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,WER
FURNACE DUST FLOWS IN SET PATTERNS ...
Simplest flow sees coarser dust drop out in first boiler pass, with bulk of stack cleaning left to dust collector ~ Partial reinjection feeds some dust back to the spreader-stoker grate, salvages heating value from waste
Total reinjection increases dust loading; collector must be more efficient
Spreader-stoker firing results in 33% of the ash leaving the furnace as flue dust, sa controls over the dust paths are one way to reduced stack discharge
Spreader Dust Responds to Controls
'.
ir
By ELMER R KAISER, Asst Director of Research, Bituminous Coal Research, Inc
:-:T.\CKOCST CO:STItOL for spreader <uken requires: (1) good design (2) ~,,\)d .operation (3) external aids in the furm of dust collectors. With each requirement playing its proper part the '-[Ireader _...ker can easily meet the
'1-\1,,: nits of dust emission set by
\,wal air pollution ordinances, In practice, parLicuiali y n;Lil uj.i;~"
already operating, these three control bctors may fall short of the idtff bale .!tll'C. Real problems like space limitJti"I1S, dollar cost of a collector in:~d\ation, penalty and nuisance of dust .li'1I')5al and, to some extent, possible !:f;!t recoyery from unburned carbon infl:!ence purchase of dust collector,.
Design. Importance of furnace space ill completing combustion ha, been recII.mized since the early work of Kreis inger. In fact, today it is quite generally Jccepted. Enough space permits estab li-hing the favorable conditions of time, temperature, turbulence and proper :lVaiiable o"Hsen.
With older, already installed boilers, the basic conditions are often not met. l'sually furnace temperature gets high enough and the proper oxygen can be made available. But often low-setting hei~hts and otherwise inadequate corn-
AUc.,UST 1951
bustion space limits the available time. The excellent work of Engdahl, :\laior" et aI, on furnace jets has gone a long way toward correcting this weakness.
They found locating jets about 12 in. above the fuel bed, and in some instances lower, gives particularly good results. For conditions requiring turbu !~-"~C ~~.~: !!";: :;!! ';!'2!~ of :! ~!0kpr, jet nozzles in opposite walls spaced about 24 in. apart meet the bill. :\lore details on jet design and location can be had from other sources.
Reinjected Dust. But there is still another control factor that effectively lengthens available combustion time and abo reduces stackdust loading. That is a system of dust or cinder re injection employing dust collectors.
A plant actually can do one of three things: (l) Trap and collect all possible dust, and dump as refuse. (2) Reinject all cinders removed in boiler passes for refiring, and dump all dust at the collector stage. (3) Return all collected dust to the furnace and have only fine ash to dispose of. Since we are concerned with increasing available
(1) R B En&:Llahl. W S ~lajor. Technical R(>port VI. BltumtnouJ Coal Rese&rch Inc. 2609 First ~I. lional Bank BItlr. Plttsbura:h 2:1, PI.
ENGINEERtNG AND MANAGEMENT SECTION
combustion time as well as controlling stackdust loading, we'll consider only the last t wo-partial or com plcte rein jection. Here are influencing elemenb,
Overfire Jets. The basic furnace allll dust.reinjection system shows in Fig. 1. By rearranging the piping from boiler pass A and dust collector B to the furnace proper, the amount of dust reinjected can be changed from partial to total. Overfire air jets are included because they are essential to good oper ation with reinjected dust. But their actual location and number can be var ied for best re,;ults.
Dust Return, Reinjeeted du,t mli-l enter the furnJee so it can land on the ~uel bed. If not. all particles will be lifte,l ri;Iht "lIt of the c'ombu:'tion cham ber in short order. Fine dust in particle form sticks to the fuel bed ill one of two ways: either by being wedged or held in place under or between larger pieces of ash or by lying adjacent to larger pieces away from high air \11l11 gasstream.velocity paths.
In the high.temperature zone near the top of the fuel bed fine ash particles tend to sillter or fuse into heavier pieces that remain on the bed.
A series of tests with Pi by 0 in.
77
-c
M
(a) Without refirlng
(b) Portiol reflYing
I
Simplified furnace shows last pass hopper, dust collector, ...Injection
2
With no reinjection at follows straightforward
011 dust path course above
3
Partial reinjection paths but promises
complicates dust some advantages
I: DUSTLOAD CHANGES
Wlthut
. . . .ed effIciellCY
mid,.., "
of dust coIle<l1Ir
70
Elficlftc:y of beller-
pan hopper
30
AlII _tent. stack dust
60
AlII _tent, collector dOlSt 45
AlII _tent of dust tn..
~er"IIIIU hopper
33
WITH REINJECTlON
PtIttiaI
Totol
,.,;'IIJfI, % ,ellrlIIg, %
74
90
29
25
65
80
48
65
34
~:..
c:.dtoa.
Ill: HDUIlY SUMMARY OF DUST
Without refiring
Partial refiring
refired, .b bt ta stack. JIj lost ta ....., Ib lost, toted, I. percent of coal fited
0
92
67
67
251
190
318
257
6.4
5.1
refired, Ib lost to stade. Ib lost to refuse, 'b lost, total, Ib retired. left oa grata, 11>
0
31
40
43
101
lJl
141
134
7
refired. Ib lost to stack, Ib lostta~ lb lost, total. refl.ed CIftd burned, tl>
0
61
27
24
150
99
177
lU
54
Carbon loss. 'lb totol itv ill cool 4.0
2.7
Steam 9QiMd. Ib
500
Toto. refiring
826 67
0 67 1.3
493 54 0 54 87
333 13 0 13 164
0.3
1510
11: HOURLY
t.... . G
F
X 0.70/0.30
E= F + G
C = E/(I.I-O.30)
D=C-E A given
M=C-A 8=C
D+F
BOILER
ToCol PIIfIIJ/Is
67 156 2U 318 95 125 193 31. 251
DUST WITHOUT
.----"-'1,---.,
Ash C-"tmlble
40
'Z1
70
86
110
113
141
177
31
64
25
100
116
77
]41
177
101
150
REFIRING
,-- i'eIaIIt-, Ash C_kstlbl.
60
40
45
55
49
51
44
56
33
67
20
80
60
40
44
56
40
60
What's the dust loading in Ib per 1000 Ib of flue gas?
How much ash is refired?
What's the percentage of carbon that's recovered?
These tables supply the answers
locomQtive tile emaer Caf]~Vler caught "');~lctqlt"'rl:I1.J~:lecting jnto the
~f the
of the
.....""'....... on the fuel
... ;:<:~,,,,. __ ;~Q,. of the
stream the collector has to increaee. its
efficiency 18-23%. Yet the price qla'"
t;lge between the double-screened'~coal and the smaller coal is such that it may
well ne worihthe extra trouble. A second and less desirable way of
meetblg the problem is to continuously withdraw to refuse a fraction of the collector catch. Wasting 50 lb an hr or one' eighth the catch of a single collec-
tor removes the e;x:tra 251b of ash an hr.
iwo'Stm anomer M8sihility would be ,~~t conecto,r~ .in series. Dust in t1utsecond coUeetl)rwcmld run at least 80% I .~h and. could be dumped to *Mu~r '.~ plants~. use . ~echan.ical ..~~.;'6ll~s.f~n(nV.ed . by electro~tatic.
~ng Rate. Once firing rate gets ~'. ve 20 Ib per hr per sq ft, each in-
. ' after that raises the dust loss from the furnace. More primary cinder and tiyash result because more coal is fired. It's as simple as that.
In addition the higher air rate through grate and furnace entrain larger and heavier particles.
Without reinjection, rate of dust emission above half load increases by about the 1.4 power of the firing rate," Increasing the excess air through the grate raiaes flow rates and shortens aVailablecoJnbU5tion time for particles
in ,tlwfJttnaee. Flow' rates through the
f~el the<l. ~t caus,e; tliepaWcles to
collec~ . ' J O t h e
o..ait , ,
>., Total "reflring Increosesloadlngs
.. to the limit of collector efficiency
1V::IIOURbYI8IlE'.RD,USl .tlHTOTAL REFIRING ,.';-
... = 4, .., ... & .
f = c!X"O.90/O.IO
E=G+F
;:.--',.
E X 0.25/0.75
,'; '8=.+D
~\-" cC::::E +D
.
,~ ' A k _ TlllbleU
M froM T..... It
J=c.f;ro..M..aTable 11
It+L=H-J
L=8.44 8-0.80 G
IL=H._J-~
V: ROURlY BOJI.ERDUSl 11111 ' .. ="''.'..--,.rc.tt----., c=a+J '
Patmtb 67 603
670 223 126 893
"l~~ 31' 575 251 87 164 191
roM Povnb
67 190 251 92 92 349 125
:'Inna 7 3,1 $.C"
It. Primory cinder
has to ~ay witlDa:that range offending the usual sUloke orl1iDlmee~';
I Totol new dust
Expected Resuhs. From a s::udy of ,..
C Total furnace dust
design factors and published data' the
D Catch of boiler-pass hopper
probable weights of dust passing through the hoiler. can be figured, fox:-
I O..t to collector
the three methods shown in Fig, 2,4. '
Collector cotch
For comparison purposes a 45,lb.
Staclt dust
per-ht spreader stoker. firing coal at: & constant rate of 5000 Ib per hr, se"es
H IefiNd dust
as the guinea pig. Its striCk-dust loading
. ,1:. Rec:irculoting dust
m\l8t stay below 0.85 l'a per 1000 Ib of .
Rue gas, corrected to fJO% excess airiro> '
K RefiJecl combustltle bul"lle4
meet the most slrin~~nt smoke
L hfjred ~h to grate
nances.
Dust'->from grate
Coal for the boiler is 13,200 Btu, PA
by 0 in. size, 10% ash content with 5%
matinaufsiri2n0g-mraetseh opfa4rt0icllbesp. eGr assq vfetlopceirti'e~s"..
(totalSOOO Ib per hr) can entrain thes&,
,--I'ound. --:-:-"I
r--~'-'-"
.AsII CCI8I6MsfiAI.
particles of minus 20 mesh. All that these particles lose in the furnace is,~
Ni CatllbvstlbI.
5.(
13
19l
211
10 65 67
20 35
moisture, most volatile, a minor amount '.
33
of fixed carbon. Because of this we as
4ot6
224
101
122
55 040
sume the particles wse half their weight
493
333
5.(7
60 61
39
on their first flight so the remaining bit
2S 116
77
20 60
10
.to
has an ash content from 10 to 20%
141
177
406
169
44 71
56 29
All heavier' coal pieces burn on the
W1
164
35
65
grate. Some flyash and carbon leave in
87
164
100
100
the gas stream in an assumed ratio of
547
346
60% ash to 40 % carbon.
KJO.40 H
carbon -..ry
At 35% excess air, 66,100 lb per hr
'.
of air flows thtough the furnace. Jets
flAt REFIRUIG
snpply 5% of this excess over the fir(;; For calculation purposes the total gas
____ POflflfl ........., AlIi' CGmburtibl.
AsII COItllustibl.
becomes 7Q,SOO lbper hr at 50% excess
.0 91
24 99
65 ... 52
35 52
air (12% CO.). For legal dust emission_
48
maximum allowable dust by weight is
11'
123
31
61
34
66 66
67 lb per ht.
31
61
165
184
34
47
'53
Now let's &ee what effect reinjection-
. 2S
lOO
tt4
77
20 60
80 40
has on staclc-dust loading. We' know'
141
In
44
7
100
55
with reinjection a more efficient dust
24
1 54
n
to2o3
collector must be used. Further, the
boiler-pass hopper removes fewer cin
k/O." H:= "" carHn '-'"
ders, as proportion of finer dust from
reinjection increases. Table I lists the \
changes in the prlneipal
we hale
OIl!![;".'U." reenters the lower poIlrt-:,o the to deal with in co_a ~ ,
short, distanee abGVediie ,fuel The collector ef&eiency,
is an
, this'd~projected;.Ilt-aa , oveNll mea.sure for the sW; consist and
the grate so.- ~n~ L,
class of dust. entering the colleCtor.Dust Flows. For our example boiler.
", '
grate and sOJlleiQf the
K, hu.rn& out ofthe..eUtden '
the loadings of each section ot Ihe dif-
J,ul:either the grate or in the tu:mace
ferent. dust paths have been calculated, . Tables 11; IV, V_ In addition. the pro-
sPaee~,Whatevet" doesn't bntnor,stay
,,'
ag~w,tb":tbo
portion!! of retired ash. L and comb~
. til:tlef.'[(,u. ':Je8ppearing, are worked
ouf. the b$i)m of Tables IV. V. Ash
1Qld, cODlhilll;ihle ,quantities plus their
make upth~
~~,------------------~i
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.J1III//I!-------------~~"..:-..,.\ \ IfI?\\ ~
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>J\. ~ P~
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:J
will be never endJ.ng."
i ~
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Here' are two important reasons why it will YOU GET THE RIGHT APPLICATIO~-
f
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Johns-Manville
INSULATIONS
More PLANT PROBLEMS __________Beqins on page
switch can be arranged to aultonlaticaillv
start a vibrator at bunker outlet wheq' i
there's no coal on scale feed belt. It' will also stop vibrator as soon as coal .
flow starts again. This eliminates noise
of continuously-running vibrator. It
also prevents packing of coal at bunker outlet. Many plants run well with this arrangement.
Similar alarm can also be installed on upper part of coal distributor; see'
sketch. As LS says, clearing bunker
VitJrator---Sw;'nq
Coal
1I i i I I ~ ,1 ~ !
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INSULATION FOR HIGH & LOW TEMPERATURE
Coal alarm
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Conical non-seqreqatinq /' coal dlstnbutor /I I
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jams will probably stop downspout jams. But alarms in both places are useful. These units are standard equipment avai lable from downs pout manu facturer.
ARTHUIt J STOCK Cleveland, Ohio
Stock Equipment Co
General Electric Co has received an or . der for a 180,OOO.hp windtunnel drive-' the greatest power output ever concentrated on a single shaft-for installation at the' National Advisory Committee on Aviation's: Ames Aeronautical Lab at Moffet Field, Calif. The drive consists of four 45,O<JO.hp .
to: 720'rpm variablegpeed induction motOT$'
mounted in tandem on a single shaft drive either of two axialflow compresson.
.1
WestiDghouse Electric Corp'!! new plant,
at Union City, Ind., for %. to 1/20th-hp
motors will produce 75,000 units a month when it goes into production. The Small :\Iotors Div has tripled its output.