Feedstock thermal conversion systems

Biomass is similar to some coals with respect to total ash content as discussed in Chapter 3, but because of the diversity of biomass, several species and types have relatively low ash and also low sulfur contents. Woody biomass is one of the feedstocks of choice for thermal gasification processes. The ash contents are low compared to those of coal, and the sulfur contents are the lowest of almost all biomass species. Grasses and straws are relatively high in ash content compared to most other terrestrial biomass, and when used as feedstocks for thermal conversion systems, such biomass has been found to cause a few fouling problems. 

Preliminary Economic Overview of Large-Scale Thermal Conversion Systems Using Wood Feedstocks... 

As alluded to in Chapter 8, the ideal biomass feedstock for thermal conversion, whether it be combustion, gasification, or a combination of both, is one that contains low or zero levels of elements such as nitrogen, sulfur, or chlorine, which can form undesirable pollutants and acids that cause corrosion, and no mineral elements that can form inorganic ash and particulates. Ash formation, especially from alkali metals such as potassium and sodium, can lead to fouling of heat exchange surfaces and erosion of turbine blades, in the case of power production systems that use gas turbines, and cause efficiency losses and plant upsets. In addition to undesirable emissions that form acids (SOx), sulfur can... 

While all pyrolysis oil production reactor systems produce similar materials, each reactor produces a unique compound slate. The first decision, especially for a potential chemical or fuel producer, rather than a reactor developer, is to determine what products to make and which reactor system to use. The operating parameters of any reactor system designed to produce pyrolysis oil, especially temperature, can be altered to change the pyrolysis oil product composition and yield. Different feedstocks will produce different pyrolysis oil compositions and by-products, e.g. amorphous silica from rice hulls or rice straw, fatty acids from pine. Finally, feedstock pretreatment and/or catalysis, or reactor-bed catalysis can be used to improve specific product yields (7). Reactor system developers need to examine what they can produce and make this information available to chemical manufacturers and suppliers/owners of biomass feedstocks. This assumes that analysis of die entire liquid product from thermal conversion can be made, including quantitative analysis for any compounds that are being considered for recoveiy. Physical characterization - pH, viscosity, solids content, etc.is also needed. However, what can be produced is of no value, if it cannot be recovered or used economically. This involves examining the trade-offs between yield and current commercial value, recovery costs, and potential commercial value,... 

Gasification and pyrolysis are the thermal conversion processes available for the thermal treatment of solid wastes. As shown in Figure 8.3, different by-products are produced from the application of these processes and different energy and matter recovery systems can be used to treat these products [16, 27]. Both pyrolysis and gasification produce three different phases a solid phase (char, 5-25 wt%), liquid phase (tars, 10-45 wt%) and gas phase [14, 22, 28]. The main disadvantages of plastic pyrolysis and gasification are the necessity to control the chloride content in the feedstock and the risk of bad fluidisation because of particle agglomeration [29]. 

The feedstock is sent to a first reactor where it is partially converted into the products then one of the products is recovered through a selective membrane separation module, while the retentate is sent to the next step or recycled to the first module. By means of a heat recovery system, the operating temperature can be reduced before the membrane unit, in order to ensure a thermal level suitable for the correct operation of the membrane unit, and then increased again before the second reactor up to the values required to support the reactions. These reaction-separation steps can be repeated until the desired natural gas conversion is achieved. 

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