Carbon Cycle

This study provides an outline of the carbon cycle for rapeseed oil derived fuels. Plant processes, fuel chemistry and combustion are examined with respect to the carbon. A diagram is presented to graphically interpret the information presented. A comparison of carbon dioxide emissions from combustion of rapeseed oil B100 and petroleum diesel is made. Complete combustion converts hydrocarbon fuels to carbon dioxide and water. The carbon cycle is fixation of carbon and release of oxygen by plants through the process of photosynthesis, then the recombining of oxygen and carbon to form CO₂ through processes of combustion or respiration. The carbon dioxide released by petroleum diesel was fixed from the atmosphere during the formative years of the earth. Carbon dioxide released by B100 is fixed by the plant in a recent year and is recycled. Many scientists believe that global warming is occurring because of the rapid release of CO₂ in processes such as combustion of petroleum diesel. Using B100 could reduce the CO₂ accumulation in the atmosphere.

INTRODUCTION
According to the Global Change Research Information Office "The Consensus of most scientists worldwide is that increasing concentrations of greenhouse gases [carbon dioxide and methane, for example] will lead to significant climate warming, shifts in precipitation patterns and rising sea levels, although the magnitude, timing, and regional patterns of these changes cannot be accurately predicted at this time" (Gibbons, 1995).

The potential for B100 (replacing petroleum diesel with a diesel engine fuel derived from animal fat or vegetable oil such as rapeseed oil) to reduce the carbon dioxide introduced into the atmosphere as a result of engine combustion has been suggested by several authors Sagar, 1995). Rapeseed oil is a renewable fuel, i.e., the oil is obtained from plant oils and animal fats. All of the carbon released by combustion of the plant oil has been fixed by the plant through the process of photosynthesis. Thus carbon dioxide from the air has been absorbed by the plant, converted to the lipids, processed into a renewable diesel fuel and then when the oil is used in the engine the carbon is again released into the atmosphere as carbon dioxide.

OBJECTIVE
To trace the carbon cycle for methyl (RME) and ethyl (REE) B100 fuels and to compare with that of petroleum diesel fuel.

LITERATURE REVIEW
Gust (1995), Encyclopedia Britannica (1993) and Thomas (1984) outline the basic principles of the photosynthesis process. Thomas (1984) presents a brief discussion of the canola plant and the photosynthesis process, discusses different factors that affect photosynthesis and how these factors affect yield. Encyclopedia Britannica (1993) gives a detailed analysis of photosynthesis, offering complex but in-depth explanations which includes an examination of the photosynthetic carbon cycle on a molecular level. General chemical equations are presented, and inputs, such as different nutrients, are correlated to respective products such as fats and other organic compounds. The effect of biomass as both an emitter and a sequesterer of atmospheric carbon is discussed by Emil and Winnett (199?)..(sic) They say that domestic policy options can produce substantial carbon benefits although some markets may be disrupted.

Shepherd and Davies (1993) examine the loss of carbon from the rapeseed plant into the soil through the roots. They conclude that 17-19% of the fixed CO₂ is transferred to the roots, with 30-34% of that released into the rhizosphere, 23-24% respirated directly as CO₂. Of the carbon released into the rhizosphere, 35-51% is assimilated by micro-organisms in the soil.

Reicosky (1994) and Galinato, et al. (1987) examine field reclamation principles and post-harvest use of excess crop material. Reicosky (1994) examines various physical and chemical processes that occur in the field after harvest, including post-harvest soil condition in which carbon plays an important role. The three important agricultural cycles (carbon, water, nitrogen) and their interdependence are discussed (Fig. 1), which includes a brief look at the processes carbon matter takes part in when it is left in the soil such as microbial decomposition. Galinato et al. (1987) discuss present and possible future uses of residual crop biomass, which present alternative pathways the carbon may take.

Ellington and Meo (1992) present a quantitative method that accounts for all greenhouse gases emitted from technical systems. They use a systems approach to compute the total warming related to the total lifetime useful output of the system to yield an index of performance that they call the Greenhouse Warming Index. They calculate 160 kg of CO₂ emitted for every 10⁹ J of methanol produced by the system and another 69 kg of CO₂ are emitted for every 10⁹ J of methanol burned in an engine.

Springer (1973), Obert (1973), Liliedahl et al. (1979), and Henein and Patterson (1972) examine the combustion processes of internal combustion (IC) engines. Engine fundamentals including combustion principles, chemistry, and differences between actual and ideal combustion are examined in these references. Formation of emissions, their composition, measurement techniques, reduction processes (emission controls), and comparison of emission output between different engine and fuel types are covered.

Peterson and Reece (1994), Geyer et al. (1984), and Mittelbach and Tritthart (1988), look at the emissions from B100 fuels and compare them to emissions from petroleum based fuel. Results from these tests show reduced CO and HC and essentially unchanged PM and NOx when using B100 compared to petroleum diesel.

Warner (1976) examines air pollutants in detail, citing causes, hazards, measurement techniques and environmental effects. Attention is also given to the natural cycle of some of these compounds, looking at both production and degradation. Stephenson (1949), Hopper (1978), and Cripps and Watkinson (1978) examine ways in which microorganisms biodegrade organic compounds, which include emissions from IC engines. The microorganism processes, required inputs, and resulting products are discussed.

Robbelen et al. (1989) and Ward et al. (1985) discuss properties of oilseed rape, although primarily producing oil for food products. Included in their material are various uses of rapeseed oil and meal. Properties of rape compared to other oilseeds are briefly discussed as well.

Peterson et al. (1995) examine the possibilities for agriculturally produced fuels. They assert that there is significant potential for energy production agriculturally, but that it will require a large initial capital investment. Stephens (1994) gives a very good description of the global warming problem and the carbon cycle.

PHOTOSYNTHESIS
One of the first principles of physics is that matter cannot be created nor destroyed. Thus, all the carbon is presently on the earth that ever has been or ever will be (space exploration and nuclear physics excepted). All naturally occurring hydrocarbons were fixed by the photosynthesis process. In the case of petroleum, the process occurred millions of years ago. It can be speculated that the earth had a carbon dioxide rich atmosphere which was removed by plants in the process of photosynthesis. These plant materials were subsequently subjected to heat and pressure to create the rich resources of petroleum found below the surface of the earth. As this petroleum is utilized, the carbon dioxide is returned to the atmosphere where it originated (Fig. 2). This freeing of carbon dioxide is one of the issues of concern with regard to global change.

Consideration of all the issues related to global change is not a simple process. In an attempt to study the full implications of the causes and effects of global change the current request for funds by the US Geological Survey Global Change Research Program is 1.8 billion dollars. This brief paper then is obviously limited in scope. We can only hope to provide insight on the concept of B100 substitution for diesel fuel and the process of fixing carbon for the B100 fuel.

B100 produced from rapeseed oil is a cyclical process (Fig. 3). The carbon dioxide released to the atmosphere was fixed by the plant in a recent year (perhaps the same year as the combustion). New rapeseed plants grown for future B100 production are fixing carbon during the currant growth cycle. According to the Arizona State University Photosynthesis center, "Photosynthesis is arguably the most important biological process on earth. By liberating oxygen and consuming carbon dioxide, it has transformed the world into the hospitable environment we know today. Directly or indirectly, photosynthesis fills our food requirements and many of our needs for fiber and building materials. The energy stored in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the energy in firewood, which is a major fuel in many parts of the world. ...If we can understand and control the intricacies of the photosynthetic process, we can learn how to increase crop yields of food, fiber, wood, and fuel, and how to better use our lands. The energy-harvesting secrets of plants can be adapted to man-made systems which provide new, efficient ways to collect and use solar energy. ...All of our biological energy needs are met by the plant kingdom, either directly or through herbivorous animals. Plants in turn obtain the energy to synthesize foodstuffs via photosynthesis. Although plants draw necessary materials from the soil and water and carbon dioxide from the air, the energy needs of the plant are filled by sunlight. Sunlight is pure energy. ...To be beneficial, the energy in sunlight must be converted to other forms. This is what photosynthesis is all about. It is the process by which plants change the energy in sunlight to kinds of energy which can be stored for later use. Plants carry out this process in photosynthetic reaction centers. These tiny units are found in leaves, and convert light energy to chemical energy, which is the form used by all living organisms.

One of the major energy-harvesting processes in plants involves using the energy of sunlight to convert carbon dioxide from the air into sugars, starches, and other high-energy carbohydrates. Oxygen is released in the process. Later, when the plant needs food, it draws upon the energy stored in these carbohydrates. We do the same. When we eat ..., our bodies oxidize or "burn" the starch allowing it to combine with oxygen from the air. This produces carbon dioxide, which we exhale, and the energy we need to survive. ...One of the carbohydrates resulting from photosynthesis is cellulose, which makes up the bulk of dry wood and other plant material. When we burn wood, we convert the cellulose back to carbon dioxide and release the stored energy as heat. Burning fuel is basically the same oxidation process that occurs in our bodies; it liberates the energy of "stored sunlight" in a useful form, and returns carbon dioxide to the atmosphere. Energy from burning "biomass" is important to many parts of the world. In developing countries, firewood continues to be critical to survival. Ethanol (grain alcohol) produced from sugars and starches by fermentation is a major automobile fuel in Brazil, and is added to gasoline in some parts of the United States to help reduce emissions of harmful pollutants.

"Our major sources of energy, of course, are coal, oil and natural gas. These materials are all derived from ancient plants and animals, and the energy stored within them is chemical energy that originally came from sunlight through photosynthesis. Thus, most of the energy we use today was originally solar energy.

"Currently, there is much discussion concerning the possible effects of carbon dioxide and other "greenhouse gases" on the environment. As previously mentioned, photosynthesis converts carbon dioxide from the air to carbohydrates and other kinds of "fixed" carbon and releases oxygen to the atmosphere. When we burn firewood, ethanol, or coal, oil and other fossil fuels, oxygen is consumed, and carbon dioxide is released back into the atmosphere. Thus, carbon dioxide which was removed from the atmosphere over millions of years is being replaced very quickly through our consumption of these fuels."

CARBON FIXATION BY THE RAPE PLANT
Rape, like all other plants, uses the process of photosynthesis to capture light energy and convert it into chemical energy that the plant can utilize. Photosynthesis is the process in which plants absorb carbon dioxide and water, and use light energy from the sun to convert them into glucose sugar (Britannica, 1993). Oxygen and water are created as secondary products and released back into the atmosphere. The plant uses the glucose, in combination with nutrients absorbed from the soil, for growth and development. The following equation describes the process in terms of a balanced chemical formula (Thomas, 1984):

sunlight energy

The important observation with regard to the carbon cycle is that the plant absorbs all of its carbon from atmospheric carbon dioxide (Britannica, 1993).

Studies show, however, that not all of the carbon dioxide fixed by the plant stays within it. Shepherd and Davies (1993) performed a study on rape seedlings (Brassica Napus L.) in which they concluded 17-19% of the fixed CO₂ was translocated to the roots over a period of two weeks. 23-24% of that was released into the atmosphere as CO₂, and 30-34% was released into the rhizosphere (soil). 35-51% of the carbon released into the rhizosphere was used by micro-organisms (Shepherd et al., 1993). This leaves 30-40 % of the carbon translocated to the roots in the soil. Thus the plant takes in more CO₂ than is accumulated in the plant biomass.

A rapeseed yield of 2.24 t/ha will yield approximately 935 L/ha (editor: Using solvent extraction, around 700 l for extruder press.) of oil and 1400 kg/ha of meal and 5,600 kg/ha of biomass (Peterson et al., 1995). Rapeseed oil, rapeseed, rapeseed meal, rapeseed biomass and wheat have been analyzed for carbon content by a commercial laboratory as follows:

Another possible use for the residue that may be considered economically is to collect the residue and use it as a biomass fuel resource. This has been studied with various other crops, including wheat, barley, and corn (Galinato et al., 1987). Rapeseed residue, as when the rapeseed meal is used for fuel, returns the carbon to the atmosphere in the form of exhaust gases from combustion.

An estimation of the carbon fixed by an average crop of rapeseed producing one ton per acre is:

Processing
During processing of rapeseed the carbon cycle becomes considerably more sophisticated and complex, due the various number of processing techniques that may be utilized. The basic initial step for processing, is the extraction of the oil from the seeds. From the above table, the seed has a carbon content of 58.4 %. Extraction can be performed mechanically, chemically, or by combination of the two methods (Peterson, 1986). After the oil is extracted, the resultant meal (with carbon content of 46.5%) may be disposed of in various ways. It can be used as a biomass fuel source, organic fertilizer, or feed for livestock although there are limitations with this last application (Ward, 1985, Robbelen, 1989; Peterson et al., 1983).

After retrieving the oil, the next step is purification. This involves removing impurities and glycerol from the raw oil. Any of a number of processing techniques may be applied, depending on exact characteristics desired in the fuel. Filtration is done by placing the oil in settling tanks, forcing the oil through elements, or both (Peterson, 1986). Degumming procedures may then be applied, such as flushing the oil with warm water (Peterson, 1986, Strayer et al., 1983).

Processing the oil into B100 requires transesterification. This consists of reacting the triglycerides in the oil with an alcohol to produce esters of both glycerols and fatty acids (Peterson, 1986). Generally, methanol or ethanol is used, forming methyl and ethyl ester, respectively, though butanol which forms butyl esters has also been used (Klopfenstein et al., 1983, Nye et al., 1983).

Certain advantages of transesterification include a higher cetane rating and lowered fuel viscosity, which leads to less trouble with the engine (Clark 1984, Peterson 1986). It is important to note that the addition of alcohol to the process also brings with it additional carbon.

COMBUSTION
Liljedahl et al. (1979) states "In general, all crude petroleum is made up of combined carbon and hydrogen in approximately the proportion of 86% carbon to 14% hydrogen. The atoms of carbon and hydrogen may be combined in many different ways to form many different hydrocarbon compounds in crude oil."

The combustion equation for any hydrocarbon is of the form (Obert, 1968):

C_nH_an+b + c O + 3.76cN - d CO₂ + eH₂O + 3.76c N
a,b,c,d,e and n are constants for a particular fuel

For example, a typical diesel fuel, C₁₆H₃₄ has a theoretical combustion equation of:

C₁₆H₃₄ + 24.5 O₂ + 3.76*24.5 N - 16CO₂ + 17H₂O + 3.76*24.5 N This equation shows that 1.4 kg of CO₂ is produced for each 453 grams of diesel used as fuel.

Reece (1995) computed the following hypothetical formulas's for the methyl and ethyl esters of rapeseed and canola oils:

Fuel          Hypothetical Formula           Molecular Weight

RME                C21H28O2                     323.38

CME                C19H35O2                     295.29

REE                C22H43O2                     340.12

CEE                C20H37O2                     309.38

C₂₂H₄₃O₂ + 34.75 O₂ + 3.76*34.75 N- 22CO₂ + 21.5H₂O + 3.76*34.75 N

This equation shows that 1.3 kg of CO₂ is produced for each 450 grams of B100 used as fuel.

In the theoretical combustion equation all of the carbon goes to carbon dioxide and the nitrogen from the air goes through the process unaffected. In the actual combustion process, part of the carbon remains as various hydrocarbons, part of the carbon forms carbon monoxide and aldehydes, and part of the nitrogen is converted to NOx. In recent tests at the Los Angeles Metropolitan Transit Authority Emissions Test Facility (LA-MTA ETF), the following emissions were measured from typical tests (Peterson and Reece, 1994):

It is easily seen that most of the carbon (99% plus) goes to carbon dioxide. While emissions of HC, CO and aldehydes constitute only a small percentage of the carbon output from combustion. These are important because of their environmental effects. B100 may help reduce both regulated and non-regulated emissions. As far as the carbon cycle is concerned these intermediate compounds constitute only a minor part of the picture. A full discussion of the emissions aspects of the carbon cycle is beyond the scope of this paper and is for practical consideration beyond the scope of currently available data. These intermediate products eventually return to CO₂ through the action of microbes and various other degradation processes. Reaction rates vary however, and for certain compounds such as the free carbon in the PM, for example, the degradation may occur over a very long time.

It can be concluded, that if combustion were complete, all of the carbon in the fuel would be converted to CO₂ and released to the atmosphere. Further, the intermediate products of combustion would also eventually return to the atmosphere as CO₂. Diesel produces 8.7 percent more CO₂ per kilogram than does B100; but diesel has 12.3 percent more energy per kilogram than B100, thus less fuel is required to do the same work. The B100 is an oxygenated fuel and according to some reports has a slightly higher thermal efficiency, however, as a practical matter, it can be assumed that per unit of work done, the amount of CO₂ produced would be nearly the same whether using diesel fuel or B100.

CARBON DIOXIDE REDUCTION FOR USING B100

Carbon dioxide exchange between diesel and B100 will be discussed on two levels. First, a simple exchange of B100 for diesel fuel and second, a discussion of the relevant factors affecting the exchange which could be considered due to production, processing, and transportation of the two fuels.

1. Carbon Dioxide to the atmosphere when Diesel is replaced by B100

   As previously demonstrated, 1.4 kg of carbon dioxide are produced for 453 grams of diesel fuel used and approximately the same amount would be produced for an equivalent energy content of B100. Peterson and Reece (1995) report that rapeseed B100 has a gross heat of combustion of 17,500 Btu/lb and Diesel 19,652 Btu/lb. This means that it takes 12.3 percent more B100 (mass basis) to produce the same energy as Diesel whereas the theoretical carbon balance shows 8.7 percent more B100 will produce the same amount of carbon dioxide. The difference is due to a) the oxygen in the B100 and b) theoretical versus actual substances.

   Typically an acre of rapeseed produces 378 litres of B100 which can replace approximately 92 gallons of diesel fuel and would replace 918 kg of carbon dioxide from petroleum with 948 kg of carbon dioxide from a renewable plant source. Further, the plants on each acre would process 5375 kg of carbon dioxide including the 18 percent estimated to be transported to the soil through the root system. Figure 3 shows the various carbon pathways. Most of this would eventually be returned to the atmosphere as carbon dioxide through the processes of biodegradation. However some may be fixed as free carbon, and some is stored in less biodegradable forms. The renewable fuel thus potentially removes more carbon dioxide than the simple exchange of fuel combustion equivalents would suggest.

2. Factors affecting the exchange which could be considered due to production, processing, and transportation of the two fuels.

   A further discussion of the carbon dioxide emissions from B100 compared to diesel fuel would require an analysis of the total system. B100 is grown on agricultural land requiring inputs of energy for tillage, chemical applications, chemical production, machinery production, harvesting, crop transport, storage and labor. Processing requires energy to operate mixers and pumps, production of storage vessels, and labour. The fuel would also require transportation to it's point of use. Auld and Peterson (1989) estimated that 4.2 units of energy were produced by rapeseed oil for every unit of energy input. Other researchers have variously estimated from 3 to 4 units of energy output for each unit of energy input for B100.

   It should also be noted that land is not bare when not growing vegetable oil crops. How does the carbon dioxide utilization for rapeseed compare to another crop such as wheat? Some crops such as sugarbeets or vegetable oil crops are very intensively cultivated. As an example consider an 80 bushel per acre wheat crop which would produce about 3628 kg of straw biomass (Wilkens et al., 1989). The following table provides a carbon dioxide processing estimate for an 80 bushel per acre wheat crop.

Assuming the biomass in wheat straw contains carbon equivalent to rapeseed biomass, the wheat crop would process about 10,341 kg of carbon dioxide compared to the 5375 kg for rapeseed. The purpose of this show is that some plants may remove more carbon dioxide than rapeseed, while at the same time a very low biomass producing crop could process considerably less.

Diesel fuel also requires energy inputs for exploration, drilling, pumping, transportation, processing and storage. No estimates of the total energy required for producing diesel fuel have been found in the literature search. If we assume it is approximately the same as for B100 the estimates of carbon dioxide exchange given above are adequate. Projected refinery energy usage for the U.S. refinery industry simulated by three clusters in three selected regions of the U.S. is given below:

Refinery Energy Usage - 1985
(Sittig, 1978 projection)

Sittig (1978) reports that the fuel consumed by a refinery in the processing of crude to finished product is in the order of 8 to 11 percent of the crude run. Additional fuel would be required for pumping, storage and transportation.

USDOE (1995) says that for every barrel of oil produced to date in the U.S., two have been left behind. The U.S. oil industry has produced 160 billion barrels, but some 350 billion barrels remain. Most of the remaining oil is difficult to produce. It is locked in complex geologic structures or is simply beyond the capability of today's recovery processes. U.S. crude oil supplies are becoming heavier and of higher sulfur content. This creates new challenges for refiners to produce light-end motor fuels and other products. The U.S. exploration and production industry faces ever tightening environmental regulations and is spending more than $1.5 billion per year to comply with EPA requirements. If environmental regulations are not made more "risk-based, site-specific and scientifically grounded" production may be reduced by 330,000 barrels per day or 120 million barrels per year in 2020.

This data demonstrates that while B100 production requires outside resources, production of petroleum diesel also requires outside resources. It is difficult to place a comparable quantity on each, however petroleum diesel probably requires 15-20 percent of the energy produced to create the product while B100 requires from 25 to 33 percent to create the product. In terms of the carbon cycle this means that an additional 5 to 15 percent B100 would be required to compensate for each gallon of diesel replaced.

CONCLUSIONS
Complete combustion converts hydrocarbon fuels to carbon dioxide and water. Diesel fuel represented by C₁₆H₃₄ releases 1.4 kg of CO₂ per 453 grams of fuel used in combustion. B100 releases 1.3 kg of CO₂ per 453 grams of fuel used in combustion. Incomplete combustion can result in small amounts of carbon monoxide and aldehydes which eventually also degrade into carbon dioxide.

In it's most simple form the carbon cycle consists of the fixation of carbon and release of oxygen by plants through the process of photosynthesis, then the recombining of oxygen and carbon to form CO₂ through processes of combustion or respiration. The carbon dioxide released by petroleum diesel was fixed from the atmosphere during the formative years of the earth. Carbon dioxide released by B100 is fixed by the plant in a recent year and is recycled by the next generation of crops.

Many scientists believe that global warming is occurring because of the rapid release of CO₂ through combustion of stored carbon such as petroleum diesel. Replacing petroleum diesel with B100 could reduce the CO₂ accumulation in the atmosphere. The reduction is not an exact one-to-one replacement. It is estimated that each 4 litres of B100 releases 1.1 to 1.2 times the CO₂ released in the atmosphere by 4 litres of petroleum diesel but the B100 CO₂ will be recycled by a future rapeseed plant.

CYCLES IN AGRICULTURAL ECOSYSTEMS

Figure 1. Carbon, water and nitrogen cycles from Reicosty (1994).

Figure 2. Carbon Cycle from Cripps and Watkinson (1978).

Figure 3. The rapeseed oil carbon cycle.

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