GAS-L: Torrefied Wood (TW)

["arcate" <arcate@email.msn.com>]

Hello Tom Reed and Gasification:

Here is something good re wood distillation:

http://getnet.com/~dickg/nmra/sigs/RR_Industries/Industry-Lineside.html

Wood Distillation - A Chemical Industry
By Stan Knotts

The chemical industry is interesting and has had little coverage as far as I
can recall. Robert Margel's request (see mailbag) has given me the impetus
to get something done on this. One chemical industry that was ubiquitous in
the early history of our country was the wood distillation industry -
usually for production of wood alcohol. Ample supplies of good quality
hardwood was about all that was required for wood distillation. This was
often a by product of the lumber industry. This article draws heavily on a
1923 book by the American Chemical Society with more modern information from
various other sources such as "The Wood Chemical Industry in the Delaware
Valley" by Frank D. Meyers III, 1986.

Very early plants (up to the 1920s) were based on small horizontal
cylindrical retorts or brick by-product kilns which were extremely labor
intensive. The principal product of these kilns was charcoal with low yields
of chemical products. Early iron furnaces and foundries were major customers
for charcoal. When the chemical products became more important, no more kiln
plants were built.

The wood used for distillation was generally cordwood 52" long with a
minimum diameter of 3 to 5 inches. Some sawmill slabs or other wood wastes
were also used. That made a wood distillation plant a natural companion
industry to a sawmill. The wood was dried for 6 to 18 months either in the
woods or in a storage yard at the plant. Artificial drying methods were
later developed using waste heat from the flues of the retort furnaces to
shorten the drying time and better control the moisture content.

The old and inefficient kilns were replaced by large rectangular retorts or
ovens by 1920. These retorts were made of steel and averaged 6' high, 8'
wide, and perhaps 56' long for a standard 10 cord oven. The ovens were
heated at one or both ends. Brick arches were installed under the ovens to
distribute the heat evenly and prevent direct contact of the flames on the
bottom of the oven. Flue gases were guided around the sides of the retort by
suitable baffles, finally passing over the top and then up the stack.

The wood is loaded up on iron cars or "buggies" with slatted ends and sides,
the latter sometimes reaching only about 2/3 of the height of the piled wood
and serving to retain the charcoal after carbonization. The sides are
removable for loading the wood and unloading the charcoal. The cars very
nearly fill the cross section of the retort and are long enough to hold
about 2 1/2 cords, four cars making one charge for a 10 cord retort. The
cars were sometimes moved by hand, but more likely with a power cable or a
small locomotive.

As soon as the wood is inside the retort, the doors are closed and the
heating started. The first distillate is almost entirely water and it is not
until about the 4th hour that the liquor slowly darkens and contains
increasing amounts of acid. The tar first appears at the 8th to 10th hour
and from this time forward, very little more heat is required to finish the
distillation at about the 22nd hour. For about 2 hours the retort is allowed
to cool and then the charge is removed and a new charge of wood run in,
making a complete 24 hour cycle.

The best layout for the wood yard, retort house, coolers, and charcoal sheds
is one in which the progress of the cars is in one direction throughout the
operation. In some cases lack of space makes this layout impossible and the
retorts may be charged and discharged at the same end. It is almost a
necessity, however, that the first coolers be in line with the retorts so
that the hot charcoal may be moved rapidly.

The vapors are taken from the retort through one to three outlets, depending
on the size of the retort. Ordinarily only two outlets are used, even on the
10 cord retorts. The vapor outlets are on the sides of the retorts near the
top and pass through the brickworks to the condensers. The condensers are of
the vertical tubular type.

Refining Processes

The liquid distillate, consisting of tar and a dark brown watery liquid
called pyroligneous acid, is pumped to wooden settling tanks where the tar
settles to the bottom. The tanks are usually arranged in series with
connecting pipes at gradually lower levels from the first, where the where
the mixture of tar and acid enters, to the last where the clear acid is
drawn off for further refining. The tar is drawn as required from the
bottoms of the tanks.

Although most of the valuable products, wood alcohol and acetic acid, is
contained in the pyroligneous acid, yet there are small quantities contained
in the tar which is distilled with steam for their recovery. A wooden still
is used for this operation since copper is so rapidly corroded by the hot
tar.

To recover the acetate, a thick milk of lime is used for the neutralization
using a covered wooden tub and a stirring apparatus. The neutralized settled
solution contains the acetic acid in non volatile form and the alcohol and
other volatile products can be separated from the acid by distillation. The
final drying of the acetate takes place on drying floors above the ovens
where the heat of the flue gases can be used. When the acetate material is
dried, it is bagged for shipment.

The wood alcohol was distilled into a concentrated crude alcohol which was
usually shipped to large refineries where the complicated refining process
could be carried out.

Later Processes

By the 1930's, sawdust and sawdust briquettes had generally replaced cord
wood, but the industry was dying due to the advent of synthetically produced
chemicals. The advantages of the sawdust was that it was cheaper, could be
more efficiently dried, and it could be handled in a continuous process.
These new processes necessitated new machinery and methods of for
decomposing the wood materials, but resulted in a shorter cycle with greater
yields. Some wood distillation plants survived into the 1940's.

Wood Distillation Products

Charcoal was used heavily in early blast furnaces for production of iron. It
was also used in the manufacture of black powder, as an ingredient in stock
and poultry food, in case hardening compounds, and as a deodorizer. The
acetic acid was heavily used in the chemical industry and in the preparation
of white lead pigment. It was also used to prepare cellulose acetate for
films, lacquers, plastics, and artificial silk. It was used in the
preparation of numerous solvents and in process of textile plants,
laundries, tanneries, insecticides, soaps, and many other industries. The
wood alcohol was used in the manufacture of varnishes and lacquers,
formaldehyde and synthetic resins such as Bakelite. The wood tar was used in
preservatives, pitch and creosote.

Modeling

For model railroads representing the eras from the late 1800's though the
1940's a wood distillation plant would provide a small chemical plant with
interesting rail traffic. Incoming would be cordwood and outgoing would be
bagged acetate of lime and tank cars of wood alcohol and tar. The use of a
wood lot and a small industrial rail line for the unique iron cars for
processing the wood through the drying cycle would be different from the
usual run of the mill industry.





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