# We Energies Biomass Energy Project
Revised Control Technology Review for Carbon Monoxide Emissions for the Biomass-Fired Boiler.
September, 2010We Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

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# Chapter 3. Carbon Monoxide (CO) Control Technology Review.

Carbon monoxide (CO) is emitted from biomass-fired boilers as a result of incomplete combustion. Incomplete combustion can also result in reduced boiler efficiency and emissions of particulate matter, volatile organic compounds, and organic hazardous air pollutants. CO and VOC emissions from the combustion of biomass fuels are normally expected to be somewhat elevated as compared to the combustion of natural gas, fuel oil, and coal because of the high moisture content and relatively low heating value of most biomass fuels. Because fuel costs represent the highest single operating cost for this boiler, this biomass-fired boiler is designed to achieve the highest combustion efficiencies practicable. CO emissions can be reduced by operating the boiler with higher furnace temperatures, higher excess oxygen levels, and longer furnace residence times. Unfortunately, techniques for reducing CO emissions can increase NO x emissions. Achieving low CO and low NO x emissions is therefore a balancing act in boiler design and operation.



## 3.1 BACT Baseline.
There are no specific Wisconsin state implementation plan (SIP) requirements or new source performance standards for CO emissions from biomass or fossil fuel-fired boilers. Therefore, baseline emissions are the uncontrolled emissions from the boiler.


## 3.2 STEP 1. Identify All Available Control Technologies.
Table 3-1 is a summary of carbon monoxide (CO) BACT emission limits for new biomass-fired boilers contained in the U.S. EPA’s RACT/BACT/LAER Clearinghouse, and in other permits issued in the United States. Emission limits range from 0.10 to 1.15 pounds per million Btu. Except for one facility, the only technology selected for all permitted biomass-fired boilers is good combustion practices. For the South Point Biomass Generation facility (Ohio, 2006), the facility was required to install an oxidation catalyst. However, this facility has not been constructed and therefore has not demonstrated compliance with a BACT emission limit. Note that from Table 3-1, of the top 6 ranked facilities, i.e., the Public Service of NH - Schiller Station, South Point Biomass Generation, Yellow Pine Energy Company, Nacogdoches Power, LLC, Mancelona Renewable, and Northern Michigan University, only the South Point Biomass boiler is a stoker boiler. The other boilers are fluidized bed boilers.We Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

- 22 - TABLE 3-1. Carbon monoxide (CO) BACT emission limits for biomass fuel-fired boilers.

Facility State Permit Date Capacity, mmBtu/hr Primary Fuel Limit, lb/mmBtu Pub. Service of NH - Schiller Station NH Oct-04 720 Wood 0.10 South Point Biomass Generation OH Apr-06 318 Wood 0.10 Yellow Pine Energy Company GA May-09 1,529 Wood 0.15 Nacogdoches Power, LLC TX Mar-07 1,374 Wood 0.15 Mancelona Renewable MI Nov-09 565 Wood/Biomass 0.17 Northern Michigan University MI Nov-09 210 Wood/Biomass 0.17 Fibrominn Biomass Power MN Feb-05 792 Poultry Litter 0.24 Virginia PWC - Laurention Energy MN Aug-05 230 Wood 0.30 District Energy St. Paul, In c. MN Nov-01 550 Wood 0.30 Sierra Pacific Aberdeen facility WA Oct-02 310 Waste Wood 0.35 S.D. Warren Company ME Nov-01 1,300 Wood 0.40 Stoneman Power Plant WI Jan-09 340 Wood 0.40 Koda Energy MN Aug-07 308 Wood / Nat. Gas 0.43 Martinsville Thermal LLC VA Sep-03 120 Wood 0.44 Wheelabrator Sherman Energy ME Apr-99 315 Wood 0.45 Northern Sun ND May-06 100 Wood / Hulls 0.63 Skagit County Lumber Mill WA Jan-06 430 Wood 0.93 Sierra Pacific Industries CA Dec-02 245 Wood 1.15



## 3.3 STEP 2. Identify Technically Feasible Control Technologies.
Available control technologies for the control of CO emissions include good combustion practices, oxidation catalysts, and thermal oxidation.



### 3.3.1 Oxidation Catalysts. Oxidation catalysts are used to reduce CO emissions from natural gas or oil-fired combustion turbines, with typical CO reductions of 50 – 90%. However, oxidation catalysts have limited demonstration on biomass-fired boilers, and we are not aware of any demonstration or commercial operation of oxidation catalysts on a fluidized bed boiler. Boilers have several characteristics that make the use of oxidation catalysts technically infeasible, including catalyst fouling and poisoning by the wood fly ash, and low excess oxygen levels in the flue gas. CO oxidation catalysts are generally noble metal catalysts which are susceptible to poisoning from sulfur and calcium bearing compounds. These catalysts may experience rapid deactivation and fouling in biomass-fired boiler flue gas streams with high particulate loading. To be effective, the oxidation catalyst must be placed in a location with gas temperatures of at least 600 o F. This location would be near the boiler economizer outlets, a location with high ash loading and low excess oxygen levels.We Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

- 23 - A research effort to assess catalyst deactivation mechanisms was presented at the 2008 Advanced Combustion Engineering Research Center conference 1 reported that SCR catalyst poisoning is the predominant deactivation mechanism resulting from biomass combustion. Note that oxidation catalysts and SCR catalysts have similar poisoning mechanisms. The study ranks the order of poisoning of the alkali-metals and alkaline-earth metals from having the most effect to lesser effect as potassium, followed by sodium, followed by calcium, and states that the “effects are proportional to basicity of the exhaust gas.” The study includes Figure 3-2 showing the difference in of coal versus biomass ash composition. Figure 3-2 shows that the relative proportion of potassium is much greater in the biomass ash than in the coal ash. Because of the presence of these base compounds and the low acid gas constituents such as sulfur dioxide (SO 2 ), biomass flue gas tends to be basic while especially high sulfur coal combustion flue gas tends to be acidic. This exacerbates the catalyst deactivation and is the primary reason that oxidation catalyst and SCR systems located in a high dust environment are not applicable to biomass combustors. This problem is worsened by the CFB boiler design which has an extremely high dust loading. Erosion, plugging, and blinding are major potential problems in fluidized bed boiler applications due to the very high particulate loading in the boiler. To reduce the fouling and catalyst poisoning problems, the oxidation catalyst can be located downstream of the particulate matter control device. However, this “tail gas” oxidation catalyst control configuration does not eliminate fouling and deactivation, since the exhaust gas stream remains highly alkaline. In addition to this severe poisoning problem, the use of oxidation catalysts in this boiler application have another problem; the lack of sufficient excess oxygen to effectively reduce CO emissions. Oxidation catalysts operate according to the following general reaction:

2CO + O 2 2CO 2 Typical excess oxygen (O 2 ) levels in combustion turbines is 12 – 15%, compared to 3 – 6% in biomassfired boilers. These low excess O 2 levels will limit the effectiveness of the oxidation catalyst. Tailgas oxidation catalyst has never been used on a similar biomass-fired circulating fluidized bed (CFB) boiler. Please refer to the attached letter in Figure 3-3 from the CFB boiler manufacturer regarding the lack of industry experience with the use of tailgas oxidation catalyst systems on fluidized bed boilers. Other concerns include severe catalyst deactivation due to the highly alkaline flue gases, and the low excess oxygen levels of CFB boilers which further reduce the effectiveness of this system. CFB boilers also have lower excess oxygen levels in the flue gas leaving the boiler than stoker-fired boilers. Because there is limited experience in utilizing CO catalyst technology on biomass fueled boilers including experience on similar CFB boilers similar to the proposed Rothschild biomass boiler, there is insufficient operating experience and information to support a determination that this technology can effectively control CO emissions from a CFB boiler.

1 Guo, X., et. al. Poisoning/Deactivation Study of V 2 O 5 /TiO 2 SCR Catalysts , ACERC Annual Conference, February 2008.We Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

- 24 - Because of these serious technical problems and the fact that oxidation catalysts have not been demonstrated on similar circulating fluidized bed biomass boilers, we have concluded that oxidation catalysts are not a technically feasible CO control technology for these biomass-fired boilers.

FIGURE 3-2. Coal and biomass ash composition showing the high levels of potassium and sodium in biomass ash relative to coal ash.

(Source: Guo, X., et. al. Poiso ning / Deactivation Study of V 2 O 5 /TiO 2 SCR Catalysts, ACERC Annual Conference, February 2008.)



### 3.3.2 Thermal Oxidation Thermal oxidation has never been required nor used on a biomass-fired boiler, and its theoretical ability to reduce CO emissions from these boilers has not been demonstrated in practice and is questionable. Thermal oxidation would involve injecting additional air into the flue gas and heating the oxygen enriched mixture to approximately 1,500 o F to oxidize CO to carbon dioxide. However, since the combustion of the reheat fuel would itself result in CO emissions, there is no evidence that thermal oxidation would result in any CO emission reductions. Since thermal oxidation has never been demonstrated on a biomass-fired boiler, and because there is no evidence that it could reduce CO emissions, thermal oxidation is not a technically feasible CO control technology for these boilers.We Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

- 25 - FIGURE 3-3. Letter from the boiler vendor regarding the use of oxidation catalysts on CFB boilers.We Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

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### 3.3.3 Good Combustion Practices Good combustion practices or combustion controls generally include the following components: 1. Good air/fuel mixing in the combustion zone. 2. High temperatures and low oxygen levels in the primary combustion zone. 3. Overall excess oxygen levels high enough to complete combustion while maximizing boiler thermal efficiency. 4. Sufficient residence time to complete combustion.

In biomass-fired boilers, good combustion practices is the only technically feasible CO control technology. Combustion efficiency is related to the three “T’s” of combustion: Time, Temperature, and Turbulence . These components of combustion efficiency are designed into utility boilers to maximize fuel efficiency and reduce the highest single operating cost of a utility boiler: FUEL. A fourth critical parameter is the level of oxygen in the boiler, referred to as the excess air or excess oxygen level. Therefore, combustion control is accomplished primarily through boiler design as it relates to time, temperature, and turbulence, and through boiler operation as it relates to excess oxygen levels. Combustion design for modern boilers is intended to simultaneously minimize formation of CO and NO x emissions. This is a difficult task, since emissions of NO x and emissions of CO are inversely related. That is, measures used to reduce NO x emissions often lead to increases in CO emissions. Therefore, the boiler design to minimize CO emissions is interrelated with the boiler design to minimize NO x formation.


## 3.4 STEP 3. Rank the Technically Feasible Control Technologies.
Based on the above analysis, the only technically feasible CO control technology for this biomass-fired boiler is good combustion practices. From Table 3-1, recent BACT determinations have CO limits ranging from 0.10 to 1.15 lb/mmBtu. The proposed CFB boiler is an advanced boiler which is expected to achieve low CO emission rates. However, boiler startup, shutdown, and load changes can causes CO emissions to increase because of unstable combustion conditions. CO emissions are also sensitive to boiler operating conditions. Changes in operating conditions, such as rapid changes in load, can have a significant, though temporary, impact on emissions. This condition is further exasperated during boiler startup because the boiler itself is relatively cool, and the low air flow rates make it difficult to obtain good air/fuel mixing. For these reasons, CO emissions can “spike” when transient conditions occur during boiler startup and shut down. Based on the analysis of this boiler, we have concluded that this boiler can achieve CO emission rates of 0.0.12 lb/mmBtu, excluding periods of startup and shutdown, averaged over any 30-day period. In addition, we have concluded that this boiler can achieve an emission rate of 0.12 lb/mmBtu including all periods, averaged over any consecutive 12-month period. CO emissions may increase for brief periods due to boiler startup and shutdown conditions, boiler load swings which may occur to address steam load requirements, and boiler upset conditions. These conditions occur for even the best controlled boilers during startup, shutdown, and load changingWe Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

- 27 - conditions. During these periods, CO emissions can increase above the normal rates during steady-state operation. Although a CFB boiler is a highly efficient boiler with low steady-state CO emissions, because of these operating conditions which are inherent to the fluidized bed boiler design, the maximum CO emission rate for the biomass-fired boiler is expected to be 800 pounds per hour. The CFB boiler is operating at normal combustion conditions when the temperature of the flue gas entering the cyclone separator exceeds 1,450 o F. At this temperature, the entire CFB bed is at or near its normal operating temperature, and combustion conditions are essentially stable at the boiler design conditions. Therefore, for purposes of defining periods of boiler startup and shutdown, startup begins with the firing of fuel and ends when the temperature of the flue gas entering the cyclone separator exceeds 1,450 degrees F. Similarly, shutdown begins when the temperature of the flue gas entering the cyclone separator drops below 1,450 degrees F and ends with the cessation of fuel firing.



## 3.5 STEP 4. Evaluate the Most Effective Controls.
Because the only technically feasible control technology for the control of CO emissions from this boiler is good combustion practices, further evaluation is unnecessary. Although we have concluded that the use of an oxidation catalyst is not technically feasible, we can also evaluate the economic feasibility of the use of an oxidation catalyst for the control of CO emissions. A tailgas oxidation catalyst system requires an oxidation catalyst reactor, a flue gas heat exchanger, a flue gas reheating system, and ductwork from the baghouse or induced draft fan to the stack. Table 3-2 summarizes the cost effectiveness of a CO oxidation catalyst for the biomass-fired boiler. If the oxidation catalyst could reduce CO emissions by 40%, from 0.12 lb/mmBtu to 0.072 lb/mmBtu, the cost effectiveness of a tailgas oxidation catalyst system, based on 8,760 hours per year of operation, would be $13,200 per ton of CO controlled. This is a very high cost of control, especially for a system which has never been demonstrated in practice on a similar, biomass-fired CFB boiler.

TABLE 3-1. Cost effectiveness of a CO oxidation catalyst for the biomass-fired boiler.

Parameter CO Catalyst CFB Boiler Only Control Efficiency 40% 0% Controlled Emission Rate, lb/mmBtu 0.072 0.12 Total Boiler Heat Input, mmBtu 800 800 Actual CO Emissions, tons per year 252 420 Total Capital Requirement, $ $10,810,000 $0 Capital Recovery Factor (CRF) 0.1098 0.1098 Annual Capital Cost, $/yr $1,186,900 n/a Annual O&M Cost, $/yr $1,034,000 n/a Total Annual Cost, $/yr $2,220,900 $0 Average Annual Cost, $/yr $2,220,900 Average CO Reduction, tons per year 168 Average Cost per Ton Reduced, $ per ton $13,200We Energies - Air Pollution Control Construction Permit Application for a 50 MW Biomass Fuels-Fired Cogeneration Facility RTP Environmental Associates, Inc. Appendix A: Control Technology Review for the Biomass-Fired Boiler. Revised September, 2010

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## 3.6 STEP 5. Proposed Carbon Monoxide BACT Determination.
Based on this analysis, We Energies has concluded that the use of good combustion practices is the best available control technology (BACT) for CO emissions from the biomass-fired boiler B01. Based on this analysis, We Energies proposes the following limits as BACT for the control of CO emissions from the biomass-fired, fluidized bed boiler B01.
(1) Carbon monoxide (CO) emissions from the biomass-fired boiler B01 shall be controlled using good combustion practices as the best available control technology. (2) Carbon monoxide (CO) emissions may not exceed 0.12 pounds per million Btu of heat input, excluding periods of boiler startup and shut down, averaged over any consecutive 30-day period. (3) Carbon monoxide (CO) emissions may not exceed 0.12 pounds per million Btu of heat input, for all periods, including periods of boiler startup and shut down, averaged over any consecutive 12-month period (4) Carbon monoxide (CO) emissions may not exceed 800 pounds per hour. (5) Carbon monoxide (CO) emissions from the biomass-fired boiler B01 shall be continuously monitored and recorded using a continuous emissions monitoring system meeting the requirements of 40 CFR Part 60 Appendix B, and NR 439.06(4), Wis. Adm. Code.