proportion. (2) The largest portion of the alkali soluble constituents boil between 200° and 2350 C. and, therefore, includes phlorol, the xylenols, guaiacol, cresol, and homopyrocatechin di-methyl ether. (3) The rest of the phenols mentioned boil above 240° C. and constitute the second largest group in total amount. The alkali insoluble constituents of the heavy oils have not been studied, but probably contain the higher members of the series found in the light oils such as ketones and "unsaturated compounds," together with the higher phenol methyl ethers.
1-3 xylenol-5 1-3 xylenol-4 Pyrocatechin Guaiacol
The composition of the pitch is unknown. A part of it is probably formed by the polymerization of complex aldehydes and ketones, and it has been suggested that part of it is of the nature of the aldehydephenol resins, since both aldehydes and phenols are among the products of wood distillation. Klason has shown, however, that there is little pitch in the primary products of distillation and that most of it must be a product of the decomposition of the primary tar. We would, therefore, expect it to consist largely of the higher members of the groups found in the oils, namely, ketones, methoxyl ethers of the phenols and possibly hydrocarbons.
A typical hardwood tar contains about n per cent water (and water soluble constituents separating with the water on distillation) and 12 per cent light oils with gravities below 1.0. The change from "light oils to 'heavy oils" takes place as the temperature of distillation reaches about 180° C. (measured in the vapors). The proportions of heavy oils and pitch vary with the conditions of distillation and the consistency of the pitch. If, for instance, the oils are distilled only to a temperature of 240° C., 25 per cent heavy oils will be obtained and ^Le pitch residue wiH be very soft. If.the distillation is carried to J s S Ut m hlSest that can be obtained without decomposition) the heavy oils wi 1 amount to 42 per cent and the pitch will be harder. If a current of air is run through the still to help carry over
KZ* Snt hfavy ?ils are ^ined and the residue is a hard brittle pitch. By coking the pitch a maximum of about 58 per cent heavy oil can be obtained. 0 p
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