Challenges in Combustion

Combustion is an applied science that is important in transportation, power generation, industrial processes, and chemical engineering. In practice, combustion must simultaneously be safe, efficient, and clean. 

Combustion has a very long history. From antiquity up to the middle ages, fire along with earth, water, and air was considered to be one of the four basic elements in the universe. However, with the work of Antoine Lavoisier, one of the initiators of the Chemical Revolution and discoverer of the Law of Conservation of Mass (1785), its importance was reduced. In 1775-1777, Lavoisier was the first to postulate that the key to combustion was oxygen. He realized that the newly isolated constituent of air (Joseph Priestley in England and Carl Scheele in Sweden, 1772-1774) was an element he then named it and formulated a new definition of combustion, as the process of chemical reactions with oxygen. In precise, quantitative experiments he laid the foundations for the new theory, which gained wide acceptance over a relatively short period. 

Initially, the number of scientific publications on combustion was very small. At that time combustion experiments were conducted at chemical laboratories. From the very beginning up to present times, chemistry has contributed a lot to the understanding of combustion at the molecular level. 

The chronology of the most remarkable contributions to combustion in the early stages of its development is as follows. In 1815, Sir Humphry Davy developed the miner s safety lamp. In 1826, Michael Faraday gave a series of lectures and wrote The Chemical History of Candle. In 1855, Robert Bunsen developed his premixed gas burner and measured flame temperatures and flame speed. Francois-Ernest Mallard and Emile Le Chatelier studied flame propagation and proposed the first flame structure theory in 1883. At the same time, the first evidence of detonation was discovered in 1879-1881 by Marcellin Berthelot and Paul Vieille this was immediately confirmed in 1881 by Mallard and Le Chatelier. In 1899-1905, David Chapman and Emile Jouguet developed the theory of deflagration and detonation and calculated the speed of detonation. In 1900, Paul Vieille provided the physical explanation of detonation 

However, a new era in the development of combustion science started with the foundation of the Combustion Institute in 1954, on the initiative of Bernard Lewis. The institute s influence was further strengthened in 1957 with the release of its journal Combustion and Flame. The creation of the institute brought about two important factors into research activity organization of the combustion commxmity and stimulation of international cooperation. 

---

Chapter 1 provides a brief introduction surranarizing the main challenges in combustion. It recalls the key events in the progress of combustion science concisely. 

At the start of the twenty-first century, efforts are underway to decrease society s dependence on fossil fuels. It is clear that alternate energy forms will bring with them their own sets of reactive radical intermediates and revisit the important intermediates seen from smaller model compounds, as we consider future challenges in combustion chemistry. We expect that advances in experimental techniques and computational approaches will correspondingly be developed in the years ahead. 

The main challenge in combustion research lies in the need to make a large number of measurements simultaneously. First, it is essential to know how well the fuel and air are mixed, together with their spatial and temporal distributions in the combustion chamber. Second, the spatial and temporal variations in temperature must be measured. Third, the large number of chemical intermediates, whose concentration changes rapidly with time, has to be determined. 

The challenge in these designs is to lower the NO without degradation in unit stability. In the combustion of fuels that do not contain nitrogen compounds, NOx compounds (primarily NO) are formed by two main mechanisms, thermal mechanism and the prompt mechanism. In the thermal mechanism, NO is formed by the oxidation of molecular nitrogen through the following reactions ... 

The challenges in this process are significant. Gasoline-fueled (and diesel-fueled) vehicles powered by internal combustion engines (ICEs) won the market competition over other motive systems and their performance has since been improved and refined for more than 100 years. Furthermore, a vast infrastructure has been established that manufactures, maintains, and fuels the current vehicular fleet in the United States. 

Challenges in Propellants and Combustion - 100 Years after Nobel, Hrsg. Kuo, KK. et al., Begell House, Inc., New York, USA, 1997... 

Measurements of total odour strength in combustion processes imply sampling challenges. Beside the chemical scrubber process, combustion of odorous air is the best odour reducing method. The disadvantage of this process is the high energy costs. Treatment at apropriate conditions, however, will destroy the odorous compounds extensively. Temperatures about 850 C and contact time up to 3 seconds are reported (2, 3). 

Instrumental methods have become more sophisticated to face these challenges. In particular, Westmoreland and Cool have developed a flame-sampling mass spectrometer that has provided several revelations in terms of relevant molecular intermediates in combustion. " Their setup couples a laminar flat-flame burner to a mass spectrometer. This burner can be moved along the axis of the molecular beam to obtain spatial and temporal profiles of common flame intermediates. By using a highly tunable synchrotron radiation source, isomeric information on selected mass peaks can be obtained. This experiment represents a huge step forward in the utility of MS in combustion studies lack of isomer characterization had previously prevented a full accounting of the reaction species and pathways. 

An additional objective of this chapter is to provide mechanistic details and timescales for the important class of reactions in organic carbonyls. Organic carbonyls are of interest due to their importance in atmospheric chemistry and related areas [1, 5-7], Carbonyls are important in addition to atmospheric chemistry in combustion, petroleum chemistry, biochemistry, and food chemistry. In several of these, the issue of photochemistry/photostability arises. Mechanistic details are known currently for relatively small carbonyls, as seen in the work by [8-30]. Large molecules are challenging for the simulation. 

Ivanov, G.V., andTepper, F. (1997) Activated aluminum as a stored energy source for propellants, in A Challenge in Propellants and Combustion 100 Years After Nobel (ed. K.K. Kuo), Begell House, Stockholm, pp. 636-645. 

Formation of Aromatic Compounds A scientific challenge comparable to that of developing oxidation mechanisms for the large hydrocarbon fuels is understanding and describing quantitatively the formation and oxidation of aromatic and polycyclic aromatic compounds (PAH) formed in combustion processes. Aromatic compounds are known to be harmful to the environment, and the emission of these species from a number of combustion systems is a significant concern. Furthermore aromatic species are important pre-... 

The concepts discussed so far indicate that the major challenge in asymmetric operation is correct adjustment of the loci of heat release and heat consumption. A reactor concept aiming at an optimum distribution of the process heat has been proposed [25, 26] for coupling methane steam reforming and methane combustion. The primary task in this context is to define a favorable initial state and to assess the distribution of heat extraction from the fixed bed during the endothermic semicycle. An optimal initial state features cold ends and an extended temperature plateau in the catalytic part of the fixed bed. The downstream heat transfer zone is inert, in order to avoid any back-reaction (Fig. 1.13). 

Challenges in Propellants and Combustion - 100 Years after Nobel, Editor Kuo, KK. et al., Begell House, Inc., New York, USA, 1997 Klingenberg, G., Knapton, J. D., Morrison, W. F., Wren, G. R, Liquid Propellant Gun Technology, AIAA, Reston, VA, 1997 Solid Propellant Chemistry, Combustion, and Motor Interior Ballistics, Editors Yang, V., Brill, T. B., Ren, Wu-Zhen, AIAA, Reston VA, 2000... 

An LHV gas may contain up to a few thousand ppm of ammonia, produced from fuel-bound nitrogen during the gasification of a solid fuel. One of the major challenges in the catalytic combustion of LHV gases is to circumvent the formation of NO from this ammonia. The selectivity for this reaction is strongly dependent on the air-fuel ratio in the catalytic combustor and on the catalyst type [102,105], Clark et al. [102] and Tucci... 

---