Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 39

Additives may catalyze or otherwise affect combustion processes. For example, salt has long been known to be of some assistance in removing soot deposits from chimneys and the carbonaceous feedstock treated with a more complex mixture of metal oxides has been reported to be activated in combustion systems but it is apparently not resolved whether the catalytic effect is with regard to carbon-oxygen reactions or whether it is more indirect since the effect resembles (to a degree) the catalysis of feedstock-steam systems in which alkali salts serve to catalyze the carbon-steam reaction to produce synthesis gas (carbon monoxide/hydrogen mixtures). This mixture may then, in turn, react to produce hydrocarbons and oxygenated materials in the presence of the multitude of trace metals that can (and do) occur in biomass feedstocks.

In addition to causing objectionable stack emissions, ash and volatile inorganic material generated by thermal alteration of mineral matter in the feedstock will adversely affect heat transfer processes by fouling heat-absorbing and radiating surfaces and will also influence the performance of the combustion system by causing corrosion, and operating procedures must therefore provide for effective countering of all these hazards.

Corrosion is mainly caused by oxides of sulfur, but in certain parts of a combustion system, specifically on furnace wall tubes with metal temperature of 290 to 425°C (550 to 800°F) and superheater or reheater tubes with temperatures in the range 600 to 700°C (1,110 to 1,300°F), corrosion can be induced by tube deposits that destroy protective surface oxide coatings.

Corrosion damage that is usually ascribed to sulfur is actually caused by sulfuric acid, which is generated from organic and inorganic sulfur-bearing compounds:

Oxidation of sulfur dioxide to sulfur trioxide occurs mostly in flames where (transient) atomic oxygen species are thought to be prevalent by interactions of hydrogen atoms with oxygen:

As well as by interactions of carbon monoxide with oxygen:

The process can be catalyzed by the ferric oxides which form on boiler tube surfaces and show excellent catalytic activity for sulfur dioxide oxidation at approximately 600°C (1110°F), i.e., at temperatures which occur in the superheater section of a boiler.

The presence of water has a marked effect on combustion (by participating in various combustion reactions) and there is evidence for the existence of active centers for chain reactions involved in the further combustion of carbon monoxide and hydrogen (which would be reaction intermediates in the combustion process).

The endothermic steam-carbon reaction is primarily responsible for cooling effects in furnaces and the presence of moisture is believed to cause heat generation at the surface of the bed and in the combustion by virtue of the (endothermic) formation of carbon monoxide and hydrogen in the bed which then burn at the surface. On the other hand, the presence of water vapor appears to assist in the formation of carbon dioxide:

Moisture appears to play a more integral role in the combustion of hydrogen-deficient carbonaceous fuels (such as coal) than has been generally recognized. The carbon-steam reaction to produce carbon monoxide and hydrogen (which are then oxidized to the final products) is an important stage in the combustion sequence as is the carbon monoxide shift reaction to yield carbon dioxide and hydrogen:

Since the whole system involves reaction (and equilibria) between the fuel (i.e., carbon), water, carbon monoxide, hydrogen, and carbon dioxide, the rapid rates of the reactions render it difficult (if not impossible) to determine precisely which of the reactions are the major rate-controlling reactions. In addition, the heterogeneous nature of the system adds a further complication.

The presence of inert gases would usually be expected to dilute the reactants and therefore diminish the reaction rates, such inert materials may actually, on occasion, accelerate the reaction(s). Indeed, the addition of nitrogen to the reaction mixture can be as effective as the addition of oxygen. The nitric oxide formed in the mixture is believed to act as a catalyst:

See also: Combustion.