i

PNL--5534

DE86 015382

ENGINEERING ANALYSIS OF BIOMASS GASIFIER PRODUCT GAS CLEANING TECHNOLOGY

E. G. Baker M. D. Brown R. H. Moore L. K. Mudge D. C. E l l i o t t

PNL-5534 UC-61F

_-

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately awned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily ,constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

August 1986

Prepared f o r t h e Biomass Energy Technology D i v i s i o n U.S. Department o f Energy under Con t rac t DE-AC06-76RLO 1830

P a c i f i c Nor thwest Labora to ry R ich land, Washington 99352

This ducumcnt is

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

I ABSTRACT

For biomass g a s i f i c a t i o n t o make a s i g n i f i c a n t cont r ibu t ion t o the energy

p i c tu re in the next decade, emphasis must be placed on the generation o f

c lean , po l lu t an t - f r ee gas products. This repor t attempts t o quant i fy l e v e l s

of p a r t i c u l a t e s , t a r s , oi 1 s , and various o the r pol 1 u t an t s generated by biomass gasifiers o f a l l types. End uses for biomass gases and appropr i a t e gas c lean- ing technologies a r e examined. Complete systems ana lys i s i s used t o p red ic t

t h e performance of various g a s i f i e r / g a s cleanup/end use combinations. Further

research needs a r e i dent i f i ed.

%

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SUMMARY

An i n t e rp re t ive l i t e r a t u r e search was done t o determine pa r t i cu la t e and

t a r emissions from various biomass g a s i f i e r s , ident i fy pa r t i cu la t e and t a r

l imi t s f o r spec i f i c end uses f o r the gas, and evaluate d i f f e r e n t gas cleaning

options which meet the spec i f ica t ions f o r d i f f e ren t end uses.

The l i t e r a t u r e search revealed a profound lack of quant i ta t ive informa-

t ion in t h i s area. In pa r t i cu la r there i s very l i t t l e information on the

propert ies of t a r s and pa r t i cu la t e s from biomass gas i f ica t ion and there i s

almost no documentation on the effect iveness of gas cleaning systems f o r

biomass derived gas, pa r t i cu la r ly f o r t a r removal.

In l i g h t of the lack of data avai lable on gas cleaning with biomass

gas i f i ca t ion systems the r e s u l t s reported here a re based heavily on informa-

t ion from other systems, primarily coal gas i f ica t ion and biomass combustion.

Numerous c i t a t i o n s on p a r t i c l e removal, b u t only l imited information on t a r

removal were found. As a r e s u l t the report addresses pa r t i cu la t e removal in

more d e t a i l . Tars a re , in most instances, ult imately removed as l iqu id drop-

l e t s so much of the da ta on pa r t i cu la t e removal i s applicable t o t a r removal.

However, the propert ies o f t a r s such as t h e i r v i scos i ty and s t i ck iness must

be considered.

A var ie ty of un i t s were encountered during the course of t h i s study. We

attempted t o keep the uni t s consis tent f o r each subject area b u t t o do so f o r

the e n t i r e report we f e l t was not appropriate. Appendix A i s a nomograph

t h a t can be used t o convert d i f f e ren t s e t s of un i t s used in the t ex t .

Gas i f ie rs t h a t have been used w i t h biomass a re fixed bed updraft , fixed

bed downdraft, f l u i d bed, and entrained bed uni ts . Fixed bed updraft un i t s

t yp ica l ly produce large quan t i t i e s of t a r s and o i l s (10-100 g/m ) . Fixed bed 3

V

downdraft and f l u i d bed uni t s generally have lower t a r emissions in the range

o f 0.05-0.5 g/m . J

3

3 Par t icu la te emissions vary from low pa r t i cu la t e loadings (0.1-1.0 g/m ) 3 f o r an updraft g a s i f i e r t o 10-100 g/m o r g rea t e r f o r a f l u i d o r entrained

bed uni t .

The type of g a s i f i e r a l so influences the physical cha rac t e r i s t i c s o f

bo th t a r s and par t icu la tes . High boi l ing, condensed aromatic t a r s a r e gener-

ated from f l u i d beds and downdraft uni ts . Lower boi l ing, highly oxygenated

wood oils a re generated from updraft and entrained f l o w pyrolysis units.

Fixed bed g a s i f i e r s emit small quan t i t i e s of very f i n e pa r t i cu la t e s (mostly

ash) entrained i n the gas stream. Fluid beds on the o ther hand re lease large

amounts o f very coarse par t iculates w h i c h are pr imari ly char and ash. Data

on pa r t i cu la t e and t a r production r a t e s and cha rac t e r i s t i c s i s 1 imi ted.

Further study which addresses quant i ta t ive pol 1 utant 1 eve1 s f o r each type o f

gasi f i e r i s needed.

End uses f o r biomass gases include use as a fue

gas. For example, pa r t i cu la t e s in burner

standards which vary depending on the s

Many s tudies have been done w i t h in ternal

f o r indus t r i a1 process

bo i l e r s , dryers o r k i lns ; as a fuel f o r diesel and spark ign i t ion engines and

gas turbines; and as a synthesis gas f o r methanol, methane, hydrocarbons and

ammonia. Each end use has spec i f i c requirements f o r the c leanl iness of the

f l ue gases must meet environmental

ze and location of the f a c i l i t y .

combustion engines. Par t icu la tes

or t a r s above 0.05 g/m are shown t o cause excessive engine wear or gum forma-

t ion on the valves. Additional research i s needed t o define l imi t s f o r gas

turbines and synthesis gas applications.

3

Gas cleanup technologies tha t a r e most applicable f o r biomass gas i f ica-

t i o n a r e cyclones, wet scrubbers, various f i l t e r s (including baghouses) and

vi

e l e c t r o s t a t i c p rec ip i t a to r s .

g a s i f i e r s has not been extensively reported.

Data on gas cleaning from operating biomass

Based on data f o r coal systems and l imited biomass da t a , pa r t i cu la t e

removal systems appear adequate f o r most biomass g a s i f i e r appl icat ions. How-

ever, t a r removal presents problems which heretofore have not been studied i n

any d e t a i l . Further research i s needed t o confirm the app l i cab i l i t y and

ef f ic iency of various gas cleanup methods coupled w i t h d i f f e r e n t g a s i f i e r

types.

Table 1 summarizes the r e s u l t s of t h i s study. Areas where ava i lab le

information i s adequate f o r se lec t ion and design of gas treatment systems are

iden t i f i ed . Areas where su f f i c i en t information i s not ava i lab le and addi-

t ional research and development a re necessary a re a l so l i s t e d in Table 1.

This should serve as a basis f o r fu r the r e f f o r t s in gas cleaning f o r biomass

gas i f ica t ion .

v i i

TABLE 1. Summary of Gas Cleaning Technology Research Needs for Biomass Gasifiers

Areas Where Avai 1 ab1 e Information i s Adequate for Selection and Design of Gas Treatment Systems

Particulate production rates

Particulate and tar limits for internal combustion eng

Particulate limits for direct-fired equipment (boilers kilns, etc.)

Tar composition

nes

dryers

Areas Where Additional Research and Development are Necessary

Tar production rates for all types of gasifiers -- effects of operating conditions, type of biomass, moisture content, etc.

Composition and size distribution of particulates

Particulate and tar limits for gas turbines and synthesis gas applications

Efficiency of tar removal devices

Effect of conditions (temperature, pressure, tar-loading) on tar deposition in transfer pipes, burners, and other downstream equipment

Volatility of ash components

viii

TABLE OF CONTENTS

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ABSTRACT i i i

SUMMARY . . . . . . . . . . . . . . . . . . . . v INTRODUCTION . . . . . . . . . . . . . . . . . . 1

1.0 BIOMASS GASIFICATION GAS STREAM CONTAMINANTS . . . . . . . . 4 1.1 G a s i f i e r Types . . . . . . . . . . . . . . . . 4 1.2 Tars and Other Condensible Organics . . . . . . . . . 11 1.3 P a r t i c u l a t e s . . . . . . . . . . . . . . . . . 16 1.4 Other Contaminants . . . . . . . . . . . . . . . 20 1.5 Pressure E f f e c t s . . . . . . . . . . . . . . . 23 1.6 Summary and Conclusions . . . . . . . . . . . . . 24 1.7 References . . . . . . . . . . . . . . . . . 26

2.0 END USES FOR BIOMASS GAS . . . . . . . . . . . . . . 3 1 2.1 Burners ( B o i l e r s / K i l n s ) . . . . . . . . . . . . . 3 1 2.2 D iese l and Spark I g n i t i o n Engines . . . . . . . . . . 35 2.3 Gas Turbines . . . . . . . . . . . . . . . . . 37 2.4 Synthes is Gas/Pipel ine Gas . . . . . . . . . . . . . 41 2.5 Conclusions . . . . . . . . . . . . . . . . . 44 2.6 References . . . . . . . . . . . . . . . . . 46

3.0 GAS CLEANUP TECHNOLOGY . . . . . . . . . . . . . . . 49 3.1 D e f i n i t i o n s . . . . . . . . . . . . . . . . . 49 3.2 P a r t i c u l a t e Removal . . . . . . . . . . . . . . 5 1 3.3 TarRemoval . . . . . . . . . . . . . . . . . 66 3.4 Opera t ing Cond i t ions . E f f i c i e n c i e s . and Cost . . . . . . 7 1 3.5 References . . . . . . . . . . . . . . . . . 75

4.0 EVALUATION AND RECOMMENDATIONS . . . . . . . . . . . . 79 4.1 Systems Ana lys i s . . . . . . . . . . . . . . . 7 9 4.2 Eva lua t ions By End Use . . . . . . . . . . . . . 83 4.3 Recommendations . . . . . . . . . . . . . . . . 87 4.4 References . . . . . . . . . . . . . . . . . 88

APPENDIX A

Nomograph f o r Conversion o f Emission Loadings . . . . . . . A . l

i x

. LIST OF FIGURES

F igure 1 Gas Cleanup Technology f o r Biomass G a s i f i c a t i o n . . . . 2 Figure 2 Schematic o f Fixed Bed Updraf t G a s i f i e r . . . . . . . . 6 Figure 3 Schematic o f Fixed Bed Downdraft G a s i f i e r . . . . . . . 7 F igure 4 Schematic o f a Crossdraf t G a s i f i e r . . . . . . . . . 8 Figure 5 Schematic o f a F l u i d i z e d Bed G a s i f i e r . . . . . . . . 9 Figure 6 Schematic of an Entra ined Bed G a s i f i e r . . . . . . . . 10 F igure 7 Typica l Tar Concentrations from Various G a s i f i e r s . . . . 12 F igure 8 Typica l P a r t i c u l a t e Concentrations from Various G a s i f i e r s . . 16 F igure 9 Al lowable P a r t i c u l a t e Concentrat ion f o r Various End Uses . . 45 F igure 10 P a r t i c l e S ize C l a s s i f i c a t i o n and Useful C o l l e c t i o n Equipment . 50 Figure 11 Cyclone Flow Pat terns . . . . . . . . . . . . . . 52 Figure 12 Two Stage Cyclone System on a F l u i d Bed . . . . . . . . Figure 13 Two Stage E l e c t r o s t a t i c P r e c i p i t a t o r . . . . . . . . . Figure 14 Typica l Reverse-Flow Cleaning Baghouse . . . . . . . . 60 F igure 15 Granular Bed F i l t e r . . . . . . . . . . . . . . 61 F igure 16 S in te red Metal F i l t e r Assembly . . . . . . . . . . . 63 Figure 17 Ventur i Scrubber w i t h Cyclone Separator . . . . . . . . 65 F igure 18 Example o f Spray Tower f o r Tar Removal . . . . . . . . 68 F igure 19 Typica l Gas Cleaning E f f i c i e n c i e s . . . . . . . . . . 73 F igure 20 Tota l I n s t a l l e d Costs f o r Par t i cu la te -Cont ro l Devices . . . 74

53

57

x i

I

LIST OF TABLES

Table 1 Summary of Gas Cleaning Technology Research Needs f o r Biomass Gasification . . . . . . . . . . . . . . v i i i

Table 2 Properties of Biomass Tars . . . . . . . . . . . . 15 Table 3 Components of Biomass Tars . . . . . . . . e . . . 15

INTRODUCTION

Biomass gasification has the potential to make a significant contribution

to the future energy supply in the United States and is already finding

commercial applications primarily in the forest products industry. One area

that has not been studied in detail is gas cleaning, primarily for particulate

and tar removal. Particulates can cause plugging and erosion of downstream

equipment and may also be an environmental problem. Plugging of downstream

equipment i s the primary problem associated with tars. Once they are separ-

ated from the gas, tars may be an environmental hazard and present a disposal

problem.

The objective of this study was to evaluate gas cleaning technology,

primarily for particulate and tar removal, as it applies to biomass gasifica-

tion, and identify potential problem areas and gaps in the technology that

could impede development and utilization of biomass gasification.

Selection of gas cleanup equipment for biomass gasification depends

primarily on two factors: 1) the type o f gasifier used and 2) the intended

end use of the gas. The type of gasifier, and t o some extent, the feedstock,

will determine the concentration of particulates and tars in the gas. The

end use will define the particulate and tar concentrations which can be tolerated. This relationship is shown in Figure 1.

Phase I of this study reviewed the state-of-the art of gas cleaning

technology as it applies to biomass gasification. The results of Phase I

which are presented here have been divided into three chapters:

Chapter 1

Chapter 2

Chapter 3 Gas Cleanup Technology

Biomass Gasification Gas Contaminants

End Uses for Biomass Gas

1

TvDe of Gasifier

Fixed Bed UDdraft

Gas Cleanup Technology

Particulate Remova

Cyclone

Bag House

Electrostatic Precipitator

Filters

Wet Scrubbers

Tar Re mova I

Wet Scrubber

Centrifugal Extraction

Electrostatic Precipitator

Cracking

End Use

gine

rbine

Direct Fuel Use

Boiler

Dryer, Ki ln

Er

T i

S - ,nthesis Gas

PiDeline Qualitv Gas

FIGURE 1 . Gas Cleanup Technology f o r Biomass Gasif icat ion

- * h

,

Phase I was based on an extensive literature survey. This survey

revealed a profound lack on data on gas cleaning systems for biomass gasifi-

cation, particularly for tar removal. Information on gas cleaning systems

for coal gasification and biomass combustion were also studied. This provided

a sizeable base of information on particulate removal, but still litt e on

tar removal. Because of this the results of Phase I focus most heavi y on

particulate removal.

Specific gas cleaning methods were applied to various gasifiers in Phase

11. The primary objective was to identify gas cleaning methods which could

meet specifications for the projected end uses o f biomass gas. Gaps in the

technology were identified and recommendations for additional research were

made. This material is included in Chapter 4 , Evaluations and

Recommendations.

i

3

1.0 BIOMASS GASIFICATION GAS CONTAMINANTS

When carbonaceous mater ia ls a re gas i f ied o r combusted, char , ash, t a r ,

and gases a re produced. The r e l a t i v e f rac t ions of these four products will

vary and depend primarily on the type of g a s i f i e r , g a s i f i e r operating condi-

t i ons , and the feedstock.

Char/ash p a r t i c l e s and higher hydrocarbon vapors or droplets a re con-

sidered contaminants in a gas stream. Gasification processes e i t h e r seek t o

minimize char and t a r production or t o maximize t h e i r removal from the gas

stream. Other contaminants o r po l lu tan ts which may be present i n biomass gas

include o l e f in s , hydrogen su l f ide , su l fu r dioxide, carbonyl su l f ide , nitrogen

o x i d e s , and t r ace metals. T h i s chapter i d e n t i f i e s the concentrations and

propert ies of contaminants produced in various types o f biomass g a s i f i e r s

with emphasis primarily on pa r t i cu la t e s and t a r s .

In the multitude of references c i t ed in t h i s repor t , researchers have

given pol lu tan t concentrations i n a wide var ie ty of uni ts . We have attempted

t o leave the c i t a t i o n in i t s or iginal form t o avoid any in te rpre ta t iona l

e r rors . For the readers ass i s tance Appendix A i s a nomograph f o r conversion

of contaminant loadings from one s e t of un i t s t o another.

1.1 GASIFIER TYPES

Over the years many d i f f e ren t types of g a s i f i e r s have been constructed

f o r experimental and commercial use. However, most f a l l i n to three d i s t i n c t

categories . The three main types are: f ixed bed, f l u i d bed, and entrained

flow, re fer r ing t o the motion of the so l id s in the g a s i f i e r . Each type o f

g a s i f i e r has d i f f e ren t cha rac t e r i s t i c s which r e s u l t in s ign i f i can t ly d i f f e ren t

y i e lds of pa r t i cu la t e s and t a r s .

-i

t

4

' 1.'1.1 Fixed Bed Updraft

"Fixed bed" i s a broad term describing a g a s i f i e r i n which the fuel i s

fed onto a r e l a t i v e l y s ta t ionary inventory of biomass already present i n the

reactor . The fixed bed category encompasses updraft , downdraft , and cross- d r a f t uni ts . Each type has d i f f e ren t operating cha rac t e r i s t i c s and emissions.

The term "fixed" r e fe r s t o the condition a t the extremes o f the bed, which do

not vary a t steady s t a t e . Most updraft biomass g a s i f i e r s a r e operated a t

atmospheric pressure b u t commercial coal g a s i f i e r s operate up t o 30 atm.

Updraft units (Figure 2) exhibi t a countercurrent flow of fuel and gas.

Solid fuel i s fed from the top by lock hoppers o r feeders. The bed of fuel

i s supported by a g ra t e a t the bottom of the reactor . The fuel flows down

th rough the drying zone, pyrolysis zone, reduction zone, and combustion zone.

Ash and unreacted fuel (char) e x i t through the g ra t e a t the bottom. Reactant

gases ( a i r , oxygen, steam) are introduced in to the reactor through the gra te .

The hot gases from the combustion zone provide energy f o r the endothermic

processes i n the upper zones and e x i t a t the t o p of the g a s i f i e r saturated

w i t h pyrolysis oi 1 s and water. E x i t temperatures range from 50-150C.

Approximately 20-25 percent of the carbon i n wood i s recovered as l i q u i d

products. The condensed l i q u i d s usually are i n two phases: an aqueous phase

containing highly oxygenated water soluble organics (pyroligneous acids) and

a separate t a r phase (Mudge, e t a1 ., 1980). Because of the low veloci ty o f the gases i n the reac tor and the f i l t e r i n g e f f e c t of the bed, the product gas

contains 1 i t t l e pa r t i cu la t e matter. Detailed descr ipt ions of fixed-bed,

updraft biomass g a s i f i e r s a r e given by Baker (1984), Fr i tz (1978), Miller

(1983) , Mudge and Rohrmann (1978), Reed (1979), and Oliver (1982).

5

Biomass

Drying Zone

Pyrolysis

Reduction

Combustion

Air/Oxygen/Stearn

Grate Ash

FIGURE 2. Schematic of a Fixed Bed Updraft Gas i f ie r

1.1.2 Fixed Bed Downdraft

Production of pyrolysis o i l s i s largely eliminated i n downdraft g a s i f i e r s

(Figure 3 ) . However

a i r , which i s used in most downdraft un i t s , i s introduced concurrently in to

the combustion zone through a d i s t r i b u t o r as shown in Figure 3 . Pyrolysis

o i l s and moisture from pyrolysis and drying a re drawn down through the high

temperature reduction and combustion zones where they undergo thermal

cracking. The product gases e x i t near the bottom of the reac tor a t 300 t o

55OoC. As

with updrafts, most operate a t atmospheric pressure.

As in updraft units so l id fuel i s fed from the top.

Ash and char leave through a g ra t e a t the bottom of the reactor .

4

1

6

Biomass

Product Gas

Alr/Oxygen/Steam

Y

Grate ~ s h

FIGURE 3. Schematic of a Fixed Bed Downdraft Gas i f ie r

Low gas ve loc i t i e s r e s u l t in low pa r t i cu la t e loadings in the same range

as updraft . Downdraft g a s i f i e r s exhibi t very low t a r y i e l d s which a re

dependent on the combustion zone temperature.

A uniform combustion area i s c r i t i c a l f o r proper operation o f t h i s type

of g a s i f i e r and sca le up of the a i r d i s t r ibu t ion system i s d i f f i c u l t . As a

r e s u l t downdraft g a s i f i e r s a r e usually small i n diameter compared t o the

o ther types of g a s i f i e r s .

Kaupp and Goss (1981) provide a de ta i led t r e a t i s e of small downdraft

Additional information can be obtained from Fr i t z (1978), biomass g a s i f i e r s .

Groenevel d (1983) , Hodam (1978) , and Reed (1983) .

7

I , 1.1.3 Fixed Bed Crossdraft

Crossdraft g a s i f i e r s (Figure 4) exhibi t many of the operating character-

i s t i c s of downdraft uni.ts. Tars and pa r t i cu la t e s a re both qu i t e low and

g a s i f i e r heat eff ic iency i s higher than a downdraft.

Air o r air/steam mixtures a re introduced in the s ide of the g a s i f i e r

tu res near

through the

l a t e s (most

near the bottom. Producer gases a re drawn off the opposite s ide a t tempera-

those o f downdraft un i t s (300-550OC). Tars and o i l s a r e drawn

reduction zone and cracked t o l i g h t e r components. Some part icu-

y ash) a r e entrained i n the product gases. More de ta i led informa-

t ion on c rossdraf t g a s i f i e r s can be obtained from Kaupp and Goss (1981) and

Miller (1983).

Biomass

Drying

Pyrolysis

Air/Oxygen/Steam

Combustion

Reduction

- Gas

Ash

FIGURE 4. Schematic of a Crossdraft Gas i f ie r

i

.

8

I

1.1.4 Fluid Bed

In a f l u i d bed g a s i f i e r (Figure 5) the incoming and evolved gases main-

t a i n the r eac to r bed in a turbulent f l u id - l ike s t a t e much l i k e a bo i l ing

l iqu id . The r e s u l t i s an expanded reac tor bed o f char p a r t i c l e s and, in most

biomass g a s i f i e r s , an iner t so l id such a s sand. Because biomass i s less

dense and has less ash and fixed carbon than coal the inert s o l i d i s used t o

maintain proper f lu id i za t ion (prevent bridging and channeling) and t o provide

addi t i onal heat capacity i n the bed.

No d i s t i n c t zones exist i n a f l u i d bed g a s i f i e r a s near isothermal opera-

t i on i s maintained w i t h good f lu id iza t ion . The product gas contains some

t a r s and o i l s depending on the bed temperature (600-900C), and does have a

f a i r l y l a rge loading of pa r t i cu la t e s (ash and char) . Operating pressures t o

Ash/ Char

Cyclone Disengagement

Section

r Fluidized Bed 1

Ash/Char

-Biomass

Air / Oxyg e n / S tea m

Ash/Char

FIGURE 5. Schematic of a Fluidized Bed Gas i f i e r

9

20 atm a re common in experimental uni ts . Depending on the design, ash &d

char may be removed from the top of the reactor with the product gases, from

the bottom of the reac tor , from the top of the bed, o r a combination of the

three. Datin (1981) , Flanigan (1983), Feldman (1983), Goldbach (1983) , Miller (1983), Mudge (1983), Murphy (1984), and Oliver (1982) provide more d e t a i l s

on design and operation of fluid-bed, biomass g a s i f i e r s .

1.1.5 Entrai ned F1 ow

I

In an entrained flow (or t ransport) g a s i f i e r (Figure 6), f ine ly sized

fuel p a r t i c l e s a re entrained in the feed gas (usually oxygen and steam) p r io r

t o entry i n t o the reactor . Gasification takes place with the feed pa r t i c l e s

suspended i n the gas phase. The product gas, ash and char leave the top of

the reactor . Limited data indicate par t icu la te and t a r loadings s imilar t o

Air/

Gas

Cyclone

As h/C ha r

'Oxygen/ Stea m

'4

FIGURE 6. Schematic of an Entrained Bed Gas i f ie r

10

. o r grea te r than those from f luidized bed units. Most entrained flow uni ts

a re low temperature short residence-time pyrolysis uni ts which operate about

500C a t atmospheric pressure. A cyclonic, entrained-flow, a i r blown g a s i f i e r

feeding sawdust operated a t 800-1 , OOOC (Cousins and Robinson 1985) .

1.2 TARS AND OTHER CONDENSIBLE ORGANICS

Tar i s a generic term f o r the higher boiling ()15O0C) constituents o f biomass gas which a re formed during the pyrolysis reactions. Depending on

the degree of cracking, the t a r may range from l i g h t , oxygenated hydrocarbons

t o heavy, polyaromatic hydrocarbons (PAH) . 1.2.1 Production Rates and Concentrations

Each type of g a s i f i e r has d i f fe ren t reaction conditions and consequently

d i f f e r e n t t a r compositions and production ra tes . An updraft f ixed bed

g a s i f i e r generates a high y ie ld of t a r ( N 20 w t % of the carbon i n wood feed i s

converted t o l i q u i d s ) due t o the low temperature a t which drying and pyrolysis

take place i n the g a s i f i e r . The drying and pyrolysis zones i n an updraft

g a s i f i e r will range from 80 t o 20OoC. A t these temperatures the t a r s a re ,

f o r the most par t , condensed droplets entrained i n the gas phase. Reported

t a r y i e l d s range from 10-100 g/m of gas (Kaupp and Goss 1981; Baker 1984;

Fritz 1978; Dravo Corp. 1976). Downdraft g a s i f i e r s pass the pyrolysis gases

through the combustion zone which thermally cracks and oxidizes much of the

t a r a t temperatures near 1200-1600OC. Reported t a r y ie lds from downdraft

units range from 50-500 mg/m (Kaupp and Goss 1981; Groenevald 1983; Reed

1983; Kumar 1984).

3

3

F1 uidized (or entrained) beds when operating properly a re nearly

Biomass feed i s pyrolyzed e i t h e r submerged i n the bubbling bed

High temperature (600-900C) and good gas-

isothermal.

o r entrained i n a hot gas stream.

11

sol id contacting resu

concentration of t a r s

and residence time in

The product gases from

I I

t s in some cracking of pyrolysis o i l s . The f ina l

i s a function of the reaction temperature, pressure, the bed and i s typ ica l ly in the range of 2-10 g/m 3 . downdraft, f 1 ui di zed bed, and en t ra i ned bed gasi f i e r s

are hot and any t a r s tha t are s t i l l present will be mostly in the vapor phase.

Typical t a r production r a t e s f o r each type of g a s i f i e r a r e summarized in

Figure 7.

1.2.2 Propert ies of Biomass Tars

A recent ly completed study o f biomass gas i f ica t ion /pyro lys i s condensates

a t PNL ( E l l i o t t 1985) concludes t h a t there i s no typical t a r composition

which can be adequately used t o represent a l l thermally produced biomass

t a r s . The t a r composition, as well as the amount, i s dependent on the

operating conditions, p r inc ipa l ly a tirne/temperature thermal severity-type

function. The proper t ies of the t a r therefore appear t o vary on a continuum

from "primary" oxygenated pyrolysis t a r col lected a f t e r a shor t residence

Fixed Bed Downdraft

Fluid Bed

10 100 1,000 10,000 100,000

Concentration mg/Norrnal rn3 FIGURE 7. Typical Tar Concentrations from Various Gasif iers

C

12

4

L

tiie at low temperatures of around 500C to highly aromatic, deoxygenated tar

which is produced at short residence time at high temperatures of around

900C. This continuum of tar cracking/condensation appears to argue against

any purely pyrolytic mechanisms for complete conversion of biomass to a char-

free and tar-free gas. The intermediate products of such a progression would

be some grade of tar with the end product being a finite amount of coke or

graphite.

This study suggests that a spectrum of tar condensates can be recovered

from entrained-flow or fluidized-bed reactors processing wood over a tempera-

ture range from 450'-950C. In these short residence time reactors the extent

of progression along a path of thermal decomposition is dependent on the

upper temperature limit achieved in the reactor. These reactions are very

fast and require only short residence times on the order of fractions of

seconds. Whether there is a smooth continuum or a sharp demarcation between

the oxygenated and deoxygenated products at some intermediate temperature

around 650C is, as yet, unknown.

The location of the organic condensates from the fixed bed gasifiers

within this tar property continuum i s most affected by specific process

configurations. A wide range o f condensate properties have been measured.

In theory the organic vapors from the downdraft gasifier should be completely

combusted; practically speaking such complete combustion i s not obtained.

The extent and type of organic contamination in the downdraft condensate is

mainly a function of the efficiency of the combustion zone and the extent of

channeling in the bed.

In the case of updraft operation the condensates appear to be very

simi 1 ar to the we1 1 -known pyrolysis condensate which i s recovered from batch

carbonization o f wood. The condensate has two phases with a heavy organic

13

phase containing the typical components found i n " se t t l ed t a r " from charcdal

manufacture. The l i g h t e r aqueous phase i s highly ac id i c and c a r r i e s a large

amount (20 t o 25 percent by weight) of dissolved organic material much l i k e

"pyroligneous acid. I'

1

The chemical compositional changes i n the t a r s suggest a general process

of deoxygenation and dehydrogenation. These changes a re noted by comparison

of elemental analyses and v o l a t i l e component i den t i f i ca t ion as well as several

forms of spectrometry. A general pathway of tar chemical functional degrada-

t i o n i s represented below. This pathway i s meant t o represent the nature of

the t a r composition as a function of thermal processing and can be viewed as

a spectrum of increasing temperature from low temperature (45OOC) on the l e f t

t o high temperature (95OOC) on the r i g h t .

mixed - phenolic - alkyl - heterocyclic - (PAH) -. l a rger oxygenates e the r s phenol i cs e thers poly-aromatic PAH

hydrocarbons

Table 2 summarizes the propert ies of t a r s from d i f f e r e n t g a s i f i e r s as

analyzed by E l l i o t t (1985). Wood was the feedstock f o r a l l of these gasi-

f i e r s . The proper t ies of t a r s from f luidized bed or entrained flow g a s i f i e r s

varied widely depending on the temperature of operation. The propert ies of

downdraft t a r s were a l so varying indicat ing the mode of operation has a

s ign i f i can t e f f e c t on t a r s from these uni t s . The analysis of the fixed-bed

t a r from the Rome, Georgia g a s i f i e r i s s imi la r t o a previous analysis of

fixed-bed t a r from PNL's fixed bed g a s i f i e r (Baker 1984).

14

3

'C

TABLE 2. Properties of Biomass Tars

Carbon, % Hydrogen, % Oxygen, % A s h , % PH v i scos i ty , cps

moisture, w t % densi ty , g/ml

Fluidized Bed/ Entrained Flow Updraft Downdraft 480'C 880C t a r aqueous t a r aqueous

52.7* 6.2

40.5 0.6 --

220- 13 00 @4OoC 16

1.26-1.28

84.0* 5.7 8.7 1.6 --

9800-26,800 @5loC 20-28 1.14-1.16

70.9* 7.2 21.7 0.2

410 @4OoC 8 1.13

--

11.3 --

0.1 2.1

67.2*-0** 7.0-0.5 5.8-0 --

25.0-0 -- 2.0-0 1.4-0.02 -- 3 .O-5.3

7500-41 , 000 -- @78OC 12-13 -- 1.16 --

* elemental and ash analyses f o r t a r s a re reported on a dry bas is ** downdraft g a s i f i e r s may o r may n o t produce a separate t a r phase depending

on the spec i f i c operation

Table 3 shows some of the major organic components of the two general

types of biomass t a r s , oxygenated and deoxygenated. Table 2 a l so gives

typical t r ace metal analysis . Iron and s i l i c a a re the most abundant followed

by calcium, potassium, and zinc.

TABLE 3 . Components of Biomass Tars ( E l l i o t t 1985)

Major Organic Components Low Temperature High Temperature Oxygenated Tars Deoxy g e n a t ed Tars

phenol methylphenols cresol s methoxyphenols dimethoxyphenol s 1 evogl ucosan

acenaphthal ene naphthalene methyl naphtha1 enes f 1 uorene phenanthrene f l uoranthrene acephenanthrylene benzanthracenes

Trace Metals (ppm)

Fe 200 - 7850 Si

1.3 PARTICULATES

The concentration, s i z e , and composition of pa r t i cu la t e s in gas from

The var ia t ions a re primarily a function biomass g a s i f i e r s vary considerably.

of the type of g a s i f i e r and the feedstock.

1.3.1 Production Rates and Concentrations

The concentration o f par t i cu la t e matter in the product gas from a biomass

g a s i f i e r depends on numerous fac tors including the type of g a s i f i e r , feed-

stock, and g a s i f i e r operating conditions. Operating conditions t h a t influence

pa r t i cu la t e emissions include gas veloci ty i n the bed, temperature, moisture

content of the b l a s t , and the r a t e of gas i f ica t ion . The concentration o f

par t i cu la t e s from various g a s i f i e r s i s summarized in Figure 8.

Fixed Bed Updraft

Fixed Bed Downdraft

Fluid Bed Entrained Bed

10 100 1,000 10,000 100,000

Concentration mg/Normal m3

FIGURE 8. Typical Par t icu la te Concentrations from Various Gas i f ie rs

16

P

c

Fixed bed g a s i f i e r s , both updraft and downdraft, employ large (2.5-5 cm,

1-2 in) chunks of feedstock t h a t a re not entrained by the gas flow; neverthe-

l e s s , the product gas does contain a s ign i f icant quantity of p a r t i c u l a t e s , 3 100-1000 mg/m (Finnie 1979, Hoenig and Cole 1981, Kaupp and Goss 1981, Katz

1983, Jacko 1983, Richey 1985, Level ton 1980). The lower Val ues were

typ ica l ly measured a f t e r the gas had been combusted i n a burner. The higher

values a r e measured d i r e c t l y from the g a s i f i e r . The values f o r downdraft

g a s i f i e r s a r e somewhat higher than updraft (Kaupp and Goss 1981) possibly

because the higher temperature of the product gas r e s u l t s i n higher gas

ve loc i t ies ex i t ing the g a s i f i e r .

Gas ve loc i t ies i n fluid-bed and entrained-flow g a s i f i e r s a r e s u f f i c i e n t l y

high t o entrain considerable par t icu la tes from the f i n e feedstock t o these

g a s i f i e r s . Typical par t icul a t e concentrations f o r f l u i d bed g a s i f i e r s range

from 10-456 g/m (Datin 1981, Datin 1983, Murphy 1984, Moreno and Goss 1983,

Oliver 1982, Mudge 1983). The higher values a r e f o r high ash feedstocks

(cotton gin t rash and c a t t l e manure) o r g a s i f i e r s where conversion i s l e s s

than 100% and char i s a s ign i f icant byproduct. For a i r blown g a s i f i e r s w i t h

a low ash wood feedstock the par t icu la te concentration ranged from 10-30

g/m3

1.3.2 P a r t i c l e Size

3

The assumed mechanism f o r p a r t i c l e e lut ion from a g a s i f i e r is entrainment

in the gas stream. The s i z e o f par t icu la tes carr ied out w i t h the product gas

will depend primarily on gas velocity exi t ing the g a s i f i e r . Only a limited

amount of data on the p a r t i c l e s i z e d is t r ibu t ion from biomass g a s i f i e r s has

been published a s shown i n Table 4. A p a r t i c l e s i z e d is t r ibu t ion f o r a fixed

bed g a s i f i e r could not be found. Jacko (1984) tested a fixed-bed g a s i f i e r

and indicated the p a r t i c l e diameters were small s imilar t o the downdraft data

17

T A B L E 4. Par t i c l e Size Distribution From Var

Microns IJacko 1985) (SERI 1979) Par t i c l e Size Downdraft Downdraft

250 74 100 50 50 30 30 18 20 99+ 11 10 97 5 96 2 95 1 89 0.5 8 1

ous Biomass Gasif

Fluid Bed (Datin 1981)

99 97 87 38 17 2

I J

e r s

reported in Table 3. The pa r t i cu la t e s from the downdraft un i t were qui te

small (89% l e s s than 1 micron). I t i s surpr is ing t o see the large p a r t i c l e

s i z e d i s t r i b u t i o n repor ted f o r Imbert downdraft veh icu la r g a s i f i e r s . This i s

probably due t o the hot gas ex i t ing through a small annular space which

r e s u l t s in a high gas velocity. Other downdraft un i t s (Hoenig and Cole,

1981) show more than 99% of the p a r t i c l e s being l e s s than 3 microns. Particu-

l a t e s from a f l u i d bed were l a rge r than those from the fixed bed un i t s , p r i -

marily between 5 and 30 microns.

1.3.3 Composition

Par t icu la tes from the d i f f e ren t g a s i f i e r types a re comprised of the

inorganic cons t i tuents in the feedstock, any unconverted feedstock, and d i r t .

In many systems the pa r t i cu la t e s in the gas stream a re col lected as a

byproduct f u e l , generally referred t o as char because they contain a large

percentage of carbon. Char i s often a byproduct from fluid-bed and

entrained-flow gas i f i e r s . Carbon content of so l id s from fixed-bed g a s i f i e r s

i s very low.

Published data on the composition of wood and agr icu l tura l residue ash

(Tab1 e 5) show considerable var ia t ion in concentrations of major const i tuents

,

18

8

(Ca, K , S i , Na, and A l ) . The usual main component i n ash i s CaO, which

accounts f o r 10 t o 60 w t % of the ash and i s typ ica l ly present a t about 40

w t % . Concentration of K20 can be 10 w t % or greater . Ash from agricul tural

residues cons is t s primarily of s i l i c a and a l k a l i . Feedstocks which have a

large percentage of d i r t o r other inorganics can r a i s e the s i l i c a content o f

any ash considerably.

Par t icu la te composition from the d i f fe ren t g a s i f i e r types i s not

reported. W i t h f l u i d and entrained bed g a s i f i e r s entrainment of sol id

p a r t i c l e s from the bed is the l i k e l y mechanism f o r p a r t i c l e e lut ion from a

g a s i f i e r . Par t icu la te entrainment, therefore , increases with increased energy

output, and the inorganic content of the feedstock.

In a fixed bed g a s i f i e r a lka l i and possibly other more v o l a t i l e com-

ponents of the ash could be vola t i l i zed in the combustion zone of the gasi-

f i e r , condensed o r s o l i d i f i e d in to small p a r t i c l e s in the cool zones, and

entrained i n the gas stream. Analysis of entrained p a r t i c l e s would determine

i f such a mechanism does occur. Corrosion products from the g a s i f i e r and

o u t l e t piping may a l so be present in the product gas as par t icu la tes .

TABLE 5. Typical Ash Compositions f o r Biomass Feedstocks (Kaupp and Goss 1981)

w t % - 2'3

CaO

Fe203 K20 MgO Na20 Si O2

Wood - 1-10 10-60 0.5 - 4 2-41

1-17 1-20

0-2

Agri cul tu ra l Residues (wheat straw, corn s tover , r i c e straw)

0-2 2-14 0-2 10-26 2-3 2-13 18-78

19

I

1.4 OTHER CONTAMINANTS

1.4.1 Sul f u r Compounds

Biomass feedstocks in general have a very low s u l f u r content. Table 6

indicates typical s u l f u r levels i n some biomass materials. The f a t e of the

s u l f u r in gas i f ica t ion depends on the temperature and the amount of water

present. Low steam r a t e s and temperatures favor the formation o f COS and

CS2. A t steam f rac t ions grea te r than 20 mole % nearly a l l of the su l fur

compounds a r e converted t o H2S. Higher temperatures a l so favor H2S formation.

TABLE 6. Sulfur Content of Biomass Fuels*

Biomass

A l f a l f a Seed Straw Almond Shel 1 s Bagasse Barley Straw Coffee Hulls Corn Cobs Corn Fodder Corn Stalks Oat Straw Cotton G i n Trash F1 ax Straw, Pel 1 eted Furfural Residue Olive Pits Manure Peach Pits Peanut Husk Peat ( F i n n i sh) Peat , General Rice Hulls Rice Straw Walnut Shel 1 s Wheat Straw Wood, Chipped Wood, General Wood, Pine Bark Wood, Green Fir Wood, Kiln Dried Wood, Air Dried

% s u l f u r , dry weight basis

0.3 l e s s than 0.02

0.03-0.12 0.14 0.2 0.001-0.007 0.15 0.05 0.23

l e s s than 0.01 0.4 0.02 0.4-0.6 0.04 0.1 0.05-0.2 1.5-2.0 0.16 0.10 0.03- .09 0.17 0.08 0.02 0.1 0.06 1.0 0.08

0.26-0.31

*Adapted from Kaupp and Goss (1981)

20

Sulfur concentrations i n gas streams a re generally low. As a comparison,

c ros sd ra f t g a s i f i e r s fueled w i t h an thrac i te (0.5% S) t yp ica l ly r u n about

1 g S/m3 (Kaupp and Goss, 1981). Baker and Brown (1984) measured 80-240 ppm

H2S (0.2-0.5 g s/m ) i n the gas from a continuous laboratory g a s i f i e r w i t h

bagasse a s the feedstock and 20-30 ppm w i t h wood. No s u l f u r was detected by

Hodam & Williams (1978) from t h e i r g a s i f i e r / b o i l e r combination. Murphy (1982) 3 measured 4-6 ppm SOX (-0.01 g s/m ) from a f l u i d bed g a s i f i e r / b o i l e r

3

5;-

I

combination. Both of these units were wood fueled.

1.4.2 Nitroqen Compounds

Fixed nitrogen in biomass feedstocks (Table 7) i s converted during gas i -

f i c a t i on t o gaseous nitrogen compounds , primari l y ammoni a (NH3) and hydrogen cyanide (HCN). Most researchers agree t h a t nitrogen i n the fuel i s the main

contr ibutor t o these compounds and t h a t l i t t l e o r no gaseous N2 i s converted

t o NH3 o r HCN.

TABLE 7. Nitrogen Content o f Biomass Fuels*

Biomass Fuel s % nitrogen dry weight bas i s

Bagasse 0.2-0.3 Barley, Straw 0.59 Corn Cobs 0.16-0.56 Corn Fodder 0.94 Cotton G i n Trash 1.34-2.09 Corn, S t a lks 1.28 F1 ax Straw, Pel 1 eted 1.1 Manure 2.5-3.1 Oat Straw 0.66 Olive Pits 0.36 Peach Pits 1.74

Prune Pits 0.32 R i ce H u l l s , Pel 1 e ted 0.57 Safflower, Straw 0.62

Peat 0.5-3.0

Walnut She1 1 s 0 26-0.4 Wood (General) 0.009-2.0

* Adapted from Kaupp and Goss (1981) 21

, i After gas i f ica t ion the spec i f ic use of the gas will a l so c rea te addi-

t ional nitrogen compounds. Any NH3 o r HCN w i l l be converted t o NO, by high

temperature burners , boi 1 e r s , o r combusti on turbines. I f the gasi f i e r i s a i r

blown then some elemental nitrogen (Np) may a l so be converted t o NO, i n the

combustion zone o r i n high temperature downstream equipment.

Airborne NO, re leases from biomass f a c i l i t i e s have been reported in the

range of 100 ppm. Hodam & Williams (1978) r e f e r t o a downdraft g a s i f i e r /

bo i le r combination as having 130 ppm NO, emissions. From a cross-flow

gasif ier /burner 0.15 lb/10 B t u ( N 300 ppm) can be expected (Miller 1983).

Fluid bed units coupled w i t h bo i le rs may show lower NO, levels . Murphy (1984)

found 58 ppm NO, from a 54 MM B t u f l u i d bed g a s i f i e r / b o i l e r combination.

Battelle-Columbus uses an air-blown char combustor t o supply heat t o a

c i rcu la t ing bed of so l id material . They (Feldman e t a l . 1983) found 0.25 l b

N0,/10

6

6 B t u ( N 500 ppm) was emitted from this combustor.

All o f the avai lable data suggests t h a t NO, emissions from biomass

f a c i l i t i e s will be a t o r below those required f o r New Source Performance

Standards (NSPS), 0.6 lb/106 Btu ( N 1200 ppm).

1.4.3 Olefins

Most gas i f ica t ion o r pyrolysis reactions generate some o l e f i n i c hydro- .

carbons. Ethylene and propylene a re the most prevalent. When the gas i s

used as a fuel o l e f i n s contribute t o the heating value of the gas; however,

i f the gas i s t o be used as a synthesis gas, o le f ins may be c a t a l y s t poisons.

They polymerize on the c a t a l y s t s ' surfaces and plug the pores. This r e s t r i c t s

the number of ac t ive s i t e s avai lable f o r the desired react ions causing

decreased o r s ign i f icant ly a l te red product yields .

'-

Olefin production depends on the type of g a s i f i e r , the temperature and

pressure, the gas residence time, the amount of H20 present, and/or the

22

presence of a c a t a l y s t . Typical o le f in concentrations a r e tabulated below in

Table 8. The highest concentrations a re found in short-residence time

pyrolysi s reactors .

TABLE 8. Olefin Production i n Biomass Gasif iers

Fixed Bed Fixed Bed Fluid Entrained Updraft Downdraft Bed F1 ow

non-catal . c a t a l y t i c 0.8-2.9% 0%

0.9-2.8% 0% 1-2.4% 0.2-0.4% 1-14% ZH4

C3H6

Reference Overend Johannson F1 ani gan , Mudge Diebold &

+

(1979) (1979) e t a l . e t a l . Scahi 11 , (1983) (1983) (1983)

1.4.4 Condensate

Cooling and scrubbing the raw gas from biomass gas i f ica t ion produces a

condensate o r wastewater stream. El 1 i o t t (1985) analyzed condensate samples

from nine experimental g a s i f i e r s . The pH of the water ranged from 2.1 f o r a

fixed-bed updraft t o 3.0-5.3 f o r downdrafts, and 5.9-8.7 f o r f l u i d bed

g a s i f i e r s . The t o t a l organic carbon dissolved i n the water was very high f o r

the fixed bed updraft (110,000 ppm), somewhat lower f o r downdrafts (5,000-

71,000 ppm) and q u i t e low f o r f luidized beds (0-400 ppm).

1.5 PRESSURE EFFECTS

The data on emissions presented i n this chapter a r e based primarily on

Advanced g a s i f i e r s may operate g a s i f i e r operation a t atmospheric pressure.

a t elevated pressures. This may have a s ign i f icant e f f e c t on emissions.

23

Calculations show much higher methane and higher hydrocarbons concentra-

t ions a t equilibrium f o r higher pressures (Mudge 1983). T h i s migh t lead t o

higher t a r concentrations as well , although this has not been substantiated.

Par t icu la tes concentrations from f l u i d beds appear t o be grea te r f o r

higher pressures. Zenz and Othmer (1960) make reference t o work by May and

Russell i n Gohr (1956) which examined entrainment as a function of pressure

and gas velocity w i t h cracking ca ta lys t s . They found f o r a given gas

veloci ty , entrainment a t 200 psig could be over 10 times t h a t a t atmospheric

pressure. Density and viscosi ty of entraining gases plays an important ro le

in entrainment. Increasing the pressure increases both the density and

v iscos i ty of gases and the resul t ing drag forces on the p a r t i c l e . L i t t l e i s

known about f ixed bed par t icu la tes vs. pressure. Most f ixed bed g a s i f i e r s

a re operated near atmospheric pressure.

Thermodynamics and reaction equi 1 i b r i a (par t ia l pressures) w i 11 a1 so

e f f e c t the p a r t i c u l a r species and r e l a t i v e concentrations of s u l f u r compounds,

nitrogen compounds, and o l e f i n s , b u t experimental data involving biomass does

not appear t o e x i s t .

1.6 SUMMARY AND CONCLUSIONS

Fixed-bed updraft units exhibi t the highest t a r and o i l emissions b u t

the lowest p a r t i c u l a t e emissions, while f l u i d beds a r e highest in par t icu la te

emissions b u t lower i n t a r and o i l loadings. Table 9 summarizes avai lable

information on contaminants from biomass gas i f ica t ion systems.

Parti cul a t e characteri s t i cs a r e not we1 1 known. Various gasi f i e r s

Par t iculates exhibi t d i f f e r e n t p a r t i c l e s i z e d is t r ibu t ions and compositions.

can be mostly char in f l u i d beds, b u t mostly ash i n fixed bed units. Wood

24

f

1 1

ash has a relatively large amount o f potassium and sodium which may volatilize

into the gas stream. Further work is needed t o evaluate alkali transport

phenomenon.

TABLE 9. Summary of Emissions from Biomass Gasifiers

Product Gas Temperature, OC

Pressure, a ta

Part icu I ates

3 Loading, g/m

P a r t i c l e s ize D i s t r i but ion, I icrons

3 Tar Loading, g/a

ppm

NO,(^), ppm

Condensate (wastewater)

PH

TOC, PP*

Fixed Bed Updraft

50-150

1

0.1-1.0

--

10-100

-- -_

2 . 1

110,880

Fixed Bed Fixed Bed Downdraft Crossdraft

300-550 300-550

1 1

0.1-10 --

3.0-5.3 -- 5,000-71,000 --

Fluid Bed Entrained Bed

600-900 800-1,000

1-20 1

10-500 --

1-50

2-10

4-6

58

5.9-8. r

0-400

a) based on combustion o f the raw gases

25

Tars and o i l s produced during biomass gas i f ica t ion a r e a l so not well

characterized. Of p a r t i c u l a r i n t e r e s t i s the dependence of chemical

const i tuents on the reaction conditions. Other areas of i n t e r e s t include t a r

deposition i n t ransport l i n e s , c a t a l y t i c destruction of t a r s , and possible

recovery/recycl ing methods.

Not enough i s known about the basic gas i f ica t ion processes t o predict

pol lutant concentrations re l iab ly . Before gas cleanup systems can be

adequately designed the ant ic ipated loadings must be known as accurately as

'-

possible.

1 .7 REFERENCES

American Cyanamid. 1981. A Feasibility Study o f the Production and Use of Wood-Derived Fuels i n a Large Chemical Plant, DOE/RA/50320, August 1981.

American Rice, Inc. 1981. Feas ib i l i ty Study f o r Alternative Fuels Production: Fluidized Bed Gasification of Rice Hulls, DOE/RA/50378, October 1981.

Baker, E. G . , L. K. Mudge, and D. H. Mitchell. 1984. "Oxygen/Steam Gasification of Wood i n a Fixed-Bed Gas i f ie r , " Ind. Enq. Chem. Proc. Des.

Baker, E. G. and M. D. Brown, 1984. Catalyt ic Gasification of Bagasse f o r the Production of Methanol, PNL-5100, Pacif ic Northwest Laboratory, Richland, Washington, a l so i n Energy from Biomass and Wastes VIII.

Cousins, J . W. and W. H. Robinson. 1985. "Gasification of Sawdust in an Air-Blown Cyclone Gasif ier . Ind. Eng. Chem. Proc. Des. Dev. 24(4) :1281-1287.

Datin, D. L. , W. A. LePori, and C. B. Parnell , J r . 1981. "Cleaning Low Energy Gas Produced in a Fluidized Bed Gasif ier" presented a t the 1981 Winter Meeting of ASAE, Dec. 15-18, 1981. Chicago, I l l i n o i s , Paper No. 81-3592.

& 23 (4) ~725-728

Datin, D. L., W. A. LePori, and C. B. Parnell , Jr. 1983. "Character is t ics of Par t icu la tes Emitted from a Biomass F1 u i d i zed Bed Gasif ier" presented a t the 1983 Winter Meeting of ASAE, Dec. 13-16, 1983. Chicago, I l l i n o i s , Paper NO. 83-3548.

Diebold, James and John Scahi l l . 1983. "Ablative Entrained Flow Fast Pyrolysis Status" i n Proceedings of the 15th Biomass Thermochemical Conversion Contractors Meeting, CONF-830323, March 16-17, 1983 i n Atlanta, Georgia.

26

Dravo Corp. 1976. Handbook of Gasifiers and Gas Treatment Systems FE-1772-11. Dravo Corp. Pittsburgh, Pennsylvania.

Elliott, D. C. 1985. Analysis and Comparison of Biomass Pyrolysis/ Gasification Condensates - An Interim Report, PNL-5555. Pacific Northwest Laboratories, Richland, Washington.

- Elliott, D. C. 1983. Analysis and Upgrading of Biomass Liquefaction Products," Vol. 4 of a Final Report to the IEA Cooperative Project D1, Biomass Liquefaction Test Facility Project. Battelle-Northwest, Richland, Washington.

. . 1 Energy Resources Company. 1982. Municipal/Industrial Waste Gasification

Feasi bi 1 i ty Study, DOE/CS/50319-1 , August 1982.

Feldman, H. F., M. A. Paisley and H. R. Appelbaum. 1983. "Gasification o f Forest Residues in a High Throughput Gasifier," in Proceedings of the 15th Biomass Thermochemical Conversion Contractor's Meetinq. March 16-17, 1983 at Atlanta, Georgia.

Finnie, G. 1979. "Halcyon Gasification Systems." Proceedings of the 10th Texas Industri a1 Wood Seminar. Technology and Economics o f Wood Residue Gasi fi cation , Lufki n , Texas. Flanigan, V. J . , M. E. Findley, and H. H. Sineath. 1983. "Steam Gasification o f Wood in a Fluidized Bed Using Indirect Heating with Fire Tubes," in Proceedings of the 15th Biomass Thermochemical Conversion Contractors Meeting, CONF-830323 in Atlanta, Georgia. March 16-17, 1983.

Fritz, J . J . , J . J . Gordon, and V . T. Nguyen. Biomass. ET-78-C-01-2854. Mitre Corporation.

Gohr, E. J. 1956. Fluidization, Reinhold Publishing; New York, NY.

1978. Status Review of Wood

Goldbach, G., K. Wilson, M. Trimble, and J . Prudhomme. 1983. Program to Develop MSW Fi red F1 uidi zed Bed Boi 1 er, DOE/CS/24321-T1 , June 1983. Groeneveld, Michael J., P..E. Gellings, and J. J . Hos. 1983. "Production o f a Tar Free Gas in An Annular Co-current Moving Bed Gasifier." Biomass and Wastes VII, Lake Buena Vista, Florida, Jan. 24-28, 1983.

Hodam, R. H. and R. 0. Williams. 1978. to Produce a Low-Btu Gas," in Energy From Biomass and Wastes, August 14-18, 1978, Washington, D.C.

Synthesis Gas With a Tube and Wire Electrostatic Precipitator" Solar Energy,

Energy From

"Small Scale Gasification of Biomass

., Hoenig, Stuart A. and Frank L. Cole. 1981. "Cleanup of Producer and

Vol. 27, NO. 6, pp. 579-580. - Jacko, R. B. 1983. "Contaminants from Biomass Gasification." Paper

presented at the Third Annual Solar and Biomass Workshop, Southern Agricultural Energy Center, Atlanta, Georgia.

27

Jacko, Robert B., Mark L. Holcomb, and John R. Ba r re t t . " A l t e r n a t i i e Energy Source Emissions - PAH, NOx, Pa r t i cu la tes and Size D i s t r i b u t i o n s f r o m Biomass Gas i f i ca t i on , " Purdue Un ive rs i t y , West Lafayet te , Ind iana

1984.

Johannson, Er ic . 1979. "Vehic le G a s i f i e r s " i n R e t r o f i t '79 - Proceedings of a Workshop on A i r Gas i f i ca t i on , SERI/TP-49-183.

Katz, L. J. J. R. Ba r re t t , C. B. Richey, and R. B. Jacko. Channel G a s i f i e r Operat ion and P a r t i c u l a t e Emissions." Trans. Am. SOC. Agr ic . Eng. 26 (2) : 1614-1618.

1983. "Downdraft

Kaupp, A. and J. R. Goss. 1981. S t a t e o f A r t f o r Small Scale ( t o 50 kw) Gas 1.

Producer - Engine Systems, F ina l Report t o the USDA/USFS on Contract #53-319R-0-141 , Uni ve rs i t y o f C a l i f o r n i a, Davis , Cal i f o r n i a Kumar, S., e t a l . 1984. "Design and Development o f a Biomass Based Small Gasi f ier -Engine System. Su i tab le f o r I r r i g a t i o n Needs i n Remote Areas of Developing Countr ies". Proceedings: Energy from Biomass and Wastes V I I I . I n s t i t u t e o f Gas Technology.

M i l l e r , B. 1983. S ta te o f t he A r t Survey o f Wood G a s i f i c a t i o n Technology, EPRI-AP-3101, E l e c t r i c Power Research I n s t i t u t e , Palo A l to , C a l i f o r n i a .

Moreno, F. E. and J. R. Goss. 1983. "F lu id i zed Bed G a s i f i c a t i o n o f High Ash Agr i cu l t u r a l Wastes t o Produce Process Heat and E l e c t r i ca l Power. I' Symposi um Papers Energy from Biomass and Wastes V I I , I n s t i t u t e o f Gas Technology, Chicago, I l l i n o i s .

Mudge, L. K,. E. G . Baker, D. H. M i t c h e l l , R. J. Robertus, and M. D. Brown, 1983. C a t a l y t i c G a s i f i c a t i o n Studies i n a Pressur ized F l u i d Bed, PNL-4594, P a c i f i c Northwest Laboratory, Richland, Washington.

Mudge, L. K., D. G. Ham, S. L. Weber, and D. H. M i t c h e l l . G a s i f i c a t i o n o f Wood, PNL-3353, P a c i f i c Northwest Laboratory, Richland, Washington.

Mudge, L e K. and C. A. Rohrmann. 1978. " G a s i f i c a t i o n o f S o l i d Waste Fuels i n a F ixed Bed G a s i f i e r , " ACS Symposium Ser ies 76 S o l i d Wastes and Residues.

Murphy, Michael. 1984. "Case H i s t o r y o f t he Design, Star tup, and Operat ion o f a 54 MM B tu /h r F l u i d Bed G a s i f i e r f o r Steam Product ion from Wood and

1980. Oxygen/Steam

Biomass Residue" i n Energy from Biomass and Wastes V I I I , Jan. 30-Feb. 2, 1984, Lake Buena Vis ta, F lo r ida . Overend, R. 1979. " G a s i f i c a t i o n - An Overview" i n R e t r o f i t '79 - Proceedings of a Workshop on A i r Gas i f i ca t i on , SERI/TP-49-183.

01 i v e r , E. D. 1982. Technical Evaluat ion o f Wood Gas i f i ca t i on , EPRI-AP-2567, Synthe t ic Fuels Associates, Palo A l to , C a l i f o r n i a , August 1982.

Po t la t ch Corporat ion. 1981. F e a s i b i l i t y Study f o r A l t e r n a t i v e Fuels Product ion: F l u i d i z e d Bed G a s i f i c a t i o n o f Wood, DOE/RA/50303, October 1981.

28

I

* Reed, T. R . , D. E. Jantzen, W. P. Corcoran and R. Witholder. 1979. "Technology and Economics f o r Ret rof i t t ing Gas/Oil Combustion Units t o Biomass Feedstock," i n Re t ro f i t '79 - Proceedings of a Workshop on Air Gas i f ica t ion , SERI/TP-49-183.

Reed, T. B. and M. Markson. 1983. "A Predictive Model f o r S t r a t i f i e d Downdraft Gasification." Conversion Contractors Meetinq. PNL-SA-11306. Pac i f ic Northwest Laboratory, Ri chl and, Washington.

Proceedings of the 15th Biomass Thermochemical

Richev. C. B., J . R. Barrett, and R. B. Jacko. 1985. "Downdraft Channel Gasifier-Furnace f o r Biomass' Fuels" Trans. Am. SOC. Agric. Enq. 598.

28 (2) :592-

Synergy Systems Management Corporation, 1981. Feas ib i l i t y Study f o r Alternative Fuels Production: Fluidized-Bed Gasification of Wood Wastes, DOE/RA/50317, Ju ly 1981.

Zenz, Fredrick A. and Donald F. Othmer. 1960. Fluidization and Fluid P a r t i c l e Systems, Reinhold Publishing, New York, N Y .

29

2.0 END USES FOR BIOMASS GAS

J

L

The extent of gas cleanup required will depend on the intended end use

of the gas. Low-Btu gas (producer gas) from biomass has been used t o f i r e

i ndustri a1 process burners i ncl udi ng d i r ec t f i red equipment such as dryers

and k i lns and ind i r ec t f i r e d equipment such as boi le rs and o i l heaters. Low-

B t u gas has a l so been used as a fuel f o r internal combustion reciprocating

engines, both gasoline and diesel , and i s being considered f o r use in gas turbine engines. Medium-Btu gas can be subs t i tu ted f o r low-Btu gas in these

appl icat ions. In addition i t can be used as a synthesis gas f o r making fue l s

such as methanol, methane (SNG), and l iqu id hydrocarbon fue l s such as gaso-

l i ne . This sect ion discusses the gas qua l i ty , primarily the t a r and par t icu-

l a t e l eve l s , t h a t a r e required fo r each of these end uses.

2.1 BURNERS (BOILERS/KILNS)

The most common use of gas f rom biomass t o date i s as a fuel f o r indus-

t r i a1 process burners. Typical appl i cat ions i ncl ude di r e c t f i red process

equipment such as dryers and ki lns and ind i rec t f i r e d equipment such as

bo i l e r s and o i l heaters. I n these appl icat ions only minimal gas cleanup i s

used. W i t h f ixed bed g a s i f i e r s the gas i s usually piped d i r e c t l y t o the

burner. With a f l u i d bed g a s i f i e r a ho t cyclone i s generally used t o reduce

pa r t i cu la t e s . The gas can a l so be cooled and scrubbed p r i o r t o burning b u t

t h i s r e s u l t s in a s ign i f i can t loss of eff ic iency.

There i s a wide var ie ty of burners ava i lab le for burning gas and/or o i l .

Gas burners a r e generally of two types. In prem x burners a i r and fuel gas

a re mixed ahead of the burner ports . Non-premixing burners use a nozzle,

31

d i f fuse r or spider t o mix the gas and a i r a t the burner t i l e .

burners used f o r biomass have been the non-premixing type.

Most of ;he

Low-Btu gas requires 6-10 times the volume of fuel delivered t o the

burner and l e s s a i r than natural gas f o r the same capacity. For this reason

a special burner i s required in most cases. One option would be the use of a

re f rac tory l ined combustor i n place of a conventional burner. The primary

advantages o f this method are minimal p i l o t f u e l usage, less wear and deposi- t ion on the bo i l e r tubes, and reduced emissions. Very l i t t l e work has been

done in t h i s area and consequently there a re no publications

l i t e r a t u r e as ye t .

Boi 1 e r and burner manufacturers general ly recommend a scrol

in the open

-type burner

f o r low-Btu gas because i t permits passage of l a r g e volumes o f gas and a i r

w i t h minimal pressure drop (Schwieger 1979). Scroll burners have a his tory

o f success on waste gases, such as blast-furnace gas, which has a heating

value of about 80 Btu/scf. A modification commonly used f o r hot d i r t y gas i s

the inclusion of self-cleaning steam j e t s (McGowan and Jape, 1981). Medium-

burners. Many references B t u gas can usually be burned in normal natural gas

on using medium-Btu gas from coal a r e avai lable . L

medium-Btu gas from bi.omass as a fuel gas was found

t t l e information on using

Table 10 l i s t s some problems encountered when burning fuel gas from

biomass. Derating and o ther problems associated with the heating value of

the gas a re not included.

Tars and pa r t i cu la t e s in the fuel gas contr ibute t o plugging, primarily

in the t r a n s f e r l i n e t o the burner. In some appl icat ions a standard natural

gas burner has been used by reducing the s i z e of the a i r ports and derat ing

the burner. In these cases plugging of the burner i t s e l f becomes more o f a

problem with the small i n l e t gas l ines .

32

TABLE 10. Problems Encountered with Biomass Gas Burner Systems

Plugging due to t a r condensation Transfer 1 i ne (Wi 11 i ams 1984) Burner head (Wi 11 i ams 1984; Wai bel 1979a)

Plugging due t o pa r t i cu la t e deposit ion Transfer 1 i ne (Wi 11 i ams 1984) Burner (Moreno and Goss 1983)

Flame out due t o low heating value (excess water in the wood and product gas) (Wi 11 i ams 1984)

Par t icu la te emissions in f l u e gas (Moreno and Goss 1983; Oliver 1982)

Erosion of furnace masonry due t o a1 kal in i t y of gas (Williams 1984)

Tar and pa r t i cu la t e s have had no noted e f f e c t on combustion in the

burner. Variations i n gas qua l i t y due t o f luc tua t ions in water vapor content

have caused ser ious problems with flameouts occurring when the heating value

of the gas i s reduced. This i s usually the r e su l t of varying moisture content

of t he feedstock, pa r t i cu la r ly in fixed bed uni ts .

The I n s t i t u t e of Gas Technology has tes ted low- and medium-Btu coal gas

on a var ie ty of indus t r ia l burners. The burners were a forward-flow ba f f l e ,

k i ln , nozzle mix, high-forward momentum, f la t - f lame, high-excess a i r , premix

tunnel , and bo i l e r burner. Their primary conclusions were t h a t rnediurn-Btu gas can be r e t r o f i t on any natural gas burner and produce s t a b l e flames and

good thermal performance, b u t low-Btu fuel gas exhib i t s more flame s tab i 1 i t y

problems and reduced thermal performance when r e t r o f i t t o natural gas burners

(Waibel 1979 a ,b ) .

IGT a l so doped the gas with char and t a r t o determine t h e i r e f f e c t on

The doping r a t e s ranged from 0.4 t o 2.7 g r / sc f (1-6 g/m ) 3 burner operation. 3 char and 1.1 t o 2.3 gr / scf (2.5-5 g/m ) t a r . T h i s represents l e s s than 1%

33

carbon conversion t o char f o r a g a s i f i e r w i t h a gas production r a t e of 2

m3/kg feed. For the most par t the t a r was consumed i n the burner although a

char-l ike residue d i d build up inside the fuel nozzle i n some of the t e s t s

when t a r was fed. I t amounted t o 3 w t % of t o t a l t a r added i n the baf f le

burner and 7 w t % i n the kiln. Data were not given f o r any of the other

burners. Most of the char was a l so consumed i n the burner w i t h the survival

r a t e ranging from 5 w t % a t the lower loadings t o 11 w t % a t the higher

1 oadi ngs.

Allowable par t icu la te loadings f o r gasif ier /burner systems will be

limited by a i r pol lut ion regulations f o r emissions i n f l u e gases. Federal

New Source Performance Standards (NSPS) require t h a t par t icu la tes be removed

to levels o f 0.03 lb/106 Btu for utility steam generators over 250 x 106

B t u / h r (Moore 1983, Ford 1980). For industrial-commercial-institutional

steam generating units over 100 x 106 B t u / h r new NSPS ru les have been proposed

which s e t the p a r t i c u l a t e emission l i m i t s f o r wood f i r e d uni t s a t 0.10 lb/106

B t u (Siege1 and Petri110 1984). However, many gasif ier /burner applications

a r e not covered by NSPS, b u t instead will be covered by various s t a t e regula-

t ions which range from about 0.1 t o 0.6 lb/106 B t u (Ford 1980). Determining

which regulations will apply i s spec i f ic t o each application and beyond the

scope of this study. A t the par t icu la te levels specified by the various a i r

pollution regulations, par t icu la tes do not appear t o cause any s igni f icant

operational problems w i t h burners, so s t r i c t e r limits due t o burner l imita-

t ions a r e not necessary. Since a substantial portion o f char p a r t i c l e s a re

consumed i n the burner, higher par t icu la te loadings entering the burner may

be permi ssi bl e.

Allowable t a r loadings will be l imited primarily by the coupling between

I f they a re close coupled and the t r a n s f e r l i n e the g a s i f i e r and the burner.

34

i s well insulated such systems can handle a heavily tar-laden gas such as

produced by a fixed bed updraft g a s i f i e r . Longer piping runs 0 2 0 m) will

cause deposit ion and possible obstruction.

the gas (compared t o a fixed-bed updraft) will be desirable .

Reduction in the t a r content of

L i t t l e quanti-

t a t i v e information i s avai lable regarding the behavior of biomass t a r s and

char in of f gas piping. Further research in t h i s area would be benef ic ia l .

2.2 DIESEL AND SPARK-IGNITION ENGINES

The heat energy o f low-Btu gas can be converted t o sha f t work o r e lec-

The four s t rokes of an O t t o t r i c i t y using diesel or spark ign i t ion engines.

cycle (spark ign i t ion) are:

1)

2) Compression -- the mixture i s compressed, igni ted and combusted, 3)

4)

The Diesel engine d i f f e r s from the Otto engine primarily in t h a t the

temperature a t the end of the compression s t roke i s such t h a t combustion i s

i n i t i a t e d spontaneously. The higher temperature i s obtained by continuing

the compression s t ep t o a higher pressure. In general , the Otto engine has a

higher e f f ic iency than the Diesel f o r a given compression r a t i o . However,

the compression r a t i o in the Otto engine i s l imited by fuel qua l i t y (pre-

ign i t ion d i f f i c u l t i e s ) so t h a t higher r a t i o s can be used in the Diesel engine,

and f o r t h a t reason higher e f f i c i enc ie s can be obtained with Diesel engines.

One measure of the fuel qua l i ty i s the octane number. The higher the octane

number of a fuel the l e s s suscept ible the fuel i s t o auto-ignition. High

octane f u e l s a re required f o r Ot to engines and low octane fue l s a re necessary

Intake -- the fue l - a i r mixture flows with the cyl inder ,

Power -- the high-pressure, high-temperature gases expand, and Exhaust -- the piston pushes the combustion gases out o f the cy1 inder.

f o r diesel engines.

35

i

Low-Btu gas has a research octane of about 100 and can be used in a

spark-igni ted gasoline engine by replacing the carburetor w i t h a mixing

chamber. Use of low-Btu gas i n a diesel engine requires t h a t 5-10 percent

diesel fuel be injected f o r igni t ion. This type of operation i s referred t o

as dual-fueling. The engine always has the potential t o run so le ly on diesel

fuel w i t h minor adjustments t o the timing.

A majority of the s ta t ionary engines i n operation i n the U.S. f o r elec-

t r i cal generation , pumps , and compressors a r e f u l l diesel engi nes. D i esel engines a r e more popular pr incipal ly because they a re less expensive than

spark-ignition engines and fuel cos ts a re somewhat l e s s (Kirkwood 1980).

I t i s usually desirable t o cool the gas before admitting i t t o e i t h e r

engine. T h e increased dens i ty o f gas increases the horsepower compared t o

using hot gas and prevents premature igni t ion. As a r e s u l t , a l iquid scrubber

of some type can be used f o r par t icu la te and t a r removal. A combination o f a

cyclone separator and a c loth f i l t e r has been used extensively, par t icu lar ly

in vehicular applications.

Some o f the problems caused by par t icu la tes and t a r s i n in ternal combus-

The f i r s t two items a r e re la ted and are t ion engines a r e l i s t e d i n Table 11.

TABLE 11. Problems Caused by Part iculates and Tars i n Internal Combustion Engines (SERI 1979, Kaupp and Goss 1981)

Engine wear due t o mechanical abrasion

Contamination of crankcase o i l

Corrosion from organic acids

Deposition i n the gas mixer and i n l e t pipes

G u m formation on valves (from t a r )

36

'i

the most ser ious problems. Par t icu la tes a re the primary cause of engine wear

and o i l contamination. Par t icu la tes come not only from the fuel gas b u t from

the a i r as well. The l a s t three items in Table 11 involve v o l a t i l e material

from the biomass and a re generally not as ser ious.

The amount of dust t h a t can be to le ra ted has been the subject of much

research in the 1930's t o 1950's. Various regulations developed during t h i s

time period ranged from 5-50 mg/m pa r t i cu la t e in the fuel gas enter ing the

engine (SERI 1979). In a recent review Kaupp and Goss (1981) summarized the

e f f e c t o f par t i cu la t e s on engine wear as follows: 3

3

Up t o 10 mg/m engine wear i s of the same order as observed

A t loadings of 50 mg/m engine wear was up t o f i v e times grea te r

DuVant , one of the l a rges t manufacturers of gas i f ie r /engine systems cur ren t ly recommends the following maximum loadings f o r t a r s and pa r t i cu la t e s

(DuVant 1981).

with gasoline.

than observed with gasol ine. 3

3 Par t icu la tes - 15 mg/m Tars (condensible vo la t i l e s ) - 15 mg/m 3

A recent study f o r the Elec t r ic Power Research , , i s t i t u t e indicated t h a t

when using low-Btu gas in engines, pa r t i cu la t e s smaller than 5 micron should

be reduced t o l e s s than 20 ppm and tars t o l e s s than 10 ppm. This corresponds

t o 20 and 10 mg/m Other possible contaminants t h a t may cause

corrosion a re H2S (should be l e s s than 750 ppm) and aluminum, sodium, and

vanadium (no 1 imits given) (Compton 1984).

3 respect ively.

-

2.3 GAS TURBINES

Low o r medium-Btu gas can be used as fuel f o r combustion gas turbines t o

A basic d i r ec t - f i r ed gas turbine system produce e l e c t r i c i t y o r shaf t work.

37

cons is t s of a compressor, a combustion chamber, and a turbine. Components

added t o the system t o improve eff ic iency include a regenerator t o recover

exhaust losses and preheat the a i r t o the combustor, an in te rcooler between

compressor s tages , and an additional reheating combustion chamber between

turbine stages.

This type of gas turbine cycle (open cycle) uses a i r as the working

medium and burns r e l a t ive ly clean fue l s such as natural gas and petroleum

d i s t i l l a t e s . Burning d i r t y fue l s such as coal , biomass, o r low-Btu gas

d i r e c t l y in a gas turbine i s s t i l l in the developmental stage. A major

problem i s erosion and corrosion o f turbine blades by pa r t i cu la t e matter and

hot gases (Pruce 1980).

The i ndi r e c t l y heated gas t u r b i n e cycl e (cl osed o r semi -closed cycl es)

has a s ign i f i can t advantage i n t h a t i t can accommodate a wide var ie ty o f

fue l s including biomass, low-Btu gas , coal , and l i g n i t e . In the ind i r ec t

cycle incoming a i r i s compressed, heated ind i r ec t ly by combustion gases using

a heat exchanger and then expanded through the turbine. The exhaust a i r i s

used t o combust the fuel outs ide of the gas turbine. The hear t of the

ind i r ec t f i r e d cycle i s the heat exchanger. Several d i f f e r e n t heat exchangers

f o r gas turbine cycles a re under development. Emphasis i s cur ren t ly on

meta l l ic and ceramic mater ia ls t o protect the heat exchanger from corrosion/

erosion and t o meet the high temperature requirements.

With both types of turbines (d i r ec t and ind i r ec t f i r e d ) , i t i s des i rab le

t o use the biomass fuel gas without cooling (and t a r removal) t o improve the

eff ic iency. Ut i l iza t ion of biomass gas in gas turbines without cooling and

cleaning presents many problems s imi la r t o those discussed with indus t r ia l

38

A

process burners such as plugging in the i n l e t l i n e , flameouts on poor qua l i ty

gas (high moisture conten t ) , and pa r t i cu la t e emissions in the exhaust gas

stream.

2 . 3 . 1 Di r e c t Fi red Turbi nes

The most severe problem w i t h d i r e c t fueled gas turbines i s l i k e l y t o be

erosion and corrosion of the turbine blades and buckets. Erosion i s a func-

t ion of the type of p a r t i c l e , s i z e of p a r t i c l e , angle of incidence r e l a t i v e

t o the surface on which i t impinges, veloci ty of impingement, pa r t i cu la t e

content of the gas stream, and the physical propert ies of the surface subject

t o erosion. Both so l id s and l iqu id droplets can cause erosion although the

mechanism i s d i f f e ren t . Erosion i s inversely proportional t o p a r t i c l e s i z e

f o r the same weight loading. Erosion i s a power function of veloci ty and can

be reduced by reducing the turbine blade t i p speed.

Corrosion of gas turbine blades i s re la ted t o erosion and deposit ion.

Erosion may abrade protect ive oxide fi lms and acce lera te corrosion. Deposits

of a lka l i metal s u l f a t e s and vanadates may react with the metal in the blades

t o produce su l f ida t ion and corrosion. Corrosion i s more severe as

temperatures exceed 70OoC.

. Extensive experience w i t h gas turbines operating not only on coal b u t

a l so with dusty i n l e t a i r and w i t h d i r t y fuel gases such as b l a s t furnace gas

and petroleum c a t a l y t i c cracker off-gas indicates t h a t the pa r t i cu la t e content

o f the hot gases fed t o the turbine must be kept t o l e s s than approximately 1

ppm t o keep turbine bucket erosion t o an acceptable level f o r turbine i n l e t

temperatures of 150OoF (815OC) o r more. DOE has adopted 0.001 gr / scf ( N 0 . 3

ppm) a s a goal f o r i t s high temperature gas turbine program (Moore 1983) .

Dropping the turbine i n l e t temperature below llOOF (593OC) makes i t possible

t o obtain turbine bucket l i ves of around 3 years with about 100 ppm o f

39

, I par t icu la tes i f a number of compromises a re made in turbine design (Lackey

1979) . The maximum a1 1 owabl e par t i cul a t e 1 oadi ng speci f i ed f o r h i g h temper- a ture gas turbines i n the COGAS process is 0.8 lb/106 scf ( ~ 1 0 ppm). The

3 l imi t f o r condensible hydrocarbons was s e t a t 0.5 lb/106 scf (N 8 mg/m )

(Robson 1974).

Corrosion of turbine blades by coal derived gas streams i s apparently

caused by a l k a l i s u l f a t e s (Moore 1983; Lackey 1979). Wood contains about the

same quantity of a lka l i as bituminous coal , about 0.1 t o 0.2 lb/106 B t u ;

however, wood has much l e s s s u l f u r which may r e s u l t in reduced corrosion.

Other s tud ies i n an ind i rec t f i r e d gas turbine using gas from biomass found

corrosion i n the hot zone o f the heat exchanger apparently caused by a lka l i

s u l f a t e s (see page 41).

The U . S. DOE Fossil Energy Gas Stream Contaminant Control Program has

established a goal of 0.02 t o 0.04 ppm a lka l i i n coal gas t o prevent corrosion

in high temperature gas turbines (Moore 1983).

Gas turbine experience most relevant t o biomass gases i s t h a t of Brown

Boveri Turbomachinery Inc. who have i n s t a l l e d more than 20 units in the l a s t

25 years t o b u r n blast-furnace gas. These machines a r e characterized by low

pressure r a t i o s (about 4.5) and turbine- inlet temperatures o f about 140OoF

(76OOC). They range i n s i z e from 2-14 MW. After extensive t e s t i n g of hot-gas

cleanup equipment, i t was found best t o cool this gas and clean i t with a

two-stage wet scrubber and/or an e l e c t r o s t a t i c p r e c i p i t a t o r p r i o r t o burning

i t i n a combustion chamber ahead of the turbine (Lackey 1979, Pfenniger 1964).

The same approach and requirement have been imposed on the low-Btu gas

supplied from Lurgi coal g a s i f i e r s t o the gas turbine of the combined-cycle

plant in Lunen, Germany (Lackey 1979).

40

2.3.2 Indi rec t Fired Turbines

. .,

Use of an ind i r ec t ly heated gas turbine i s one means of avoiding erosion

and corrosion of the turbine blades. This cycle depends on a high temperature

heat exchanger t o t r a n s f e r heat from the hot d i r t y combustion gases t o a clean

gas, t yp ica l ly a i r which passes through the turbine. There i s very l i t t l e

data ava i lab le f o r these systems as they a re in the ear ly s tages of develop-

ment; however, biomass gas has been used in an ind i r ec t ly heated gas turbine

cycle.

Gas from f lu id ized bed gas i f ica t ion of high ash agr icu l tura l wastes was

used t o r u n a small (30 kw) ind i r ec t ly f i r e d gas turbine in t e s t s by Advanced

Energy Applications, Inc. The major d i f f i c u l t i e s encountered were corrosion

in the hot zone (160OOF) o f the heat exchanger due t o a lka l i s u l f a t e s and

fouling in the hot zone from ash deposits (Moreno and Goss 1983). No infor-

mation was given on the type of gas cleaning t h a t was done, b u t the conclusion

was t h a t development of new high temperature f i l t e r would be necessary t o

prevent ash fouling. Corrosion could be reduced by material changes.

2.4 SYNTHESIS GAS/PIPELINE GAS

A po ten t i a l ly major use f o r biomass medium-Btu gas in the long term i s

as a synthesis gas for methanol, hydrocarbons, o r methane (SNG). In these

appl icat ions the gas goes through several processing s teps between the gasi-

f i e r and the c a t a l y t i c synthesis uni t . These include:

Pa r t i cu la t e and Tar Removal

S h i f t Conversion

Acid Gas Removal

Compression

Trace Gas Removal

41

I , The downstream processing streams and the synthesis c a t a l y s t s used f o r

these process a r e a l l qu i te s imilar so they will be considered together.

Methane t o be transported by pipeline will have t o meet additional specif ica-

t ions f o r the f ina l product gas.

Potential contaminants f o r methanol synthesis c a t a l y s t s a re shown in

Table 12. For some contaminants such as t a r s and o i l s the contaminant l imi t s

a re not well defined. To date most commercial synthesis gas plants a r e based

on natural gas and many of these contaminants have not been a problem.

Catalysts f o r hydrocarbon synthesis (Fi scher-Tropsch) and methanation (SNG)

are q u i t e s imi la r t o methanol c a t a l y s t s so the contaminant problems should be

simi 1 a r .

Some po ten t i a l problems other t h a n c a t a l y s t poisoning o r fouling t h a t

can be envisioned a r e l i s t e d i n Table 13.

TABLE 12. Potential Methanol Synthesis Catalyst Contaminants (White 1983, Bennett 1980, Connor 1982)

Synthesis Gas Component Contamination Level and Potential

Potential Catalyst Poison @ 3 ppm C2H2

C2H4, Higher Olefin Presently Unknown b u t Possible Catalyst Poisonous a t Higher Concentration

CH4, Higher Paraffin Iner t

Sulfur (H2S, COS, CS2) Poison @ 0.03 t o 0.2 ppm f o r Cu based c a t a l