Partial oxidation cycles

A simple PO plant (DI). Fig. 8.18, after Newby et al. 112], shows a simple PO plant, of the type listed as Dl in Table 8.ID. In this plant insufficient air is supplied to the PO reactor, less than that required for producing stoichiometric combustion. After expansion in the PO turbine the fuel gas is fed to the main turbine combustor where additional air is also supplied for complete combustion. 

A feature of this cycle is the reduction in compressor air flow for the same size of main expansion turbine. The figure shows air for the PO turbine taken from the discharge of the main compres.sor, but it may be taken straight from atmosphere. Note also that steam is raised for injection into the PO reactor and Newby et al. suggested that some of the steam raised in the HRSG may also be used to cool the PO turbine. The chemical reactions for the PO reactor of this case were described in Section 8.5.3. 

Newby and his colleagues provided some calculations of the performance of this partial oxidation cycle. They show that a major parameter in the performance of the PO cycle is the PO turbine inlet pressure, and listed calculations for three values of this pressure 45 bar, 60 and 100 bar. Their results for the composition of the gas streams round the plant (from the 60 bar calculation, which gave 49.3% for 335 MW) are given in Table 8.2. 

Stream PO reactor outlet PO turbine outlet Combustion turbine outlet Slack 

Of course, there is no methane at exit from the PO reactor, and no oxygen. The hydrogen content is quite high, over 15% and comparable to that in Lloyd s example of the steam/TCR cycle, but the CO content is also nearly 8%. It is interesting to note that the calculated equilibrium concentrations of these combustible products from the reactor are reduced through the PO turbine (because of the fall in temperature) before they are supplied to the gas turbine combustor where they are fully combusted, but it is more likely that the concentrations would be frozen near the entry values. 

---

In conventional cycles, combustion is the major source of irreversibility, leading to reduction in thermal efficiency. Some novel plants involve partial oxidation (PO) of the fuel in two or more stages, with the temperature increased before each stage of combustion, and the combustion irreversibility consequently reduced. In other plants full oxidation is employed which makes CO2 removal easier. 

I.J] Lxizza, G. and Chiesa, P. (2002), Natural gas decarbonisation to reduce COt emission from combined cycle—Part I Partial oxidation, ASME J, Engng Gas Turbines Power 124(1), 82-88. 

Several of the gas turbine cycle options discussed m this section (intercooling, recuperation, and reheat) are illustrated in Figure 4. These cycle options can be applied singly or in various combinations with other cycles to improve thermal efficiency. Other possible cycle concepts that are discussed include thermochemical recuperation, partial oxidation, use of a humid air turbine, and use of fuel cells. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

The resulting radicals are not usually observed, but thermal desorption products indicate the nature of the surface intermediates. Molybdenum(V) dispersed on silica also gives rise to 0 and O2 ions when exposed to N2O and O29 respectively. The 0 ion on this surface may be used to activate methane and ethane in a catalytic cycle which leads to their partial oxidation. 

Partial Oxidation of Methane. As described previously, 0 ions on Mo/Si02 react with CHi, forming methyl radicals, which in turn give rise to methoxide ions. The methoxide ions further react to methanol. These reactions form part of a catalytic cycle which leads to the partial oxidation of methane (8,18). 

Steam reforming feeding gas and naphtha in Cases 3A and 3B, 6A and 6B. Partial oxidation feeding cycle oil and SRC-11 oil in Cases 3C and 6C. 

Intermediates in Oleic Acid Oxidation What is the structure of the partially oxidized fatty acyl group that is formed when oleic acid, 18 1 (A9), has undergone three cycles of /3 oxidation What are the next two steps in the continued oxidation of this intermediate ... 

---