Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June 2009)13Reduction of CO2 Emissions in Conventional Boilers

YOSHIYUKI WAKABAYASHITOKUSHI MARUTA

MASAHIKO HOKANOYUUICHIROU KITAGAWAHIROSHI SUGANUMA

KOUTARO FUJIMURA

Contemporary society relies heavily on fossil fuels and thus faces environmental problems
that include global warming and the depletion of resources. It is essential to develop a new
high-efficiency coal-based power generation system to ensure a stable energy supply. Mitsubishi
Heavy Industries, Ltd. (MHI) has vast experience with new power generation technologies basedon coal-fired conventional boilers. This paper describes three related developing technicaltrends: advanced biomass energy technology, A-USC (advanced ultra-super critical) boilers,and oxy-fuel combustion.

|1. Introductionn
Contemporary society relies heavily on fossil fuels including coal, crude oil, and natural gas
as primary energy sources. This has caused environmental problems including global warming andthe depletion of resources.
The development of a new high-efficiency power generation system is essential to reducing
environmental burdens and ensuring sustainable economic development. A stable energy supply
has become a key issue, given the recent fluctuations in crude oil prices. Therefore, thedevelopment of a high-efficiency power generation system with a stable and secure supply of coalis very important.
MHI has developed various high-efficiency power generation systems including theIntegrated Gasification Combined Cycle (IGCC) and Integrated Coal Gasification Fuel Cell
Combined Cycle (IGFC) systems. This paper describes MHI’s developments of three othertechnologies: advanced biomass energy technology, which is a CO2 emission reduction technology
based on conventional coal-fired boilers; advanced ultra-super critical boilers (A-USC); andoxy-fuel combustion.

|2. Development of CO2 emission reduction technology
2.1

Advanced biomass energy technology

Biomass is part of the carbon cycle in which carbon dioxide in the atmosphere is fixed inbiological matter by solar energy. The amount of CO2 in the atmosphere does not increase as longas the balance between consumption and production is maintained, even if biomass is consumed.
The use of biomass is thus considered to be an effective measure for reducing CO2 emissions. The
perspective of forest management and cultivation or effective use of biomass-derived wastes,development and implementation of advanced energy utilization technology for biomass resources
is now essential.
Table 1 compares the main biomass utilization technologies. This paper discusses trends infuel mixing combustion in large-capacity conventional coal-fired boilers.

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Table 1 Comparison of biomass energy utilization technologies
ItemDirect firing

Multi-fuelcombustion incoal-fired boiler

Production of
gasified liquidfuelPyrolysis system

Ethanolfermentation

Methanefermentation

Rawmaterial

All plants

Wood chip

All plants

All plants,Sludge

Sugar, amyloid

Compost, sewage
sludgeElectric power

Methanol,Dimethyl ether

Pyrolysis gas

Ethanol

MethaneHigh（electricpower）

High（liquid fuel）

Low-medium（liquid fuel）

Low-medium（liquid fuel）

Applicable tohigh-efficiency
plant.

Valuablebyproduct(methanol anddimethyl ether.)

Product
Energyconversion
efficiency

Features

Steam, electricpower
Low（electricpower）
High（steam）
Close to thebiomass
production site..
Small scale.

High（solid or fuelgas）
Applicable tohigh-efficiency
plant

Applicable to highValuable
water content
byproduct
biomass.
(methanol).
Limitation on
Limitation on
biomass material. biomass material.

(1) Efforts to develop biomass–coal mixing combustion system
High-efficiency power generation through the use of biomass will be achievable byburning woody biomass with coal directly in the existing coal-fired boilers of high-efficiency
power plants. These fuels can be mixed in two ways. The first is to pulverize both the woodybiomass and the coal in a coal pulverizer and then inject the mixture into the furnace via a coalburner. The second is to pulverize the woody biomass in a dedicated pulverizer for woody
biomass and then inject the pulverized biomass into the furnace through a dedicated burner.
The first method requires only a simple modification of existing facilities because it uses
the existing coal pulverizer. However, the coal pulverizer’s capacity is limited whenpulverizing woody biomass, whose primary component is elastic material. This puts an upper
limit on the mixing ratio of biomass with a heat input ratio of 1–5%.
Using the dedicated pulverizer, on the other hand, enables to achieve a further increased
biomass mixing ratio.
(2) Pretreatment equipment for raw materials
Pretreatment equipment such as crushers may not be required for woody biomass in chipor pellet form, depending on the size or type, although dryers or magnetic separators areinstalled as required.
(3) Supply and combustion equipment
・ Mixing pulverization method using a coal pulverizer
Three methods are available for mixing the biomass with coal: mixing in a coal yard,mixing on a coal conveyor, and mixing in a coal feeder. It is desirable to supply a constantamount of biomass at the coal feeder to control the fluctuations in the mixing ratio that result
from poor mixture in a coal bunker caused by differences in the specific gravity of the fuels.
When the mixing ratio is 1–5%, the normal practice is to mix the coal and biomass on theconveyor.
・ Pulverization method using a dedicated pulverizer
This method requires a dedicated pulverizer for woody biomass. The pulverized
woody biomass is normally carried to the dedicated burner by conveying air. The biomass ismore flammable than is coal because of its high-volatile matter content, although its
grindability is less than that of coal due to its elasticity. This causes excessive power
consumption in the pulverizer. Selecting the optimal grinding particle diameter is therefore akey issue.
(4) Environmental characteristics
The nitrogen content of biomass is lower than that of coal, and NOx emission are
generally less in proportion to the mixing ratio, or at least at the same level. Characteristics area function of the manner of mixing and the grinding particle diameter. Because the ash contentof biomass is less than that of coal, the production of soot and dust show a downward trend inproportion to the biomass mixing ratio.
(5) Development efforts
MHI has plenty of experience with circular firing boilers using wood waste or bagasse as
fuel in its subsidiaries in Brazil, where the use of biomass has been widespread. We haveMitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (May. 2009))
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developed based on the production, shipment, related know-how and knowledge about themixed combustion for various types of woody biomass in both our domestic and overseas.
In cooperation with the Shikoku Electric Power Co., MHI conducted mixing combustion
tests using woody biomass in the 156-MW Saijo No. 1 boiler. This was the first such
experiment with Japanese domestic electric power companies and now full-scale operation witha mixing ratio of 2% is underway.
MHI also conducted biomass mixing combustion testing of the 900-MW Maizuru No. 1boiler in collaboration with the Kansai Electric Power Co. where full-scale operation started
using biomass combustion with an average mixing ratio of 3% in 2008. Furthermore, alongwith the Hokuriku Electric Power Co., MHI has operated the 700-MW Tsuruga No. 2 boiler
with a mixing ratio of 3%, and confirmed its excellent operational performance. Each of theseunits used the mixing pulverization method utilizing existing coal pulverizers.

2.2

Development of the next-generation advanced ultra super critical (A-USC)
boilerIncreasing efficiency by raising the temperature and steam pressure of thermal electric power
plants is an effective means of reducing CO2 emissions. Governments in Europe, the United States,and Japan have all promoted basic research into next-generation 700°C A-USC plants with thegoal of attaining a net thermal efficiency of 46% on the basis of higher calorific value.
Commercializing the A-USC, however, has presented some challenges, such as thedevelopment and practical application of new materials suitable for high-temperature andhigh-pressure use, the development of related manufacturing technologies, and the establishment ofa design method that is reliable and economically feasible.
Figure 1 shows the planned development schedule. This covers the nine-year period

from the basic development such as material development to verification tests such as fieldtests. Five years have been allotted for demonstration tests. Securing Japan’s energy futureusing coal-fired thermal power generation is an urgent priority. At the same time,
continued research and development are imperative to establish the new materials andmanufacturing technologies required to put the high-steam conditions of the A-USC into
practical use.

Figure 1 Government project schedule for A-USC technological development(1) Material development
The development of materials that can withstand the 700°C steam conditions is essential;
these are unlike conventional materials and must maintain their strength at high temperatures.
The use of Ni or Fe-Ni-base alloys with sufficiently high tensile strength in thehigh-temperature zone is essential. In addition to Ni-base alloys such as Alloy617，CCA617，Haynes230，Alloy740，and Nionic263, an Fe-Ni-base alloy referred to as HR6W is currently
under development in Japan as a candidate for boiler materials.
Because expensive Ni-base alloys are used, it is important to control the scope andestablish a design method that ensures economic efficiency.

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16(2) Development of manufacturing technology
Overall verification is required for Ni-base or Fe-Ni-base alloys that are used inhigh-temperature and high-pressure steam conditions. This includes the development ofbending technology; the development of welding materials and methods for joining thesematerials together and with dissimilar metals; the optimization of welding conditions; theacquisition of knowledge of the microstructure and of joint strength and toughness; and thedevelopment of heat treatment processes and inspection methods for welded areas.
(3) Structural considerations
The thermal expansion in main and reheating steam pipes must be considered, and thestructural reliability of valves installed in high-temperature zones must be examined.

2.3

Oxy-fuel combustion

Capture and storage is the most direct and effective method of reducing CO2 emissions into
the atmosphere. CO2 can be captured in two ways. The first is to absorb and separate the CO2 fromexhaust gases using an absorbent. The second method is a new oxy-fuel combustion method that
captures CO2 emissions, where the nitrogen is separated from the air used in the combustion andCO2-based exhaust gases recirculated from the boiler outlet are mixed and burned (Figure 2).
We discuss the development trends of this oxy-fuel combustion method below.

Figure 2 CO2 capture methods in conventional boilers(1) Oxy-fuel combustion method
Burning coal using the O2 separated from the air can increase the CO2 concentration in
the exhaust gas to more than 90%. This makes the separation of CO2 from exhaust gas
unnecessary, and at the same time, it enables to capture the CO2 using compression and coolingprocesses.
In a power generation system using this concept, the oxy-fuel combustion recirculates
CO2-based exhaust gases and mixes them with the O2 used for combustion, settling theconcentration of the O2 to the appropriate value. Thus, the increase in flame temperatures canbe controlled. If a low-cost and low-motorization air separator can be developed, the oxy-fuelcombustion will be a promising method that is more economical than other CO2 capturingsystems and presents lower technological hurdles.
(2) Oxy-fuel combustion tests
To determine the optimum oxy-fuel combustion properties, MHI constructed an
exhaust-gas recirculating system that included a test furnace with a 1/100 scale burner andevaluated the oxy-fuel combustion properties. Figure 3 compares sample test results ofoxy-fuel combustion with ordinary air combustion. We confirmed that the flame brightness in
oxy-fuel combustion was higher than that of air combustion, and that the combustion itself wassmooth and stable.

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17(a) Oxy-fuel combustion(b) Air combustion

Figure 3 Comparison of combustion conditions

We found that the NOX emission in the oxy-fuel combustion were 1/2 to 1/3 that of air
combustion due to the effect of returning the circulation gas into the reduction zone in the furnace.
We will continue to test larger scale burners.

|3. Conclusion
Amid growing demand for a fewer CO2-burden power generation toward low-carbon society,the price of coal rarely fluctuates, unlike that of crude oil or natural gas, and a stable supply of coalis available. Reducing CO2 emissions from coal-fired thermal power generation is a top priority inmaintaining a stable future power supply. By combining next-generation developments such as
IGCC and IGFC with CO2 emission reduction technologies using conventional boilers as describedin this paper, MHI hopes to promote a wide range of technological improvements for coal-fired
power generation and offer clean-coal utilization technology harmonizing with global
environmental priorities.

References
1.
2.

Hashimoto, E. et al., Latest Technology of Highly Efficient Coal-Fired Thermal Power Plants and Future
Prospects, Mitsubishi Heavy Industries Technical Review Vol. 45 No. 1 (2008)
Kobayashi, Y. et al., Advanced Technology of Biomass Energy Utilization, Mitsubishi Juko Giho Vol. 40
No. 4 (2003)

Authors
Yoshiyuki Wakabayashi
General Manager
Boiler Engineering Department
Power Systems Headquarters

Tokushi Maruta
Deputy General Manager
Boiler Engineering Department
Power Systems Headquarters

Masahiko HokanoManager
Boiler Engineering Department
Power Systems Headquarters

Yuuichirou Kitagawa
Manager
Boiler Engineering Department
Power Systems Headquarters

Hiroshi Suganuma
Manager
Boiler Engineering Department
Power Systems Headquarters

Koutaro FujimuraManagerNagasaki Research & Development
Center, Technical Headquarters