Use of ROFA and Rotamix to Reduce NOx in a Riley Turbo

Use of ROFA and Rotamix to Reduce NOx
in a Riley Turbo-Fired Boiler
Burning Landfill Gas and Natural Gas
John Ralston, Jamie Fessenden, Geoffrey Green, and Brian Higgins
Mobotec USA
and
Gil Espinosa
Glendale Public Service Department
ABSTRACT
A MobotecSystem was installed on Glendale Water and Power’s Grayson Unit 5 during spring
2003 for NOx control. Grayson Unit 5 is a 44-MW Riley turbo-fired boiler with induced flue gas
recirculation (IFGR) burning a blend of landfill gas (LFG) and natural gas (NG). The installed
MobotecSystem consists of ROFA and Rotamix. The MobotecSystem reduced NOx emissions
by 48% with a 30%-LFG and 70%-NG mixture. A 58% reduction in NOx occurred with
100% NG. While burning the LFG/NG blend at full load the NOx emission was 61 ppm without
the MobotecSystem and 32 ppm with the MobotecSystem. The Grayson Power station is load
limited by a fixed (lb/day) NOx emission rate. With the NOx reduction from the
MobotecSystem, Unit 5 is able to nearly double its daily generation capacity.
INTRODUCTION
Due to tightening air quality regulations imposed by the South Coast Air Quality Management
District (SCAQMD), the City of Glendale was required to reduce their yearly, daily, and hourly
NOx emissions. In December 2002, a contract was signed between the City of Glendale and
Plant Maintenance, Inc, for the installation of a MobotecSystem™ ROFA© and Rotamix© System
at the Grayson Steam Plant Unit #5 to reduce NOx at 40 MW, while maintaining an ammonia
slip of less than 10 PPM.
Grayson Unit #5 is 44 MW Riley Turbo-Fired forced draft boiler. In 1996, Glendale Water and
Power (GWP) modified the unit to burn a 30/70 blend of land fill gas (LFG) or 100% natural gas
(NG). Unit 5 has six burners, 3 on the front wall and 3 on the back wall specifically designed for
LFG and NG. LFG has approximately 30% of the heating value of NG and greater than 20 times
more fuel-bound nitrogen. Table 1 presents the gas composition of LFG and NG. Burning
landfill gas is desirable for economic and environmental incentives. Also in 1996, GWP installed
an induced flue gas recirculation (IFGR) system. The IFGR provides a NOx reduction of 35-40%
over and above Mobotec’s NOx reduction.
1
Table 1: Landfill gas and natural gas composition
% by Volume
Landfill Gas
Natural Gas
CH4
50
96.33
C2H6
0
1.57
C3H8
0
0.25
N2
16
0.71
CO2
31
1.14
O2
H2O
3
0
0
0
TOTAL %
100
100
HHV (Btu/SCF)
300
1009
BACKGROUND
The MobotecSystem is a three tiered solution to multi-pollutant control. The MobotecSystem
consists of ROFA, Rotamix, and an SCR. ROFA (Rotating Opposed Fired Air) directly affects
combustion and NOx emissions by staging the furnace and turbulently mixing the flue gas with
high velocity secondary air. Rotamix is an advanced selective non-catalytic reduction (SNCR)
system that makes use of the well mixed flue gas and high velocity air to evenly distribute
chemicals into the flue gas. Both SNCR reagent injection (NOx control) and sorbent injection
(SOx and Hg control) can be utilized in a Rotamix system1 . An SCR (selective catalytic
reduction) in-duct catalyst can be added with the MobotecSystem at reduced costs over a
standard SCR due to the size reduction available by the NOx reduction from ROFA and
Rotamix.
With ROFA, the gas volume in the furnace is set in rotation via special asymmetrically placed air
nozzles. The flue gases mix well with the added air, due to increased turbulence mixing and
rotation in the entire furnace. This improves temperature and species distributions and improves
particle burnout in the upper furnace.
Mixing and rotation prevents the formation of stratified laminated flow, which enables the whole
furnace volume to be used more effectively for the combustion process. The ROFA swirl reduces
the maximum temperatures of the combustion zone and increases heat release, which may
improve boiler efficiency. Mixing the combustion air more effectively with ROFA can also
reduce surplus excess air. This results in less cooling of the furnace due to unused combustion air
thereby increasing the heat absorption.
Some the documented advantages of the ROFA technique are:
•
•
Less temperature variation in the cross-section of the furnace.
A more even distribution of combustion products in the cross-section of the furnace (e.g.,
O2 , CO, and NOx).
1
Haddad et al., “Full-Scale Evaluation of a Multi-Pollutant Reduction Technology: SO2 , Hg, and NOx”, MEGA
Symposium #117 (2003).
2
•
•
Lower CO levels which means reduced excess air.
Less excess air (O 2 ) which means less NOx and higher overall efficiency.
Rotamix is a third-generation SNCR and sorbent injection system. The turbulent air-injection
and mixing provided by ROFA allows for the effective mixing of chemical reagents with the
combustion products in the furnace. The result is the efficient introduction of reducing chemicals
directly into a well-distributed, rotating mixture. The Rotamix system is tuned to adapt to
changes in load and temperature in the furnace, and only introduces reducing chemicals to the
furnace where the temperature is most favorable for pollution reduction. This reduces chemical
consumption and lowers chemical slippage by increasing the reaction efficiency. Relative to
other SNCR systems, Rotamix can decreases recurring chemical costs.
While NOx reduction with typical low-NOx burners (LNB) and OFA systems can sometimes
exceed 50%, ROFA NOx reduction routinely exceeds 60% and Rotamix NOx reductions
routinely exceed 35%. The minimum NOx reduction achievable with the MobotecSystem is
75% and often approaches 90%.
The third tier to the MobotecSystem is the installation of an SCR. As shown by Haddad et al.2 ,
every dollar spent on the ROFA and Rotamix system SCR costs by $2.05.
MOBOTECSYSTEM
The MobotecSystem installed on Grayson Unit #5 consists of:
•
•
•
•
•
•
•
A ROFA Fan and inlet damper
ROFA and Rotamix ductwork
Individual flow-controlled ROFA & Rotamix boxes (each with several directed nozzles)
An ammonia storage tank and pump skid
A water storage tank and pump skid
A Rotamix flow-control cabinet for ammonia and water delivery
A Moboview control system
ROFA Process: The ROFA Fan draws air from the ductwork between the FD fan and the air preheater inlet. The ROFA fan delivers pressurized air at 120 to 140°F to the ROFA and Rotamix
boxes strategically positioned on the furnace side walls. The amount of secondary air diverted to
the individual ROFA and Rotamix boxes and into the furnace varies with load and is controlled
based on steam flow.
The Rotamix system consists of a reagent
tanks, water and reagent pump skids, and
water and ammonia to the Rotamix cabinet,
The water and ammonia are mixed inside
delivery system including reagent and water storage
delivery lines. The delivery lines supply pressurized
located near the Rotamix boxes at the upper furnace.
the Rotamix cabinet and are delivered to individual
2
Haddad et al., “The Viability and Economics of Adding a ROFA/Rotamix MobotecSystem to a Selective Catalytic
Reduction (SCR) Installation,” NETL/DOE 2003 Conference on SCR and SNCR for NOx Reduction
Pittsburgh, PA October 29-30, 2003.
3
injectors. The Rotamix injectors consist of delivery air, cooling air, and diluted reagent mixture.
The locations of the ROFA and Rotamix boxes are determined from CFD modeling and field test
data.
CFD MODELING
The performance of a MobotecSystem is strongly dependant on the design phase. A proven
method to evaluate and validate the design from a fluid flow and combustion perspective is to
use computational fluid dynamics (CFD). To perform a CFD analysis of a MobotecSystem, a
baseline case is defined with full-load operating conditions including fuel flow rates, air flow
rates, inlet temperatures, etc. Upon the completion of the baseline case a ROFA case is defined
and incorporated into the baseline CFD model. Several key parameters such as O2 , CO, NOx,
unburned fuel, and temperature are evaluated to validate the predicted performance. The CFD
design methodology is an iterative process which includes multiple case studies. The
finalized/optimized CFD ROFA model is used as the design basis for the installed system.
The objective of applying CFD modeling to Grayson Unit 5 is to provide better understanding of
how the unit behaves and to provide a determination of how the ROFA technology affects NOx
production and unit operation. Model results can be helpful in providing improved understanding
of furnace combustion. Because models inherently contain physical assumptions, results are best
used to indicate trends in combustion behavior in comparison to modeled baseline levels. In
addition, furnace measurements and observed furnace behavior are essential to understand model
results.
The computer model used for this study is a CFD-based reacting flow code, GLACIER,
developed by Reaction Engineering International (REI). The code couples the effects of
turbulent fluid mechanics, gas-phase combustion chemistry, finite-rate NOx chemistry, and
convective and radiative heat transfer. The REI combustion code assumes that the flow field is a
continuum field that can be described locally by general conservation equations. The flow is
assumed to be steady state and gas properties are determined through local mixing calculations.
The fluid is assumed to be Newtonian and dilatation is neglected. The comprehensive model uses
an Eulerian-Lagrangian framework and handles either Reynolds- or Favre-averaging. The code
couples the turbulent fluid mechanics and the chemical reaction process, using progress variables
to track the turbulent mixing process and equilibrium chemistry to describe the chemical reaction
process.
The CFD model predicted a slight increase in exit temperatures and CO emissions and a
significant decrease in NOx. The CFD results as compared with actual results are presented in
Table 2.
Note from Table 2 that the predicted NOx concentrations are lower than the actual measured
NOx concentrations. This is due to the fact that at these very low NOx concentrations, a high
degree of accuracy in the computational modeling is expensive in terms of the number of nodes
and the level of chemical kinetic complexity required to accurately model the furnace and the
burners. While the NOx concentrations are not necessarily accurate, the trends between the two
models are considered to be accurate.
4
Table 2: CFD model results versus actual results at 43.5 MW
NOSE
Temp (K)
Baseline
Model
1545
ROFA
Model
1588
Baseline*
Measured
-
ROFA**
Measured
-
Temp (°F)
2321
2399
-
-
CO (dry, ppm)
5585
6057
-
-
O2 (dry, %)
2.89
2.65
-
-
25
10
-
-
Baseline
Model
1169
1645
ROFA
Model
1195
1691
Baseline*
Measured
ROFA**
Measured
CO (dry, ppm)
388
555
O2 (dry, %)
2.38
2.37
NOx (dry, ppm)
EXIT
Temp (K)
Temp (°F)
NOx (dry, ppm)
24
9
61
32
lb-NOx/MMBtu
0.027
0.010
0.063
0.035
% NOx reduction
*Baseline data at 40.4 MW
**ROFA data at 42.9 MW
63%
48%
On the following pages are NOx, temperature, and O2 profiles. These can be used to further
understand the flow through the furnace and how ROFA affects this flow to reduce NOx. The
ROFA condition as-shown defined the actual design case. That is, this case was used to specify
the box locations, nozzle sizes, nozzle angles, and exit velocities as built and constructed. This
case was arrived at after analyzing multiple cases and making design adjustments to affect NOx,
temperature, and oxygen distributions.
The first two figures show the NOx profiles for the baseline case without ROFA (Fig. 1) and the
“as-designed” case with ROFA (Fig. 2). The difference in NOx concentrations between the
baseline and ROFA case are considerable.
The baseline case has more NOx throughout in the furnace. This is because NOx reduction with
ROFA occurs throughout the furnace, not just at the ROFA ports. In the baseline case, it appears
that there is a significant build up of NOx near the front left corner. This is due to high
temperatures in that region (which is also evident in Fig. 3).
For the ROFA NOx figure (Fig. 2), the overall NOx quantity is lower and it is isolated in the
center of the furnace. The NOx in the exit plane is far more uniform for the ROFA case than for
the base case.
5
Figure 1: Baseline NOx levels (0 to 50 ppm)
NOx (ppm)
Figure 2: ROFA NOx levels (0 to 50 ppm)
NOx (ppm)
6
Figures 3 & 4 contain the predicted temperature for the baseline and ROFA cases. The hot spot
in the front left corner of the base case is clearly discernable. The effect of ROFA is also seen as
cold jets propagate across the furnace and mix with the combustion products coming up the
center of the furnace from the burner level. Due to high rates of heat transfer in the convective
backpass, the temperature profiles at the exit of the furnace are similar.
Figure 3: Baseline temperature (0 to 2800ºF)
Temp (°F)
Figure 4: ROFA temperature (0 to 2800ºF)
Temp (°F)
7
In Figures 5 & 6, the O2 levels are represented. Note the oxygen stratification in the baseline
case as the products round the nose. The ROFA jets are clearly delineated in the bottom figures.
Also, reduce O2 at the burner level is visible in the ROFA figure.
Figure 5: Baseline O2 levels (0 to 10%)
O2 (%)
O2 (%)
Figure 6: ROFA O2 levels (0 to 10%)
8
INSTALLATION
Detailed (post-CFD) engineering of the installation began in December 2002. The technical
engineering was performed by Mobotec USA and the Balance of Plant (BOP) Engineering was
performed by Industrial Design Services. UCI, Inc. successfully completed the physical
installation of the MobotecSystem under the direction of Plant Maintenance, during spring 2003.
Commissioning and start-up were finished the first week of June, 2003. MobotecSystem tuning
started immediately after commissioning and was concluded in two months.
DATA AND RESULTS
The Mobotec System achieved significant reductions at Grayson Unit #5 with the NG, LFG, and
the previously installed IFGR system. Tables 3 & 4 list the performance test results for the unit
fired with natural gas (NG) only. Tables 5 & 6 list the performance test results for the unit fired
with a blend of landfill gas (LFG) and NG.
The NOx reduction achieved with 100% NG is 59%. The NOx reduction achieved with LFG and
NG is 48%. The difference in NOx emission levels with and without LFG lies partly in the
difference in fuel nitrogen content. With LFG, the total amount of fuel bound nitrogen increases
from 0.7% to 5.0%. Also because LFG has a lower heating value than NG, the stoichiometric air
requirement increases with LFG. Increased total air flow has two drawbacks: (1) thermal NOx
increases and (2) total IFGR quantity decreases.
An induced flue gas recirculation (IFGR) system is typically employed to allow a plant to run at
low load and still achieve reheat temperatures. However, at this site, the IFGR was installed to
achieve lower NOx through dilution of fuel and air during combustion. The MobotecSystem was
tested with and without IFGR in service. The IFGR system improved the NOx reduction when
coupled with the MobotecSystem. The NOx reduction achieved from the IFGR system was 35 to
40% when coupled with the MobotecSystem. Because NG has very low fuel nitrogen content,
the majority of the NOx formed during combustion is thermal NOx. The IFGR dilutes the
oxygen and thus reduces peak combustion temperature. Fundamentally, the NOx reduction from
the IFGR and the MobotecSystem are additive.
9
Table 3: Natural Gas Only without Mobotec System
Test Load, MW
Stack Flow, Kdscfm
NOx ppm, @ 3% O2
NOx lb/hr
NOx lb/MW-hr
Min Load
13.6
90
37.6
8
0.585
50% Load
23.1
52.8
44.7
14.4
0.625
75% Load
31.9
70.5
60.5
26.5
0.831
Table 4: Natural Gas Only with Mobotec System
Test Load, MW
Stack Flow, Kdscfm
NOx ppm, @ 3% O2
NOx lb/hr
NOx lb/MW-hr
Reduction %
Min Load
13.0
39
12.6
2.6
0.203
65.3%
50% Load
22.1
55.5
19.6
6.4
0.290
56.2%
75% Load
31.3
75.1
24.7
11.1
0.356
59.2%
100% Load
42.7
103.3
42.1
25.9
0.607
*
*Baseline (Figure 1) 100% Load not measured due to hourly and daily NOx limit
Figure 7: Baseline and ROFA NOx emission for 100% NG
30
25
NG without MobotecSystem
NOx emission rate (lb/hr)
NG with MobotecSystem
Expon. (NG with
MobotecSystem)
20
15
59%
NOx Reduction
10
5
0
0
5
10
15
20
25
Load (MW gross)
10
30
35
40
45
Table 5: Landfill Gas with Natural, without the Mobotec System
Test Load, MW
Stack Flow, Kdscfm
NOx ppm, @ 3% O2
NOx lb/hr
NOx lb/MW-hr
Min Load
17.7
51.0
24.3
7.0
0.397
50% Load
23.0
64.8
31.5
11.9
0.516
75% Load
32.2
80.6
40.9
20.2
0.625
100% Load
40.4
96.3
61.2
36.7
0.908
Table 6: Landfill Gas with Natural Gas, with the Mobotec System
Test Load, MW
Stack Flow, Kdscfm
NOx ppm, @ 3% O2
NOx lb/hr
NOx lb/MW-hr
Reduction, %
Min Load
18.0
52.7
13.2
4.1
0.228
45.8
50% Load
23.6
61.7
15.6
5.9
0.249
50.3
75% Load
33.0
83.9
23.9
12.1
0.365
41.6
100% Load
42.9
108.6
32.1
20.9
0.487
47.6
Figure 8: LFG and NG baseline and ROFA comparison
40
LFG and NG without Mobotec System
LFG and NG with MobotecSystem
35
Expon. (LFG and NG with
MobotecSystem)
NOx emission rate (lb/hr)
30
48% NOx
Reduction
25
20
15
10
5
0
0
5
10
15
20
25
Load (MW gross)
11
30
35
40
45
CONCLUSIONS
The Mobotec ROFA/Rotamix System is an effective low-cost solution for NOx reduction in NG
and LFG/NG fired units. In the case presented, NOx emissions were reduced nearly 48% at
Grayson Unit #5 for the LFG/NG blend. NOx emissions were reduced substantially more when
burning NG only. The quick turnaround time, from contract signature to operational unit, makes
the Mobotec System even more attractive when coupled with total cost.
GWP’s Grayson Unit 5 can be operated at twice the capacity factor while staying under the NOx
emission limit with the installation of the MobotecSystem. The IFGR system worked very well
with ROFA system. The NOx reduction achieved with the IFGR in addition to the Mobotec
System was 35 to 40%.
CONTACT INFORMATION
Eric B. Fischer
Mobotec USA Inc.
[email protected]
PO Box 2025, Orinda CA 94563
925-962-9040 ext 11
Edwin Haddad
Mobotec USA Inc.
[email protected]
8 McCarthy Circle, Framingham, MA 01702
508-872-1610
12