The use of a mixed feedstock in combustion, pyrolysis, gasification, plasma technology, liquefaction, and supercritical technology is becoming more and more prevalent and popular. Going forward, there are, however, sev­eral issues that need to be addressed to make the use of mixed feedstock in each of these technologies more economical, environmentally accept­able, and technologically feasible. Some of these issues are briefly outlined below.

1. Although there are several successful commercial co-firing instal­lations, the effects of increased biomass/coal ratio and the use of low-quality biomass on the effectiveness of co-firing need to be further examined. Some of the issues include transportation, han­dling, storage, milling and feeding problems, slagging and fouling due to deposit formation, agglomeration, corrosion or erosion, and ash utilization. There are several methods to address these issues. One method is the cleaning of deposits by soot blowing or exchange of agglomerated materials. It is also possible to avoid the effects of agglomeration and deposit formation by adding chemicals to reduce corrosion and increase the ash melting point. It is also possible to create an independent infrastructure for feeding, milling, storage, and conveying biomass just like one for coal. More expensive paral­lel and indirect firing modes where both feed treatments and gas­ification infrastructure can be separate at different levels are also options that need to be further examined for specific types of mixed feedstock. For each combustion process and mixed feedstock, an optimum logistic detail for biomass transportation, storage, and pre­treatment needs to be examined.

2. Unlike combustion, co-gasification is more complex because the process of gasification can be used to generate fuels, chemicals, and materials along with heat and electricity. Syngas generated via gas­ification has many downstream applications, but it needs to be pure and of the right composition. It is generally accepted that a future gasification reactor, even for a mixed feedstock, is most likely to be a pressurized oxygen-blowing entrained bed reactor [6]. For this type of reactor, an optimum burner design for solid biomass feeding and optimum gasification conditions with respect to biomass particle size (1 mm or less), maximum efficiency, maximum heat recovery, minimum flux use, minimum inert gas consumption, complete con­version, production of biosyngas with low CH4, and no tars will need to be obtained for each type of mixed feedstock [6]. In a slagging gas­ifier the ash and flux are present as a molten slag that protects the inner wall against high temperature. The slag must have the right viscosity and flow behavior at the temperature in the gasifier and its behavior as a function of gasification temperature, biomass ash properties, and selected flux should be known. The reactor should be operated at the lowest temperature possible to get high cold gas efficiency and oxygen consumption [6]. In a gas cooling and puri­fication system, maximum energy efficiency is desirable. Multiple cooling steps should be further optimized. The reactor should also be capable of processing feedstock of a wide variety of properties and produce no waste. The bottom ash should be recoverable as non — leachable slag with a value as construction material whereas fly ash needs to be used for mineral recycling [6]. This goal will require a continuous and steady flow of mixed feedstock for safe and steady operation. An appropriate pretreatment of feedstock, which may include leaching, torrefaction, and pelletizing, and an optimum process configuration are critically important to achieve the desired process objectives.

As shown in Section 7.5 [6], there are numerous process options possible. Milling of woody biomass and pressurization in a piston compressor with negligible inert consumption and feeding to the gasifier with a screw feed system (or a stamet type of system) is one option. The use of flash pyrolysis for grassy and straw biomass mate­rials to produce bioslurry that can be fed to the gasifier is another option. The torrefaction can allow the generation of fine and homo­geneous particles (with low energy consumption). This can then be homogeneously mixed with coal and pressurized by a piston com­pressor and fed to the reactor by a screw or a pneumatic feeding system. Finally, all feed materials can be gasified in a fluidized bed gasifier and the product gas then be processed in an entrained bed gasifier. These are some of the options when using coal and biomass of different properties. For other mixtures, which include materi­als such as oil shale, plastics, rubber tires, tar sand, and the like, additional modifications of the above-described options may be required.

3. Just as with combustion and gasification, the use of a plasma reac­tor for a variety of feedstock needs to be optimized. Plasma reactors often generate valuable chemicals along with fuel gas. The reactor design and operating conditions will need to be such that maximum production of valuable chemicals and fuel gas is possible. More experience with a larger scale plasma process is needed.

4. For high severity pyrolysis, the above-described statements for gas­ification apply. For low severity pyrolysis, a focus needs to be on the liquid production rate and its quality. As shown earlier, there is no synergy between pyrolysis of coal and biomass although there is a significant synergy between pyrolysis of biomass and plastics. Thus, an appropriate optimization of the operating conditions for the pyrolysis process will need to be examined based on the nature of the feedstock mixture. More experience with a larger scale pyroly­sis operation using mixed feedstock is needed.

5. An optimization of liquefaction of mixed feedstock is challenging and interesting. There seems to be very little synergy between liq­uefaction of coal and biomass. There is, however, synergy between liquefaction of biomass and polymeric products. This is true for both organic (hydrogen donor) solvents as well as water. More data on synergy for other types of feed mixture are needed. At the present time, just as in direct coal liquefaction, the liquefaction process for the mixture is, in general, economically unattractive for the produc­tion of high-quality fuels. More research is needed to test the lique­faction process which uses less pressure, less gaseous hydrogen, and recycled solvent. More markets for lower quality liquefaction prod­ucts need to be found.

6. Supercritical technology, both for gasification and liquefaction, is becoming more popular. The application of this technology using water, carbon dioxide, and some alcohols for mixed feedstock needs to be fully explored. The technology may be very attractive for selected feedstock mixtures and products.

Until now at larger scales, co-utilization of waste and coal has received considerably more attention than co-utilization of coal and biomass. This has been true for all thermochemical processes. The pricing factor for the raw material and government subsidies often make the use of waste for energy and products more attractive. New concepts of sustainable resource management and enhanced landfill management will make thermochemical conversion of mixed waste more popular. Investigated waste has been (a) municipal solid waste that has had minimal presorting, (b) refuse derived fuel that has had significant pretreatment such as mechanical shredding and screening, and (c) shredded rubber tires, paper and pulp waste, and plastic waste.