EXERGY ANALYSIS OF THE COAL GASIFICATION PROCESS IN EX-SITU CONDITIONS

In this article the possibilities of implementing exergy analysis of coal gasification processes in ex-situ conditions was presented. The analysis was performed in order to detect the sources of exergy loss. The experimental results of the coal gasification process are also presented and was used as input data to perform the exergy analysis of the coal gasification process.


INTRODUCTION
Coal gasification is the chemical processing of fossil fuels through the thermo-chemical conversion of coal seams to a syngas due to interaction with gasification agents (air, oxygen or a mixture of oxygen and steam) (Białecka 2008;Bielewicz, Prus, Honysz 1993;Stańczyk 2008).Syngas is a result of the thermo-chemical process (Białecka 2008;Stańczyk 2008; Stańczyk et al. 2012).Energy devaluation is mainly related to the occurrence of certain exergy value losses of the gasification reaction by-products that are apparently only dissipated in natural surrounding of the georeactor; in the analysed case in the rock mass (Łączny 2011).
The success and the commercialization of technology of underground coal gasification process (UCG) depend on the effective use of chemical energy accumulated in fossil fuels (Białecka 2008;Stańczyk 2008;Szargut, Guzik, Górnik 1979).
The main goal of the work is to detect the causes of reduction of the excellence of the UCG process in the form of the value of the exergy loss, and thus by obtaining information concerning the possibilities of its improvement (Szargut, Petela 1965;Szargut 2007;Szargut, Guzik, Górnik 1979;Szargut 1976).
Article presents the practical benefits of the implementation of exergy analysis for examining the coal gasification process under ex-situ conditions (Szargut, Petela 1965).The analysis is carried out in order to detect the main sources of exergy loss of the georeactor.The results of coal gasification process in ex-situ conditions were obtained and developed by the research employees of Department of Energy Saving and Air Protection of the Central Mining Institute (Stańczyk et al. 2012).

DESCRIPTION OF THE EX-SITU TESTS
The experiment was carried out in a block of coal, which was built to imitate the phenomena accompanying underground coal gasification process.The tests was carried out in the time interval of 170 hours.The main elements of the reactor were shown in Figure 1.The coal gasification process tests were realized under normal conditions 5. Along the gasification channel, about dimensions 0.1 × 0.1 × 2.5 m, the coal gasification process was realized.The gasification channel was built along a solid block of coal about dimension 0.7 × 0.7 × 2.5 m.Surrounding of the coal seam was a layer of rock.The rock layer was created to imitate the natural surrounding of the georeactor.The georeactor was insulated from the surrounding without the loss of gas.The chemical composition of syngas and temperature were measured at the outlet of the reactor.The pressure of gasification was equal to the ambient pressure (Stańczyk et al. 2012).For the purposes of tests was decided to divide the georeactor into two volumes.The results were shown in Figure 2.

Exergy analysis of the gasification agents
The equation ( 1) was adapted to the examined process conditions.In the first order, the process of mixing the gasification agents was analyzed.The components of the gasification agents delivered to the reactor were presented in Figure 3. Change of the gasification agents were shown in Figure 4.In connection with the above, equation ( 1) can be formulated in the following form: The graphic interpretation of the exergy balance of analysed volume I was shown in Figure 5.In accordance with the above, the internal exergy loss was formulated in the following form (4): After the appropriate transformations, the following form of equation was developed based on work (Szargut 2007): where: Under the conditions of the process, the phenomenon of intensive mixing of gasification agent components occurs.This phenomenon is the result of an exergy loss due to the diffusion of mixture components.The exergy loss was estimated based on the following equation (7) given in work (Szargut 2007): The heat loss was estimated as the amount of heat lossed by the gas in interaction with the surrounding.The exergy efficiency was expressed in the form of (9): In Figures 6-9 the results of the analysis were presented.The exergy efficiency of the gasification agents transport process was shown in Figure 9.The analysis shows that the primary sources of the exergy losses are phenomena of diffusion and dissipation in the first 30 hours of the process.

Exergy analysis of the reactor
Under the conditions of the UCG process along the gasification channel, three gasification reaction zones are separated, namely (Białecka 2008): oxidation zone reduction zone pyrolysis zone The oxidation zone includes exothermic reactions between the gasification agent and the coal seam.The reduction zone includes endothermic reactions between products of the oxidation zone and the gasification agent.The pyrolysis zone includes the thermo-chemical processes in which gaseous and liquid products are produced (Białecka 2008;Stańczyk 2008).A graphic interpretation of the volume II was shown in Figure 10.The moisture and sulphur mass fraction were taken based on the ultimate/proximate analysis of cola seam (Table 1)  The chemical composition of coal seam was shown in Table 1.The mass loss rate of coal was defined based on the equation (15) and shown in Figure 11: where:  2. The composition of syngas was presented in Figure 12 and the mass flow was presented in Figure 13.
The tests also included the monitoring of temperature of the syngas at the outlet of the georeactor system.The measured parameter was shown in Figure 14.
where: The value of changes of internal exergy losses of the analysed thermal system, in the interval of 170 hours, is described in Figure 15.

DISCUSSION OF RESULTS
Within the framework of this work, an analysis of coal gasification process was carried out under ex-situ conditions.Model studies were based on performing an exergy analysis (a gasification agent and syngas) of two separate volumes.As input data for the analysis the results of ex-situ experimental tests were used, which included information regarding the changes of the temperature and chemical composition of the syngas, the mass flow and mass fraction of gasification agent components as well as the mass loss rate of coal.
The exergy analysis shows that the initial loss of exergy appears at the stage associated with supplying the gasification agent to the georeactor.Also the sources of loss are the phenomena of diffusion between the gasification agents.
The results of the research allow formulating the following conclusions: 1. Due to the losses of gasification factor exergy, it was better to create conditions in which the irreversible phenomena of diffusion and heat loss will be eliminated, 2. The exergy losses of liquid (tar) and solid (ash) products are determined by the value of chemical exergy.

Fig. 1 .
Fig. 1.Model of ex-situ reactor developed for simulation the gasification process

Fig. 3 .
Fig. 3. Changes of the oxygen and water vapour volume fraction in the time interval of 170 hours (Stańczyk et al. 2012)

Fig. 4 .
Fig. 4. Mass flow of a water vapour and oxygen in the time interval of 170 hours (Stańczyk et al. 2012) due to flow resistance[J]

Fig. 5 .
Fig. 5. Flow diagram of the volume I The system, described in Figure 2 as volume I, is an open system, where mass and energy change enables to interpretate the equation (2) in the following form, namely exergy of products [J•s -1 ].The exergy of components of the gasification agent were expressed in the following form of equation (5) based on work (Szargut 2007): o , S oenthalpy and entropy in the ambient pressure and temperature [J] H, Senthalpy and entropy [J] T o ambient temperature [K]

2 m 2 m 2 h 2 s
of oxygen [kg•s -1 ] O H -mass flow of water vapour [kg•s -1 ] O H mass flow of the mixture [kg•s -1 ] T o ambient temperature [K] 2 O h enthalpy of oxygen [J•kg -1 ] O H enthalpy of water vapour [J•kg -1 ] 2 O o h enthalpy of oxygen under normal conditions [J•kg -1 ] O H 2 o h enthalpy of water vapour under normal conditions [J•kg -1 ] 2 O s entropy of oxygen [J•kg -1 ] O H entropy of water vapour [J•kg -1 ] 2 O o s entropy of oxygen under normal conditions [J•kg -1 ] water under normal conditions [J•kg -1 ] losses as a result of mixing of gasification agents [J•s -1 ] k n the moles of the k component of gasification agents [mole•s -1 ] ik z initial mole fraction of the i component of the gasification agents [-] im z the final molar fraction of the i component in the gasification agents [-]The mixing process also includes an irreversible process due to the transport of heat into the surrounding area.The value of these losses was determined based on the following equation given in work(Szargut 2007): losses due to heat loss [J•s -1 ] T o ambient temperature [K] T temperature of the gasification agents [K]

Fig. 6 .Fig. 7 .Fig. 8 .
Fig. 6.The exergy loss due to heat transfer in a time interval of 170 hours

Fig. 9 .
Fig. 9. Exergy efficiency of the gasification agent transport process in the time interval of 170 hours

Fig. 10 .
Fig. 10.Flow diagram of volume II Exergy of the volume II was expressed in the following form of equation: s op tar ash g w c B B B B B B B

Fig. 15 .
Fig. 15.Exergy losses of the coal gasification process in the time interval of 170 hours