EXECUTIVE SUMMARY
4.3 CONVERSION TECHNOLOGY OPTIONS .1 Direct Combustion
4.3.5 Fuel Methanol
Biomass gasification to a synthesis gas consisting of CO, H2 and CH4, provides opportunities for methanol production.
All the technology stages after syngas production are well known and proven through the conventional natural gas process route.
The production of methanol from biomass requires the production of a clean syngas from a gasification process. Whilst biomass has the advantage over conventional fuels of having a low sulphur content, it also has a high oxygen content.
BIOMASS IN THE ENERGY CYCLE
TABLE 4.7 Cost Data : Cogeneration via Gasiflcation
BIOMASS IN THE ENERGY CYCLE
As a result, the biomass derived gas will have ratios of hydrogen to carbon oxides significantly lower than the optimum (2:1) required for efficient synthesis and the process for must therefore be modified.
Two routes are available for achieving this: direct oxygen blown gasification which requires an air separation plant or indirect steam- blown gasification, which produces a syngas rich in hydrogen and utilises a portion of the syngas produced to supply heat to the reactor. Both routes end up with a syngas with too much carbon monoxide and need a shift reaction step to adjust the chemical ratios. In addition, all gasifiers generate about a 20-40% energy loss due to the formation of methane and higher hydrocarbons in the gasification process. These compounds must be removed before or in the methanol synthesis plant. Overall efficiencies of conversion lie in the range 57 - 65% for biomass to methanol, with the higher conversion relating to indirect gasification processes.
Typical methanol plants using natural gas as feedstock, operate at relatively low pressures (50 - 100 atm) and at capacities of 1500 - 2500 tonnes a day to be commercially viable. Technology for more that 90% of these commercial facilities has been supplied by ICI of the UK and Lurgi of Germany.
A new ICI methanol synthesis route, called the Leading Concept Methanol process, is to be demonstrated in a BHP project in Victoria Australia for the first time. This 160 tonnes a day methanol plant will utilise an oxygen fired reformer instead of the conventional large direct fired reformer. Methanol synthesis will also take place at relatively high temperatures of 1200- 1500°C. It is expected that this new technology could reduce the economical size of methanol plants to 1000 tonnes a day or less.
Lurgi has also developed a low pressure process quite different from the ICI route, and over 20 plants using this technology have been built since 1969 using natural gas feedstock in the capacity range 22 to 2200 tonnes a day methanol.
The Hydrocarb process, originally developed by Brookhaven National Laboratories, is now being developed by the Hydrocarb Corp in the US. It comprises three stages: hydrogasification of the biomass, pyrolysis of the methane to hydrogen and carbon, and methanol synthesis. In the process natural gas is added to enhance the production of synthesis gas, excess hydrogen is recirculated to the gastfier, energy is recovered from the high temperature step and the shift conversion, and the cold gas clean-up to remove C02, sulphur and alkalis has been eliminated. Alt of these process components have, to some extent, already been proven in various applications and testing of the concept on a bench-scale plant is presently underway in Southern California.
B I O M A S S IN T H E E N E R G Y C Y C L E
Work being carried out at the National Renewable Energy Laboratories and Batelle Columbus in the US to demonstrate catalysts for hot gas cleanup and methane conversion to useful syngas components. Prove out of long term performance is underway.
Improved methanol synthesis routes are also under investigation, and a major improvement would be the development of a "once through" system to eliminate recycle loops. Brookhaven National Laboratory has developed a low temperature liquid phase catalyst that can convert 9 0 % of the CO in a single pass, combined with an efficient heat recovery system. Other technologies are looking at methanol removal on synthesis and development of a single step synthesis gas/ methanol conversion process.
Reviews by the U S D O E indicate optimism to substantially increase efficiencies and yields from the present 51 to 6 2 % and from 575 to 704 litres of methanol a tonne respectively by the year 2000, thereby almost halving the cost of methanol production by that date.
Cost data for ranges of methanol process technologies are given in Table 4.8.
4.3.6 Fuel H y d r o g e n
Capital and operating cost data for the leading gasification routes to hydrogen are given in Table 4.9.
Hydrogen can be produced as a fuel gas from biomass utilising the indirect gasification process, followed by conditioning and a shift reaction step to convert as much as possible of the carbon monoxide to carbon dioxide, removal of water from the gas now containing some 6 0 % hydrogen, and then finally a purification step, usually achieved by a pressure swing adsorption unit, which produces 9 9 . 9 % purity hydrogen.
Biomass to hydrogen conversion efficiency will range from 6 8 % (direct gasification) to 7 8 % (indirect gasification).
No commercial facilities for hydrogen production from biomass are currently in operation, the major barrier being the development of a viable gasification process. All other process steps are commercially proven.
In the only demonstration of its kind, Veba Oel of Germany is in the process of developing a 1 tonne an hour plant to produce hydrogen from elephant grass using fluidised bed gasification.
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BIOMASS IN THE ENERGY CYCLE
TABLE 4.8 Cost Data : Methanol from Biomass
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BIOMASS IN THE ENERGY CYCLE
TABLE 4.9 Cost Data : Hydrogen from Biomass
BIOMASS IN THE ENERGY CYCLE
In a more advanced development, a laboratory scale process is being evaluated to produce hydrogen from wet biomass by gasifying in supercritical water. The University of Hawaii is studying hydrogen production from a marine derived biomass as feedstock.