135 Fischer Tropsch Synthetic Fuels

The Fischer-Tropsch (FT) gas-to-liquid (GTL) process essentially involves three catalytic processes. In the first reaction, natural gas is combined at specific temperature, pressure, and ratio with air, oxygen-enriched air, or oxygen and a small amount of water to manufacture synthesis gas. The resulting synthesis gas consists primarily of carbon monoxide and hydrogen that is diluted with nitrogen to the extent that air was used in the feed. Table 13.2 illustrates the conversion process.

TABLE 13.2

Conversion of Natural Gas to Synthesis Gas

TABLE 13.2

Conversion of Natural Gas to Synthesis Gas

Synthesis Gas

(diluted with nitrogen if

Natural Gas Air

Steam

Catalyst

air instead of O2 is used)

Water

CH4 + O2 +

(N2 +) h2o

>

CO + h2 + (N2 + ...)

h2o

In the second process, commonly referred to as the Fischer-Tropsch reaction, the synthesis gas flows into a reactor containing either an iron or cobalt-based catalyst. As the synthesis gas passes over the catalyst, it is converted into hydrocarbons of various molecular weights, with byproduct water and carbon dioxide also produced. The produced hydrocarbons and water are subsequently separated.

Cobalt catalysts are well suited for the conversion of natural gas because they work most efficiently with hydrogen to carbon monoxide ratios of approximately 2:1, which happens to be the ratio of synthesis gas from methane. Iron FT catalysts are better suited for synthesis gas feedstocks with hydrogen to carbon ratios of less than 2:1, such as refinery coke and coal. The many differences between iron-based and cobalt-based FT processes are beyond the scope of this discussion. However, one drawback of iron-based FT processes is that when natural gas is used as a feedstock, iron-based FT processes produce significantly more CO2 per unit volume of synthetic fuel produced than cobalt-based FT processes. Additionally, synthetic oils from iron-based FT processes are undersaturated with respect to hydrogen and, thus, must undergo additional hydroprocessing if saturated paraffins are the desired products.

The following chemical notation illustrates the FT reaction:

Synthesis Gas

(if diluted with nitrogen)

Catalyst Hydrocarbons

Nitrogen

Water

h2 + CO (+ n2 )

-----> CnH(2n + 2) +

(N2 +)

h2o

FT catalysts typically produce a very waxy synthetic crude oil. More than 50% of a barrel of synthetic crude oil is solid at room temperature due to the high wax content. Thus, the third process to make FT synthetic fuels is the conversion of these waxy hydrocarbons into fuels (gasoline, kerosene, and diesel) using conventional refining technologies (hydroisomerization). Just as in a conventional refinery, yields of diesel, kerosene, and naphtha are controlled by hydrocracker severity — the more severe the hydrocracker reaction, the lighter (smaller) the average molecule in the product from the hydrocracker. The hydrocracker product is fractionated into constituent fuels, diesel, kerosene/jet fuel and naphtha, which are defined by product boiling point ranges. Fuel yields are dictated by fuel and oil market conditions.

Gasolines from FT processes have unique characteristics when compared to typical gasoline that consists of the C5 to C9 fraction. Without further upgrading, the octane of FT gasoline is too low for internal combustion engines. However, it has been demonstrated to be an excellent fuel cell fuel. Additionally, FT diesel and perhaps gasoline may be a viable fuel when emulsified with water and minor additives to yield a diesel substitute that is anticipated to significantly reduce emissions and improve engine efficiency. This technology, analogous to water injection, is in the engine testing phase.

The FT distillate (C10 to C20) stream includes both the jet/kerosene range (C10 to C14) as well as traditional diesel (C13 to C20). Due to its strong paraffinic character, FT distillate has a very high cetane number, which lies in the mid-70s. Also due to its chemical nature, FT distillate has a relatively high pour point that may restrict its appeal in more northern regions. Further isomerization of the paraffin and the use of additives can reduce pour point and cloud point temperatures.

Blending of low sulfur level FT distillate with cheaper, high sulfur crude-derived stocks can allow a refiner to both meet finished product specifications and reduce refining costs. The level of allowable sulfur in diesel is dropping in many parts of the world. Future changes in sulfur specifications are imminent including 0.005% in Western Europe and parts of the Asia/Pacific region. A low sulfur blending-stock is attractive in these countries as an alternative to more severe and costly hydrotreating to remove the sulfur. .

With the primary exception of the U.S. (except California), almost all countries have relatively high minimum cetane number/ index requirements for diesel. In the Asia Pacific and Western Europe regions, cetane specifications range from 45 to 53. Latin America cetane requirements run from 42 to 47. The U.S. requires a 40 to 42 cetane depending on the pipeline, except for California where CARB specifications call for a cetane of 48. Numerous recent engine testing programs have demonstrated that FT diesel significantly reduces several criteria emissions (particulate matter, NOx, CO, HC).

FT synthetic oil may also become a viable alternative to LNG for new and existing power generation. It can be produced and delivered for less cost per Btu, less safety risk, fewer handling and capital investment issues, and still provide all of the advantages of LNG for power generation. Virtually all power generation facilities that use LNG can also use alternate fuel oil to cover shortages of LNG. FT synthetic oil contains no sulfur, no aromatics, and no particulates or metals. Where local markets offer high prices for premium quality diesel fuel or kerosene/jet fuel, either component may be separated from the commingled product stream for sale, with the balance of production shipped on to power plants.

13.6 Biomass

In the U.S., biomass-fueled facilities account for about 7000 MWe of installed capacity. A diverse range of interests including utilities, independent power producers, the pulp and paper industry, and other forest product businesses operate facilities at more than 350 locations.

Essentially all electricity from biomass is generated using conventional steam-cycle processes. Biomass is burned, the heat is used to generate steam, and the steam is used as a working fluid in an expansion turbine to provide mechanical power for electrical generators. While these processes are effective and commercially available, they operate at relatively low efficiencies. Typical biomass steam-cycle processes are approximately 20 to 25% efficient at converting the energy in biomass into electricity. The newer systems have efficiencies of at least 25 to 30%.

In the U.S., commercial biopower facilities are typically in the size range of 20 to 30 MWe, with a few as large as 50 MWe. This size range is dictated by the economies of scale of the steam-cycle process and the availability of biomass. Smaller facilities are not cost-effective for commercial power generation. Larger-scale facilities are also difficult to build because of feedstock availability. Present facilities use waste as a source of biomass. Obtaining reliable supplies of these feedstocks at scales above 50 MWe may be difficult. The upper size limit may be substantially increased by using dedicated energy feedstocks. However, those fuels cost more than waste.

In situations where there are needs for both heat and power, biomass facilities may be profitable at smaller scale. Many CHP (combined heat and power) facilities exist in the range of 1 to 10 MWe in size, both in industrial applications and in district heating facilities in Europe.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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