Cofiring - Types of Cofiring

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 combustion 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:

• 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 gas must be treated separately.
• Gasification systems: the secondary fuel is transformed into gas (with heating value) by means of a gasifier. The resulting syngas is either directly or with a previous treatment, injected in the original combustion chamber or boiler through new dedicated ducts.
• Pyrolysis: biomass is transformed into a mixture of gas, bio-oils and char by means of pyrolysis. Fractions may be separated and introduced into the boiler in different sites.