Natural gas is a widely available fuel which is primarily methane. The prices of natural gas has declined in recent years and are expected to possibly remain low for decades, natural gas has become a favored form of energy. As with any hydrocarbon based energy source, questions are raised about its contribution to greenhouse gases.
Greenhouse gases are part of the ‘greenhouse effect’ that presents an environmental issue associated with the potential for global climate change. A number of gases in our atmosphere regulate the amount of heat retained close to the earth's surface. Water vapor, carbon dioxide, methane, and nitrogen oxides are the most common naturally occurring greenhouse gases. As the concentration of greenhouse gases increase, temperatures throughout the environment will rise significantly.
The continually increasing use of fossil fuels has caused an increase over the naturally occurring amounts of these gases and ways are sought to reduce the man-made influence in the generation of such gases. Carbon dioxide is seen as a major source of greenhouse gas. Even though carbon dioxide has less of an effect on trapping heat than the other naturally occurring gases, its prevalence from the increased reliance on hydrocarbons makes it suspect as a significant contributor to trapping of heat in global warming. For this reason those concerned with climate change are especially interested in limiting the generation of carbon dioxide.
From the standpoint of CO2 production from hydrocarbon usage, natural gas offers important environmental benefits relative to the combustion of other fossil fuels. In rough numbers combustion of treated and unconverted natural gas will produce 30% less CO2 emissions than oil and almost 45% less CO2 emissions than coal. However, methane in the form of an emission is over 20 fold more potent in its greenhouse gas effect than carbon dioxide.
While there are many applications for using natural gas as an energy source in its gas phase, attention has focused on how to utilize this energy source in the most efficient manner. Major drawbacks to the use of natural gas in traditional applications are problems of its storage and transportation as long as it remains in a gas form.
One way in which those seeking increased the natural gas utilization have attempted to overcome these problems is by employing processes for converting natural gas to denser and more easily transportable forms. Natural gas is typically converted into a liquid form by gas to liquid (GTL) processes, the primary of which either produces syncrude that directly converts the gas or produces synthesis gas that indirectly converts the natural gas. Although the combustion of natural gas in the unconverted gaseous form has green-house gas advantages, further processing of natural gas to liquids to transform it to more easily transported forms increases its greenhouse gas impact.
One solution to the problem of CO2 production from industrial processes is to sequester CO2 that is produced by such processes. Effecting carbon dioxide (CO2) capture and sequestration (CCS) has been proposed by various technologies that can reduce CO2 emissions from facilities such as coal-fired and gas-fired power plants, and large industrial sources. Most CCS technologies employ three steps. The first step will capture CO2 from oil refining operations, treatment of well head gas, powerplants, renewable fuel production or other industrial processes. Compression of the captured CO2 prepares it for transfer, in tanks, pipelines or via other means, to permanent sequestration facilities. These facilities usually comprise geologic formation located a mile more underground that hold the CO2 in a porous layer of rock situated beneath a gas impermeable layer of rock. The impermeable layer of rock prevents an upward migration of the CO2. This type of CO2 sequestration is expensive, used infrequently and impractical for major sources of CO2 emissions.
Another major source of CO2 emissions is in the use of hydrocarbons as motor fuels. An approach to reduce the effect of CO2 emissions from hydrocarbon fuel combustion is to produce motor fuels from renewable resources such as plant materials. This approach reduces the use of non-renewable, i.e. fossil, hydrocarbons that when combusted put carbon dioxide into the atmosphere by releasing the fossil hydrocarbons from their naturally sequestered state. In contrast, using renewable sources for feedstocks, i.e. plant materials, maintains a closed CO2 system where the carbon dioxide released by hydrocarbon combustion balances with the carbon dioxide captured by the plant material that goes into the feedstock for producing the fuels and chemicals.
Plant materials for use in such feedstocks can come from almost any plant source. One general type of plant material, such as corn, rice, cane, wheat, sorgum etc., have high carbohydrate concentrations that are readily converted to sugars. At this time essentially all of the production of ethanol for motor fuel purposes relies on the saccharization of plant materials into sugar and its fermentation to alcohols. These fermentations produce CO2 directly as a by-product of the fermentation process. In such processes the amount of CO2 produced directly can equal the mass of ethanol. Moreover, concerns about plant sources can arise since their use requires utilization of land and water resources for growth of these corps or trees and is subject to limitations posed by alternative uses of these land and water resources for food and other agricultural and forest product production.
Another way to produce alcohol motor fuels or other chemicals is by generating syngas from plant material or natural gas. In the case of plant material, its gasification will produce a syngas. In the case of natural gas it may be converted to reformed gas. The syngas obtained by gasification or by conversion of natural gas are suitable substrates for anaerobic fermentations of hydrogen and carbon monoxide. Anaerobic fermentation processes involve the contact of the substrate gas in an aqueous fermentation menstruum with microorganisms capable of generating alcohols such as ethanol, propanol, i-butanol and n-butanol. The production of these alcohols requires significant amounts of carbon monoxide and carbon dioxide together with hydrogen.
In addition to the aforementioned plant and natural gas sources, a suitable source of the substrate gas for carbon monoxide and hydrogen conversions can be derived from the gasification of carbonaceous materials other than those specifically mentioned such as the partial oxidation of natural gas, biogas from anaerobic digestion or landfill gas, coal coking and industrial steel manufacture. The substrate gas containing, carbon monoxide, hydrogen, and carbon dioxide, usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like. (For purposes herein, all gas compositions are reported on a dry basis unless otherwise stated or clear from the context.)
In any of the above cases certain anaerobic bacteria, especially those from the genus Clostridium can produce various alcohols, particularly ethanol, from the resulting substrates comprising CO, CO2 and H2 via the acetyl CoA biochemical pathway. Different strains of Clostridium ljungdahlii that exemplify producing ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. Abrini et al, Archives of Microbiology 161, pp 345-351 (1994)
The effectiveness of converting CO2 to other products will depend on the amount of CO and H2 that is present with the CO2 in the converted gas stream. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O→C2H5OH+4CO2 6H2+2CO2→C2H5OH+3H2O.As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. The microbiological processes that take place have been expressed for convenience in the formulas shown above. It is known that in the microbiological process the production of the alcohol requires a carbon source and a biological energy source. The formulas show that the biological energy source is the available electrons in the hydrogen and carbon monoxide of the substrate gases. As a result gas streams with high CO will produce more CO2 or those with low H2 will limit the conversion of CO2 and in either case may make the anaerobic process a net producer of CO2.
Regardless of originating source, the substrate gases are typically more expensive than equivalent heat content amounts of the carbonaceous feedstock fuel. Hence, a desire exists to use these gases efficiently to make higher value products. The financial viability of any conversion process, especially to commodity chemicals such as ethanol, will depend, in part, upon the costs of the feedstocks, conversion efficiency and operating and capital costs for generating the substrate gases; and upon the capital costs, the efficiency of conversion of the carbon dioxide, carbon monoxide and hydrogen to the sought products, the process energy costs and the extent of the electron availability and utilization to effect the conversion of the substrate gases to the higher value products.
In a bioreactor, hydrogen and carbon oxides pass from the gas phase to being dissolved in the aqueous menstruum, and then the dissolved hydrogen and carbon oxides contact the microorganisms for bioconversion. Due to the low solubilities of carbon monoxide and, especially, hydrogen in aqueous media, mass transfer can be a factor limiting rate and conversion in the bioconversion to alcohol.
The off gases from bioreactors contain substrate that was not bioconverted and diluents such as methane and nitrogen. Although off gases can be recycled to the bioreactor or passed to another bioreactor, challenges can exist. For instance, the substrate gases may contain diluents that, if recycled to a bioreactor, can build-up and reduce the partial pressure, thus reducing the driving forces for mass transfer of hydrogen and carbon monoxide to the aqueous menstruum.
Overall, the net effect of the processing as gas into liquids such as alcohol or hydrocarbons will produce an increase in the CO2 emissions when compared to a liquid to liquid conversion such as petroleum crude to gasoline. No one to date has disclosed a process that can improve the overall greenhouse gas emissions from the processing of natural gas to produce ethanol.
Bell in United States Published Patent Application No. 20100105118 discloses an integrated process for making alcohols from natural gas which provide a high bioconversions of carbon monoxide in fermentations in the absence of oxygen. Bell notes at paragraph 0013 that, in theory, carbon dioxide may be used as a reactant for the production of higher alcohols such as ethanol. However, he states that in practice the fermentation route to higher alcohols tends to be a net producer of carbon dioxide. In his disclosed process, the gas from the bioreactor which contains carbon dioxide is fed to a reaction section of a steam reformer. The reformer is either operated dry or with a mole ratio of water to carbon dioxide of less than 5:1. Bell confirms the low conversion of hydrogen in the examples. Although Bell may have reduced carbon dioxide emissions as compared to the use of autothermal reforming or traditional steam reforming, the low conversion of hydrogen detracts from the commercial viability of the disclosed process.
Processes are therefore sought that can reduce the carbon footprint in the utilization of the non-renewable resources.
Processes are also sought that can reduce emitted greenhouse gases by utilizing CO2 released in the fermentation of renewable resources.
Processes are also sought that can maximize the production and availability of electrons for microorganism growth and alcohol production while also reducing CO2 emissions for the production of alcohol in commercial-scale, continuous operations.