ATTRITION OF LIGNITE CHAR DURING FLUIDIZED BED GASIFICATION: EXPERIMENTAL AND MODELING STUDIES. Paola Ammendola and Fabrizio Scala

Transcription

1 ATTRITION OF LIGNITE CHAR DURING FLUIDIZED BED GASIFICATION: EXPERIMENTAL AND MODELING STUDIES Paola Ammendola and Fabrizio Scala

2 Introduction The The urgent need need to to capture and and sequester CO CO 2 in 2 in particular emitted from from the the use use of of coal coal has has triggered the the development of of new new processes like like chemical looping combustion and and sorption enhanced gasification. Both Both these these processes require that that fuel fuel gasification is is carried out out at at a relatively low low temperature, limiting the the types of of fuel fuel that that could be be used used to to those with with highly reactive chars, such such as as low-rank coals coals and and biomass. Lignite is is an an abundant fuel fuel and and makes up up approximately 40% 40% of of coal coal reserves in in the the world. Its Its commercial use use is is forecast to to grow grow substantially. However, lignite usually has has high high water and and oxygen content, low low energy density, and and it it is is expensive to to transport. All All these these characteristics have have limited its its utilization.

3 Introduction Fluidized bed bed (FB) (FB) gasification is is acknowledged to to have have great great flexibility in in conversion of of several several solid solid fuels, fuels, including low low rank rank coals, coals, into into synthesis gas. gas. Fuel Fuel attrition attrition and and fragmentation phenomena are are well well known known to to affect affect the the reliability and and efficiency of of FB FB combustion and and gasification processes. Attrition/fragmentation cause cause the the elutriation of of fine fine material material from from the the bed bed that that results results in in the the loss loss of of unconverted carbon. carbon. Attrition/fragmentation may may also also significantly change change the the particle particle size size distribution of of the the materials in in the the bed bed which which influences the the bed bed fluid-dynamics, heat heat and and mass mass transfer transfer coefficients and and reaction reaction rates. rates. Several Several attrition attrition studies studies are are reported under under FB FB combustion conditions, but but only only limited limited activity activity is is reported reported under under gasification conditions, and and no no study study could could be be found found in in the the literature on on attrition attrition of of lignite lignite during during gasification.

4 Introduction The The authors authors started started a a study study on on attrition attrition of of lignite lignite char, char, pointing pointing out out that that fragmentation and and attrition attrition of of the the char char particles during during gasification were were significant, and and suggested a a gasification-assisted attrition attrition enhancement effect. effect. This This mechanism, associated to to the the low low reactivity of of the the generated fines, fines, made made the the loss loss of of carbon carbon by by fines fines elutriation during during char char gasification more more significant than than that that typically found found under under combustion conditions. t = gasification time, X C = carbon conversion degree T= 850 C, U= 0.4 m/s, Gas = 100% CO 2

5 Scope of the work C.N.R. In In this this work work the the attrition behaviour of of char char from from an an Italian Italian lignite lignite (Sulcis) was was studied under under CO CO 2 gasification 2 conditions in in a a lab-scale FB FB apparatus. In In particular, attrition was was characterized as as a a function of of the the char char conversion degree. The The influence of of bed bed temperature and and CO CO 2 concentration 2 on on attrition and and carbon conversion was was investigated. On On the the basis basis of of the the experimental results, a a kinetic model model of of lignite lignite char char under under CO CO 2 gasification 2 conditions was was developed in in order order to to predict the the time time resolved profiles of of carbon conversion. The The mechanical resistance of of the the char char particles was was also also characterized at at different stages stages of of char char conversion by by specific attrition experiments. A semi-empirical model model was was finally developed to to simulate the the experimental profiles of of carbon elutriation rate rate as as a a function of of time. time.

6 Two-exit head FB 1) gas preheating section; 2) electrical furnaces; 3) ceramic insulator; 4) gas distributor; 5) thermocouple; 6) fluidization column; 7) head with three-way valve; 8) sintered brass filters; 9) hopper; 10) SO2 scrubber; 11) stack; 12) cellulose filter; 13) membrane pump; 14) gas analyzers; 15) personal computer; 16) manometer; 17) digital mass flowmeters ; 18) air dehumidifier (silica gel). ID = 40 mm T= C U= 0.4 m/s CO 2 = % d fuel = mm W sand = 180 g d sand = mm C.N.R. Experimental Sulcis lignite properties Proximate analysis, % (as received) Moisture 6.3 Ash 16.9 Volatile Matter 49.7 Fixed Carbon 27.1 Ultimate analysis, % (dry and ash free basis) Carbon 71.7 Hydrogen 5.7 Nitrogen 1.8 Oxygen 13.1 Sulphur 7.7 LHV, kcal/kg 5137

7 Char attrition tests results temperature effect Elutriation rate A 1.0 Carbon conversion B E C /W C0, min C 850 C 800 C X c, U= 0.4 m/s W batch = 1 g Gas = 100% CO Time, min Time, min 900 C 850 C 800 C Operating Temperature Gasification time (min) Gasified carbon (%) Elutriated carbon (%) Unconverted carbon (%) T = 800 C T = 850 C T = 900 C

8 Char attrition tests results CO 2 effect Elutriation rate A 1.0 Carbon conversion B E C /W C0, min Time, min 20% 50% 100% X C, Time, min 20% 50% 100% T= 900 C U= 0.4 m/s W batch = 1 g Gas = % CO 2 in N 2 CO 2 inlet concentration (in N 2 ) Gasification time (min) Gasified carbon (%) Elutriated carbon (%) Unconverted carbon 20% % % (%)

9 Kinetic model Char gasification in the present operating conditions is controlled by intrinsic kinetics. The concentration of produced CO is negligible (< 1% of CO 2 concentration). dx dt C C 0 a CO CO A X k exp E /RT C C A X a b X c X C C C a (-) b (-) c (-) k 0 (s -1 ) E a /R (K) (-) ,0006 U= 0.4 m/s, Gas = 100% CO 2 T= 900 C, U= 0.4 m/s 0,0006 0, , % 50% 20% 0,0004 0,0004 dx C /dt, s -1 0,0003 dx C /dt, s -1 0,0003 0,0002 0,0002 0,0001 0,0001 0,0000 0,0 0,2 0,4 0,6 0,8 1,0 0,0000 0,0 0,2 0,4 0,6 0,8 1,0 X C, - X C, -

10 0,0006 C.N.R. Kinetic model - results U= 0.4 m/s, Gas = 100% CO 2 T= 900 C, U= 0.4 m/s 0,0006 0,0005 0, modello 850 modello 800 modello 0,0005 0, % 50% 20% 100% modello 50% modello 20% modello dx C /dt, s -1 0,0003 dx C /dt, s -1 0,0003 0,0002 0,0002 0,0001 0,0001 0,0000 0,0 0,2 0,4 0,6 0,8 1,0 0,0000 0,0 0,2 0,4 0,6 0,8 1,0 X C, - X C, -

11 E k UU W d C mf C W W 1X C C0 C C.N.R. Elutriation model - 1 Attrition rate Carbon mass In order to understand the relationship between the char attrition constant (k) and carbon conversion (X C ), attrition experiments were carried out under inert conditions (100% N 2 ) using char samples pre-gasified for different times (ranging from 10 to 50 min). 1.6e-5 Initial values k(x C ), - 1.4e-5 1.2e-5 1.0e-5 8.0e-6 6.0e-6 Experimental data Fitting curve 6 k exp 2.31X C 4.0e-6 2.0e-6 b X C, - Pre-conversion of the char significantly enhances the mechanical attrition of the particles.

12 Elutriation model - 2 Only limited secondary fragmentation occurs at low carbon conversions, while significant shattering is evident at conversions larger that 50%. Char particle size 3 d X d 1 n X C 0 C Number of char particles * * C max C C C n X 1 n 1 X X 1 X X C * = 0.5 n max = fitting parameter Comparison between experimental and model time resolved carbon elutriation rates obtained during CO 2 gasification experiments at different operating conditions E E E E E C /W C0, min -1 C /W, C /W C 0 min-1 C /W, C0, min -1 C /W C0, min -1 C 0 min Time, min a b c d e

13 Conclusions - 1 C.N.R. A gasification-assisted attrition attrition mechanism mechanism is is proposed. proposed. Internal Internal reaction reaction progressively weakens weakens the the structure structure inside inside the the particle, particle, enhancing enhancing particle particle attrition. attrition. The The low low reactivity reactivity of of the the generated generated fines fines under under gasification gasification conditions conditions makes makes the the loss loss of of carbon carbon by by fines fines elutriation elutriation significant. significant. The The total total elutriated elutriated carbon carbon increased increased when when the the bed bed temperature temperature is is decreased. decreased. Two Two reasons reasons are are likely likely to to determine determine this this behaviour: behaviour: i) i) at at lower lower temperatures the the residence residence time time for for complete complete gasification gasification increases; increases; ii) ii) the the extent extent of of in-bed in-bed conversion conversion of of the the attrited attrited fines fines increases increases with with the the bed bed temperature. The The total total elutriated elutriated carbon carbon increased increased when when the the CO CO 2 concentration 2 was was decreased. decreased. At At lower lower CO CO 2 concentrations 2 the the residence residence time time of of particles particles in in the the bed bed increases increases and and the the extent extent of of in-bed in-bed conversion conversion of of the the attrited attrited fines fines decreases. decreases. On On the the whole, whole, the the carbon carbon loss loss by by elutriation elutriation is is certainly certainly one one of of the the critical critical factors factors during during the the gasification gasification process, process, especially especially at at low low gasification gasification rates rates (i.e. (i.e. at at low low CO CO 2 concentration 2 and and bed bed temperature).

14 Conclusions - 2 C.N.R. A char char gasification kinetic kinetic model model was was proposed which which was was able able to to correctly predict predict the the carbon carbon conversion as as a a function of of time time found found in in the the experiments, including the the effect effect of of temperature and and CO CO 2 concentration 2 on on carbon carbon conversion. A semi-empirical carbon carbon elutriation model model was was also also presented, where where the the char char attrition attrition constant was was expressed as as a a function of of the the carbon carbon conversion degree. degree. The The change change of of the the average average char char particle particle size size due due to to secondary fragmentation was was taken taken in in account. Model Model results results were were in in good good agreement with with the the curves curves obtained from from the the experimental tests. tests. Particle Particle fragmentation appears appears to to be be a a key key phenomenon in in determining the the attrition attrition rate rate of of the the char char in in the the bed. bed. Nevertheless, the the presence of of a a fitting fitting parameter, i.e., i.e., the the maximum number number of of generated fragments, implies implies that that the the model model is is not not yet yet predictive at at this this stage. stage.