Development of Black Liquor Char Properties during Conversion. Kevin Whitty University of Utah

Transcription

1 Development of Black Liquor Char Properties during Conversion Kevin Whitty University of Utah Colloquium on Black Liquor Combustion and Gasification May, 2003 Park City, Utah, USA Outline Low temperature conversion Starting material Particle size Particle structure Composition High temperature conversion Particle structure Transition to smelt bead Practical implications Research needs

2 Low Temperature Conversion Low Temperature Conversion Below melting point of inorganics (~725 C) No molten phase throughout conversion Applies primarily to fluidized bed gasification systems Liquor not necessarily injected as droplets Char particles crushed in FB environment

3 Stages of Low Temp BL Conversion Starting Material Low Temp Char Source: Järvinen (2002)

4 Starting Material Low Temp Char Starting Material Low Temp Char Swelling/density depend on Temperature Pressure Gas composition Liquor type Pyrolysis rate Char density ranges: g/cm 3 (1 atm) g/cm 3 (20 atm)

5 Atmospheric vs. Pressurized Char 1 atm 5 atm 20 atm Atmospheric vs. Pressurized Char 1 atm 20 atm

6 Particle Size Development Low Temp mm 8 SV max Hupa's stages of droplet burning Drying Devol Char burn Time, s Particle Size Development Low Temp 7.4 mm 212 mm 3 mm 8 6 SV max 82x mm 2.6 mm Time, s Char burn

7 Particle Structure Development Low T Data on microstructure (porosity, surface area) during BL char conversion is limited Can be inferred from shape of rate versus conversion curve Assume rate is function only of available surface area for reaction Typically, max rate achieved at ~40% conoversion Development of Microstructure (Gasification at 700 C) 0% 26% 77% 100%

8 Particle Structure Development Low T (Data from Gasification at 650 C) Composition Low T H 2 O Gasification 1 atm 20 atm

9 High Temperature Conversion High Temperature Conversion Above melting point of inorganics (>800 C) Final product is molten Applies to high temperature systems Recovery boilers Entrained-flow gasifiers

10 Particle Size Development High Temp 7.4 mm (100%) mm 8 SV max 800 o C in air mm (34%) mm (1.2%) 2 0 Time, s Char burn Kev's Theory Given that Chemrec-style gasifiers have all the following: Small particles High heating rates High volatiles yields (up to 70% C at 1000 C) Relatively high inorganic concentrations in the char Char exposed to high temperatures then perhaps there never really is a "char" in the classical sense, much less char gasification ~60% of conversion through pyrolysis ~30% of conversion by inorganic decomposition rxns ~10% of conversion by heterogeneous gasification Most post-pyrolysis conversion occurs in molten phase

11 maybe not Observations of chars from LEFR indicate that chars formed at high temp (1000 C) are "fluffy" even at high conversions Chars from pyrolysis in N 2 are black and "weightless" Chars from pyrolysis/gasification in environment with CO 2 are still "fluffy" despite being nearly fully converted (gray color) Apparently, even very low concentrations of carbon in char affect melting behavior Or could it be that the chars simply look "fluffy" to the naked eye, but that they are indeed largely molten and fixed carbon allows it to maintain its structure? Deserves quantitative investigation More Questions than Answers At what conversion does melt develop in the char? How does the presence of carbon influence the structure of the char when molten phase is present? At what conversion will the particle "collapse" and is the surface area after collapse essentially the external surface area? How does the collapse occur? Is there a "trigger"? How efficient are inorganic decomposition reactions in this "pseudo molten" state? What degree of sulfur reduction, if any, is observed in chars from LEFR pyrolysis experiments?

12 Practical Implications Particles contacting wall may "bounce" rather than "stick" If they "stick," will they stick? Or flow? Or stick until a certain conversion is reached, then flow? What is the optimum droplet size in terms of impaction, sulfate reduction, flow? Research Needs Measure development of internal surface area throughout conversion for both LT and HT chars Identify (confirm) relation between gasification rates and internal surface area Is there a way to introduce BL so as to maximize swelling/surface area generation? How does char attrition in a fluidized bed environment impact surface area? Identify fate of elements throughout conversion Released early, late, or evenly with respect to carbon release?

13 Research Needs, continued Study physical development of high temp chars in more detail Char structure maintained despite presence of melt? How does internal surface area change once a molten phase is present in the char? Study the "collapse" of the char in more detail Occurs rapidly once a particular conversion is achieved, or more gradually? Occurs at what conversion for different temps?