Reduction of carbon dioxide emissions from the metallurgical industry

Another sector that consumes a lot of electricity and heat and thus is also responsible for a large share of anthropogenic greenhouse gas emissions is the metallurgical industry. It was estimated (in Steinfeld and Meier, 2004) that by replacing the non-renewable energy in the technology used for processing of aluminum (which requires very high temperatures ~2200°C) by energy deriving from solar irradiation, the CO2 emissions produced would be reduced by ~90%. In that sense, solar energy either directly (by solar furnaces) or indirectly (through the production of solar fuels) could be applied to multiple industrial processes that demand high amounts of energy (usually electricity), high process temperatures or a combination of both.

20.6 Conclusions

The high power density, ease of transportation and storage, and many years of development of internal combustion engine technologies have put hydro­carbon fuels at a privileged position in our energy mix. While for many years we have been accustomed to consider hydrocarbon fuels as a primary energy source, we must today adopt a different point of view, in order to mitigate the environmental, political and other consequences of today’s fossil hydrocarbon-based economy: reducing our dependence on fossil hydrocarbon fuels is necessary. However, this should not prevent hydrocar­bons from being used as a preferred energy carrier.

In order to invest in a sustainable future, the alternative energy sources explored should be not only environmentally friendly but also capable of meeting the continuously growing world energy demand. The most promis­ing candidate is solar energy, which can be employed in various applications and can be exploited either directly as a heat source in several processes or indirectly through energy carriers.

The direct use of solar energy, though, comes with several limitations (e. g., solar energy is not always available, should be consumed in a narrow area from its production point, etc.) that could be easily overcome when storing it as an ‘energy carrier’ such as hydrogen or a hydrocarbon. In order to maximize the ‘eco-friendly’ nature of solar energy, the hydrogen should be derived from carbonaceous-free or at least renewable carbonaceous sources (such as water or bio-fuels). However, in order to progress an orderly transition, there is a strong argument for combining solar thermal processing with fossil fuel feedstocks or biomass to produce hybrid solar fuels.

Finally, solar energy could be exploited in hydrocarbon production pro­cesses. In these processes, CO2 emissions are captured at the point of gen­eration and are treated as a valuable feedstock that reacts with hydrogen and is recombined back to solar liquid fuels. In this way both the issues of the high cost of implementing the new and appropriate infrastructure to utilize an alternative energy carrier (i. e. pure hydrogen) into an almost fully fossil fuel dependent world and CO2 storage, can be overcome simul­taneously, ensuring clean and cost-effective energy sufficiency in a carbon-neutral future.

[1] Developments and innovation in carbon dioxide (CO2) capture and storage technology Volume 1: Carbon dioxide (CO2) capture, transport and industrial applications

Edited by M. Mercedes Maroto-Valer

[2] Geological repository systems for safe disposal of spent nuclear fuels and radioactive waste

Edited by Joonhong Ahn and Michael J. Apted

[3] Wind energy systems: Optimising design and construction for safe and reliable operation

Edited by John D. S0rensen and Jens N. S0rensen

[4] This discussion applies most directly to CSP systems with thermal energy conversion; con­centrating photovoltaic (CPV) systems are discussed briefly in Section 2.7.5 and then further in Chapter 10.

[5] A black body is an ideal surface that absorbs all radiation incident on it and radiates a defined spectrum and intensity of radiation according to its own temperature. See, for example, Bergman et al. (2011).

[6] Intensity is also sometimes used in discussing solar irradiation; however, strictly speaking, intensity is not a radiant flux but radiant power per unit solid angle from a source.

[7] The area that is referred to here is the useful active absorbing area, often defined by an aperture.

[8] The angle в is now generalized to be the acceptance angle which could be more or less than es.

[9] Having established this limit under the condition of thermal equilibrium, we can then argue that it also applies away from equilibrium, since the reflections and refractions that lead to the optical concentration at the receiver are not a function of the receiver temperature. Hence this limit on concentration ratio is general.

[10] This result is known in non-imaging optics as the principle of conservation of etendue. A rigorous proof is given by Chaves (2008).

[11] An exception is the compound parabolic concentrator (CPC) used in certain non-tracking concentrators, such as for backing reflectors in solar hot water systems. CPCs also play a role in the secondary optics in certain types of systems (Meinel and Meinel, 1976).

[12] Consideration of a precise width in this way is based on an assumed pill-box sun shape, an approximation.

[13] This is the case with astronomical telescopes; they cannot have a large rim angle or else their imaging quality will be lost.

[14] This analysis ignores the effect of mirror shading by the receiver, and also considers only a single-sided receiver. For a fuller treatment, see Rabl (1976).

[15] In practice, lower angles of the order of 80° are commonly used, because of increasing cosine losses per glass area and increasing amplification of surface errors due to the great distance from mirror to receiver.

[16] A special case is the Lambertian surface, for which the apparent brightness of reflected radiation is equal in all directions. Such surfaces are useful in methods for characterizing concentrator performance, since they allow a photograph to be taken to record the irradiance in the focal plane of a dish, heliostat, etc.

[17] Note that CSP ray-tracing is quite distinct from the ‘backward’ ray-tracing used in CGI, animated movies, etc., which considers possible origins/luminous intensity of rays emanating from the eye of the observer in different directions.

[18] Note that evaluations can sometimes be presented based on aperture with or without the effect of receiver shading or total mirror area rather than aperture area, so results must be interpreted carefully.

[19] For low concentration systems, this is an experiment that can be performed, but for high concentration point focus systems, destruction of the receiver is likely before an empirical stagnation temperature could be established.

[20] Exergy is also sometimes referred to as availability. This should not be confused with avail­ability as a term used in the power industry for the fraction of time that a piece of equipment is available to function on demand.

[21] In a real system, operating temperature can be controlled through variation of HTF flow rate, for example.

[22] This is equivalent to treating optical efficiency and energy transport and/or storage efficiency as 100%.

[23] Whilst being closer to the equator improves the performance in inland Australia, note that equatorial regions generally offer poor performance due to tropical cloud and humidity.

[24] This is the most commonly recognized form of NPV on the assumption of annual compound­ing. Compounding can actually be done on any time scale including continuously. Also, in a strict mathematical sense, DR is a fraction per unit time and is multiplied by the compounding time interval (in this case 1 year).

[25] In a literal sense, a plant may be decommissioned at the end of its useful life. However, if a conservative assumption has been made, it may prove to be the case that the plant’s life actu­ally exceeds the value assumed.

[26] Dr David Mills has not worked with AREVA Solar since June 2010 and has no direct knowl­edge of Areva Solar’s present-day technology.

[27] Dr David Mills has not worked with AREVA Solar since June 2010 and has no direct knowl­edge of Areva solar’s present-day technology.

[28] The term heat transfer fluid is used because the fluid transfers heat from the solar field to the point of use and typically (for power generation) is different from the power cycle working fluid.

[29] 0

9.24 Typical input-output diagram of a dish Stirling system.

[30] In the language of thermodynamics, direct losses of energy are quantified via a ‘thermal’ or ‘first law’ efficiency. The effects of unavoidable temperature drop are quantified with a ‘second law’ or ‘exergetic’ efficiency.

[31] Whilst natural gas has been used nearly exclusively for backup purposes to date, in principle any fuel could be used, including biomass.

[32] Markets in some jurisdictions (Eastern Australia, for example), have actually moved to the concept of a continually varying cost of electricity based on supply and demand estimates for short future time intervals.

[33] In many cases, such scenarios are based on quantitative models and analysis, although in the case cited here the quantitative basis for the projections is not given.

[34] Two units are commonly used for carbon prices, per unit mass of carbon and per unit mass of carbon dioxide. $1/tonneC = $0.27/tonneCO2.

[35] Based on a tubular receiver with 70 mm diameter. The radiation heat loss increases about 50 W/m in the relevant temperature range 350°C when emissivity changes from 11% to 8%.

Rank = should material be investigated for CSP applications 1 = Yes, 2 = No, 3 = Maybe a TSSS = thickness-sensitive spectrally selective; b SS = stainless steel.

[37] In this analysis all heliostats in a field are assumed to be identical; any second order effects, such as the possibility that heliostats experience different average wind loads depending on location within the field, are neglected.

[38] ‘Range compression’ is used to make such images appear more natural to the human eye. Some consumer grade cameras can produce ‘raw’ files without range compression, but special­ist ‘machine vision’ cameras are nevertheless preferable for flux mapping work.

[39] Here ‘vertical’ refers to the projection of the global vertical axis onto the plane of the receiver aperture.

[40] So-called ‘dewar’-type evacuated tube units as used in stationary hot water heating arrays are available but have not been successfully applied with PT concentrators.

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