Atmospheric entrained-flow gasification of biomass and lignite for decentralized applications

Table 3 Flow rates of the gasification agent air, feeding rates of the fuel, calculated O 2 concentration at the reactor inlet, equivalence ratio, and gas residence time at 950 ≤ T ≤ 1100

Research article

decentralized applications

Jens Schneidera, Christian Grubea, André Herrmanna, Stefan Rönscha,b,⁎

a Syngas Technologies, Department Biorefineries, Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Torgauer Str 116, 04347 Leipzig, Germany

b Department of Industrial Engineering, Ernst-Abbe-Hochschule Jena, Carl-Zeiss-Promenade 2, 07745 Jena, Germany

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 4 February 2016

Received in revised form 9 May 2016

Accepted 31 May 2016

Available online xxxx

The present study deals with the development of a small-scale entrained-flow gasification technology for the decentral use of biomass Gasification experiments with woody biomass in a wide range of particle diameters

dSin the fractions 0.04b dSb 0.11 mm, 0.20 b dSb 0.25 mm, 0.25 b dSb 0.50 mm, and 0.50 b dSb 1.0 mm were carried out in an atmospheric electrically-heated entrained-flow gasifier at temperatures between 950 and

1100 °C Power plant lignite in the fraction 0.05b dSb 0.08 mm was gasified as well for comparison These low temperatures were chosen in order to verify that an entrained-flow gasification technology operating at mild conditions can be developed Low investment costs combined with the production of a tar-free syngas make this technology option attractive especially for decentralized applications (b5 MW fuel input power) The pro-duction of a high syngas quality during autothermal operation has still to be demonstrated

A short review of studies prepared for entrained-flow gasification of biomass since 2006 points out the state of the art and most importantfindings The concentrations of H2, CO, CO2and N2together with carbon conversion, cold gas efficiency and syngas yield resulting from the present work are reported and compared to the respective literature values Carbon conversion, cold gas efficiency, and specific syngas volume varied strongly with temper-ature and particle diameter showing values between 63 and 100 wt%, 14 and 61%, and 0.6 and 1.4 m3kg−1(STP), respectively With the present set up, high cold gas efficiencies were only obtained at temperatures of 1100 °C and particle sizes of less than 0.2 mm

Particle residence times in the gasifier were measured at 25 °C for three sawdust fractions and varied between 1.4 and 3.3 s These measurements indicate that the particle residence time is not equal to the gas residence time in general A model for the calculation of particle velocities and residence times at ambient and gasification condi-tions is presented The interrelacondi-tionships between particle residence time, particle diameter, carbon conversion, and temperature are discussed

© 2016 Elsevier B.V All rights reserved

Keywords:

Gasification

Entrained-flow

Biomass

Syngas

Decentralized

1 Introduction

1.1 Background

For the transition of the energy system from fossil to renewable

en-ergies, economic, environmental and supply security concerns have to

be considered[1,2] The use of bioenergy technologies provides a

possi-bility to meet these concerns Bioenergy technologies allow a

substan-tial reduction of net CO2 emissions [3,4], while providing energy

reliably and weather-independent[5], which cannot be guaranteed by

wind and solar power plants alone

Especially the gasification of lignocellulosic biomass is a versatile biomass conversion pathway producing energy-rich gas that can be used for the generation of electrical power[6–8]and basic chemicals

[9–14] However, for a competitive market implementation and use in small-scale bioenergy plants (b5 MW fuel input power), the investment costs of this technology have to be reduced Especially downstream pro-cesses like gas cleaning are complex and related to high specific invest-ments in small-scale facilities For a reduction of the gas cleaning equipment[15,16]and the related investment costs, the production of

a tar-free syngas is a crucial factor

Entrained-flow gasifiers are able to generate a tar-free syngas that is nearly free of hydrocarbons[17] They are commercially available e.g as large scale coal gasifiers typically constructed for a few hundred mega-watt fuel input power and operated at extreme pressures (40–80 bar) and temperatures (1400–1600 °C)[17,18] Commercial entrained-flow gasifiers for biomass are under development

⁎ Corresponding author at: Syngas Technologies, Department Biorefineries, Deutsches

Biomasseforschungszentrum gemeinnützige GmbH, Torgauer Str 116, 04347 Leipzig,

Germany.

E-mail address: Stefan.Roensch@dbfz.de (S Rönsch).

http://dx.doi.org/10.1016/j.fuproc.2016.05.047

Contents lists available atScienceDirect

Fuel Processing Technology

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / f u p r o c

1.2 Objective of this work

Comparably low investment costs for a gasifier operating at mild

conditions and being constructed of less expensive materials motivate

this work Pressures above 40 bar and temperatures above 1400 °C

ne-cessitate not only heavy and durable materials but also intensive safety

precautions, which are expensive and seem to exclude the operation of

small-scale bioenergy plants Hence, this work is aimed at investigating

an entrained-flow biomass lab-scale gasifier working at comparatively

mild operation conditions (atmospheric pressure, temperatures

≤1200 °C) and to prove its applicability for future small-scale facilities

(b5 MW fuel input power) Experiments were carried out in order

to identify a working point, in which the electrically heated lab-scale

gasifier reaches high carbon conversions and cold gas efficiencies

Nomenclature

h -1

2 Compact review concerning entrained-flow gasification of

biomass

Several studies concerning the gasification of various biomasses in

entrained-flow reactors were conducted, especially since 2009[19–

42] The following presentation of research results inSection 2.1focuses

on the influences of the process conditions on gas composition and

de-duced measures The conditions studied and majorfindings are

present-ed chronologically Entrainpresent-ed-flow gasification technologies that apply

biomass and are commercialized or at an advanced development stage

are presented inSection 2.2

2.1 Selected research results since 2006

Zhang et al conducted gasification experiments with steam and

ox-ygen in nitrogen as well as pyrolysis experiments with N2in an

entrained drop-tube furnace at temperatures between 600 and 1400 °

C [20,26] Hinoki cypress sawdust with particle diameters below

1 mm was applied at gas residence times between 2 and 4 s H2yields

increased with temperature and were high with high H2O feed

(≈39 mol kg−1fuel (dry) at 1400 °C) and low with high O2feed

(≈12 mol kg−1at 1400 °C) as gasification agent Dehydrogenation of

the fuel's constituents (cellulose, hemicellulose, and lignin) during

py-rolysis and of the pypy-rolysis products (char, tar and hydrocarbons)

dur-ing gasification led to the formation of H2 The decomposition of

pyrolysis products was promoted at TN 1000 °C CO yields increased

from 600 to 800 °C and at T N 1100 °C in all atmospheres At

900b T b 1100 °C with H2O feed, CO yields decreased likely due to the

water-gas shift reaction (CO + H2O→ CO2+ H2) At TN 1000 °C with

O2and N2as well as at TN 1100 °C with H2O as gasification agent,

in-creasing CO yields were explained by the Boudouard reaction

(C + CO2→ 2CO) and the steam gasification (C + H2O→ CO + H2) re-action CO2 yields were low with N2 feed at all temperatures (≤2 mol kg−1fuel (dry)) and increased with temperature with H2O feed due to the water-gas shift reaction CO2yields were high with O2

as gasification agent (≈17 mol kg−1fuel (dry)) at Tb 1000 °C and de-creased at higher temperatures

Furthermore, Zhang et al.[20,26]showed that the concentrations of tars and char decreased with increasing temperature in all atmospheres and resulted in tar yields below 0.8 g kg−1fuel (dry) at a temperature of

1200 °C[26] Coke (soot) yields increased at temperatures between 800 and 1100 °C and decreased at higher temperatures in all atmospheres The activities in tar, char, and soot destruction followed the order with respect to gasification agent: O2N H2ON N2 Between 600 and 900 °C, carbon conversion increased with temperature with all atmospheres, decreased up to 1000 °C with H2O and O2, decreased up to 1100 °C with N2, and increased again at higher temperatures The intermediate decline of the carbon conversion was justified by the competition be-tween hydrocarbon decomposition (forming coke) and carbon consum-ing reactions with H2O, O2and CO2[26]

The effect of steam addition in the gasification of dealcoholized marc

of grape (particle diameters below 0.5 mm) with air was studied by Hernández et al.[29] The effects of the steam-biomass ratio (between

0 and 3.2 mol mol−1) on gas yields as well as of the operation temper-ature (750 °C≤ T ≤ 1150 °C) on gas yields in air (between 0 and 2.6 mol air per mol biomass) and air-steam (0 to 100 wt% H2O) gasification are demonstrated Majorfindings of Zhang et al.[26]are supported Qin et al studied the air and air-steam gasification of beech sawdust (median diameter d50= 280μm) and wheat straw (d50= 170μm) at temperatures between 1000 and 1350 °C in an atmospheric electrical-ly-heated entrained-flow reactor[28] Molar steam-to-carbon ratios varied between 0 and 1, while excess air ratiosλ varied between 0.25 and 0.50 Gas residence times between 2 and 3 s were applied The ef-fects of temperature, steam-to-carbon ratio, excess air ratio and biomass type on gas and soot yields are demonstrated

The pyrolysis and steam gasification behaviors of beech sawdust

in the fractions 0.313b dSb 0.400 mm and 0.730 b dSb 0.900 mm were studied by Septien et al in an atmospheric, electrically-heated drop-tube reactor at temperatures of 1000, 1200, and 1400 °C[33] Gas residence times varied between 2.2 and 4.4 s It was shown that soot yields decrease with temperature in a wet atmosphere (75 vol% N2, 25 vol% H2O) but increase with temperature in an inert atmosphere (amounting up to 22 wt% of the dry biomass) Tars were present at 1000 °C but completely converted at T≥ 1200 °C Char yields were below 5 wt% at all conditions studied and decreased with tem-perature in the wet atmosphere due to steam gasification Conse-quently, H2and CO2yields increased with temperature in the wet atmosphere, while C2H2, C2H4and C6H6yields decreased with tem-perature in both inert and wet atmospheres No significant influence

of the particle size on product yields were found at the conditions studied (gas residence times of 2.2 and 4.4 s) It was speculated that particles with diameters of about 1 mm could be gasified effec-tively in an entrained-flow reactor, which would decrease the pre-treatment costs relative to lower diameters

Yu et al investigated the gasification of rice straw with particle di-ameters below 0.3 mm and oxygen-enriched air[38] The influences

of O2concentration (up to 60 vol%), equivalence ratio (0.15≤ λ ≤ 0.35) and gasification temperature (800 ≤ T ≤ 1200 °C) on gas composition, carbon conversion, lower heating value and tar yield are demonstrated Concentrations of H2and CO2increase with increasing O2 concentra-tions, while CO and CH4concentrations decrease Considering the lower costs for the provision of oxygen-enriched air relative to pure air,λ = 0.25 and cO2= 40 vol% are proposed as reference operation conditions in order to achieve nitrogen-diluted syngas with a lower heating value of 8.2 MJ m−3(STP) and tar yields of 2.6 mg g−1 Yu et

al suggest that membrane technologies could provide oxygen-enriched air with up to 40 vol% O in an economic way[38]

Weiland et al studied an oxygen-blown autothermal pilot plant

gas-ifier at 2 and 7 bar with (thermal) fuel loads between 200 and 600 kW,

excess air ratiosλ between 0.25 and 0.51, and commercial stem wood

pellets produced from sawdust of pine and spruce[41] The pellets

were milled to particle sizes with characteristic size distribution

num-bers d50between 125 and 180μm and d90between 230 and 410μm

Weiland et al found carbon conversions of 100% within

0.35 b λ b 0.50 (equivalent to process temperatures of

1300b Tpb 1500 °C) and of 95 and 80% at λ = 0.30 (Tp≈ 1150 °C)

and 0.25 (Tp≈ 1090 °C), respectively The variation in fuel particle

size did not have a statistically significant effect on the gasification

re-sults This could result from the low variation in particle diameters

The highest CO yield as well as low CH4and benzene yields below

1 vol% and 100 ppm (on dry, N2-free basis), respectively, were received

atλ ≈ 0.425 (Tp≈ 1400 °C) The gasifier was optimized for the

produc-tion of syngas, which is applicable in downstream synthesis reactors,

e.g for fuel production

Detailed analyses of tars formed during pyrolysis and gasification of

biomasses with steam and oxygen at temperatures between 600 and

1400 °C were conducted by Zhang et al.[26]and Hernández et al

[34] More than 20 individual tar species were detected and quantified

The total amount of tars included in the product gas decreases with

in-creasing temperature and inin-creasing oxygen-to-fuel ratio in the feed

Higher temperatures enhance reaction rates of tar cracking and

reforming reactions O2was found to be more effective in tar cracking

than H2O or CO2due to its higher reactivity It was found that the BTX

group of tars (being represented by benzene, toluene and o-, m- and

p-xylene) is the most abundant one and needs temperatures above

1200 °C to be completely destructed [26] Phenol, naphthalene,

biphenylene and acenaphthylene could also represent a significant

share of the tars produced The amount of individual species depends

strongly on temperature, gasification agent, oxygen-to-fuel ratio and

fuel applied as demonstrated by Zhang et al and Hernández et al

In order to produce a syngas with low tar amounts and high heating

values, entrained-flow gasifiers should be operated at T ≥ 1200 °C[26]

and steam contents below 50 wt% in the gasification agent[34]

2.2 Technologies commercialized or at advanced development stage

Entrained-flow gasification technologies that apply biomass and are

commercialized or at an advanced development stage are summarized

inTable 1

Chemrec is involved in thefield of pulp and paper mills and offers

at-mospheric as well as pressurized entrained-flow oxygen-blown

gas-ifiers for the conversion of black liquor into syngas[43] A pressurized

demonstration plant started up in 2005 in Piteå

The Energy Technology Centre (ETC) in Piteå constructed an

autothermal oxygen-blown entrained-flow gasifier for woody biomass

[40,41] It is a pilot gasifier with a maximum thermal throughput of

1 MW and participated in the SUPRABIO projectfinanced by the

Europe-an Commission within the 7th Framework program It is the target to

demonstrate the economic processing of biomass in highly integrated biorefineries

KIT applies a gasifier that is based on first experiments of the Gaskombinat Schwarze Pumpe (GSP) technology[44]and built by Air Liquide in the bioliq project[45] Heated, homogenized bioslurries made by fast pyrolysis of lignocellulosic biomass are fed to the gasifier with screw or plunger pumps and atomized with oxygen Extreme pres-sures up to 80 bar are targeted for the large-scale gasifier

Linde acquired the Carbo-V technology from Choren Industrietechnik in 2012 and redesigns it since then[46] It combines horizontal stirred-tank pyrolysis as pretreatment, oxygen-blown entrained-flow gasification of pyrolysis gas and chemical quenching of the syngas with the char from pretreatment Choren's original design was sophisticated and had several drawbacks Linde expects their im-proved design to show better robustness and availability[47] ThyssenKrupp Uhde offers the PRENFLO technology, which was de-signed for the oxygen-blown entrained-flow gasification of coal, but can

be fed with pretreated biomass as well[48] Its applicability in a full bio-mass-to-liquids (BTL) process will be demonstrated in the BioTfueL pro-ject in France[49]

All entrained-flow gasification technologies described inTable 1 be-sides the one of ETC apply thermally pretreated biomass These pre-treatments change the properties of the original biomass and shift them closer to the properties of lignite or generate slurries with high heating values This facilitates the application of coal gasification tech-nologies At least the PRENFLO technology originates from coal gasifiers Most designs aim at large-scale applications with thermal input powers

of more than 100 MW and are typically operated at pressures above

25 bar This is justified by the economy-of-scale especially if Syngas-to-Fuel applications are the target[45] Nevertheless, small-scale appli-cations of entrained-flow biomass gasifiers investigated in this work are

of interest especially in order to decentralize the energy system

3 Experimental 3.1 Test facility The experimental setup included gas supply, fuel feeding, entrained-flow reactor, coarse gas cleaning and gas analysis (Fig 1) Compressed air, oxygen and carbon dioxide at typicalflow rates _VSTP between 2 and 4 m3h−1at standard temperature and pressure (STP) could be used as gasification agents at varying nitrogen dilutions

A screw-conveyor fed fuel at a rate between 1 and 2 kg h−1into a venturi nozzle A nitrogenflow (2 m3h−1(STP)) transports the fuel from the venturi nozzle to the top of the reactor (inner diameter:

160 mm) in a tube with an inner diameter of 20 mm An electrical oven heats the reactor tube in three independent sections to a temper-ature of up to 1200 °C The reactor tube was made of stainless steel Eight type N thermocouples measured the temperature at the vertical axis of the reactor tube with a length of 2.1 m Theflow rates of

Table 1

Entrained-flow gasification technologies for biomass that are commercially available or at advanced development stage [40,41,43,45,47,48,50,51]

Gasification (BLG)

mill Energy Technology Centre,

Sweden

torrefaction Karlsruhe Institute of

Technology, Germany

500 °C

pretreatment

4–5 bar, N1400 °C, quenching with char to b1000 °C

Pyrolysis at b500 °C ThyssenKrupp Uhde, Germany PRENFLO Pulverized torrefied material from wood, straw,

and energy crops

gasification agent and transport gas (N2) were set by massflow

control-lers Bothfluid streams entered the hot reactor tube without preheating

In order to reach approximately isothermal conditions during gasi

fi-cation,first and last section of the oven were set to 30 K higher

temper-atures than the middle section Typically, maximum deviations between

the target temperature of the middle oven section and temperatures

measured inside of the reactor tube amounted to less than 30 K within

the region 0.3b x b 1.7 m at steady-state gasification conditions (Fig 2)

The temperature distributions measured for beech wood at the four

tar-get temperatures are displayed inFig 2 Higher deviations from the

tar-get temperature are present close to the end of the reactor tube

(xN 1.7 m) since this part was outside of the oven and connected via

flanges to the water quench (injection of ca

2.6 l min−1at 15 °C) The tubing above and below the oven is

insu-lated only with ceramicfiber tape without external heating

Raw gas, ash, and non-converted fuel particles passed a water

quench downstream to the reactor tube, where four hollow cone

noz-zles (1.1 mm drilling with 0.66 l min−1throughput at 6 bar water

pres-sure, spray cone of 60°) were placed and injected water with a

temperature of about 15 °C The gas trajectory included a 180° curve

in the water quench Thereby, a fast cooling to temperatures between

40 and 60 °C occurred and solids as well as liquid droplets were

separat-ed from the gasflow Most of the tars produced by the gasifier were

con-densed here and pumped out of the quench together with water

Neither weighing nor compositional analysis of these tars were carried

out Further particles were separated by a cyclone following the water

quench Finally, a fraction of the gas passed afiber filter prior to the

gas analysis by a Fourier transform infrared (FTIR) spectroscope and a

gas chromatograph with thermal conductivity detector (GC-TCD)

3.2 Gas analysis

FTIR spectroscope and GC-TCD were the main devices for analyzing

the gas composition The FTIR spectroscope was of the type Gasmet

CX4000 from the company ANSYCO Analytische Systeme und

Componenten GmbH It was connected to the test facility behind the

cy-clone (compareFig 1) via a heated tube (T = 150 °C) The GC-TCD was a

GCM Microbox II from the company Elster GmbH It was also connected

to the heated tube Additionally, detectors for H2(TCD CONTHOS 2 from

the company LFE GmbH & Co KG) and O2(paramagnetic oxygen

analyzer PMA100 from the company M&C TechGroup Germany GmbH) were combined with the FTIR spectroscope The gas phase spe-cies CO, CO2, N2, O2, H2, H2O, CH4, C2H6, C2H4, benzene, toluene and xy-lene were detected and quantified by the quoted techniques The concentrations of the most important species (CO, CO2, H2, CH4) were measured by FTIR spectroscope and GC-TCD independently

3.3 Fuels Two types of woody biomass were used in the experiments: afine beech wood (BW) fraction with particle diameters of 0.04≤ dS≤ 0.11 mm and waste wood (WW) from a local joinery, which was a mixture of beech and spruce The waste wood was fraction-ated with a sieving unit into samples with particle diameters of 0.2≤ dS≤ 0.25 mm, 0.25 ≤ dS≤ 0.5 mm, and 0.5 ≤ dS≤ 1.0 mm Addition-ally, power plant lignite (LI) with particle diameters of 0.05≤ dS≤ 0.08 mm served as a reference Results of proximate and ul-timate analyses are shown inTable 2 Moisture content, ash content, fixed carbon, and lower heating value mentioned in proximate analysis were determined by procedures regulated in DIN EN 14774–1, DIN

51719, VDI 2465 (part 2) and DIN EN 14918, respectively The applied methods for the determination of carbon, hydrogen, and nitrogen as

Fig 1 Experimental setup of the atmospheric entrained-flow test facility.

Fig 2 Measured temperature distributions at the vertical axis of the reactor tube at steady-state gasification conditions with beech wood (BW) and four target temperatures.

well as of sulfur (and chlorine) stated in the ultimate analysis are

regu-lated in DIN EN 15104 and DIN EN 15289, respectively Chlorine

con-tents were below 200 ppm in all cases While the values from

proximate analysis for BW and WW were comparable, lignite contained

about 12% (absolute) more carbon and 24% (absolute) less oxygen than

the wood samples

Ash, volatiles andfixed carbon contents for beach wood (BW) and

waste wood (WW) were comparable at values of about 1%, 87%, and

12%, respectively The WW had a higher moisture content of 7.6%

rela-tive to 5.1% for BW and a slightly smaller lower heating value of

18.1 MJ kg−1 Lignite had much higher ash andfixed carbon contents

of 15 and 43%, respectively, a higher moisture content of nearly 12%,

and a higher lower heating value of 24.6 MJ kg−1than the two wood

fractions Comparable results for beech wood are presented in[33]

3.4 Test series

Gasification experiments were carried out with air as gasification

agent at oven target temperatures between 950 and 1100 °C Flow

and feeding rates, calculated O2concentration as well as the equivalence

ratio (excess air ratio, i.e mass of oxygen applied to mass of oxygen

needed for total combustion) for the different fuel fractions are

summa-rized inTable 3

3.5 Data evaluation

The measured H2O concentrations were affected by the injection of

water in the quench at the reactor tube outlet and did not represent

the water content of the product gas They varied between 3.5 and

9 vol% in all cases without showing a clear trend No dependences for

the gas species CH4, C2(C2H4, C2H6) as well as for BTX tars (benzene,

tol-uene and xylene) are discussed since their concentrations were low or

affected by the water quench CH4, C2 and BTX tar concentrations

were measured by the FTIR spectroscope and amounted to values

below 1.0, 0.2 and 0.2 vol%, respectively, in all cases

A carbon mass balance was formulated based on the measured

spe-cies CO, CO2, CH4and BTX tars, while all other gaseous hydrocarbons

(C2) were omitted Since a certain but not quantified amount of tars

was condensed in the water quench (compareSection 4.4) and

uncon-verted char was pumped out of the water quench without weighing, the

carbon mass balance could never be closed The missing value to 100%

was attributed to char and tars washed out in the water quench The

molarflow rate of the product gas was determined by the ratio of the

N2concentrations present in educt and product gas Based on this, the

mass of carbon-containing species in the product gas was deduced

The carbon conversion was derived by calculating the ratios of the

car-bon mass present in the product gas and in the raw fuel Furthermore,

the volumeflow rates of H2, CO and CO2were calculated, summed up

and divided by the fuel'sflow rate yielding the specific syngas volume

(volumes of H2, CO and CO2per mass of wet fuel) The cold gas efficiency

was determined by dividing the chemical energy (product of mass-basedflow rate and heating value) in the produced syngas by the

ener-gy input (product of feeding rate and heating value) from the fuel (wet basis) Lower heating values were applied in all cases

4 Results and discussion 4.1 Particle residence time and slip velocity Prior to gasification experiments, measurements of the particle res-idence time were carried out with a modified setup The steel reactor was replaced by a quartz tube of comparable size to allow a visual obser-vation of injected biomass particles The motion of a particle fraction through the tube was monitored by dosing 2 g of biomass into the ven-turi nozzle The times when particles entered the top and reached the bottom of the quartz tube were measured These measurements were reproduced ten times for each fraction Standard deviations of up to 11% of the mean value for each fraction were realized Since big particles have a higher velocity than small particles[23,33], the residence times

of the bigger particles in each fraction were determined

Particle residence times measured for sawdust fractions with parti-cle diameters of dS b 0.25 mm, 0.25 b dS b 0.5 mm and 0.5b dSb 1.0 mm at a total flow rate of appr 3 m3h−1(STP) are displayed inFig 3(A) The residence times of 1 mm and 0.25 mm parti-cles amounted to 1.4 ± 0.1 s and 3.3 ± 0.2 s, respectively, while the gas residence time was about 50.7 s These measurements indicate a strong dependence of the particle residence time on the diameter of the fluid-ized particles and support theoretical predictions of Dupont et al.[23], Umeki et al.[52]and Kirtania and Bhattacharya [53] This will be discussed in the following with the help of a general model

In the model, the gas velocity uGis determined by Eq (1)from the volumetricflow rate at standard temperature and pressure _VSTP, actual temperature T and pressure p in the tube, and the cross section A of the tube Particles were injected into the reactor tube byfluidization in a N2

flow with _VN2¼ 2:0 m3h−1through a tube with an inner diameter of

2 cm in the lab-scale gasifier Assuming that the particles move with the velocity of thefluidization agent at the end of the feeding tube, their starting velocity at the top of the reactor tube amounts to uS,0= 1.93 m s−1(Eq (1)) Since the reactor tube is eight times wider than the feeding tube, the gas decelerates quickly, e.g to uG= 0.07 m s−1 when 2.3 m3h−1of air (total gasflow rate of 4.3 m3h−1assumed in

Fig 3A) are applied as gasification agent at 25 °C

uG¼ _VSTP

T pA

101325 Pa

The progress of the particle velocity from the top to the bottom of the reactor tube is governed by the attractive gravitational force and a decelerating drag force Fdas a simple assumption in this model The

Table 2

Proximate and ultimate analyses of beech wood (BW), waste wood (WW) and lignite (LI).

Proximate analysis (wt%)

Ultimate analysis (wt% dry, ash-free basis)

drag force is calculated by Eq.(2)while the drag coefficient Cdis

de-scribed by Eq.(3) [54] Spherical particles are assumed in Eq.(2)for

simplicity More advanced models, which can describe the motion of

non-spherical particles explicitly, are presented by Chhabra et al.[55]

and Loth[56]and need the aspect ratio or the shape factor as additional

input data Eq.(3)includes thefitting parameters α and β Eq.(3)

rep-resents the Schiller-Naumann expression[56,57]when the values

α = 0.15 and β = 0.687 are used The residence times calculated with

these values were significantly lower than the measured values at 25 °

C That is why the values of thefitting parameters were evaluated

from our data and determined toα = 0.20 and β = 1.0 Using these

values in the model presented (Eqs 2–4), residence times determined

by experiment and by modelling agree well with each other The stated

values ofα and β are specific for the grinded biomass used and should

not be generalized Furthermore, Eq.(3)includes the slip Reynolds

number Re of fuel particles, which is displayed in Eq.(4) Therein,ρG

andμGare representing density and dynamic viscosity of the gas sur-rounding the particles

Fd¼18πd2

Cd¼ 24 1 þ α Re β

Re¼ ρGjuS−uGj dSμ−1

Particle residence times and slip velocities uslip= uS− uGat the re-actor end at 25 and 1100 °C without fuel conversion were simulated with the model described above They are included also inFig 3(A)

In order to discussFig 3(A), the spatial development of the velocity

of particles inside of the reactor tube is shown in the right part Particles enter the reactor tube at its top with a velocity of

uS≈ 1.9 m s−1, where the gas phase decelerates suddenly As a con-sequence, particles are affected by a drag force that decelerates them at the top of the reactor tube 0.25 mm particles decelerate within thefirst 0.2 m of the reactor tube (Fig 3B) They do not reach the velocity of the gas because the drag force equals the gravitational force at a higher par-ticle velocity A noticeable parpar-ticle slip velocity is a result thereof (Fig

3A) 1.0 mm particles decelerate within 0.4 m of the reactor tube at

25 °C since the drag force per mass acting on them is lower than for 0.25 mm particles (Fig 3B) Furthermore, a higher slip velocity than for 0.25 mm particles is needed for 1.0 mm particles in order to compen-sate the accelerating gravitational force This is justified as the gravita-tional force grows with dS, while the drag force grows with dSonly (Eq (2)) At 1100 °C, gas velocities are increased by a factor of 4.4 and gas densities are decreased accordingly 1.0 mm particles accelerate to about 2.5 m s−1until the end of the reactor tube in this case since the drag force is strongly reduced

Eqs (1) to (4)demonstrate the relationships between particle ve-locity, particle diameter, temperature, and pressure mathematically

Fig 3shows these relationships in a more comprehensive way Particle residence times increase as the particle diameter decreases (Fig 3A) This is a consequence of increasing drag forces per mass with decreasing particle diameters At diameters below 0.1 mm, the particle slip velocity gets close to zero so that particles have the same velocity as the sur-rounding gas phase For dSN 0.35 mm, uslipis typically higher at 1100 °

C than at 25 °C, while it depends only to a minor extend on temperature

at dSb 0.35 mm (Fig 3A)

The particle slip velocity was modelled also by Dupont et al.[23]for a comparable entrained-flow reactor at 150–300 K lower temperatures Dupont et al quoted slip velocities for 0.2 and 1.1 mm particles as high as 0.20 andN2 m s−1at the end of their reactor tube, respectively These values are lower than the ones displayed inFig 3(A) at 1100 °C amounting to 0.26 and 2.4 m s−1,respectively The principal depen-dence of the particle slip velocity on the particle diameter is reproduced well, while there is some uncertainty about the exact height of the slip velocity

Table 3

Flow rates of the gasification agent (air), feeding rates of the fuel, calculated O 2 concentration at the reactor inlet, equivalence ratio, and gas residence time at 950 ≤ T ≤ 1100 °C with beech wood (BW), waste wood (WW), and lignite (LI).

Fuel fraction Flow rate gasification agent in m 3

h−1 (STP)

Feeding rate fuel in kg

h−1

O 2 concentration at inlet in vol%

Equivalence ratio

Gas residence time in s

0.04 ≤ d S ≤ 0.11 mm

(BW)

0.2 ≤ d S ≤ 0.25 mm

(WW)

0.25 ≤ d S ≤ 0.5 mm

(WW)

Fig 3 A: Measured and simulated residence times of fuel particles as well as the particle

slip velocities at 25 and 1100 °C in dependence of the particle diameter without fuel

conversion at a total flow rate of 3 m 3

h−1 B: Development of the particle velocity at 25

flow rate of 4.3 m 3 −1

Umeki et al.[52]and Kirtania and Bhattacharya[53]also applied Eq.

.2–4but usedα = 0.15 and β = 0.687 (Schiller-Naumann expression

[57]) in order to calculate the drag force coefficient Residence times

of 9 and 0.7 s for 0.1 mm and 1 mm particles, respectively, were

calcu-lated for a reactor length of 1.9 m at 1000 °C by Umeki et al.[52]

These residence times are higher than the ones presented inFig 3(A)

since the gas velocity was lower in the setup of Umeki et al.[52] The

values of the residence time in[53]is in the same range as in[52]for

particle sizes between 0.1 and 1 mm but the shape of this function

dif-fers The latter results from the incorporation of accurate pyrolysis

ki-netics into their model (consisting of particle motion, heat transfer

and pyrolysis) and the changes in particle motion caused by density

var-iations due to pyrolysis This approach is beyond the scope of the

pres-ent article but of major importance when modelling the gasification

process

Deviations between the results of different models may not only be

attributed to the partly different process conditions but also to deviating

particle shapes, which influence the drag force and the fitting

parame-tersα and β consequently Hence, it is recommended to measure the

particle velocity or residence time at low temperatures and calculate

the parametersα and β with the model described above Particle

veloc-ities at gasification temperatures can be determined subsequently

4.2 Gas concentrations

Gasification experiments were carried out at the conditions listed in

Table 3in order to study the effects of temperature and particle

diame-ter on product gas composition and carbon conversion The gas

concen-trations measured for H2, CO, CO2and N2after the cyclone are presented

inFig 4 The concentrations varied between 2 and 10% for H2, 4 and 16%

for CO, 6 and 11% for CO2as well as 67 and 83% for N2depending on

tar-get temperature and particle diameter Decreasing N2concentrations

are a direct consequence of pyrolysis, char gasification, and gas-phase

reactions [26,41], which release gases and dilute the educt gas

consisting of 87.2–89.5% N2with other species A discussion of the var-iations in gas concentrations is complicated since pyrolysis was not complete in all cases (especially for particles with dSN 0.1 mm and tem-peratures below 1050 °C)

Nevertheless, basic trends found by Zhang et al.[22]for partial oxi-dation conditions could be reproduced by this work E.g CO and CO2 concentrations were nearly independent on target temperature at

T≤ 1000 °C for biomass, while CO2concentrations decreased and CO concentrations increased at higher temperatures These dependencies were more pronounced with lignite (LI) than with biomass over the whole temperature range studied H2concentrations generally in-creased with increasing target temperature and decreasing particle di-ameter due to improved dehydrogenation of the fuel (cleavage of

C\\H or C\\O bonds) and decomposition of primary products (hydro-carbons, char, and tar)[20,26] They were similar for biomass and lignite particles with comparable particle sizes

With oxygen or air as gasification agent, it is characteristic that H2

and CO concentrations increase with reactor temperature and decrease with particle diameter, while CO2concentrations decrease slightly with reactor temperature and increase slightly with particle diameter This behavior is observed in entrained-flow reactors[21,22,25,26,28]as well as in other reactor types[58,59]when biomass or coal are applied

as fuel Higher temperatures and lower particle diameters support the complete pyrolysis and char gasification of solid fuels and the approach towards the chemical equilibrium of all reactions involved especially of the water-gas shift reaction

The calculated H2/CO ratio is displayed inFig 5 Typically, the H2/CO ratio increased with target temperature and varied between 0.55 and 0.81 for all biomass fractions Such values are typical for gasifiers oper-ated with relatively dry biomass and oxygen or air as gasification agent[25,26,41] No trend relating the H2/CO ratio with the particle di-ameter is observed Interestingly, the H2/CO ratio decreased with target temperature for lignite (Fig 5) from 0.82 at 950 °C to 0.63 at 1100 °C This might be attributed to the higher C/H ratio present in lignite

(12.5, mass basis) relative to biomass (8.2) (Table 2) and an improved

carbon conversion with increasing temperature

4.3 Carbon conversion, cold gas efficiency, and gas yield

At 950 °C, all waste wood (WW) fractions and lignite (LI) showed

carbon conversions between 63 and

66 wt% displayed inFig 6(A), which indicate that pyrolysis was not

completed yet At the same time, beech wood (BW) showed a carbon

conversion amounting to nearly 80 wt%, which suggests complete

devolatilization of the fuel into permanent gases, tars, water, and char

[20,26,33]due to its low particle diameters Carbon conversions

in-creased for all fuels applied with target temperature At 1100 °C, the

largest WW fraction and the veryfine BW fraction showed carbon

con-versions of 72 and 93 wt% BW with particles sizes below 0.12 mm was

fully pyrolyzed and the resulting char and tars were partly converted

into gas Higher amounts of char and tars were generated with the

larg-er biomass fractions At the same time, lignite with particle sizes below

0.09 mm was fully converted into permanent gases and water with

minimal tar concentrations

At 950 °C, cold gas efficiencies varied between 15% and 36% for the

largest WW and the BW fraction inFig 6(B), respectively Increasing

the target temperature, cold gas efficiencies increased steadily for all

fuels applied At 1100 °C,finest and largest biomass fraction showed

cold gas efficiencies of 54 and 27%, respectively, while lignite reached

a value of 61% Cold gas efficiencies steadily increased with decreasing

biomass particle diameters at all target temperatures Hernández et al

found cold gas efficiencies between 35 and 38% with dealcoholized

marc of grape with particle diameters below 0.5 mm studying air gasi

fi-cation in the same temperature range[29] Weiland et al reported cold

gas efficiencies (assuming fuel applications) between 62 and 70% in

their oxygen-blown pilot gasifier at a comparable equivalence ratio

(lambda value of ca 0.4 equivalent to a temperature of ca 1400 °C)

and increased pressures (up to 7 bar)[41] These comparisons

demon-strate that the gasifier works relatively well at 1100 °C with very fine

wood particles (dS≤ 0.11 mm) With particle sizes of 0.2 mm and

above as well as at temperatures below 1100 °C, cold gas efficiencies

are relatively low and hamper a commercial application of the

technol-ogy The results of Weiland et al.[41]indicate that a few hundred kelvin

higher temperatures (equivalent to lambda values of around 0.4 in

autothermal gasification) are necessary to reach commercially viable

cold gas efficiencies

The specific syngas volume (gas yield) is another measure to

evalu-ate the gasification process The obtained values between 0.6 and

1.4 m3kg−1(STP) inFig 6(C) are in the range reported by other authors

[58,60–63] Principally, the specific syngas volume shows the same

de-pendences on temperature and particle diameter as carbon conversion

and cold gas efficiency (Fig 6A and B) The highest specific syngas

volumes are observed for lignite and the veryfine beech wood fraction

at 1100 °C which correlates with the highest carbon conversions and cold gas efficiencies

4.4 Discussion of the results

As shown inTable 4, the particle residence time depends only

slight-ly on temperature for veryfine particles (dS≤ 0.11 mm), while it is prac-tically independent of temperature for larger particles Hence, the higher carbon conversions obtained at higher temperatures are clearly attributed to enhanced pyrolysis, char gasification and tar decomposi-tion reacdecomposi-tions Although both surface-to-volume-ratio (assuming spherical particles with a mean diameter) and minimal residence time (Table 4) increase significantly as particle diameters decrease, the in-crease in carbon conversion is less pronounced A significant increase

Fig 5 H 2 /CO ratio resulting from the measured gas concentrations for beech wood (BW),

waste wood (WW) and lignite (LI).

Fig 6 Dependence of carbon conversion (A), cold gas efficiency (B) and specific syngas volume (C) on temperature and particle diameter d S for beech wood (BW), waste wood (WW) and lignite (LI).

is observed only for thefinest biomass fraction, which is attributed to

doublings of residence time and surface-to-carbon-ratio At the same

time, the increase in cold gas efficiency can be clearly attributed to

lower particle diameters and higher gasification temperatures

These results supportfindings made in entrained-flow[25],

fluid-ized-bed[64], andfixed-bed gasifiers[65], after which temperature

and particle diameter are the most important parameters influencing

their performance As the gasification temperature in entrained-flow

re-actors increases to temperatures above 1200 °C, the influence of particle

diameters on carbon conversion disappears progressively for

submilli-meter particles[33,41] This is true for gasifiers that guarantee a

suffi-ciently high particle residence time since pyrolysis and char

gasification are close to completion (no solid residues containing

car-bon) In this case, gasification temperature, gasification agent

(composi-tion and amount), gas residence time, and possibly available catalytic

materials determine the final gas composition (permanent gases,

water, and tars)

Particles with diameters of ca 0.2 mm and above can be subject to

extra- und intra-particle heat and mass transfer limitations at gasi

fica-tion condifica-tions, which decrease pyrolysis and char gasification rates

[64,66] Taking this into account together with the decreasing residence

time of particles with increasing diameter (Table 4), complete carbon

conversion during entrained-flow gasification can be obtained only

with particles of a certain size Septien et al speculates that this limit

lies at a particle diameter of about 1 mm[33] At this limit, a carbon

con-version of 72 wt% was obtained at 1100 °C in the test facility Studies

with torrefied biomass show that this pretreatment can enhance the

carbon conversion efficiency of biomass[40,67,68]due to the

decompo-sition of hemicellulose and restructuring of the pore system[69,70]

Hence, the particle size applicable in entrained-flow gasifiers can be

en-hanced by torrefaction Additionally, grinding costs are strongly

re-duced[71,72]

The tar concentrations measured by FTIR in this work (including

benzene, toluene and xylenes (BTX) only) after water quench and

cy-clone varied between 400 and 2000 ppmv (equivalent to 1.6–8 g m−3

(STP)) while commercial co-current and counter-currentfixed-bed as

well asfluidized-bed gasifiers show values in the ranges of 0.5–2, 50–

150 and 7–10 g m−3(STP)[73,74], respectively A significant fraction

of the tars from gasification condensed by the water that was injected

into the quench The amount of these tars was not quantified

Considering the works of Zhang et al.[26]and Hernández et al.[34],

BTX are the dominating tar components at the conditions studied with

lower amounts of naphthalene, phenol, and biphenylene Regarding

statements made in[26,34], BTX concentrations should be by a factor

offive to 20 higher in the product gas of the test facility in front of the

water quench than were measured after the cyclone These

consider-ations indicate that most of the tars generated during mild

entrained-flow gasification can be separated from the product gas by injection of

water This represents a simple condensation technique at the expense

of gas temperature and cold gas efficiency

5 Summary and conclusions

A technology for the atmospheric decentralized entrained-flow

gas-ification of biomass was investigated at temperatures between 950 and

1100 °C and equivalent ratios between 0.38–0.43 With reference to the

aim of this work– developing an entrained-flow biomass gasification technology that operates at mild conditions and makes decentralized applications more likely– important findings were made:

• Maximum carbon conversion at T ≤ 1100 °C was 93 wt% with very fine biomass particles (dSb 0.12 mm) and amounted to ≥72 wt% for parti-cles with dSb 1 mm,

• Entrained-flow fuel feeding with N2and application of air as gasi fica-tion agent resulted in N2concentrations of≥67 vol% (dry),

• Particle residence times resulting from the applied fuel feeding were slightly higher than from free fall techniques,

• Tar concentrations behind the cyclone were below 8 g m−3(STP) due

to water quenching, but a factor offive to 20 higher in front of the water quench indicating insufficient reactor length or gasification temperature, and

• H2/CO ratio of 0.75 was obtained at the highest carbon conversion

This work reveals high nitrogen dilutions of the product gas and short residence times as main obstacles of the gasifier concept studied This situation could be improved if the gasification agent is used as flu-idization agent at the same time Alternatively, instead of a pneumatic feeding, screw or belt conveyors can be used since they are inexpensive and have low energy consumption[75]

Increasing the length of the reactor tube is another option to in-crease carbon conversion and dein-crease tar concentrations by improving the residence times of particles and gas High residence times are espe-cially important for particles with diameters of 0.2 mm and above since they can be subject to significant intra- and extra-particle heat and mass transfer limitations[64], which counteract fast gasification Measures for thermal insulation have to be improved when longer reactor lengths are applied

By considering the improvement options discussed above, the devel-opment of an entrained-flow biomass gasifier for decentralized applica-tions seems to be feasible

Abbreviation Description

BLG Black liquor gasification BTL Biomass-to-liquids ETC Energy Technology Centre FTIR Fourier transform infrared GC-TCD Gas chromatograph with thermal conductivity detector GSP Gaskombinat Schwarze Pumpe

KIT Karlsruhe Institute of Technology

Acknowledgement This work wasfinanced partly by the European Regional Develop-ment Fund (ERDF) (100116093) provided by the Sächsische AufbauBank (SAB) We thank ERDF and SAB for thefinancial support

of the project

Table 4

Comparison of surface-to-volume-ratio (S2VR), carbon conversion (η C ), cold gas efficiency (η CG ) and minimal particle residence time τ min (calculated with the model presented in Section 4.1 ) of all biomass fractions applied at 1000 and 1100 °C.

Biomass fraction S2VR in m−1 η C , 1000 in wt% η CG , 1000 in % τ min , 1000 in s η C , 1100 in wt% η CG , 1100 in % τ min , 1100 in s

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