Time to tackle another major climate warming culprit – CH41 January 2009
As a result of natural gas, crude oil and coal production huge amounts of methane, a far more effective GHG than CO2, are entering the atmosphere. Maarten J. van der Burgt analyses possible solutions.
When it comes to studying greenhouse gas (GHG) emissions in relation to to the use of fossil fuels the attention is, currently, fixed mainly on carbon dioxide (CO2). This is logical enough, as most fossil fuels are directly or indirectly combusted and when this is carried out efficiently the carbon in the fuel ends up in the atmosphere as CO2. What is often forgotten is that during the production and processing of all fossil fuels large amounts of methane (CH4) are produced that per mole has a greenhouse effect eight times as great as that of CO2 for a time horizon of 100 years. This implies that even when methane is disposed of by being fully combusted this would be beneficial for the environment because the greenhouse impact would be reduced by almost an order of magnitude.
Methane emissions are often blamed on ruminants and decomposition emissions from agriculture, marshland, etc. Governments and industries responsible for fuel-related methane production are all too happy to allow this misconception to continue whereby we blame the methane production on unavoidable emissions partly associated with our milk and meat consumption rather than to our fossil fuel consumption.
The reality is that as a result of natural gas, crude and coal production huge amounts of methane are entering the atmosphere. An infrared overview of the world (Figure 2) already shows much of the anthropogenic wrongdoings in terms of GHG emissions. Apart from the fact that we consume far too much fossil fuel in industrialised countries and some oil producing regions in the world, we can clearly see the flaring of associated gas in countries such as Nigeria, Gabon, Venezuela, the Middle East and Russia. Proper flaring that is required in order to convert all methane and other hydrocarbons into CO2 and water is not an easy job in a remote area. Mixing large amounts of often-fluctuating gas supplies with air requires fairly complex burners and steam assistance and this is often no option in remote fields. As a result of incomplete combustion large amounts of methane are entering the atmosphere. Moreover the associated gas is often vented.
Figure 2 also shows bushfires in large parts of Africa, South America and Southeast Asia. All these fires involve incomplete combustion that is another source of methane production.
Emissions from production and transport of fuel
What the infrared picture does not show are the emissions from leaky pipelines and processing equipment. Although pipelines are generally reasonably well-maintained leakage is nonethlessl estimated at 2% of the total natural gas production of 2600 billion m3/year.
As on average 90% of the natural gas is methane, the emission from it is 47 billion m3/year, which corresponds to emissions of about 380 billion m3/year of CO2 or 750 million ton/year.
The methane emission related to the production of crude oil is mainly caused by associated gas that is co-produced with the crude in regions where there is no market for the gas. In the continental part of the USA this gas is mostly added to the natural gas pipeline network that is locally available. However, in other areas in the world this option is often not available and the gas is mostly flared. The quantities of gas flared are very substantial. Delucchi gives a figure of 3–4 trillion cubic ft/year or 85 – 113 billion m3/year (Delucchi M, A Lifecycle Emissions Model (LEM), www.its.ucdavis.edu/publications.html, 2003). Compared with a total crude oil production of 88 million bbl/day corresponding to 4.6 billion ton/year even the highest figure would imply that associated gas constitutes only 2% of the combustion energy leaving the crude oil well (Simmons, New Scientist 28 June 2008). This figure is a factor 5 lower than mentioned in other sources (van der Burgt, MPS, April 2004). There are various causes for this discrepancy. Models are intrinsically unreliable because much depends on the meteorological conditions, types of flares, surveillance, etc. Admittedly some efforts have been made to reduce flaring and venting by re-injection of the associated gas into the oil fields. The reason is not so much environmentally driven but because it enhances oil production. Further, oil companies have little interest in measuring the amount of associated gas, and if they do they have even less interest in giving the results to outsiders. However, the most important reason is that both governments and oil companies are for political reasons keen to downplay the amount of associated gas flared or vented. Some governments close their eyes to venting or even promote it because they do not like the sight of big flares. A (possibly conservative) compromise figure for associated gas being flared or vented is 5% of the combustion energy leaving an oil well corresponding to 280 billion m3gas/year. As about 90% of the gas is methane this corresponds to about 250 billion m3 methane/year.
Delucchi reasons that maybe as much as 20% of the associated gas is being vented. Moreover for flaring of associated gas no sophisticated combustion equipment is used and therefore it is reasonable to assume that 3-5% of the methane in the gas is not combusted. The above figures imply that world-wide 24% of the associated gas is entering the atmosphere, corresponding to methane emissions of 60 billion m3/year.
Nor is the methane emission associated with coal mining shown in Figure 2, because the available statistics differ too widely. The problem is the same as with associated gas. On the one hand mining companies are keen to report low figures. On the other hand they are in the best position for producing the methane emissions. Implicitly Delucchi gives figures of both 70 and 140 billion m3/year methane. Another estimate, by the US EPA in 2008, has it at about 100 billion m3/year of methane.
The exploitation of coal bed methane is a source of some encouragement. In the USA 10% of gas production is already derived this way. In Australia, Arrow Energy is very active in this field and Shell is also very interested in this development. All these emissions are summarised in Table 1.
There are in principle four solutions to the methane problem.
• Reduce leakage. This is possible in all cases but is most important for natural gas handling plants and pipelines.
• Assure complete combustion by proper flaring where over 99% of the methane is combusted to CO2. This of course is only a solution for associated gas from crude oil production. It is relatively low cost and almost certainly lower cost per GHG equivalent, in the industrialised world, than removal and storage of CO2. Almost as important as the lower costs is the fact that the NIMBY aspect of CO2 transport is absent. Moreover as CH4 has per mole an 8 times stronger GHG effect than CO2 proper combustion would on its own reduce the overall GHG effect of associated gas by over 60%. This is the major GHG contribution of the ‘well to car’ GHG emissions that oil companies are responsible for. Also in refineries the ‘well to car’ efficiency could be improved by co-generating electricity and thus making better use of the exergy potentially present in the fuels used in furnaces in a refinery (Van der Burgt, Modern Power Systems, April 2004. This option becomes more and more attractive with the processing of the ever-heavier feedstocks and the demand for lighter and essentially sulphur free products. In relation to the above mentioned points, heavy tar sands oils are an interesting feedstock as it is the only fossil fuel where hardly any methane is co-produced during its production. But combustion of methane is not much of a solution for leakage of methane from natural gas pipelines or from coal mining. For ‘removing’ methane from ventilation air from coal mining the best solution is to use this air as combustion air in furnaces or gas turbines. Unfortunately this option, where you even make use of the heat of combustion of methane, is seldom used.
• Conversion of methane into liquid hydrocarbons by Fischer-Tropsch synthesis or into methanol (gas-to-liquid, GTL, processes). The problem is that there are as yet no small-scale plants that can be economically used for these syntheses. It is not the synthesis per se that with existing processes cannot be economically carried out on a small scale but the conversion of the methane into the synthesis gas preceding the process. Combiforming of methane into synthesis gas comprising steam methane reforming followed by partial oxidation with air could be a possible solution. If such small-scale plants are not an option the methane has to be gathered requiring expensive two-phase pipelines for oil and gas or separate pipelines for oil and gas. Then large scale plants can be used to convert the methane into excellent motor fuels in GTL plants, methanol or into LNG. Dual-phase pipelines do exist, eg in North Sea fields where the associated gas is separated from the oil on land and used in the natural gas grid. To install such more expensive pipelines on land requires political stability to ensure a safe operation. Understandably oil companies are reluctant to make the investments required because of the political instability of many of the oil producing countries.
• Using methane to raise electricity. The big advantage of this investment is that it can be done economically on a small scale provided there is a market for the power. Low mass small gas turbines such as the OPRA turbines that are available in the range of 2-10 MW would be very interesting for this purpose. In countries like Nigeria such solutions could be attractive in supplying nearby villages with electricity and thus co-generating some goodwill among the population.
It may be concluded that apart from good housekeeping the most promising options for reducing methane emissions are power generation and small scale Fischer-Tropsch or methanol plants. The problem with GTL plants is that such a process is as yet not commercially available. LNG always requires large plants. Converting all oil associated gas in the world into LNG, power and/or liquid hydrocarbons would reduce the anthropogenic GHG emissions related to fossil fuel production by about 10% (Knoef and van der Burgt, Handbook of biomass gasification, BTG Biomass Technology Group BV, 2005). For coal related methane emissions the only solution is to build, wherever practical, power stations on top of a mine and use the mine gas as combustion air, or if it is sufficiently concentrated use it as fuel in such a power station, but this solution will only be practicable at a relatively small number of mines.