Бурение грунтовых зондов, установка энергетических колодцев

The immediate and apparent effects that sulfur and particulate matter have on human health and SOx and NOx have on the world around us (forests dying from acid rain), have provided a strong motivation for the substantial progress that has been made in reducing these emissions from both the power and the transport sectors. In contrast, the potential damage of C02 emissions is a long-term issue for which the mechanisms are not fully understood, and this has led to the fact that no clear strategy has emerged to counter the problem despite the intensive debate over the last 10 or 15 years. Nonetheless, there is no disputing the correlations between global average temperatures and atmospheric C02 concentrations (as determined from Antarctic ice cores) over hundreds of thousands of years, and between anthropogenic emissions and atmospheric C02 concentrations over the last 250 years (Figures 9-5A and 9-5B). And whether we understand the mechanisms in detail or not, it is certainly wise to take measures to reduce man-made contributions to global warming.

Before discussing what place gasification could have in any such strategy for C02 emissions reduction, it may be useful to have a look at the overall greenhouse gas effect as related to the use of fossil fuels.

As can be seen from Figure 9-6, the two biggest contributors to C02 emissions are the electric power and transport sectors. Given the millions of small moving emission sources involved in transport, any significant reductions in C02 emissions is only likely to emerge through the change to a less carbon-intensive fuel such as natural gas or hydrogen. Although the use of natural gas (at least while it lasts) would still result in large, even if lower, emissions from millions of individual sources, the use of hydrogen offers the possibility of bundling C02 emissions at fixed locations in a manner that would allow fixation or sequestration. Further discussion of this aspect is discussed after first looking at the largest C02 emitter, the power sector.

A major contribution to lowering C02 emissions in the power industry could be realized almost immediately simply by increasing the efficiency of the power park. Replacement of a 30-year-old coal-fired unit operating at, for example, 32% efficiency by a modern plant (whether with IGCC or ultra-supercritical PC technology) using the same fuel with an efficiency of 43% would drop the C02 emissions per produced kilowatt-hour by 25%. The barriers here are not of a technological but of a financial nature given the limited capital and incentive for such replacement projects. Again, the reductions offered by cogeneration (or combined heat and power), although ultimately limited by the available heat sinks, face at present financial rather than technical hurdles.

Commercial Residential

Beyond the above, serious consideration is now being given to recovery of the C02 from the energy conversion process and putting it to use or simply sequestering it. In such a scenario there are some natural advantages to gasification over combus­tion technologies. In fact, C02 capture and usage is already standard practice in almost all gasification-based ammonia plants, where the C02 is recovered from the synthesis gas and used to manufacture urea.

Another example of C02 recovery from a coal gasification plant is the Great Plains SNG plant, which sells C02 for enhanced oil recovery (EOR) in the Weyburn field in Canada. EOR is particularly attractive for C02 usage in that besides the avoidance of adding to atmospheric C02, it provides a means of extending the life of existing energy resources. EOR has been practiced in the Permian basin oil fields in Texas for over 30 years, although much of the C02 used comes from natural under­ground C02 reservoirs and therefore does not as such contribute to greenhouse gas abatement. Nonetheless, it points the way to one potential route for sequestration. Other alternatives have been or are being investigated as well, such as sequestration in underground saline aquifers (e. g., Statoil’s Sleipner project in the North Sea), storage in coal seams, and others (White 2002; Beecy 2002).

As mentioned above, gasification-based processes have a natural advantage over combustion technologies when it comes to C02 capture. In one study based on using gasification with total water quench technology at 80 bar, the addition of C02 capture was shown to add only a small increment on capital costs (about 5%) and an efficiency penalty of only 2% (O’Keefe and Sturm 2002). The block flow scheme is shown in Figure 9-7.

This should be compared with the impact of C02 capture on a ultra supercritical PC combustion technology power block. This is extremely heavy even if one assumes the prior existence of FGD and SCR flue gas treatment, which are a prereq­uisite for C02 recovery from the flue gas, for which at present amine scrubbing is

universally proposed. The key fact is that the volume of gas from which the C02 has to be extracted is in the case of combustion technology 150-200 times larger than is the case in an IGCC plant. The add-on capital cost has variously been estimated at 60-80%. The steam requirement for amine regeneration is lost to the final turbine stages, and this causes a drop in efficiency of some 9.5-14 percentage points (Simbeck 2002; Koss and Meyer 2002).

Clearly there is an incentive to find some use for the C02 and “recycle” it. Although there may be local markets where this is possible on a small scale, a review of the data in Table 9-5 will show that the “chemical” scope for such a solution on a global scale is very limited. Apart from oxygen and hydrogen, there are no potential reactants with relative mass flows approaching that of carbon. Even iron usage is an order of magnitude smaller. It is further observed that the only mass in the world that is of the same order of magnitude as fossil fuels is waste biomass flow (though this is not valid for the energy, which is an order of magnitude less) (Shell 2002).

The only major application of waste C02 is for the enhanced recovery of oil and gas, but most of the C02 will have to be sequestered underground in depleted gas fields, in aquifers, and in deep-sea basins (van der Burgt, Cantle, and Boutkan

1992) .

An alternative option for removing C02 from flue gases from power stations that has been proposed is combustion with pure oxygen. The flue gas will then contain only water and C02, and in the case of coal and heavy oil fractions, also S02, NOx, and other contaminants such as mercury. The advantage is that in this case all contaminants can be sequestered together, and apart from the ASU, no gas separations are required. However, with the present cryogenic separation of air, this solution is unlikely become attractive for both economic and efficiency reasons. Moreover, in

the case of CC power stations, this route would require the development of gas turbines that is optimized to handle pure tri-atomic gases.

It should be noted that the IGCC variant provides a source of hydrogen that, with a combination of membrane and PSA technologies, can be extracted during periods of low electric power demand and stored for onward sale into the transport sector. The importance of this possibility is discussed further in Chapter 10.

Methane. On a mole for mole basis, methane contributes 20 to 25 times more to the greenhouse effect than C02. Anthropogenic methane from fossil sources enters the atmosphere in the form of vented or incompletely combusted associated gas, leaks in natural gas pipelines, and methane emissions related to coal mining.

The largest source of methane emissions resulting from fossil fuel production and refining derives from the associated gas. On average about 10% of the energy leaving an oil well is in the form of associated gas, and assuming that 5% of this gas is not combusted either because people do not want to see flares or because flares are not working properly, it is conservatively estimated that about 8% of the green­house gas effect related to the use of crude oil is due to methane emissions during production. Adding to this the effect of the C02 from the 95% of the associated gas that is properly combusted in operating flares, this figure increases to 15%. Schaub and Unruh (2002) have estimated that the quantity of associated gas flared in Nigeria alone would be sufficient to supply 30% of Germany’s natural gas supply. Clearly, there is potential here for a reduction in greenhouse gas emissions, to which synfuels production via partial oxidation of the gas and subsequent Fischer-Tropsch synthesis could make a significant contribution. Global application of such a solution could reduce the 15% of the greenhouse gas effect related to the use of crude oil to a mere 2-3%.

Комментарии запрещены.