Biomass cofiring at Seward Station

Biomass cofiring at Seward Station

Biomass and Bioenergy 19 (2000) 419–427 Biomass co ring at Seward Station Joseph J. Battista Jr.a; ∗ , Evan E. Hughesb , David A. Tillmanc a Coÿring ...

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Biomass and Bioenergy 19 (2000) 419–427

Biomass co ring at Seward Station Joseph J. Battista Jr.a; ∗ , Evan E. Hughesb , David A. Tillmanc a Coÿring

Alternatives, 219 South Phaney Street, Ebensburg, PA 15931, USA Power Research Institute, 3412 Hillview Avenue, Palo Alto, CA 94304, USA c Foster Wheeler Development Corporation, Perryville Corporate Park, Clinton, NJ 08809-4000, USA b Electric

Received 22 November 1999; accepted 1 February 2000

Abstract Sithe Energies, under a cooperative agreement with EPRI, the US Department of Energy, the Biomass Interest Group (BIG) and the Upgraded Coal Interest Group (UCIG), has developed a demonstration of co ring biomass with pulverized coal at the Seward Generating Station on Boiler #12. This demonstration, constructed and tested by Foster Wheeler Development Corporation (FWDC) included construction of a facility to receive, screen, store and transport sawdust into a front- red pulverized coal boiler. This facility has several distinctive features including the sawdust receiving system, the silo storage for processed biomass, and the injection system using the centerpipe of the coal burners. The biomass is prepared separately and transported separately from the coal. The wood waste is then injected into the center of the coal ame and burned there for optimum biofuel performance. This demonstration, involving an extended period of co ring testing, is based upon successful parametric tests conducted in December, 1996 and July, 1997. These tests documented the impact of co ring, in the short term, on capacity, eciency and emissions. Those results are now being evaluated over a long-term period. Additionally, operational and maintenance issues are being addressed with this demonstration and modi cations have been made to improve the handling of the fuel. c 2000 Elsevier Science This paper describes the co ring system, and reviews progress to date with the combustion tests. Ltd. All rights reserved. Keywords: Sawdust; Separate injection; Wall- red PC; NOx ; Tangential ring

1. Introduction Co ring of biomass with coal has been shown to be the lowest cost method for generating “green power” and, at the same time reducing the emissions of fossil-based CO2 . Simultaneously, co ring can provide a service to customers by providing an end ∗ Corresponding author. Tel.: +1-814-471-6689; Fax: +1-814-471-6689. E-mail addresses: [email protected] (J.J. Battista Jr.), [email protected] (E.E. Hughes), david [email protected] (D.A. Tillman).

use for low value or negative value products such as sawdust and other ne wood wastes generated by sawmills, furniture mills, and related industries. Co ring can also be applied to non-woody biofuels such as agribusiness wastes given the proper fuel preparation and feed systems. About 56% of the electricity generated in the US comes from coal, and about 50% of the electrical energy generated in the US comes from pulverized coal (PC) boilers. Consequently co ring can be most successful in the utility market if systems are developed to co re biofuels with coal in wall red and tangentially red PC units.

c 2000 Elsevier Science Ltd. All rights reserved. 0961-9534/00/$ - see front matter PII: S 0 9 6 1 - 9 5 3 4 ( 0 0 ) 0 0 0 5 3 - 2


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Previous parametric testing by GPU Genco, Tennessee Valley Authority (TVA), New York State Electric and Gas (NYSEG), Southern Company Services (SCS) and others have evaluated co ring in PC boilers using one of two techniques: blending biomass with coal on the coal pile and feeding the blend to the pulverizers and then the boiler; and preparing the biofuel separately from the coal and injecting it directly into the boiler without impacting the coal mills. With the latter approach, coal burners can be adapted to accept biomass instead of coal, or adapted to accept biomass as a separate stream with coal. Alternatively, additional boiler penetrations can be used to inject the biofuel into the PC boiler. Blending biomass with coal in the fuel yard is the least expensive approach to co ring. At the same time, however, there is an upper limit to the quantities or concentrations of biomass employed. Generally 6 5% biomass ( 6 2% biomass, heat content) can be injected without impacting the sieve analysis of the pulverizer product. Further, if the boiler is pulverizer limited, co ring using this technique can cause signi cant derating of the unit as a consequence of lower bulk density (lb=ft3 ), lower calori c content (Btu=lb), and increased moisture. Separately preparing and ring the biomass with coal, without impacting the delivery of fossil fuel to the boiler, has been shown to have the following advantages based upon parametric testing at the Seward Generating Station of GPU Genco: • Reducing NOx emissions by up to 15%. • Increasing boiler capacity when the unit is ring wet coal and is consequently derated. • Reducing fossil CO2 emissions. The results of parametric testing have led GPU Genco to a long term demonstration of biofuel co ring at the Seward Generating Station. 2. Coÿring biomass in utility boilers The impetus for the biomass co ring technology development comes from the following factors: • Co ring is a low-capital cost-low-risk strategy for electric utilities to develop renewable energy resources in response to the proposed portfolio standards and to the incipient green power markets.

• Co ring is a low-cost approach for utilities to reduce greenhouse gas emissions–fossil based CO2 emissions and equivalent gases — by substituting renewable fuel for coal and by removing some organic matter such as wood waste from land lls; this supports the voluntary Global Climate Challenge program and other voluntary e orts. • Co ring provides a mechanism for utilities to support local industries, creating a market for residues that could otherwise end up in land lls at some cost to the generators; co ring also increases demand for trucking and other transportation services moving biofuels to utility power plants. • Co ring biofuels with coal o ers a proven technique for reducing NOx emissions by 10 –30% depending upon boiler type, base coal, biofuel, and co ring technique. • Co ring biofuels with coal o ers a proven method for reducing SO2 emissions as a function of fuel substitution. Initial activities in the development of biomass co ring involved engineering studies at wall- red pulverized coal boilers, tangentially red pulverized coal boilers, and cyclone boilers. These initial studies documented the economic and environmental promise of this partial fuel substitution technology. Those studies were conducted at generating stations owned and operated by such utilities as Tennessee Valley Authority (TVA), General Public Utilities (GPU Genco) and Northern Indiana Public Service Company (NIPSCO). They paralleled work being performed by Southern Company Services at generating stations of Georgia Power Company and Savannah Electric Company. Parallel and fundamental work was being undertaken, independently, by TVA also; this work was in addition to the EPRI-related co ring activities of TVA. Parametric tests followed engineering studies. Initial testing occurred at the Allen Fossil Plant of TVA. This was soon followed by testing at such generating stations as Kingston Fossil Plant (TVA), Colbert Fossil Plant (TVA), Shawville Generating Station (GPU Genco), Seward Generating Station (GPU Genco), Michigan City Generating Station (NIPSCO), Blount St. Station (Madison Gas & Electric), Lee Station (Duke Power), LaCygne Generating Station (Kansas City Power & Light) and other locations. An initial

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demonstration was con gured for the Greenidge Station of New York State Electric and Gas (NYSEG) — now owned by AES. Most of the fuel red in these tests was wood waste — typically sawdust — from either sawmills or from secondary processors such as furniture and ooring plants. Other fuels tested included urban wood waste, railroad ties, switchgrass, non-recyclable paper and cardboard, and lm plastics. 3. Seward generating station Seward Station is located in western Pennsylvania about 70 miles east of Pittsburgh. The station was recently acquired by Sithe Energies from GPU, Inc. Seward Generating Station is comprised of three boilers (#12, #14, and #15). Boiler #12 is a 1950 vintage Babcock and Wilcox (B&W) front wall- red boiler, with a capacity of approximately 37:7 kg=s (300; 000 lb=h) ◦ ◦ of 45:9 atm=446 C (675 psig=835 F) steam. Along with Boiler #14, a twin to #12 except it has been modi ed with low NOx burners, it feeds steam to a common header which, in turn, feeds a 64 MW (net) Westinghouse steam turbine (Unit #4). The net heat rate for these units is approximately 14:98 MJ=kW h (14; 200 Btu=kW h). The third boiler (#15) is a 147 MWe (gross) Combustion Engineering (CE) boiler built in 1957 which is a tangentially red pulverized coal boiler. This boiler has four CE Raymond bowl pulverizers. Each pulverizer supplies one level of the furnace on four corners. Boilers #12 and #14 are each equipped with a pair of pressurized EL-56 ball and race mills to pulverize the coal to a neness of greater than 75% passing 200 mesh. Each boiler has six burners (two rows of three) on the front wall with each mill supplying coal to one row of three burners. Boiler #12 is equipped with conventional burners with controlled air registers such that the operator can regulate individual burner fuel/air ratios. The gaseous combustion products exiting the boiler and air heater pass through a mechanical separator for dust collection and are then ducted to electrostatic precipitators (ESPs) for removal of the ner

yash particles. Following the ESPs, the ue gases from the twin boilers are ducted to a common stack. During the parametric tests, Boiler #12 was equipped with a biomass surge bin, metering augers, lock hoppers and transport pipes. The transport pipes connected the biomass delivery system to the unused


centerpipes of the three top burners. Each burner was equipped with a separate metering auger, a separate lock hopper, and a separate blower. Each such system could deliver 2:72 ton=h (6000 lb=h) of sawdust to the boiler. As a practical matter, however, the unit was typically operated in the 2 ton=h range. The current project consists of co ring sawdust with pulverized coal in a 32 MWe wall- red pulverized coal boiler (#12) by utilizing separate injection of the wood at a rate of approximately 2 ton=h (up to 10% on a heat basis) for an extended period of time. The objectives of the Seward test program are to: quantify the impacts of co ring wood waste with pulverized coal by separate injection with respect to boiler eciency, ame stability, operability and the formation of airborne emissions including SO2 , NOx and opacity; demonstrate that the co ring technology can be applied to wall- red pulverized coal boilers at moderate percentage; evaluate the economics of co ring to improve the capacity factor of the unit and provide a basis for evaluating the infrastructure needed to deliver biomass to Seward Station. 4. Project history Parametric testing was performed by Foster Wheeler on #12 Boiler in December, 1996 and July, 1997 to prove the concept. These tests evaluated biomass co ring up to 10% by heat of wood wastes with various moisture and ash contents and with varying fuel volatility. Fuel moisture ranged from 13% to over 50% and ash contents ranged from less than 1% to 5%. The biomass fuels varied from kiln dried sawdust to over two-year old sawdust that had begun the process of devolatilization. Installation of a continuous, semi-permanent, demonstration system was completed by FWDC in March, 1999. Initial testing began in April, 1999 and was suspended shortly, thereafter, due to unit unavailability during the ozone season. Additional testing is to resume upon unit availability in the early Winter subject to the weather and dispatching costs. 5. Design basis for the system The overall system design (see Fig. 1), developed and subsequently modi ed by Foster Wheeler, is to


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Fig. 1. Schematic of the Seward Demonstration System.

receive raw sawdust from the sawmills by walking-bed trailer. The unloading system then delivers the sawdust to a trommel screen where the sawdust is screened to minus 6:35 mm (1=400 × 0). The trommel screen is capable of screening 20 –30 ton of sawdust per hour. The properly sized sawdust is then conveyed to a silo for storage. From the silo, the fuel is delivered by screw and paddle conveyors to a surge bin. From this bin, the sawdust is metered by a weighbelt feeder. The metered sawdust is then picked up by three separate sets of screw conveyors and rotary air lock feeders. From the airlock feeders, the sawdust drops into three separate three inch diameter delivery tubes being supplied with air from three separate blowers. The delivery tubes then transport the sawdust to the burner front and into the boiler past inverted cone distributors which are inserted into the center of the coal burners. The sawdust, transported down the centerpipe of the conventional PC burner, had a velocity exceeding 25:6 ms=s (5000 ft=min). This transport velocity was required to overcome the ame speed of sawdust. The centerpipe of the burner was tted with a di user that created a wood waste ame within the coal ame, creating a more severe reducing condition at the base of the ame while ensuring early ignition and combustion of both fuels. This system maximized the com-

bustion bene ts of sawdust while keeping the fuel delivery systems separated. The biofuel can only be fed when the unit is operated above 50% of capacity or when the ame scanners do not cause a boiler trip when co ring wood waste and coal. The system must be controlled from the existing boiler control room, with local controls for start up and shut down. The fuel receiving=processing area must also provide for dust management. The system has since been modi ed to utilize the two center burners of each level of three in order to maximize the combustion eciency. 6. Biofuel receiving, processing and storage details The biofuel receiving and processing activities are housed in a pole barn, protecting equipment from weather and minimizing dust emissions on site. Biofuel is delivered by truck to the biofuel receiving and processing area. There is no provision for a truck dump; trucks supplying fuel must be self-unloading walking oor vans. The sawdust passes through spiked rollers to break up clumps of material, ensuring its

ow through the process and discharges the material into a hopper. Subsequently, the biomass is fed to the surge hopper of a trommel screen.

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The pole barn houses a POWERSCREEN Model 615 truck-mounted trommel screen with receiving hopper and conveyor, and a receiver bin with rotary airlocks and fan for pneumatic transfer of the biofuel to storage. A conveyor feeds the trommel screen from the truck unloading hopper. The trommel screen is electrically powered. Rejected material is conveyed to a dumpster for disposal. Storage is accomplished in a glass-lined silo 7.7 m (25 ft) diameter × 16:9 m (55 ft) high, which will enable a 2-day storage capacity to be attained. This silo is equipped with a top mounted cyclanet for dust control, with a bin vent lter located at ground level for easy access for maintenance. The cyclone is also equipped with a bottom of pile reclaim system — a discharge auger. 7. Biofuel injection details Biofuel is removed from the storage silo by discharge auger, which transfers the fuel onto an incline conveyor. This conveyer in turn dumps the fuel into a surge chute designed for minimum storage capacity. Biofuel then ows onto a weigh belt feeder for accurate control of the feed rate. The weigh belt feeder dumps the regulated amount of biofuel into an injection vessel for pneumatic transfer to the Unit #12 furnace. The injection vessel is a surge bin capable of holding 20 min of fuel. It is equipped with three metering screws at the bottom of the bin. Each screw feeds a rotary airlock which transfers the biofuel to pressurized pneumatic lines. Pneumatic transfer occurs by way of three separate 7.6 cm (300 ) pipes, each served by independent rotary airlock and blower. The blowers are sized to supply a constant stream of air, ∼ 0:14 m3 =s (∼290 actual cubic feet=minute, or ACFM) to the boiler. This stream of air equates to a fuel velocity at the burner tip of 25.6 m=sec (5000 ft=min). This speed exceeds the

ame speed of woody biomass. The biofuel enters the Unit #12 furnace through each of three existing front wall burner ports. This has since been modi ed to utilizing two burners (the center burner of each level of three) to improve the turn down and combustion eciency. This system, then, provides for separate feeding of the biomass to the Seward #12 boiler. This feeding


is accomplished using the existing interface between the wood waste handling system and the existing coal burners. The interface consists of ring the wood down the 7.6 cm (300 ) centerpipe of the burners. The surge bin as modi ed, the lock hoppers, the blowers, and the metering augers were recovered from the previous tests; they were used as an integral part of this design. 8. Construction and expansion activities The facility was constructed during the last quarter of 1998 and the rst quarter of 1999. At that time GPU Genco expressed signi cant interest in expanding the project to Boiler #15, a 147 MWe tangentially red unit. Foster Wheeler designed special inserts for pneumatic injection of sawdust through unused oil gun ports between the A and B rows of coal burners. These inserts permit the sawdust injectors to follow burner tilts as controlled by the operators. Additional piping was installed. A fourth transport line was installed to provide for reaching all corners of the T- red boiler. The rotary air locks and blowers were replaced with larger, more robust systems to facilitate pneumatic transport to all corners of Boiler #15. The entire facility was originally constructed for $987; 739. The expansion cost $687; 000. The original system, depicted in Fig. 1, was capable of ring 2.72 ton=h of sawdust to boiler #12. The sawdust is capable of delivering 23.7 GJ=h (25×106 Btu=h) to the boiler. Seward Generating Station boiler #12 is a 32 MW unit that was installed in 1946, and has a net station heat rate of 14.98 MJ=kW h (14,200 Btu=kW h). The system can support the generation capacity of 1785 kW of electricity. For this installation, the capital cost is $553=kW of biomasssupported capacity. A modern generating station would have a net system heat rate of about 10.5 MJ=kW h (10,000 Btu=kW h). On that basis, the installation would have a capital cost of $395=kW supported by biofuel. Note, however, that the boiler is very small, and consequently the capital cost is very high. Several items were purchased at a minimum size, regardless of the boiler capacity. Consequently, as noted previously, engineering for modi cation of this system to co re in Boiler #15 was initiated.


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The data presented above document the signi cant di erence in system complexity and capital cost associated with co ring using separate injection rather than co ring with fuel blending in the coal yard. At the same time such systems have more operational

exibility and control; the level of co ring can be increased or decreased — instantaneously — to meet the needs of a given plant. Re nements were made during construction with particular emphasis on selection and installation of the silo reclaim system and the design of the surge chute and bin. Based upon the success to date of both the parametric test program and the construction of the biomass handling facility, modi cations have been made to expand the capacity of the handling system and improve the unloading and sawdust handling capabilities. Expansion of the Seward program involves the following: Engineering and installing an expansion to the fuel handling and transport system, including piping from the existing system to Boiler #15 for potential future testing. Detailed evaluation of the impact of co ring at a larger scale on economies of scale, fuel availability, fuel cost, and overall project economics • Detailed evaluation of the environmental impact of co ring on a larger scale. The design for Boiler #15 (see Fig. 2) includes a modi ed truck unloading system to increase the surge capacity of truck unloading and to reduce truck unloading time from ¿ 1 h=truck to ¡ 20 min=truck. Further, the truck unloading system as modi ed will have the capacity to receive trailer dump trucks as well as walking oor vans. This design includes increased capacity blowers and rotary airlocks. It adds a fourth blower and airlock in order to co re in all four corners of the T- red boiler. The design includes additional piping, and specially designed co ring sawdust injectors for the T- red unit. A wood grinder is incorporated at the discharge of the trommel to convert oversized biomass into useful fuel. The Seward #15 project is sized for 10 ton=h or the support of 10 MWe of capacity. Operationally the unit may re 6 –8 ton=h of biofuel. The modi ed system increases the capital cost to $1; 646; 000. This is equivalent to $165=kW supported by biofuel. Note the dramatic di erence in capital cost on a $=kW basis; this is the consequence of economies of scale between 32

and 147 MWe . The program involves both engineering and installation, and detailed performance and environmental testing. The results will complement both the work on Boiler #12, and the companion demonstration at New York State Electric and Gas (NYSEG). It will provide for incremental scale-up of the co ring process in PC boilers. 9. Operations & testing of the Seward demo system Once the Seward system was constructed (see Fig. 1) and was ready for testing, diculties arose in boiler availability. Because of a warm winter, there was little need for the capacity of Boiler #12. This is an old unit and it is low on the dispatch ladder. When the unit was ready for ring, additional problems were encountered with the ame scanners and combustion eciency, necessitating the change from injecting biofuel in the three top burners to injecting biofuel in the two center burners. Some delays were also encountered with the original truck unloading design (trucks required ¿ 1 h to unload). This required additional modi cations. When those issues were addressed, the system was ready for intensive testing. Capacity implications could not be tested because of the warm weather. However, eciency and emissions could be addressed. Such testing occurred in April, 1999. The parameters of those tests are shown in Table 1. The testing was conducted co ring up to 15% by mass (7% by heat) in Boiler #12. Unlike the parametric tests, there was no attempt to vary the quality of fuel being red. The quality of fuel was essentially constant. Load was varied between about 65 and 100%. Excess O2 varied between 3 and 4.5%. From an operations perspective, co ring sawdust with separate injection had no impact on boiler capacity. Its impact on boiler eciency was modest (about 0.5%) as shown in Fig. 3. The major impact was on emissions, where co ring had favorable results. SO2 emissions decreased proportional to the co ring percentage on a heat input basis. Since Seward Station has no scrubber, the 7% SO2 reduction had signi cant economic bene ts. NOx emissions decreased dramatically when co ring sawdust with the results conforming to equation: NOx = 18:92 − 647:4(Wm ) + 9:66(L) + 59:9(EO2 ); (1)

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Fig. 2. Table 1 Co ring levels tested at Seward demonstration Test no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Main steam ow (kg=s) (kpph)

39.03 37.82 37.46 36.10 37.72 40.22 40.28 32.17 29.68 34.93 34.38 35.68 36.08 35.67

(309.76) (300.14) (297.30) (286.49) (299.36) (319.20) (319.70) (255.29) (235.52) (277.25) (272.88) (283.16) (286.37) (283.08)

Sawdust ow to boiler Ton=h (ton=h)

GJ=h (106 Btu=h)

Mass co ring percentage

Heat co ring percentage

0.00 0.68 1.36 1.91 2.05 2.18 2.33 0.00 1.36 1.50 1.64 1.77 2.10 2.05

0.00 7.96 15.91 22.36 23.96 25.55 27.26 0.00 15.91 17.57 19.16 20.76 22.36 23.96

0.00 4.89 9.63 13.60 13.86 13.89 14.66 0.00 11.74 11.10 12.19 12.62 13.43 14.42

0.00 2.16 4.37 6.32 6.45 6.47 6.86 0.00 5.39 5.08 5.62 5.83 6.24 6.74

(0.75) (1.50) (2.10) (2.25) (2.40) (2.56) (1.50) (1.65) (1.80) (1.95) (1.91) (2.25)

where NOx are the oxides of nitrogen, ppmvd at 3% O2 (dry basis), L the load measured as main steam ow in kg=s, EO2 the excess O2 reported in the control room (total basis) and Wm the wood co ring percentage, mass basis. The equation is quite robust. The probability that the results are a random occurrence is 4:3 × 10−6 , the probability that the Wm term is a random event is

( 7.58) (15.15) (21.21) (22.73) (24.24) (25.86) (15.15) (16.67) (18.18) (19.70) (21.21) (22.73)

8:3 × 10−7 , the probability that the L term occurs randomly is 2:1 × 10−5 , and the probability that the EO2 term occurs randomly is 2:3 × 10−5 . These probabilities indicate the highly signi cant relationship established, and the signi cance of co ring as a NOx trim measure in wall- red pulverized coal- red boilers. A similar equation can be developed based upon biomass co ring as a heat input percentage.


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Fig. 3. The impact of co ring on boiler eciency at Seward Generating Station.

10. Conclusions The Seward demonstration has produced results consistent with previous parametric tests. They document the desirability of continuing with the demonstration in a more long-term mode. At the same time they highlight future directions for commercializing this technology including: • Optimizing fuel blending with locally available opportunity fuels to achieve speci c operational and environmental objectives; this may include alternative opportunity fuels and alternative base coals. • Optimizing the design of separate injection systems to maximize the consequences of co ring through combustion modi cation and combustion technique. • Optimizing the condition of the speci c biofuels being burned to maximize their operational and environmental potential to the electricity generating station. The long-term co ring testing and demonstration at Seward Generating Station has signi cant potential to

facilitate commercialization of this technology. Further, with the expansion of this technology, scale-up of co ring using separate injection can be addressed. The program is designed to address both technical and institutional issues. It will signi cantly advance the commercialization of co ring biofuels in PC boilers. The Seward Demonstration, then, documented the capital costs associated with co ring in the 32 MWe capacity arena, and in the 150 MWe capacity arena. It showed that minimum sizes can be costly; however at boiler capacities ¿ 100 MWe , capital costs can be held to ¡ $200=kW supported by biofuel. From a capacity perspective, the Seward Demonstration again showed that separate injection does not reduce boiler capacity. From an eciency perspective, the demonstration highlighted that eciency losses can be modest and managed. The largest single problem was unburned carbon in the bottom ash, a result that could be reduced through burner design. Emissions reductions were a dominant result of the Seward co ring demonstration. Of critical significance was the favorable impact of co ring on NOx emissions. This result parallels results at all previous

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parametric tests. Biomass oods the combustion region with volatiles, creating more substantial fuel staging at the base of the ame; this reduces NOx emissions. At $1200=ton–$3000=ton as the value of NOx , these results are economically very promising.


The Seward Demonstration illustrates that co ring can be a low-capital cost approach to biomass usage. It can have favorable impacts economically, particularly when considering the cost of emissions management.