Carbon dioxide: key to boosting revenues and limiting emissions from biofuels and allied ventures

November 16, 2012 |

By Sam A. Rushing, President, Advanced Cryogenics, Ltd. 


Perhaps the most logical approach to boosting revenues from biofuels projects, that being those which yield a CO2 by-product, such as fermentation, is the recovery, and subsequent refinement, and properly assessing the best suited market for the commodity. The same is applicable to the other so-called traditional by-product sources of CO2 feedstock from other chemical and energy processes and sources. From a merchant CO2 perspective, the majority of carbon dioxide is a by-product of a chemical reaction, or combustion; outside of the one source which is natural in origin, that being from geological sources. The latter refers to CO2 supplies generally termed ‘CO2 wells’, or ‘natural’ sources.

These sources are derived from specific geological formations in many world markets. In the United States, these natural sources are typically found in the States of New Mexico, Colorado, and Mississippi; and are termed the Bravo Dome, Sheep Mountain site & McElmo Dome; and the Jackson Dome. The well head pressure of these natural sources can exceed 2,000 psig, or be as low as 20 psig, for example.

Sources of CO2 from the (generally) concentrated natural and chemical by-product projects usually  have a CO2 percentage by volume within the raw gas of approximately 97% – 99+%; while exhaust gas from power plants and other combustion sources are probably from 3 to 15% by volume.

The combustion sources then require concentration via processes such as a solvent recovery, often MEA; which then means a very expensive plant for  concentrating this raw and dilute flue gas, then constructed upstream of a liquefaction/purification plant, if the latter is used for the market or process sought to serve.

Carbon dioxide grades sought, applications, and sequestration challenges

The majority of refined CO2, produced from this variety of sources, would be sold to the food and beverage industries. More specifically, in places such as the US, a wide variety of applications in food processing (from chilling, to cooling, to modified atmosphere packaging) generally consume the most merchant product, when sold as a liquid (gas under pressure), or as dry ice.

Then in many developed markets the beverage grade and applications for use carbonating soft drinks, beer, and use in soda dispensing is probably the most stringent in the industry, short of perhaps specific medical grades (i.e. USP); and beverage grade can often follow, or even exceeding food grade usage. Therefore, new production from merchant sources, sourced from a variety of raw gas types often must meet beverage grade quality; that being, once again, the most stringent of the larger usage grades consumed in the merchant markets.

Food grade definitions are often written for a specific producer of a food product, or perhaps they adhere to the beverage grade, which is often called ISBT (International Society of Beverage Technologists), which can be interchanged with the gas business trade group named CGA (Compressed Gas Association). Otherwise, European and other international markets have their own trade group definitions for quality. Therefore beverage grade is essentially the benchmark of purity, which can be affiliated with food applications; and to follow, when beverage grade is produced in a CO2 plant, then this is usually the one and only grade of product is sold to all industries served by a given plant.

Exceptions can be dedicated plants which produce a so-called industrial grade which serve a captive chemical manufacturing operation, for example; or are serving oil and gas markets, often referred to as EOR (enhanced oil recovery) and frac (fracturing in gas and oil projects, kin to recently discussed ‘hydraulic fracturing’ techniques). If the so-called USP or medical grade is sought, usually the major players such as Linde Gas set aside various regional plants for the production of this grade, which essentially requires a great deal of monitoring and a mountain of paperwork to achieve USP.

Industrial grade or more specifically frac grade CO2 has been set aside by some of the major gas firms in the US over the years, where this product was dedicated to oil and gas service alone. 

Captive sources of CO2 from natural and energy projects including natural sources owned by Denbury  Resources, known for their Mississippi natural sources which are being delivered via pipeline to energy projects in the Mississippi and Louisiana EOR markets; then the New Mexico Bravo Dome is supplying EOR projects for companies such as Hess in the regional Southwest.

Next, Dakota Gasification of Beulah, ND is supplying CO2 directly from their coal gasification and ammonia plant projects via a large pipeline with a capacity up near 10,000 tons per day; and this is fed into the Saskatchewan EOR projects operated by Encana and Apache for the Weyburn and Midale fields. Ethanol was evaluated and even planned as raw gas for possible oil and gas projects in places such as Kansas and Texas in the past; however, such EOR opportunities remain viable when sourced from ethanol in the future.

Today, the United States is entirely confident they will be the largest oil and gas producer by 2020, largely due to shale based production, partly made possible by hydraulic fracturing techniques; which today do not use CO2, but may in the future.

Pure carbon dioxide sequestration projects have been evaluated by my office for large US and Brazilian ventures, primarily fed by CO2 from ethanol projects. In the future, it is entirely likely that more CO2 will actually be dedicated to subterranean oil and gas injection techniques, and dedicated carbon sequestration requirements.

The latter represents CO2 injected into aquifers, in some cases, when the mission is simply carbon sequestration, and not oil and gas production or enhancement. The problem today with subterranean CO2 sequestration or any other form of sequestration outside of possibly algae production and EOR is a dire shortage of funding for these projects, and the U.S. a government unwilling and unable to foster these carbon sequestration ventures; all due to the economic crisis, politics, and the recession.

However, these sequestration projects such delivery as into aquifers will be resurrected once environmental thinking restarts, and the economy improves. Simply put, it is very expensive to sequester even a highly enriched CO2 by-product; and even less viable when derived from power plants and other combustion projects.

Raw CO2 source options

As with most developed CO2 market supplies from a variety of carbon dioxide raw gas sources, the United States for example has most of the CO2 by-product sources originating from anhydrous ammonia production, reformer operations – found in some oil refineries; then natural sources, ethanol production; and to follow a smaller percentage from the production of ethylene oxide, titanium dioxide, and flue gas recovery projects. Dominant sources for mature, developed markets such as the US include ammonia, ethanol, natural, and reformer by-product.

At one time, ammonia was the largest source type by percentage of US CO2 plants; which was once surpassed by ethanol sources during the height of corn based ethanol production. Ethanol may remain approaching the primary source type, despite some plant closings. Then natural sources and reformer were the balance of the primary sources by percentage of plants, and perhaps volume as well. Volumes in this case represent a sum of raw CO2 by-product gas which is being recovered, and generally liquefied and purified for merchant use.

Specific to the purest forms of raw gas among those dominant in the discussion above, this has typically been anhydrous ammonia by-product and sometimes natural CO2 sourcing; however natural sources are not always the most pure.

More than likely, on a fairly constant basis, the purest or easiest raw gas to refine from the major source types discussed would be anhydrous ammonia production. One example of ammonia by-product analysis indicates 99.2 -99.6% CO2, less than 1% of hydrogen, oxygen, nitrogen; and trace methane.

Natural can be nearly as simple as this summary, such as a sample for one source showing 99.99% CO2; then trace methane, oxygen, nitrogen, ethane, and trace aromatic compounds. On the other hand, natural sources have sometimes been plagued by more methane and heavier hydrocarbons, sulfur compounds, benzene, and even radon gas in some cases. Captive industrial use such as oil production could endure some of these compounds, in a limited manner; however, food and beverage grade production cannot allow impurities such as benzene, and radon gas, the latter mostly due to consumer fear.

Further, it is problematic or prohibitively expensive to remove certain compounds. Some of these compounds like radon make a well unsuitable for use in the food and beverage market.

Next, sources derived from reformer operations, often found in oil refineries, sometimes sell raw CO2 gas to gas companies for refinement and liquefaction; and these sources can be quite clean, as mentioned earlier.

One such specification I have indicates 98% minimum CO2 content, plus small percentages of oxygen, carbon monoxide, methane and hydrogen.  In the US, there are now, or have been a few merchant CO2 plants sourced from cogeneration based flue gas (two operating now); and the raw CO2 content derived from coal combustion is probably near 14% by volume; plus constituents which can be removed to produce a quality food grade product.

Then a couple of plants sourced from processes such as ethylene oxide and titanium dioxide production are found in North America. In the case of ethylene oxide, what is particularly problematic are compounds such as ethyl chloride and vinyl chloride, which then requires upgrades in the metallurgy used in the CO2 plant construction; thus more expense.

Ethanol based raw CO2 by-product is a very significant factor within the overall North American CO2 supply network; in the US, probably over one – third of the total number of plants that refine and liquefy CO2.

When looking at typical dry mill ethanol production as the raw gas source, the CO2 content can typically be a minimum of 99% by volume. Then an assay may show ethanol, methanol, and other alcohols measured in parts per million (ppm), maybe near 100 ppm for  ethanol and methanol content; as well as similar levels of acetaldehyde and ethyl acetate, perhaps. Much less content of other constituents are also found.

In the end, ethanol based sourcing is so well proven, that building plants for CO2 based on this type of feedstock, which are dedicated to the food and beverage markets, are very much a matter of course. Raw gas assays vary somewhat among locations, and the first step when pricing a CO2 plant specific to the ethanol project in mind will require taking raw CO2 gas samples for analysis; then the plant is usually custom tailored to fit the specific feed gas.

Advanced biofuels, and raw gas summary comments

Thus far, we are looking forward to recovery of CO2 from viable advanced biofuel operations. Many processes which use enzymes for example in cellulosic processes have defined very little in terms of raw gas specifications. Some of this reason is the lack of world scale advanced biofuels operations, or those ready for near term implementation; and proprietary issues. On the other hand, it is logical to assume future CO2 for food, beverage and industrial use will be a part of the network for supplying the North American and other markets; however, as a consultant, I look forward to working on such advanced biofuel ventures. I am confident the future of the merchant CO2 markets, including the lion’s share of merchant service going to food and beverage service will eventually be supplied by advanced biofuel operations. 

In any event, CO2 by-product from fermentation represents a major share of the overall supply chain of plants sourced by all concentrated feed stocks. This means generally over 98 – 99% CO2 content is found in this raw gas from ethanol; as it would from the other two or three dominant source types mentioned which represent the majority of the network of CO2 plants found in North America, and most of the other developed markets.

The developing markets often have combustion sources and some chemical and natural as well operations. However, many of the CO2 markets in the developing countries are primarily beverage carbonation. Once again, the quality benchmark would also need to be met when serving the multinational beverage concerns.

About the author:

Sam A. Rushing is president of Advanced Cryogenics, Ltd., a CO2 and cryogenic gas consulting firm, supplying all source types for CO2 and allied gases; with a great deal of specialty in the biofuels industry. Mr. Rushing is a chemist, and also has a vast merchant CO2 industry background with the former Amerigas CO2 Division, with decades’ long consulting expertise. Regarding the specialty in CO2 work, all subjects and project types are covered from technical, process, purity; through markets, contracts, and business development. Expert witness work is also available. To contact, please call 305 852 2597, [email protected];

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