Intraparticle effects in char combustion steady state analysis

Abstract

A model is developed to study the intraparticle effects of char combustion which incorporates the reactions, C + 1/202 → CO, CO + 1/202 → CO2 and CO2 + C → 2CO with appropriate intrinsic kinetics. Based on some limiting cases a feasibility region, inside which the solution should lie, is developed. In almost all cases the major product is CO with CO2 production 2 to 3 orders of magnitude lower. A maximum of three steady states is observed for the parameters chosen. The combustion rate decreases with increase in particle size and increase in boundary layer thickness. The solution structure depends strongly on the radiation conditions which obtain and on the transport properties inside the particle. The purpose of this study is to determine the effect of intraparticle transport without the confounding effects of other phenomena.

Diffusion and reaction in a char particle and in the surrounding gas phase. A continuous model

“…In general, it was found that at the ambient conditions used the multiplicity of the problem manifests itself, whenever multiple solutions arise, in a 1-3-1 pattern. The above result is at variance with the results from the shrinking core models (Caram and Amundson, 1977; Mon and Amundson, 1978; Sundaresan and Amundson, 1980) that revealed the existence of up to five steady states for some Figure 5. Behavior of the Tc, TB vs. Tb and Tp vs. Tb loci at higher intraparticle diffusional limitations.…”

“…The rate expressions used in this study and the references in which they are reported appear in Table I. Undoubtedly, the kinetics and mechanisms of the reactions are more complex (Laurendaeu, 1978), but on the one hand, there are few data in the literature regarding intrinsic reactivities of char or coal to gaseous reactants, and on the other hand, we want to make our results comparable with the results of other researchers who used similar reaction rate expressions (e.g., Caram and Amundson, 1977;Mon and Amundson, 1978;Sundaresan and Amundson, 1980;and Srinivas and Amundson, 1980). However, the development of the mathematical model uses arbitrary reaction rate expressions, and hence the same model can be used with any other rate expressions.…”

“…Therefore, the radicals formed in the void space of the particle may preferably react with the carbon or be quenched at the solid surface. Srinivas and Amundson (1980) found in their study of an isothermal particle with intraparticle diffusion and reaction only that their results were hardly sensitive to the occurrence of the homogeneous reaction in the interior of the particle. In the present study the above effect will be reexamined since higher amounts of heat released inside the particle will cause higher intraparticle thermal gradients which in turn may modify severely the behavior of the solution.…”

Diffusion and reaction in a char particle and in the surrounding gas phase. Two limiting models

“…Most of the investigators concentrated their attention on the boundary layer diffusion and reaction effects. In order to isolate the boundary layer problem and study it independently, they either considered the carbon particle impervious to diffusion or, as in Caram and Amundson (1977), Mon and Amundson (1978), and Sundaresan and Amundson (1980), lumped, mostly arbitrarily, the internal diffusion and reaction effects on the gross external surface area of the particle. A hierachy of models of varying complexity was used above for the study of the boundary layer problem and extensive computations revealed some previously unsuspected features of the solution, such as the existence of up to five steady states and quenching and lack of ignition for some range of the model parameters.…”

Πειραματικη Μελετη Και Μαθηματικη Μοντελοποιηση Αντιδραστηρων Στερεας Και Κινητης Κλινης Caco3/Cao-Λιγνιτη Για Τη Δεσμευση Του So2

“…Αυτό έχει επισημανθεί στις εργασίες των Cwiklinski, Βαγενά, Γεωργάκη και Wei[134,135], που μελέτησαν θεωρητικά την επίδραση των ομογενών αντιδράσεων οξείδωσης του CO και του Η^ στην κινητική συμπεριφορά της καύσης και γενικότερα της αεριοποίησης ρωματιδίων άνθρακα.Ο Γαβαλάς[45] πρότεινε ένα μοντέλο, που χρησιμοποιεί γραμμικές εξισώσεις, για την αεριοποίηση του άνθρακα με οξυγόνο. Το μοντέλο προβλέπει μέγιστο στο ρυθμό της αντίδρασης συναρτήσει της μετατροπής, σε συμφωνία με σχετικά πειραματικά αποτελέσματα[136].Επίσης οι Srinivas και Amundson[137] πρότειναν ένα μοντέλο για την επίδραση της εσωτερικής διάχυσης στην καύση του άνθρακα, που Η εγγενής ταχύτητα της αντίδρασης καύσης ποικίλλει ανάλογα με τη σύσταση και την προέλευση του λιγνίτη[3]. Οι εργασίες των Sergeant και Smith[138] και του Arthur[139], ο οποίος πρώτος προσδιόρισε πειραματικά την εξάρτηση της αναλογίας των παραγόμενων CO και C0 2 από τη θερμοκρασία, είναι κλασσικές πειραματικές μελέτες για την εγγενή ταχύτητα της αντίδρασης καύσης διαφόρων γαιανθράκων.…”