Proceedings of the 18th Annual North American Waste-to-Energy Conference
NAWTEC18
May 11-13, 2010, Orlando, Florida, USA
NAWTEC18-3567
HIGH EFFICIENCY WASTE TO ENERGY POWER PLANTS COMBINING MUNICIPAL
SOLID WASTE AND NATURAL GAS OR ETHANOL
Sergio Guerreiro Ribeiro
University of Brasil – COPPE-UFRJ
Rio de Janeiro, RJ 21945-970, Brazil
Tyler Kimberlin
Omega Energy Consulting
Fort Collins, CO 80525, USA
ABSTRACT
1. INTRODUCTION
A new WTE (Waste-to-Energy) power plant
configuration combining municipal solid waste and gas
turbines or landfill gas engines is proposed. The system
has two objectives: increase the thermodynamic
efficiency of the plant and avoid the corrosion in the MSW
(Municipal Solid Waste) boiler caused by high tube metal
temperatures. The difference between this concept and
other existing configurations, such as the Zabalgarbi plant
in Bilbao, Spain, is lower natural gas consumption,
allowing an 80% waste contribution to the net energy
exported or more. This high efficiency is achieved
through four main steps: 1. introducing condensing heat
exchangers to capture low temperature heat from the
boiler flue gases; the stack temperature can drop to 70°C;
2. high steam temperatures in external superheaters
using hot clean gases heated with duct burners; 3. mixing
the exhaust gases of a small gas turbine with hot air
preheated in a specially designed heat exchangers. The
resulting temperature of this gas mixture is almost the
same as a standard gas turbine but with the flow similar
to that of a large machine with a higher O2 content; 4.
After the duct burner and heat exchangers, the oxygen
content of the clean gas mixture is still high, nearly 18%,
and the temperature is approximately 200°C. The gas is
then used as combustion air to the MSW boiler such that
all the energy stays in the system. The efficiency can be
as high as 33% for the MSW part of the plant and 49% for
the natural gas system. Since the natural gas
consumption is almost ten times less than the existing
designs, it can be replaced by landfill gas or gasified
ethanol or biodiesel. Currently an 850 ton/day plant is
being designed in Brazil in partnership with a large power
company. Other advantages include, self generation of
internal power and lower steam superheating
temperatures in the MSW boiler. This concept can be
used with any grate design.
Conventional WTE plants burn waste on specially
designed grates and the hot flue gases generate steam in
a boiler. Due to the very corrosive nature of these flue
gases, [1], the steam temperature and pressure are
limited to 400°C / 40 bar resulting in low thermodynamic
efficiencies, around 22%, for power generation. One way
to overcome this difficulty is to combine a natural gas
turbine with a waste incinerator in such a way that the
superheated steam produced in the MSW boiler is further
heated using the “clean” exhaust from a gas turbine in an
external superheater. Many WTE plants have been built
using this concept, the most important one being the
Zabalgarbi plant, Figure 1. This power plant generates
100 MWe gross and the thermodynamic efficiency for the
MSW portion of the fuel is approximately 30%. For natural
gas the efficiency is around 50%. The disadvantage of
this scheme is that 75% or more of the electric energy
produced comes from natural gas. Although in some
cases, this can be a good solution from an energy point of
view, it is not as environmentally desirable since natural
gas is a fossil fuel and contributes to global warming,
cancelling the benefits of landfill diversion. Also natural
gas prices can vary unpredictably and it may not be
economical to dispatch such plants. However, WTE
plants have to run with a high availability which poses
additional problems to the grid operator.
2. OPTIMIZED COMBINED CYCLE – OCC
The proposed concept, named Optimized Combined
Cycle - OCC, greatly reduces the amount of natural gas
needed to increase the efficiency of MSW combustion.
With OCC, 80% or more of the net energy comes from
MSW allowing the natural gas to be replaced by fuels not
commonly available in large amounts, including landfill
gas or biogas from anaerobic digestion. Another
possibility is to replace natural gas with gasified bio-fuels
such as ethanol or biodiesel using the LPP Combustion,
1
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LLC
C, a Maryla
and-based ccompany, process
p
[2]. The
efficciency of the
e MSW can reach values of more than
33%
% and the na
atural gas e
efficiencies are
a higher th
han a
gas turbine if it was
w used in a standalone
e combined cycle
he natural ga
as efficiency approachess 50%
without MSW. Th
n for small gas turbiness around 5 MWe. The OCC
even
(Opttimized Co
ombined Cycle) conccept has other
adva
antages succh as being specially suited for high
moissture MSW as well as for small in
ncinerators using
refra
actory walls. Neverthelesss large wate
erwall boilers
s can
emp
ploy the sche
eme with ma
any advantag
ges as discu
ussed
here
ein.
between 600
b
0°C and 700
0°C, with du
uct burners (11)
(
and
(12) to adjusst the steam
m superheatting tempera
ature. To
in
ncrease the overall efficiency of the plant, the am
mount of
n
natural
gas used
u
in the d
duct burners
s must be op
ptimized,
the steam cyycle efficiencyy increased (higher presssure and
emperature and reheating), the stack losses minimized
m
te
b lowering the waste b
by
boiler flue ga
as temperature, and
lo
owering the combustion excess air .
F 2 – Optimized Combined Cycle
Fig.
e Scheme
arbi Plant Concept in Bilbao, Spain
n [4]
Fig. 1 – Zabalga
D
ON OF THE PROCESS
3. DESCRIPTIO
Consider Figure
F
2. The
e power is generated
g
byy one
sma
all gas turbine (10), a HP (High Pressure) steam
s
turbine (17) and a LP (Low Pressure) stteam turbine (18).
The HP steam iss superheate
ed in the MSW
W boiler (6) up to
a co
orrosion safe
e temperaturre and, optio
onally, to a higher
h
temp
perature in the
t external superheaterr (3). Similarlly the
LP steam
s
is reh
heated in (5) below 400
0°C first by MSW
M
flue gas then furrther reheate
ed in the exte
ernal reheate
er (2).
After the extern
nal superheater, (3) the
e flue gas is at
perature T2, above 400°C, and can be
b used in (1
13) to
temp
preh
heat the air, from (9), beffore being mixed
m
with the
e gas
turbine exhaust (Y). This ha
as two effects: it reduce
es the
ount of naturral gas in the
e duct burne
ers and incre
eases
amo
the O2 content of the gas turbine
t
exha
aust. After th
he air
heater (13), the flue gasses from the gas turbine
e may
preh
preh
heat the boiler feedw
water in the
e optional heat
exch
hanger (25)) and then be used as part off the
com
mbustion air in
n the MSW boiler.
b
Corrosion iss avoided byy using one or more extternal
erheaters (2)) and (3) hea
ated by the clean
c
gas exh
haust
supe
com
ming from the gas turbine (10) mixed with
w preheate
ed air
at (9
9) and (13). This mixturre is heated
d to tempera
atures
Since the flue gas te
emperature leaving
l
the APC
A
(Air
Pollution Con
P
ntrol) with drry scrubbers is 140 to 17
70°C, we
c
can
recover this ene
ergy using condensin
ng heat
e
exchangers
(
(CHX)
made
e of glass tu
ubes, teflon tubes or
te
eflon coated
d steel tube
es, built by Swiss comp
pany Air
F
Fröhlich
[3].. Combustio
on air can
n be prehe
eated to
te
emperature Tair2 usin
ng a CHX (Condensin
ng Heat
E
Exchangers)
air heater (9
9) and CHX economizer (8) used
o preheat the
to
t
feedwate
er close to the deaera
ator (23)
te
emperature. In a good de
esign, the sta
ack (16) tem
mperature
c be as low
can
w as 70°C allowing not only
o
the senssible heat
r
recovery
but also the lattent heat from water con
ndensing
in
ncreasing the heat transferred from the
e waste
c
combustion
in the boiler ((14) to the stteam. The flu
ue gas at
T which is cooler and has a lowerr O2 contentt can be
T9
p
partially
recirculated as combustion
n air to con
ntrol the
w
waste
comb
bustion temperature an
nd to reducce NOx
fo
formation
in the
t MSW furrnace. We ca
an also run the
t plant
w
without
the gas
g
turbine (10) by incrreasing the pure air
f
flow
Y, and natural gas duct firing (11)
(
and (12
2) during
m
maintenance
e periods. In this case, th
he amount of
o energy
p
produced
by the natural gas approx
ximately matcches the
p
plant
parasitiic load and most of the energy exp
ported by
the plant will come from the wastte. The natural gas
e
efficiency
willl be lower since it is liimited by the steam
c
cycle
efficien
ncy, howeve
er, such a plant without the gas
turbine would
d be a good
d solution iff landfill gass can be
u
utilized,
particularly beca
ause it is ge
enerally ava
ailable in
liimited quanttities. This iss also a go
ood solution from an
2
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b ASME
environmental point
p
of vie
ew, because
e all the power
p
prod
duced will come
c
from waste, inc
cluding the plant
para
asitic load.
In some ca
ases it is be
etter to use a gas engin
ne as
osed to a gas
g
turbine. Gas engines differ from
m gas
oppo
turbines with re
espect to th
heir use in combined cycle
t
ways: allmost all the
e heat rejectted in
appllications in two
gas turbines goe
es to the exh
haust flue gas
s. In gas eng
gines,
ubstantial pa
art of the he
eat loss occ
curs in the water
w
a su
cooling the cylinders. Thus w
we can introd
duce an addittional
dwater prehe
eater before
e or after th
he optional heat
feed
exch
hanger (25),, to capture
e the heat from the en
ngine
cooling system to increase
e the efficiency of the plant
whic
ch at the sam
me time redu
uces the nee
ed for a heatt sink
to co
ool the engin
ne. Gas engine exhaust has
h an O2 co
ontent
of 7-11%,
7
comp
pared to a g
gas turbines 13-16%. Mixing
M
hot ambient air from heat exchanger
e
(1
13) increase
es the
c
of th
he gas engine exhaust to that of a gas
O2 content
turbines exhaus
st, usually hig
gher. In con
ntrast with na
atural
c
plants, where only gas turbines are
gas combined cycle
used
d, we can employ either gas engines or turb
bines,
choo
osing the be
est solution ffor each parrticular case. This
has special adva
antages for small machines, say below 2
MWe, where ga
as engines are more efficient
e
than
n gas
turbines.
In the propo
osed scheme, the comb
bustion air fo
or the
MSW
W boiler is preheated
p
be
etween 200°C
C and 230°C
C and
the O2 content is
s close to 18
8%. This help
ps to reduce
e NOx
form
mation and to
o vaporize th
he water in the
t MSW ea
arly in
the combustion grate. This is particularrly advantag
geous
ure waste th
hat otherwis
se would re
equire
for high moistu
o promote continuous com
mbustion.
addiitional fuel to
4. NUMERICAL RESULTS
The actual design of a WTE plant using OCC
requ
uires extensiive calculatio
ons in order to optimize
e the
para
ameters gov
verning the project.
p
Des
sign requirem
ments
vary
y for diffe
erent locations as well as MSW
M
charracteristics. For
F example, in Brazil, th
here is a tax cut in
elec
ctricity sales for power pla
ants up to 30 MWe. Alth
hough
tipping fees are very low, un
nder US$ 20
0/ton, power costs
as is almost twice
are higher than in the USA. Natural ga
m
be redu
uced to a po
oint in
the international price and must
ch the MSW
W efficiency is optimized
d with respe
ect to
whic
econ
nomic feasib
bility. The ssmall amoun
nt of natural gas
need
ded opens the door ffor a WTE//gas plant 100%
1
rene
ewable, exce
ept for plastic
cs, allowing the
t use of eth
hanol
[2] or
o landfill gas
s.
To reach the optimum design point, we have
eloped speciific OCC plan
nt software making
m
it pos
ssible
deve
to quickly
q
run hundreds o
of cases, varying
v
not only
thermodynamic quantities but
b also plan
nt configurations,
MSW
W properties
s, as well ass economic parameters. This
softw
ware was validated
v
(An
nnexes A and
a
B) using
g the
Gate
eCycle comp
puter program showing an
a almost pe
erfect
agreement between
a
b
the two calculattions for a particular
p
c
case
of OCC configuratio
on.
To emphasize the ad
dvantages off the concep
pt, Figure
3 represents
s a case where the Zab
balgarbi MSW
W boiler,
7
792
TPD (m
metric tons p
per day) of 1,850 Kcal//Kg LHV
(Low Heating
g Value) was
ste correspon
nding to 71 MWth,
M
is
c
combined
wiith a 5.5 MW
We General Electric (G
GE) GE5
g turbine, instead
gas
i
of a GE LM6000 (46 MWe) as
a shown
in
n Figure 1. The
T LM6000
0 has an ope
en cycle effic
ciency of
4
41%
while th
he GE5 value is 30.7%. In a pure combined
c
cycle,
Generral Electric liists the effic
ciencies as 51%
5
and
4
43%,
respec
ctively. Con
nsidering tha
at the natu
ural gas
e
efficiencies
in
n the MSW/g
gas plant are the same as if the
s
same
amoun
nt of gas wa
as used in a standard combined
c
cycle
plant, as
a described by Korobitsy
yn [1], we ha
ave:
Z
Zabalgarbi
 MSW appa
arent efficienc
cy = 31.66%
%
Total Nat G
Gas consump
ption = 152 MWth
M
O
OCC

MSW appa
arent efficienc
cy = 34.51%
%
Total Nat G
Gas consump
ption = 21.84
4 MWth
It can be
e seen that th
he natural ga
as needed de
ecreases
by a factor of
b
o seven and the MSW efficiency is
s almost
1
10%
higher. The actual e
efficiencies fo
or the OCC case
c
can
b calculated
be
d and are sho
own below:
OCC MSW actual
O
a
efficien
ncy = 32.65%
%
O
OCC
Nat Gas efficiency
= 49
9.06%
A naturall gas efficien
ncy of 49% on this scalle, 21.84
MWth, is no
M
ot achievab
ble in any internal com
mbustion
m
machine
ava
ailable. Also, a pure com
mbined cycle
e system
fo this amo
for
ount is not e
economical and in prac
ctice, the
m
maximum
effficiency tha
at can be obtained us
sing gas
e
engines
this size is under 40%. Of co
ourse the nattural gas
c
consumption
ecreased with a corres
sponding
can be de
lo
ower value for
f the MSW
W efficiency as a functio
on of the
d
design
requirrements. Forr an apparen
nt MSW effic
ciency of
3
30%,
approximately 80%
% of the net power
p
will co
ome from
w
waste.
These
e results are summarized
d in Table 1.
T
Table
1 – Orriginal Bilbao Plant x Sa
ame MSW boiler
witth OCC.
3
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b ASME
For this case, it is feasib
ble to replace
e natural gass with
dfill gas, gasified ethanol,, or biodiese
el [2]. We can
n use
land
a sm
maller gas tu
urbine, gas engine
e
or jusst the duct burner
depe
ending on th
he economicss and the avvailability of these
t
fuelss.
The
T
Optimiz
zed Combine
ed Cycle co
oncept has been
exam
mined by the
e Austrian P
Patent Office
e in Vienna under
u
the Patent Co
ooperation Treaty (PC
CT) Internattional
08/000347. It has been given
appllication No. PCT/BR 200
the statements of Novelty (N), Inventiv
ve step (IS)) and
ustrial applica
ability (IA).
Indu
F 3 - OCC Applied to Zabalgarbi Boiler.
Fig.
4 CO2 EMISSIONS
4.
The nexxt item to co
onsider is ho
ow the use of fossil
natural gas will affectt the globa
n
al warming related
e
emissions.
F Italian wa
For
aste, Consonni et al [6] showed
that the total carbon conttent is 27.6%
% and the re
enewable
f
fraction
is 16.0%,
1
i.e., 58% of the
t
total ca
arbon is
r
renewable.
T
This
seems to
o be true for several locations.
Considerr a conventional WTE plant,
p
without natural
g
gas,
burning 792 TPD of a 1.850 Kc
cal/Kg LHV, with the
s
same
carbon
n characterisstics as the Italian waste
e, and a
c
capacity
facttor of 90%. This corres
sponds to a thermal
in
nput of 71 MWth.
M
With 22% efficien
ncy, the total electric
p
power
produ
uced will be 15.62 MWe
e. The annu
ual fossil
C 2 emission
CO
n from MSW
W burning wou
uld be:
CO2 from MSW
W (fossil) = 792
7 x 365 x 0.9
0 x 0.116 x (44/12)
= 110,660 TP
PY (Tons perr year)
onsider the same MSW
W boiler with OCC
Now co
cconsuming 21.84 MWtth of naturral gas (N
NG) and
p
producing
33
3.89 MWe. Considering, for
f simplicity,, that NG
iss equal to pure
p
methan
ne (CH4), burning
b
one MWh of
n
natural
gas will
w produce 0.2 ton of CO
C 2. The ann
nual CO2
e
emission
from
m NG would be:
C 2 from NG
CO
G in OCC=21.84 x 0.2 x 7884
7
= 34,43
37 TPY
Since the 21.84 MW
Wth of NG spent
s
would result in
33.89 - 15.62
3
2 = 18.27 MWe additional power, ge
enerating
the same po
ower using NG alone with 40% efficiency
e
or this amount), one wou
uld need
(maximum achievable fo
o burn 18.27
7 / 0.4 =45.68 MWth of NG.
N This wou
uld result
to
4
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b ASME
in 72,028 TPY of CO2. The difference of 37,591 TPY of
avoided CO2 is due to the efficiency improvement of
OCC. This corresponds to 34% of the CO2 emissions of
the fossil fraction of the MSW. If we replace natural gas
with landfill gas or ethanol, this will increase to 65%.
Additionally, the avoided methane from landfill diversion
will correspond to approximately 338,000 TPY of CO2
meaning that, even for the NG case, the annual net CO2
sequestration would be 265,000 TPY of CO2 for a 792
TPD WTE plant.
5. CONCLUSIONS
This process allows WTE to be feasible at very
modest tipping fees. Developing Countries that could not
afford the costs of landfill diversion will be able to stop
burying their organic wastes. Also Europe, North America
and Japan could benefit from this concept and apply the
surplus of resources from lower tipping fees in other ways
to mitigate global warming.
The OCC concept can be generalized to other types
of thermal electric power plants such as sugarcane
bagasse fuel for which the efficiency improvement can
surpass 50% with very modest increase in the
investment.
ACKNOWLEDGMENTS
I would like to express my gratitude and deepest
admiration for Professor Nickolas J. Themelis, Chair of
WTERT, who probably does not realize that the seeds he
planted when he visited Rio de Janeiro in 2006 are about
to germinate into large trees.
REFERENCES
[1] Korobitsyn, M.A., “New and Advanced Energy
Conversion Technologies. Analysis of Cogeneration,
Combined and Integrated Cycles” – Laboratory of
Thermal Engineering of the University of Twente –
1998.
[2] LPP Combustion, “Dispatchable Renewable Energy:
Gas Turbines Can Burn Liquid Biofuels as Cleanly as
Natural Gas”- Renewable Energy World March 10 12, 2009.
[3] Air Fröhlich - Flue Gas Heat Exchangers Catalog
(http://www.airfrohlich.com/).
[4] Martin, J., “Global Use and Future Prospects of
Waste-to-Energy Technologies” - Fall Meeting
Columbia University, Oct.7-8, 2004.
[5] Alison Smith, Keith Brown, Steve Ogilvie, Kathryn
Rushton, Judith Bates, “Waste Management Options
and Climate Change” - Final report to the European
Commission - DG Environment - July 2001
[6] S. Consonni, M. Giugliano, M. Grosso, “Alternative
strategies for energy recovery from municipal solid
waste Part A: Mass and energy balances” - Waste
Management 25 (2005) 123 135.
[7] Reference Document on the Best Available
Techniques for Waste Incineration, Integrated
Pollution Prevention and Control - EUROPEAN
COMMISSION – August 2006.
.
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ANNEX
A
A
PLANT SPECIFIC
C OCC MODE
EL
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ANNEX B
GAT
TECYCLE O
OCC MODEL
L
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