The Role of Structure and Composition in Gasification Reactivity of Biomass Char

Transcription

1 The Role of Structure and Composition in Gasification Reactivity of Biomass Char Gautami Newalkar 1, Kristiina Iisa 2, Carsten Sievers 1 and Pradeep K. Agrawal 1 1 School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 2 National Renewable Energy Laboratory, Boulder, CO September 4, 2013 tcbiomass2013 Conference 1

2 Goals and Objectives To obtain experimental data on the rates of hydrocarbons and tar formation during pressurized gasification of biomass To develop kinetic models that describe carbon gasification rates To study the evolution of char structure and morphology as a function of temperature, pressure, and heating rate during the pyrolysis stage 2

3 Experimental approach Complementary reactor types: Pressurized Entrained Flow Reactor (PEFR) Pressurized Thermo-gravimetric Analyzer (PTGA) Operating Parameter PEFR PTGA T ( C) Limits P (atm) Limits Heating rate C/s Below 1 C/s Solid Residence Time 2-40 s Up to Hours Gas Residence Time 2-40 s s 3

4 Pressurized Entrained Flow Reactor 4

5 Elemental Composition of Biomass Feed Element Loblolly Pine Switchgrass Cornstover C H N O Ash Volatile Matter Fixed Carbon Lignocellulosic biomass have comparable C,H,N,O values, volatile matter, and fixed carbon content. The differences appear to be in the ash content and composition. 5

6 Elemental Analysis of Feed Biomass (ICP) Element Loblolly Pine Switchgrass Cornstover Ca Fe K Mg

7 Evolution of Char Morphology in PEFR Effect of temperature at constant pressure (5 bars) 600 C 800 C 1000 C Effect of pressure at constant temperature (600 o C) 5 bar 10 bar 15 bar 7

8 8 Gas-Filled Pockets Formed at High Pressures Pine Char pyrolyzed at 15 bars and 1000 C

9 9 Comparison of Char Reactivity

10 CO2 Area (m 2 /g) CO 2 versus N 2 Surface area measurement C 800 C 1000 C N2 Area (m 2 /g) CO 2 adsorption at 0 0 C gives ultramicropore ( 1 nm) distribution using DFT model. N 2 adsorption at 77 o K gives mesopore and macropore distributions, using BJH model. 10

11 Distribution of total surface area in micro-, meso- and macro-pores Micropores(<2 nm) Mesopores(2-50 nm) Macropores(>50 nm) _10 800_ _10 11

12 Initial Reactivity (1/min) Reactivity vs. Micropore Area C 800 C 1000 C Area of Micropore (m2/g) 12 Chars formed at a lower temperature are more reactive than those formed at a higher temperature even if they possess the same area. This indicates the presence of another parameter along with/ other than surface area which influences reactivity

13 Mol % in Product gas Product Gas Composition Gas composition at 800 C, 10 bar C 2 -C 4 Hydrocarbon variation with T and P C 2 -C 4 0.1% 8.00 CH 4 13% 600 C H 2 20% CO 51% C 1000 C CO 2 16% Pressure (bar) 13

14 Tars Increase in Residence Time Glycoldehyde dimer Dibenzofuran Pyrene 4-Methyl-5H-furan-2-one Fluorene Primary Secondary Tertiary 14

15 Intensity XRD Spectra of Pine Chars 1000 _10_30s 800_5_30s 800_5_15s 800 _5_4s θ Cellulose in Pine loses its crystallinity as biomass falls through the reactor. Melting of bio-polymers causes re-arrangement of carbon skeleton developing graphite-like structure at severe pyrolysis conditions. Feed 15

16 Initial Reactivity (1/min) Reactivity vs. Inorganic Content C 800 C 1000 C Total inorganics (%) Chars with similar ash content, but formed at different temperature, possess different reactivity. It is therefore necessary to measure the content of active ash 16

17 Initial Reactivity (1/min) Reactivity vs. H/C ratio C 800 C 1000 C H/C Ratio in Char The H/C ratio might be an indicator of graphite-like tendency of char. Chars with lowest H/C ratios are least reactive; cause vs. effect is not clearly understood presently. 17

18 Initial Reactivity (1/min) Catalytic Gasification of Avicel Char K2CO3 CaO Metal/Carbon ratio K 2 CO 3 reduces to K 2 O at ~900 C in presence of Carbon. K 2 O reduces to metallic K during gasification. K migrates inside carbon matrix to enhance reactivity. CaO does not reduce to metallic Ca at reaction conditions. CaO is known to sinter as conversion increases 18

19 Conclusions Surface area, inorganic content in char, and the chemical character of carbon in char together determine the char gasification reactivity. Studying these parameter in isolation is not feasible and all of the parameters need to be considered for each char Pyrolysis operating conditions, mainly heating rate, temperature, pressure, leads to drastic differences in char morphology and reactivity Finding an optimum range of these operating condition is key for gasifier design High temperature char is mainly carbonaceous with large aromatic/ graphene like sheets and appears to correlate with the lowered H/C ratio of char. High heating rate at high temperature and long residence time (30 sec) leads to very low reactivity Higher pressure makes reactivity even worse. 19

20 What does it mean for gasification strategy? Higher pyrolysis pressure reduces char surface area and leads to more graphitic/ polyaromatic char lowering the reactivity. 5 bar processing pressure seems appropriate for gasification reactor. Need to look at overall gasification complex for optimum gasification reactor pressure Higher pyrolysis temperature leads to decrease in gasification reactivity Lowered total surface area of char at higher pyrolysis temperature Increased graphitization at higher temperature and pressure Lower residence time even at high temperature (1000 ºC) leads to highly reactive char There is potential in blending recycled ash with biomass to achieve catalytic effect during gasification. Future study will include evolution of char morphology at different stages of conversion during the gasification. 20

21 Acknowledgements This work is supported by the US DOE Bioenergy Technologies Office, Golden, CO. PSE Graduate Fellowship from IPST is thankfully acknowledged. 21