Mixed feedstock process options for gasification technology depend on the nature of the technology.

7.4.4.1 Combustion

There are basically three process configurations for co-combusting coal and biomass [1]. The most popular option is direct co-firing where biomass and coal are fed together in the same combustor. This is because with this method an existing coal power plant can be converted to a co-firing plant with a rela­tively low financial investment. In this method, both coal and biomass are either fed together or separately in the same combustor. When they are fed together, biomass feed can either be prepared jointly with coal or prepared separately and then injected in the combustor using the coal injection and burning system. When they are injected separately, a new and dedicated bio­mass feed preparation and sometimes also burning equipment is used. Thus there are three separate subconfigurations for feed preparation and burning within a co-firing system. Ultimately, the common combustor unit produces steam from both coal and biomass for power generation.

Many different types of biomass can be co-fired with coal. These include wood, residues from forestry and related industries, agricultural residues, and various biomass in refined forms such as pellets (RDF). Energy crops except oil, sugar, and starch can also be used for co-firing [1]. Constraints in the use of co-firing originate from feedstock properties. Raw biomass has high moisture content and low bulk and energy densities compared to coal, a low ash melting point, higher chlorine and oxygen content, and a hydro­philic and nonfriable character. The constraints related to co-firing include fuel preparation, handling and storage, possible decrease in overall effi­ciency, deposit formation (slagging and fouling) agglomeration, corrosion or erosion, and ash utilization. The degree of these constraints depends on the quality and percentage of biomass in the fuel blend, type of combustor used, co-firing configuration of the system (as mentioned above), and the prop­erties of coal. The importance of these problems increases with increased biomass/coal ratios and the use of poor quality biomass without a dedicated biomass preparation infrastructure.

Biomass pretreatment can help alleviate several concerns. As mentioned earlier, leaching, pelletizing, and torrefaction are preferred pretreatment methods; however, they can be expensive. Another interesting pretreatment option is fast pyrolysis to produce pyrolysis oil of high energy density. This oil can be mixed with coal and the slurry can be injected in the combus­tor. This method, however, requires a dedicated infrastructure for transport, storage, and feeding as well as a separate conversion unit. Herbaceous bio­mass is presently not considered a suitable biomass for co-firing. It can prob­ably be used with pretreatment by torrefaction and pelletization.

The second method is parallel co-firing (hybrid system) where coal and biomass are fed into separate combustors (with a separate set of operating conditions) producing steam that can be combined in a common header before being sent to the turbine for power generation. The advantage of this method is that ash coming from coal and biomass is kept separate and can be used separately, thus allowing the use of biomass with high chlorine and alkali content. The disadvantage of this method is that it is more expensive, and it may require a larger capacity for the steam turbine. This method is also particularly popular in the paper and pulp industries.

The third and final method is indirect co-firing. In this method biomass is gasified in a separate gasifier or combustor and the gases are injected to the coal combustor thus providing additional heat for the combustion pro­cess. The method once again keeps the ash from coal and biomass separate, but the overall process is more expensive than methods one and two. This method with pregasification is currently practiced in a number of demon­stration plants in Austria, Finland, and the Netherlands [1].