Temperature profiles for air gasification

Temperature profiles are measured every 60 seconds along the gasifier axis. A typical gas analysis is presented in Figure 3.44a for an experiment at ER = 3.18 and S:F = 0.8. It is apparent that the temperature profile achieves almost state steady in the last ten minutes; therefore, it is appropriate to assume steady state conditions during the last 10 minutes of each gas analysis. The temperature profiles discussed in this paper correspond to the average measured during the last ten minutes. As discussed before, in a fixed bed gasifier, the oxidation of char (heterogeneous oxidations) occurs near to the bottom of the bed where mostly char reacts with the oxygen and steam to produce CO, CO2, H2, and the heat required for driving the gasification process is released. Because under gasification conditions char oxidation of large particles is almost diffusion controlled, the char oxidation rate is dependent upon the availability of O2in the gas stream. The temperature in the combustion zone (Tpeak) depends upon the concentrations of O2, H2O, and CO2. Above the combustion zone, the temperature decreases since oxygen concentration is negligible and most of the reactions occurring there are endothermic. Below the combustion zone, the temperature is lower because it corresponds to ash temperature. It is apparent from Figure 3.44a that the peak temperature occurred at ~5 cm above of the grate indicating no ash accumulation.


Increase of ER, at fixed S:F ratio implies a decrease in the oxygen supplied; thus, heat generation due to char oxidation decreases resulting in lower Tpeak and hence results in a lower temperature

profile (Fig. 3.44b). Due to the presence of oxygen at the bottom of the bed, the peak temperature occurs near the bottom. The temperature of the particle under the assumption of negligible char — steam reaction and diffusion-controlled combustion can be derived as (Annamalai and Puri,

2007) :

cp(Tp — Toq )

-^-p——- — = B (3.42)


where hc = hcI for CO, hc = hcII for CO2 produced, Tp = particle temperature, B = {YO—/vO2}, Vq2 = 1.33 for CO, 2.33 for CO2 produced, YO2— = Oxygen mass fraction, and cp specific heat of the gases. In particular, for ER = 1.59 and S:F = 0.68 the peak temperature measured is about 950°C (Fig. 3.44b); however, this value is lower compared to (1191 °C) obtained with the equa­tion 3.42 (cp of air = 1.15kJ/kgK, cp of the steam = 2.3kJ/kgK, cp of mixture = 1.28kJ/kgK, YO2— = 0.203, and hcI = 9204kJ/kg). The lower experimental temperature compared to that of the model indicates that (i) the char may react with both O2 and steam at the bottom of the bed to produce CO and H2 and (ii) combustion may not be diffusion controlled. On the other hand, if the steam carbon reaction was included in the model and if diffusion limited hetero­geneous reactions was assumed, the estimated Tp would be lower than the estimated using equation 3.42.

Figure 3.44c shows the effect of change in ERs and S:F ratio on the peak temperature (combus­tion temperature zone). Also are presented two Tpeak (1098 and 998°C) obtained for gasification with only air at ER = 2.12 and ER = 3.18. At lower ERs, the effect of the S:F ratio is higher. For instance, at ER = 1.59 the peak temperature difference between the curves of S:F = 0.35 and

0. 80 is 185°C while at ER = 6.36 the difference between the same curves is 91°C only since oxygen availability is limited. The curves from Figure 3.44c suggest that at constant S:F, the peak temperature is affected almost linearly by changes on the ER. Increased S:F causes the Tpeak to decrease. This can occur due to (i) decreased amount of air, (ii) change in the cp of the mix­ture, (iii) regimes of combustion: kinetics vs. diffusion controlled, and (iv) steam-char reaction. At ER = 2.12, the peak temperature for gasification with air only is 147°C (15.45%) higher as compared to that of gasification with air-steam at ER = 2.12 and S:F = 0.35 while at ER = 3.18, the difference in peak temperature between gasification with air and gasification with air-steam is ~132°C (15.24%). In general, for the range of operating conditions (ER and S:F) investigated the Tpeak ranged between 519 (ER = 6.36, almost pure pyrolysis) and 1015°C (ER = 1.59). Temperature profiles for enriched air gasification and CO2:O2 gasification For the gasification experiments with higher oxygen percentages, at ER = 2.1 and S:F = 0, the temperature profiles obtained are plotted in Figure 3.45. The peak temperatures obtained can be compared to that of the theoretical values obtained using the B number.

From Figure 3.45, the peak temperature obtained when using enriched air mixtures is observed to increase with increased oxygen concentration. The numbers obtained experimentally were almost same as the values calculated theoretically using B number calculations.

Enriched air results in the presence of nitrogen in syngas, which lowers the heat value of gases. Also, carbon dioxide (CO2) can be separated easily from products compared to nitrogen (N2) in the event CO2 sequestration is necessary to enhance the heat values. Hence, experiments were performed using carbon dioxide-oxygen mixture as the gasification medium instead2 of air. In this case, carbon dioxide is substituted for nitrogen in the air mixture. Also the carbon dioxide produced as a result of gasification can be separated and circulated again into the reactor at high temperatures (e. g. as cooling medium for the gasifier) in order to increase the efficiency of the reactor and also to sustain the reaction within the gasifier. This will also increase the upper limit on ER. This in turn helps to reduce the amount of carbon dioxide released into the environment. CO2 has a slightly higher specific heat (cp) than N2 at higher temperatures. The cp of the mixture of CO2 and O2 is higher than cp of the mixture of N2 and O2. Hence, Tpeak, using CO2 instead of N2, is expected to be low. The difference in peak temperatures can be observed in the temperature profiles (Fig. 3.46) obtained using carbon dioxide in the gasifying medium instead of nitrogen (Thanapal et al., 2012).


Figure 3.45. Steady state temperature profile, ER = 2.1, S:F = 0 (adopted from Thanapal, 2010).


Figure 3.46. Temperature profile, 21% oxygen, ER = 4.2, S:F = 0 (adopted from Thanapal, 2010).