Cofiring
Encyclopedia
Cofiring is the combustion
of two different types of materials at the same time. One of the advantages of cofiring is that an existing plant can be used to burn a new fuel, which may be cheaper or more environmentally friendly. For example, biomass
is sometimes cofired in existing coal
plants instead of new biomass plants. Cofiring can also be used to improve the combustion of fuels with low energy content. For example, landfill gas
contains a large amount of carbon dioxide
, which is non-combustible. If the landfill gas
is burned without removing the carbon dioxide, the equipment may not perform properly or emissions of pollutants may increase. Cofiring it with natural gas
increases the heat content of the fuel and improves combustion and equipment performance. As long as the electricity
or heat
produced with the biomass
and landfill gas
was otherwise going to be produced with non-renewable fuels, the benefits are essentially equivalent whether they are cofired or combusted alone. Also, cofiring can be used to lower the emission of some pollutants. For example, cofiring biomass with coal results in less sulfur
emissions than burning coal by itself.
of two different fuels in the same combustion system. Fuels can be solid fuels, Liquid fuels
or gaseous, and its nature either fossil
or renewable. Therefore, use of heavy fuels assisting coal power stations may be considered technically co-firing. However the term cofiring is used in the present technological framework to designate combined combustion of two (or more) fuels sustained in the time, as a normal daily practice.
The interest for cofiring and the use of this term sprung in the 80's in the U.S. and Europe, and referred specifically to the use of waste
solid residues (paper, plastic, solvents, tars, etc.) or biomass
in coal power stations that were initially designed for combustion of sole coal, and attempted, because of existence of those new opportunity fuels, to carry out a combined combustion in order to increase benefit margins. As a matter of fact, this interest on cofiring has grown in the last decade mainly due to the increasing social concerns on global warming
and greenhouse gas
(GHG) emissions. Consequences of this concern are the new policies on energy and environment aiming at reducing emissions. Cofiring is regarded as a great opportunity for replacing coal (solid fossil fuel
) used for power generation easily with renewable fuels (biomass
) with low costs and a direct repercussion in the decrease of greenhouse gas emissions. During the last decades research has provided very diverse solutions for cofiring biomass
in coal power stations with a limited impact in efficiency, operation and lifespan.
Under this present context the definition of cofiring could be as next: together use of two (or more) fuels being the principle fossil and the secondary from other nature (renewable or residual) into the boiler originally designed for the fossil fuel
, either using same combustion system or additional devices.
and grate firing
). Cofiring in cement kilns is already a quite widespread solution for valorization of waste
materials mostly, as well as for biomass
. The iron industry (blast furnace
) and domestic sector (coal stoves) are also sectors where cofiring could be implemented.
The use of a secondary fuel (biomass
or waste
) replacing a share of the original fossil fuel may involve from trivial changes in the facility to a complete retrofitting with important reforms. Modifications will depend on the characteristics of the fuels, the original combustion technology, the plant layout and the type and location of auxiliary systems. The percentage of original fuel replaced, also known as cofiring rate (either expressed in mass or in energy basis) is furthermore a definitive parameter limiting the technical solutions valid for a specific plant.
The co-firing systems, according to the current state of the art and the future perspectives, can be classified into direct and indirect cofiring technologies. The former refer to those systems where combustion of both fuels takes place at the same combustion device or into the same boiler simultaneously. The secondary fuel (biomass, waste) may be either mixed with coal before the combustion starts or fed by a separate device, e.g. specific biomass burners. Indirect cofiring, on the contrary, separates the combustion of both solid fuels, though Combustion Gasses may be mixed afterwards.
Direct biomass cofiring systems entail advantages of simplicity and economics. However direct cofiring systems are also more sensitive to variations in fuel quality and heterogeneity. Additionally other problems limit the rate of secondary fuel replacing the original fossil fuels. In example ash deposition (fouling
and slagging) and corrosion usually increase with the use of biomass and wastes replacing coal, what may shorten the lifespan of diverse devices in contact with Combustion Gasses like superheaters, heat exchangers, selective catalytic reduction
(SCR), etc. Direct cofiring systems include next technological solutions:
Indirect cofiring systems imply usually more complex and expensive solutions, but they reduce usually problems related with corrosion, fouling, slagging, etc. This, a priori, allows cofiring rates larger than direct systems, that is, larger percentages of coal substituted by biomass or waste. In addition, indirect cofiring systems are in general better for fuel mixtures where secondary fuel may include potential contaminants like heavy metals or other dangerous inorganic compounds.
Main indirect cofiring systems are listed next:
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
of two different types of materials at the same time. One of the advantages of cofiring is that an existing plant can be used to burn a new fuel, which may be cheaper or more environmentally friendly. For example, biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
is sometimes cofired in existing coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
plants instead of new biomass plants. Cofiring can also be used to improve the combustion of fuels with low energy content. For example, landfill gas
Landfill gas
Landfill gas is a complex mix of different gases created by the action of microorganisms within a landfill.-Production:Landfill gas production results from chemical reactions and microbes acting upon the waste as the putrescible materials begins to break down in the landfill...
contains a large amount of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, which is non-combustible. If the landfill gas
Landfill gas
Landfill gas is a complex mix of different gases created by the action of microorganisms within a landfill.-Production:Landfill gas production results from chemical reactions and microbes acting upon the waste as the putrescible materials begins to break down in the landfill...
is burned without removing the carbon dioxide, the equipment may not perform properly or emissions of pollutants may increase. Cofiring it with natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
increases the heat content of the fuel and improves combustion and equipment performance. As long as the electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
or heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
produced with the biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
and landfill gas
Landfill gas
Landfill gas is a complex mix of different gases created by the action of microorganisms within a landfill.-Production:Landfill gas production results from chemical reactions and microbes acting upon the waste as the putrescible materials begins to break down in the landfill...
was otherwise going to be produced with non-renewable fuels, the benefits are essentially equivalent whether they are cofired or combusted alone. Also, cofiring can be used to lower the emission of some pollutants. For example, cofiring biomass with coal results in less sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
emissions than burning coal by itself.
Origin of cofiring and meaning according to present technology framework
Cofiring (also referred usually as co-firing or co-combustion) is the combustionCombustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
of two different fuels in the same combustion system. Fuels can be solid fuels, Liquid fuels
Liquid fuels
Liquid fuels are those combustible or energy-generating molecules that can be harnessed to create mechanical energy, usually producing kinetic energy; they also must take the shape of their container...
or gaseous, and its nature either fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
or renewable. Therefore, use of heavy fuels assisting coal power stations may be considered technically co-firing. However the term cofiring is used in the present technological framework to designate combined combustion of two (or more) fuels sustained in the time, as a normal daily practice.
The interest for cofiring and the use of this term sprung in the 80's in the U.S. and Europe, and referred specifically to the use of waste
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...
solid residues (paper, plastic, solvents, tars, etc.) or biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
in coal power stations that were initially designed for combustion of sole coal, and attempted, because of existence of those new opportunity fuels, to carry out a combined combustion in order to increase benefit margins. As a matter of fact, this interest on cofiring has grown in the last decade mainly due to the increasing social concerns on global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
and greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
(GHG) emissions. Consequences of this concern are the new policies on energy and environment aiming at reducing emissions. Cofiring is regarded as a great opportunity for replacing coal (solid fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
) used for power generation easily with renewable fuels (biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
) with low costs and a direct repercussion in the decrease of greenhouse gas emissions. During the last decades research has provided very diverse solutions for cofiring biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
in coal power stations with a limited impact in efficiency, operation and lifespan.
Under this present context the definition of cofiring could be as next: together use of two (or more) fuels being the principle fossil and the secondary from other nature (renewable or residual) into the boiler originally designed for the fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
, either using same combustion system or additional devices.
Types of cofiring
The concept of cofiring is quite simple. It consists in the use of two or more fuels inside the same combustion device. It is applicable to all kind of combustion systems traditionally used for power generation (pulverized fuel, fluidized bed combustionFluidized bed combustion
Fluidized bed combustion is a combustion technology used in power plants. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective...
and grate firing
Grate firing
Grate firing was the first combustion system used for solid fuels. It now is used mainly for burning waste and biomass, but also for smaller coal furnaces.* Capacities 0.3 to 175 MWth in industry and CHP...
). Cofiring in cement kilns is already a quite widespread solution for valorization of waste
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...
materials mostly, as well as for biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
. The iron industry (blast furnace
Blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...
) and domestic sector (coal stoves) are also sectors where cofiring could be implemented.
The use of a secondary fuel (biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
or waste
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...
) replacing a share of the original fossil fuel may involve from trivial changes in the facility to a complete retrofitting with important reforms. Modifications will depend on the characteristics of the fuels, the original combustion technology, the plant layout and the type and location of auxiliary systems. The percentage of original fuel replaced, also known as cofiring rate (either expressed in mass or in energy basis) is furthermore a definitive parameter limiting the technical solutions valid for a specific plant.
The co-firing systems, according to the current state of the art and the future perspectives, can be classified into direct and indirect cofiring technologies. The former refer to those systems where combustion of both fuels takes place at the same combustion device or into the same boiler simultaneously. The secondary fuel (biomass, waste) may be either mixed with coal before the combustion starts or fed by a separate device, e.g. specific biomass burners. Indirect cofiring, on the contrary, separates the combustion of both solid fuels, though Combustion Gasses may be mixed afterwards.
Direct biomass cofiring systems entail advantages of simplicity and economics. However direct cofiring systems are also more sensitive to variations in fuel quality and heterogeneity. Additionally other problems limit the rate of secondary fuel replacing the original fossil fuels. In example ash deposition (fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
and slagging) and corrosion usually increase with the use of biomass and wastes replacing coal, what may shorten the lifespan of diverse devices in contact with Combustion Gasses like superheaters, heat exchangers, selective catalytic reduction
Selective catalytic reduction
Selective catalytic reduction is a means of converting nitrogen oxides, also referred to as with the aid of a catalyst into diatomic nitrogen , , and water, . A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed...
(SCR), etc. Direct cofiring systems include next technological solutions:
- Co-milling (in case of solid fuels): blending of primary (coal) and secondary (biomass or waste) fuel, combined milling (in original system) and injection through the coal burners (or feeding system)
- Co-feeding: separate treatment of primary and secondary fuels (milling in case of solid fuels), and incorporation of secondary fuel to the main flow. In case of solid fuels the mixture takes place downstream the coal mill.
- Combined burner: fuels are treated separately (milled in case of solid fuels) and transported to the burner, where primary fuel uses the original ports and secondary fuel uses new ports or unused ducts. In this case, though feeding does not involve fuel physical mixing, combustion stages takes place simultaneously and with similar aerodynamics to original design.
- New burners: fuels use independent feeding lines. Primary fuel uses original injection system, whereas secondary fuel is transported to specific dedicated burners or inlet ports penetrating into the combustion chamber. New burners (injection systems) may replace former burners of primary fossil fuel, or may be installed in new positions in the combustion chamber. This option may involve the use of different combustion systems. For example in a pulverized coal power station a grate firing system may be installed at the bottom of the combustion chamber, though this solution is rare.
Indirect cofiring systems imply usually more complex and expensive solutions, but they reduce usually problems related with corrosion, fouling, slagging, etc. This, a priori, allows cofiring rates larger than direct systems, that is, larger percentages of coal substituted by biomass or waste. In addition, indirect cofiring systems are in general better for fuel mixtures where secondary fuel may include potential contaminants like heavy metals or other dangerous inorganic compounds.
Main indirect cofiring systems are listed next:
- Separated burning: burning of secondary fuel in a separate boiler or system and introduction of flue gases downstream the radiant section of the original boiler.
- Coupled plant: separate burning in a new boiler specially designed and built for firing the secondary fuel. Original and new system couple their heating fluid circuits. Combustion gases are not mixed and Exhaust gasExhaust gasExhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
must be treated separately. - GasificationGasificationGasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures , without combustion, with a controlled amount of oxygen and/or steam...
systems: the secondary fuel is transformed into gas (with heating value) by means of a gasifier. The resulting syngasSyngasSyngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
is either directly or with a previous treatment, injected in the original combustion chamber or boiler through new dedicated ducts. - PyrolysisPyrolysisPyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
: biomass is transformed into a mixture of gas, bio-oils and char by means of pyrolysisPyrolysisPyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
. Fractions may be separated and introduced into the boiler in different sites.
Advantages of cofiring
Use of biomass in co-firing incorporate additional environmental, socio-economic and strategy advantages regarding the use of biomass in dedicated biomass plants. In case of waste residues there are no additional benefits, however the combustion of waste may change the emissions regulations to satisfy more strict regulations. For example, limits in emissions from environmental regulations for large scale combustion facilities are more permissive than regulations for incineration plants. Except for the previous drawback related to waste cofiring, the following advantages are common for waste and biomass cofiring:- Specific investment (per unit of installed power): reduced in comparison with conventional biomass facilities since plant using fossil fuel already exists and only diverse modifications are required
- Power generation with better efficiency: generally biomass power plants produce electricity with relative low efficiency (18 to 22%) compared with the huge coal units (32 to 38%) with optimised cycles given the economy of scale
- Flexible operation: original plant can operate still at 100% load with fossil fuel. Co-firing facility is less sensitive to seasonality in biomass production and to biomass availability and price
- Carrot for development of biomass markets: diverse European countries have proven the promotion of co-firing is a key for the development of biomass markets as well as for the creation of expertise on biomass handling and combustion
External links
- USDOE United States Department of Energy
- EUBIA - European Biomass Industry Association
- IEA - International Energy Agency
- ACVCOCO Project: Determination of the real potential of greenhouse emissions reduction in Spain by means of the co-firing implementation
- CIRCE - Co-firing of low rank coal and biomass: a chance for biomass penetration in the renewables