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close this bookAlcohol Fuels: Options for Developing Countries (BOSTID, 1983, 101 p.)
View the document(introduction...)
View the documentAcknowledgments
View the documentPreface
View the documentOverview
View the document1 Production and Use
View the document2 Biomass Sources
View the document3 Ethanol Production
View the document4 Methanol Production
View the document5 Social, Economic, and Environmental Implications
View the document6 Conclusions and Recommendations
View the documentAdvisory Committee on Technology Innovation
View the documentBoard on Science and Technology for International Development

4 Methanol Production

Methanol was first produced as a by-product in the manufacture of charcoal through the destructive distillation of wood, with yields of 12-24 liters per ton of wood. Most methanol today is produced from natural gas. In principle, many carbon-containing materials may be substituted for natural gas as starting materials. These include (in addition to wood) coal, lignite, and even municipal wastes. Each of these raw materials, however, must first be converted to syngas; for this step, each alternative feedstock requires process modifications that increase capital investment costs over those required for natural gas.

Descriptions follow of the processes for converting natural gas and wood to methanol.

METHANOL FROM NATURAL GAS

The production of methanol through the conversion of natural gas to syngas is used in conventional methanol plants throughout the world. Typically' natural gas-primarily methane-is catalytically reacted with steam and carbon dioxide to yield hydrogen and carbon monoxide in a ratio of 2:1.

3CH4 + ........2H2O + ....CO2 ----> 8H2...............+ 4CO
methane...... water...... carbon............ hydrogen ® carbon
................................. dioxide.............................. monoxide

This gas mixture (syngas) is then compressed and converted to methanol:

8H2...............+ 4CO ----> 4CH3OH
hydrogen......... carbon...............methanol
.....................monoxide

METHANOL FROM WOOD

The production of methanol through the conversion of wood to syngas is being examined in several countries. In terms of converting carbon to methanol, wood is inherently less efficient than natural gas. The initial gasification step in producing syngas from wood yields a mixture of CO and H2 deficient in hydrogen; to bring the ratio of H2 to CO to 2:1, part of the CO is reacted with steam to yield additional hydrogen:

CO......... + H2O ----> CO2............ + H2
carbon...... water............ carbon............ hydrogen
monoxide...........................dioxide

This carbon dioxide is then removed from the process stream and discarded. Approximately 50 percent of the carbon in the wood entering the process is nonproductively released to the atmosphere. This also means that 50 percent of all the forestry and transport activities to provide the wood to the plant are wasted. About 2.25 kg of natural gas are required to produce 4 liters of methanol as opposed to about 9 kg of dry wood.

Figure 25 shows the sequence of operation suggested by Hokanson and Rowell for the production of methanol from wood.

Suggested Steps for Wood Conversion Gasification

In the first step, wood is charged at the top of the reactor and ash discharged from the bottom. Air and steam are charged near the base of the reactor.

The sequence of reactions occurring in wood gasification are as follows:

Drying (100° - 200°C)
Moist wood and heat ----> Dry wood and water vapor
Pyrolysis (200°-500°C)
Dry wood and heat ----> Char + CO + CO2 + H2 +CH4+ + tars and pyroligenous acids
Gasification (500°C+)
Char + O2 + H2O... ----> CO + H2 + CO2

The raw gas typically contains hydrogen (18 percent), carbon monoxide (22.8 percent), carbon dioxide (9.2 percent), methane (2.5 percent), other hydrocarbons (0.9 percent), oxygen (0.5 percent), and nitrogen (45.8 percent).

Gas Purification and Shift Conversion

The raw gas is then purified to remove all but hydrogen and carbon monoxide. This mix is reacted to convert part of the CO to H2 So that the final mixture contains a 2:1 ratio of H2 to CO. In this conversion, additional CO2 is formed and must be removed before methanol synthesis.

In detail, the raw gas from the reactor passes through a scrubber (2) cooling the gas to about 32° C and removing tars and acid.

The gas is then compressed to about 100 psig (3) and treated in two stages to remove carbon dioxide. In the first stage (4), a hot potassium carbonate solution reduces the CO2 content to about 300 ppm. In the second stage (5), monoethanolamine (MEA) is used to reduce the CO2 content to about 50 ppm.

The gas then passes through a cryogenic system (6), which removes the residual CO2 and water vapor, plus methane and other hydrocarbons and finally nitrogen.

The purified gas is a mixture of hydrogen (approximately 44 percent) and carbon monoxide (approximately 56 percent). It requires further processing to provide the 2:1 ratio of H2 to CO needed to produce methanol.

Following cryogenic purification, the gas is compressed (7) to 400 psig for shift conversion. Part of the CO reacts with water vapor in the presence of an iron catalyst to form additional hydrogen (8) so that the exit gas contains a 2:1 ratio of H2 to CO.

Since the shift reaction (equation 3) also produces CO2, it is necessary to rescrub the gas with a potassium carbonate absorption system (9).


Figure 25. Methanol synthesis from wood waste. Process steps: (1) partial oxidation of wood waste, (2) clean and cool crude gas, (3) compress to 100 psig, (4) remove residual carbon dioxide formed in shift, (6) remove nitrogen and hydrocarbons, (7) compress to 400 psig, (8) shift gas to two parts hydrogen and one part carbon monoxide, (9) remove carbon dioxide formed in shift, (10) compress to 2,500 psig, (11) convert hydrogen and carbon monoxide to methanol, (12) refine crude methanol into specification grade product. (A. Hokanson and R. Rowell)

Methanol Synthesis

The syngas is then compressed (10) to 2,000-4,000 psig and passed into the methanol synthesis reactor ( 1 1). In the reactor, approximately 95 percent of the gas is converted to methanol over a zinc-chromium catalyst. The unreacted gasses are separated and recycled and the methanol purified by distillation (12).

Processing Alternatives

Other investigators have examined alternative methods in the conversion of wood to methanol.

Earl describes a process in which oxygen instead of air is used in the gasification step. This requires an air separation plant at the beginning of the process, but eliminates the cryogenic step described by Hokanson and Rowell.

Mundo and Wehner suggest that locating a wood-to-methanol plant at a hydroelectric site would have several advantages. Electrolytic oxygen could be used in gasification and electrolytic hydrogen could be used to adjust the H2:CO ratio for syngas production instead of shifting carbon monoxide. Location at a dam might also allow less expensive water transport for wood.

Osler, reporting on methanol production options in Canada, compares simple gasification of wood with hybrid systems that also utilize hydrogen or methane. To produce 1 ton of methanol, either 2.33 tons of wood or 0.9 tons of wood with 0.12 tons of hydrogen or 0.4 tons of wood with 867 cubic meters of natural gas would be required.

Arbo describes a process in which low-grade coal is briquetted with cellulosic wastes and gasified as a possible first step for methanol production.

Rooker details the requirements for methanol production from Brazilian eucalyptus wood. For a 2,000-ton-per-day methanol facility, the total plant investment would be $275 million.

Haider covers methanol production considerations, including thermodynamics, catalysis, and kinetics, as well as a description of the low-, medium-, and high-pressure processes for converting syngas to methanol.

Laboratory studies at Battelle-Northwest have defined conditions and catalyst systems for the steam gasification of wood to synthesis gas. These results have been used by Davy-McKee, Inc., to prepare a conceptual plant design for methanol production from wood. Capital and operating costs for methanol plants with annual capacities of about 2.5, 10, and 100 million gallons have been projected and are shown in Table 10.

TABLE 10 Methanol Production Costsa


Methanol

Operational Characteristics

(in millions of gallons per year)

Production

Level


2.50

9.91

98.81

Wood capacity (dry tons per day)

50

200

2,000

Capital required (millions of dollars)

16.4

41.2

145.6

Annual operating costs (millions of dollars)

2.9

7.8

36.9

Methanol cost (dollars per gallon)




Utility financing

1.99

1.30

.55

Private financing

2.62

1.70

.69

a Based on wood at $20 per dry ton.

SOURCE: Batelle-Northwest, Richland, Washington, USA.

International Harvester is developing a small-scale methanol plant that would be factory built in major modules and then transported to the production site. Because of the savings made possible by producing identical units in quantity in a factory, a package plant could produce methanol for the same price as a plant 15 times its size built by on-site construction. Capital costs of about $0.25 per annual liter are projected for plants with capacities of 25-30 million liters per year. Such plants would utilize 100-150 dry tons of wood per day.

The Energy Transition Corporation of Santa Fe, New Mexico, is planning a peat-to-methanol project in North Carolina. A gasifier developed by Koppers Co. and Babcock and Wilcox is expected to convert approximately 700,000 tons per year of peat to 250 million liters of methanol.

Sweden's Board for Energy Source Development has spent $4.5 million to build and test a pilot plant that can convert 12 dry tons per day of peat or wood (via syngas) to about 10,000 liters of methanol.

REFERENCES AND SELECTED READINGS

Methanol Production

Anonymous. 1979. Brazil out to show methanol grows on trees. Chemical Week March 14:38.

Anonymous. 1981. EEC approves $2.6-million program to produce methanol from
wood. Synfuels December 4:5.

Anonymous. 1981. Methanol boom will require new technology. Chemical and Engineering News October 5:32-33.

Anonymous. 1982. Sweden's cool new plant. International Agricultural Development September:3.

Arbo, J.C. 1980. Energy from Biomass: The Simplex Process for the Gasification of Coal and
Forest Pulp. New York State Energy Research and Development Authority, Albany, New York, USA.

Earl, W.B. 1978. Methanol from wood in New Zealand. Alcohol Fuels Conference, August 9-11, 1978, Sydney, Australia.

Frank, M.E. 1982. Methanol: emerging uses, new syntheses. Chemtech 12:358-362.

Green, L.A. 1977. Methanol: A Selective Cross-Disciplinary, Bibliography. Prepared for U.S. Energy Research and Development Administration under contract No. W-7405-Eng48. Lawrence Livermore Laboratory, University of California, Livermore, California, USA.

Haider, G. 1977. Methanol production from organic waste. In: Fuels from IV Waste, edited by L.L. Anderson and D.A. Tillman. Academic Press, New York, New York, USA.

Hokanson, A.E., and Rowell, R.M. 1977. Methanol from Wood Waste: A Technical and Economic Study. USDA Forest Service General Technical Report FPL 12. Department of Agriculture, Forest Products Laboratory, P.O. Box 5130, Madison, Wisconsin, USA

Klier, K., Herman, R.G., Kobylinski, T.P. 1977. Methanol Synthesis: Catalysts and Mechanism Sinclair Lab., Gulf Research and Development Co., Pittsburgh, Pennsylvania, USA. Available from NTIS (Order #PB-288318).

Konig, A., Menrad, H., Lee, W., and Bernhardt, W. 1978. Alternativkraftstoff methanol: Auswahlkriterien and Stand der Forschung. Erdol and Kohle-Erdgas-Petrochemic 31(8):360-367.

Linder, C., and Reimert, R. 1982. Gasification of wood in the circulating fluidized bed methanol production route. Pp. 115-125 in: Energy from Biomass, edited by W. Palz and G. Grassi. D. Reidel Publishing Company, Dordrecht, Holland.

Lowenheim, F.A., and Moran, M.K. 1975. Methyl alcohol. Pp. 524-529 in: Faith, Keyes, and Clark's Industial Chemicals, 4th edition. John Wiley & Sons, New York, New York, USA.

Mitchell, D.H., Mudge, L.K., Robertus, RJ., Weber, S.L., and Sealock, Jr., LJ. 1980. Gasohol/biomass developments: methane/methanol by catalytic gasification of biomass. Chemical Engineering Progress September:53-57.

Mundo, KJ., and Wehner, H. 1979. Production of methanol From wood: processes foresting and economics. Third International Symposium on Alcohol Fuels Technology, May 28, 1979, Asilomar, California. Available from NTIS (Order #CONF-790520).

Osler, C.F. 1979. Canadian Scenario for Methanol Fuel. Third International Symposium on Alcohol Fuels Technology, May 28, 1979, Asilomar, California. Available from NTIS (Order #CONF-790520).

Pasternak, A. 1977. Methyl alcohol-a potential fuel for transportation. In: Energy Technology Handbook, edited by D.M. Considine. McGraw-Hill Book Company, New York, New York, USA.

Reed, T;B., Grabowski, M., and Markson, M. 1981. Synthesis Gas from Biomass: Operating Data on a One Ton/Day Oxygen Gasifier. Paper presented at the 13th Thermochemical Conversion Contractors' Meeting, Arlington, Virginia, October 27-29, 1981. To be issued as SERI/PR-234-1455.

Reed, T.B., and Lerner, R.M. 1973. Methanol: a versatile fuel for immediate use. Science 182(4119):1299-1304.

Rooker, J.H. 1980. Methanol via Wood Gasification. Paper presented at the Energy from Biomass and Wastes IV Symposium, January 21, 1980, Lake Buena Vista, Florida. Institute of Gas Technology, Chicago, Illinois, USA.

Seattle Office of Management and Budget. 1974. Feasibiliy Study: Conversion of Sol*' Waste to Methanol or Ammonia. Mathematical Sciences Northwest, Bellevue, Washington, D.C., USA. Available from NTIS (Order #PB-255-449).

Wentworth, T.O., and Othmer, D.F. 1982. Producing methanol for fuels. Chemical Engineering Progress August:29-35.

World Bank. 1982. Emerging Energy, and Chemical Applications of Methanol: Opportunities for Developing Countries. World Bank, Washington, D.C., USA.

Yang, V. 1980. Wood Gasification for Methanol Production. M.S. Thesis, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.