A. Feedstock Composition Impacts

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 also form compounds that deactivate methanol synthesis catalysts, whereas chlorine can be transformed into toxic chlorinated organic derivatives as well as acids.

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.

The high moisture contents of aquatic and marine biomass species make it unlikely that they would be considered as feedstocks for thermal gasification processes. However, a few processes can be performed with aqueous slurries or do not require dry biomass feedstocks as described earlier. As harvested, aquatic and marine biomass species often have moisture contents greater than 90% of the total plant weight. In addition to the relatively high ash contents of herbaceous feedstocks, the nitrogen content is an important factor. Grasses are higher in protein nitrogen than woody feedstocks and can increase nitro­gen oxide (NO) emissions on gasification.

The compositions of wood compared to those of other potential biomass feedtocks make woody biomass a preferred feedstock for thermal gasification. Although not shown here, most woody biomass species, especially those indige­nous to the contiguous United States, are similar in composition. It is important to emphasize that quantitative ash analyses of biomass feedstocks sampled at the plant gate and from storage should be carried out periodically and some­times continually to provide real-time data needed for process control. There can be large differences in the amounts of specific mineral components in biomass.

A major mechanism of the fouling of heat exchanger surfaces with biomass feedstocks, particularly the straws and herbaceous residues, is the formation in the thermal conversion zone of low-fusion-point alkali metal salt eutectics such as the alkali metal silicates. The problems caused by these salts and the control methods for combustion and thermal gasification systems were discussed in Chapter 8. Several experienced designers of biomass gasifiers and the operators of commercial plants operated on biomass feedstocks have indicated that the problem is usually not severe with gasification systems, but can be with combustion systems. Temperature control to reduce slagging and the formation of molten agglomerates and equipment designs that avoid contact of the internals with hot gases that may contain low-fusion-point particulates are the preferred control methods for minimizing these problems. For biomass gasifiers that are used to supply fuel for gas turbines, the control methods are similar. Some biomass, although high in minerals, may be low in alkali metals. Fouling by sticky particulates is therefore much less with this type of feedstock.

Some gasification process designers claim to have developed proprietary gas processing systems that yield product gases from biomass gasifiers “cleaner than natural gas” using conventional desulfurization processes for sulfur re­moval and cyclones and proprietary filters to remove ash and char fines. Electrostatic precipitators are not used, and scrubbers are claimed to be optional for some of these systems. These statements are difficult to support without public dissemination of full-scale test results. They are probably true, however, because there are many emissions — and ash-removal systems that have been installed and effectively operated in large-scale commercial biomass combus­tion plants that meet all requirements. Some of these plants are designed to meet California’s stringent South Coast regulations. Much of this experience and technology can be drawn upon to design environmentally clean biomass gasification plants.

Many of the commercial or near-commercial biomass gasification facilities that have been built and operated use green or partially dried feedstocks in which the moisture content of the feedstock to the gasifiers is not specified. The steam-carbon reactions that occur are undoubtedly one of the main reasons for variation in product gas compositions from these systems. Since the carbon content of dry biomass is about 45 wt %, green wood contains about 2.2 kg moisture/kg of carbon. Table 9.10 shows the effects of the moisture content of poplar wood when gasified in an air-blown, downdraft gasifier. As the moisture content of the wood decreases from 34 to 13 wt %, thermal efficiency, product gas heating value, dry gas yield, and the proportion of the combustible components in the dry gas each increase. These data illustrate the importance of specifying feedstock moisture content. Feedstock dryers are essential for some biomass gasification plants depending on the feedstock’s moisture content and variation, as well as on the end uses of the product gases.