Wood is a biomass fuel

Wood is a biomass fuel, meaning it originates from living matter. Fossil fuels also originate from living matter but have been chemically changed and concentrated. Both burn with the same basic net chemical reactions:

C + O2 > CO2 + heat

2H2 + O2 > 2H2O + heat

Wood bums in three stages. First, the moisture in the wood is driven off. Wood contains a significant amount of water. It ranges from less than 10 percent to more than 60 percent on a wet basis. During the drying phase, the temperature of the wood remains at a constant 212°F as the water absorbs energy and is converted to steam. There is a significant energy loss during this stage. The water also takes the place of combustible material, further reducing the net energy released from a pound of wood.

The second stage of combustion takes place as the wood is heated, and the complex dry wood molecules break down. From 60 to 80 percent of the dry wood is evolved as combustible gasses which burn when mixed with combustion air.

The final stage of burning is the oxidation of the carbon left after the volatiles have been driven off. This carbon is referred to as fixed carbon. It is, in effect, charcoal composed mostly of elemental carbon mixed with inorganic ash. The carbon is oxidized as oxygen somes in contact with the exposed surface of each fuel particle.

There are several methods of measuring the energy potential, or heating value, of a fuel. Each method is defined differently and yields a substantially different value. The 3ROSS HEATING VALUE (GHV) is the total heating potential of a unit of wood as delivered and reflects the displacement of fibre by water present in the fuel. The HIGHER HEATING VALUE (HHV) is the maximum potential energy contained in a sample of dry fuel. The NET HEATING VALUE (NHV) considers the energy lost to the evaporation and superheating of the water in a sample of wood fuel. This energy loss accounts for most of the lower efficiency of boilers burning wet wood compared to dry wood or fossil-fueled burners. Higher heating values for wood and bark generally range from about 7,900 to slightly more than 10,000 BTU/lb.

The moisture content substantially reduced the energy available from a unit of wood fuel. It has the added effect of reducing flame temperature. If the moisture content exceeds 67 percent, there is not enough energy available to maintain the ignition temperature, and the fire will not be self-sustaining. Net heating values of wood fuels range from approximately 3,500 BTU/lb to 8,000 BTU/lb.


R. C. Farley1 and M. Zeemont1

Gasification dates back to the mid-19th century and interest, research and development, and use of gasifiers increased during World War II, particularly in Europe. Gasification development in the United States has generally lagged behind similar work in Europe, but this technology was used in American cities for "gasworks" in the 1930s.

The wood gasification process can be summarized by the following steps:

(a) Some of the latent chemical energy in wood is released by combustion to produce sensible heat.

(b) Most of that sensible heat is reconverted to latent energy by evaporating water, by breaking down and volatilizing complex wood molecules, and by reducing CO-2 and H2O to produce CO and H2.

(c) The reaction products pass out of the gasifier vessel as a relatively cool gas with an energy value of from 100 to 900 BTU/scf depending upon the gasification process used. This gas can be stored, burned, or used as a chemical feedstock external to the gasifier.

There are three major methods for converting solid biomass fuels into gaseous fuels:

(a) AIR GASIFICATION in which air is fed into the gasifier to partially oxidize the biomass and generate heat producing a low-energy gas with a heating value of from 100 to 150 BTU/scf.

(b) OXYGEN GASIFICATION in which pure oxygen is used in place of air to react with biomass yielding a medium-BTU gas (250 BTU/scf or more).

(c) PYROLYSIS in which biomass is heated in the absence of air, oxygen, or any other gas. This process produces a higher energy gas (600 BTU/scf or more).

Air gasification has the most immediate commercial potential for use with industrial boilers. Three types of air gasifiers appear most viable at this time. Those are updraft, downdraft, and fluidized-bed gasifiers. There are currently about a dozen gasifier manufacturers in the United States. Energy outputs of units range from 60,000 to 50 million BTU/hr.

Cost data on gasification systems are largely site specific. Since few of these units have been put into industrial operation to date, there are no firmly established data on capital investment and operating costs. However, the principal theoretical economic advantage of gasifiers is that they can be retrofitted to existing gas- and oil-fired boilers, thus saving some of the costs associated with switching to new wood-fired boiler systems. Also, they are expected to produce clean combustion products similar to the output of oil burners.

Source: Decisionmaker's guide to wood fuel for small industrial energy users - Michael P. Levi, Michael J. O'Grady, Solar Energy Research Institute, North Carolina State University. Extension Wood Products Section - Google Books
Address : http://books.google.com/books?id=6AIK-vcrqKwC&pg=PA2&dq=wood+gasification&output=text#PA2

Generator gas has been used extensively since the middle of the 19th Century in the iron working industry for the firing of furnaces. A Swedish design which first attracted attention was the gas generator invented by Gustaf Ekman at Lesjofors and named "Ekman's Coal Shaft Furnace." [21] It was described in Jernkontorets Annaler 1843 (Annals of the Swedish Ironmasters Association, 1843). The "downdraft principle" (gases passing down through the firebed) of this generator is, on the whole, the same as that used for most gas generators for fueling engines. The first proposal for the use of generator gas for engines appeared in 1877 and the engine was run for the first time around 1881. It was used for stationary operation, and because the gas is sucked by the engine through the generator, the gas was usually called "suction gas." The first gas generator specifically constructed for stationary engine operation did not come into existence until the beginning of the 20th Century. Something similar, however, was described in an English patent as early as 1859.

With the outbreak of war in 1939, the use of generator gas for vehicular operation increased, and the head of the Swedish Board of Trade, Axel J. Enstrom, proposed the Swedish name "GENGAS,"* which soon was accepted in this field. Gas for firing industrial furnaces, however, is still usually called generator gas.

Not until around 1920 were there portable gas generators worth mentioning which could be used for motor vehicles although experiments had been carried out much earlier. [55] At this time, portable gas generators were attached to trucks as well as to tractors. There, was interest in France in this development, and the firm of Panhard & Lavassor manufactured the first gas generator for practical use. Another French design was the Gohin-generator, which can be considered the forerunner of generators not enclosed in brick and with water vapor added. In Germany there was also great interest in generator gas operation; the best known is Imbert's gas generator for wood.

In 1918 Axel Swedlund of Sweden designed an updraft charcoal generator, and in 1924 the first of his downdraft designs was manufactured. [58] During 1923 and 1924, a few Austrian charcoal gas generators of Julius Heller's design were imported to Sweden and tried on trucks, buses, and railcars. These updraft gas generators produced a gas with a rather high tar content, which was difficult to remove. Although the best available beech charcoal was used, during the first long drive (625 kilometers) [50] the truck motor used for the experiment had to be taken out after about 300 kilometers and thoroughly cleaned of tar. Trial runs during 1925 to 1926 showed that starting and driving using only charcoal gas was possible, but not convenient. For example, starting directly on coal gas was time consuming and troublesome even when the fire in the gas generator had been lit

*In English, approximately "wood gas," "generator gas," or "manufactured gas," but there is no exact equivalent.

beforehand. In those cases where there was a starting fan it was hand operated and not sufficiently effective. Generally, no modifications had been made to the engines, which had relatively little power even when run with gasoline; and when run on generator gas, down-shifting of the gears was required on even moderate inclines. Therefore, gasoline was preferred for starting and warming up the engine; gasoline was also used to increase the climbing ability of the car when necessary.

By the end of the 1920s the first charcoal gas generators were made in Sweden. They used a downdraft design and were mainly intended for farm tractors, and were soon followed by designs for trucks and cars. At the same time, an interest developed in wood as a gas generator fuel, and the first wood gas generator in Sweden was designed by the Widegren brothers and A. B. Svenska Flaktfabriken (The Swedish Fanfactory, Inc.). This gas generator used a downdraft fire and was double jacketed with the rectangular section enclosed in brick. This wood burning gas generator was tried out on trucks in the early 1930s, but there was considerable tar residue deposited in the motor and corrosion of the crankshaft necks. Since the results of the experimental operations did not match expectations and there was a lack of consumer interest, the work was not pursued.

At the Swedish Riksdag (Parliament) in 1930, bills were introduced to support generator gas operation of motor vehicles; and in the following year a government committee was appointed on the initiative of Ingeniorsvetenskapsakademien (The Swedish Academy of Engineering Sciences) to perform scientific and practical experiments with generator gas driven vehicles. The committee received a government grant of 15,000 kronor*, and experiments were carried out with three different types of gas generators.

In 1932 the Riksdag (Parliament) appropriated 200,000 kronor ($53,600 U.S.) for a loan fund for car owners who wished to install gas generators. In addition, the vehicle tax was reduced to half that imposed for operation on liquid fuel. The vehicle tax reduction amounted in practice to 33%, due to the weight of the gas generator. These measures led to a rapid increase of generator gas operation in 1932, and the total number of generator gas operated cars in Sweden during the first half of 1933 reached about 250; practically all of these cars were equipped with charcoal gas generators. This increase, however, was followed by a drastic decrease mainly due, on the one hand, to the fact that many hastily produced gas generators were introduced on the market by inexperienced manufacturers and proved to be seriously defective; and on the other hand, to the fact that in the transition to generator gas operation, insufficient attention was paid to whether the vehicle and type of traffic in question were actually suited for generator gas operation. To this must be added the shortage of personnel knowledgeable in generator gas technology, imperfect service, and the difficulty of obtaining suitable charcoal for which there was no organized distribution.

The disappointment with generator gas operation was undoubtedly justified in many cases because the technical conditions for satisfactory operation were not met. But even in cases where the conditions for good results existed, the generator gas operation met with resistance among most consumers. It was quite natural, of course, that to those concerned with driving and car upkeep, the more convenient and cleaner operation with

*The exchange rate (January 4,1932) was one kr equalled $0,268.

liquid fuel was more attractive than was generator gas operation. It was also understandable that the big oil companies did not welcome new competition.

Another factor of importance was that the generator gas operation was not necessitated by an obvious emergency but was supported by motives less apparent to the average man—reasons that had to do with military preparedness, forestry, and national economy. However, a small number of car owners, who either drove their own generator gas cars or encouraged their hired chauffeurs with bonuses, continued with generator gas operation and had good results, especially when they were able to solve the fuel problem by burning charcoal from their own forest.

Military authorities showed an interest in generator gas as early as 1925 by buying the first Swedish truck adapted to such operation, which was tested with both charcoal and wood gas generators. In 1933, at the initiative of the Generator Gas Committee of the Academy of Engineering Sciences, thorough experiments on a large scale were started by the Army on a number of different types and sizes of trucks equipped with modern (for that time) types of gas generators. Civilian interest does not seem to have been particularly stimulated by these tests. A contributing factor which was particularly unfavorable for generator gas operation was the relation between gasoline and charcoal prices in Sweden; the approximate ratio between gasoline price per liter and charcoal price per kilogram in Sweden was 2:8; but in France, 4:3; in Austria, 6:4; and in Italy, ll:4 (as of 1937).

The increased interest for military preparedness caused the head of the Royal Ministry of National Defense to engage three experts from the government to assist with an investigation concerning generator gas operation of motor vehicles. Under the name "The Gas Generator Committee of 1937" the investigators started their work in February 1937 and delivered partial reports on December 9, 1937, [55] and a final report on July 8, 1939. [57]

When wood gas operation was still considered to be in the experimental stage the Committee initially planned on generator gas operation based on charcoal. It was established that the gas generators had been significantly improved since the experiments in 1931 to 1932. Among other things the heat of combustion had risen from 550-580 kcal/Nm3* (66-70 Btu/scf) since the early tests to 620-650 kcal/Nm3 (75-79 Btu/scf). The starting time had been reduced considerably by addition of an electric starting fan and a central air nozzle into the gas generator. Thus direct starting on generator gas without the use of liquid fuel became possible and practical. Improvement in the design of gas exhausts had brought about greater dependability, and interference by slag formation or carbonization was minimized. Also, better nozzle designs eliminated the slag formation on the hearth sides. Motor operation with generator gas was improved; idling characteristics were almost equal to gasoline operation, and engine power per liter of cylinder capacity had been improved. Generator gas operation was considered most suitable for long nonstop driving, but the experiments had shown that satisfactory results also could be obtained in very intermittent operation. With regard to the engines, the opinion was that relatively large cylinder capacity and low rpm were of considerable importance for obtaining favorable results.


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