Results and Discussion

In the study the main gaseous products mixture flows and percentage composition of the gaseous product mixture in the steam gasification process were determined using mass flowmeter and GC, respectively. The influence of addition of Fe2O3 and CaO on the amount of gaseous product and its composition was analyzed. The temperature is an important parameter which strongly influences the product gas yield in the gasification process. In Fig. 17.4 the total yield of H2, CO, CO2, and CH4 and the product mixture composition in 1 h tests of steam gasification of coal are presented. The total gas yield at the temperature of 1173 K was about 1.7 times higher than at 923 K. The total amount of hydrogen produced in 1 h gasification tests varied between 2669 ml at the temperature of 923 K and 4836 ml at 1173 K.

Table 17.2 The proximate and ultimate analysis of the tested coal.

Parameter

Value

As-received

Total moisture Wtr [%]

8.58

Ash Ar [%]

5.54

Total sulfur Str [%]

0.49

Calorific value Q;r [kJ/kg]

26,798

Analytical

Total moisture Wa [%]

6.02

Ash Aa [%]

5.69

Volatiles Va [%]

31.12

Heat of combustion Qsa[kJ/kg]

28,805

Calorific value Q;a [kJ/kg]

27,616

Ash melting point - oxy:

990

Sintering point tj [°C]

Softening point tA [°C]

1270

Melting point tB [°C]

1350

Flow temp. tC [°C]

1370

Total sulfur Sta[%]

0.5

Carbon Cta[%]

70.64

Hydrogen Hta [%]

4.08

Nitrogen Na[%]

0.98

Dry

Ash Ad [%]

6.05

Total sulfur Std [%]

0.53

Volatiles Vdaf [%]

35.25

Heat of combustion Qsdaf [kJ/kg]

32625

In the process of steam gasification of coal (see Fig. 17.4) the concentration of hydrogen in the syngas increased from 59% at the temperature of 923 K to 62% at 1173 K, while the CO2 concentration increased from 20% at 923 K to 24% at 1023 K. In a temperature range of 1073-1173 K the CO2 concentration decreased and reached 16% at 1173 K. The CO concentration in the produced syngas varied between 19% at the temperature of 923 K and 22% at 1173 K. Moreover, in a temperature range of 923-1023 K the total concentration of CO in the synthesis gas decreased from 19 % to 15%, whereas in a temperature range of 1073-1173 K it increased to 21-22%. The increase in CO and decrease in CO2 concentrations in the gaseous products mixture in a temperature range of 1073-1173 K can be explained by a reverse Boudouard reaction during which an amorphous carbon and CO2 are consumed which results in CO enrichment of the gaseous product and by the absence of the water-gas shift reaction at temperatures over 1073 K. Similar results were reported by Mondal et al. (2005). The concentration of CH4 was low in a temperature range of 923-1023 K and at higher temperatures (1073-1173 K) it decreased to zero. It can be explained by steam and dry reforming of CH4.

Fig. 17.4 (a) The total gas yield and (b) the gas composition obtained in 1 h tests of the steam gasification of coal.

The simultaneous addition of Fe2O3 and CaO significantly increased the hydrogen content in the product gas. In Figs. 17.5 and 17.6 the total yield of H2, CO, CO2, and CH4 and the composition of the gaseous products mixture in 1 h tests of steam gasification of coal with addition of Fe2O3 and CaO in layers and mixed, respectively, are presented. Fe2O3 oxidized CO to CO2 and enhanced the water-gas shift reaction which resulted in an increase in H2 content and in an effective reduction of CO content. Practically all CO was oxidized to CO2 both in series with Fe2O3 layered on and mixed with coal samples.

Fig. 17.5 (a) The total gas yield and (b) the gas composition obtained in 1 h tests of steam gasification with addition of Fe2O3 and CaO layered on coal samples.

Addition of Fe2O3 and CaO in the process of steam gasification of coal enhanced the yield and the content of H2 in the gaseous product mixture. CaO was added to remove CO2 produced in the water-gas shift reaction, the Boudouard reaction and in the oxidation reactions with Fe2O3. A significant increase in hydrogen content was observed both with addition of Fe2O3 and CaO in layers and mixed, but the CO2 capture was more effective when CaO was mixed with coal and Fe2O3. At the temperature of 923 K the hydrogen yield was 3959 ml for layered and 4446 ml for mixed samples, whereas at the temperature of 1173 K the hydrogen yield was 5295 ml and 5901 ml for layered and mixed samples, respectively.

In case of steam gasification of coal with Fe2O3 and CaO put in layers in a temperature range of 973-1173 K the total concentration of CO2 was higher than without iron and calcium oxides (see Figs. 17.5 and 17.6). It can be explained by by the fact that the whole amount of CO was oxidized by Fe2O3 to CO2 and its absorption by CaO was hindered as the contact surface between agglomerated CaO and CO2 was limited. Mixing CaO with Fe2O3 and coal ensured better contact between CaO particles and CO2 and improved the efficiency of CO2 absorption.

Fig. 17.6 (a) The total gas yield and (b) the gas composition obtained in 1 h tests of steam gasification with addition of Fe2O3 and CaO mixed with coal samples.

The process of CO2 absorption was more effective at lower temperatures (923-1023 K). At the temperature of 923 K the CO2 content was reduced to 16% and 9% in tests with additives layered and mixed, respectively. In the tests of steam gasification with Fe2O3 and CaO layered on coal samples the concentration of CO2 increased to about 32, 43, and 39% at the temperatures of 973 K, 1023 K, and 1073 K, respectively. This was probably caused by CaO agglomeration (not sufficient amount of CaO available for absorption of CO2) as this disadvantage was overcome in tests with Fe2O3 and CaO mixed with a coal sample. In the latter cases the concentration of CO2 was effectively reduced to 11-12% in a temperature range of 973-1073 K. At the temperatures of 1123 K and 1173 K an increase in CO2 concentration was observed both in tests with additives layered and mixed with coal samples. It can be explained by the fact of CaCO3 calcinations which, at higher temperatures, is favored over the carbonation of CaO.

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