Gasification in Detail - Types of Gasifiers - Gasifiers for Special Applications
Biomass and Municipal Solid Waste (MSW) Gasification
The gasification of biomass and municipal solid waste (MSW) differ in many ways from the gasification of coal, petcoke, or natural gas. This page will discuss these differences, the technology used to gasify biomass and MSW, and give a brief overview of some operating plants.
Characteristics of Biomass and MSW
While the gasification technologies used with biomass or MSW are fairly standard (and already discussed in the section on Types of Gasifiers), performance depends greatly on the unique characteristics of the biomass or MSW feedstock. Biomass and MSW as feedstock are discussed in other sections (biomass and MSW), but in general, these feedstocks have much higher moisture content and less heating value by volume than coal. In addition, the non-uniformity of the feedstocks and the variability of the specific compositions over time require flexible and robust gasifiers.
Gasifiers for Biomass
A 2002 NETL study on various biomass and MSW gasifiers analyzed published information about demonstration and operating biomass gasifiers. Operating conditions, syngas composition, other required systems, and other parameters were compared to the optimum conditions for electricity, fuel, chemicals, and hydrogen production to determine which gasifier technologies best fit a certain product application. Some significant findings of this study are summarized below.
- Bubbling Fluidized-Bed (BFB) gasifiers, discussed in general here, are the most demonstrated of the biomass gasification technologies reviewed. The BFB technology has been operated over a wide range of temperatures, pressures, throughput, and a variety of biomass types. Fuel, chemicals, and hydrogen production benefits from high temperatures, like those seen in coal gasification, because at temperatures over 1,200-1,300 °C little or no tar, methane, or higher hydrocarbons are formed, while syngas (hydrogen [H2] and carbon monoxide [CO]) production is maximized. Several BFB gasifiers have been operated at the high pressures (>20 bar) that would be advantageous for fuel and chemical synthesis. While this eliminates the need for a compressor following the gasifier, it does necessitate a more complex feed system. BFBs may require the feed to be chopped, pulverized, or otherwise reduced in size, and would most likely need to be dried to allow for the higher operating temperatures.
The choice of oxidant—some combination of air, oxygen, and/or steam—has a substantial effect on the output syngas composition. Air introduces nitrogen, which dilutes the product gas and is detrimental to synthesis processes. For this reason, an oxygen plant is usually required. Varying steam to oxygen ratio input is a way to adjust the H2/CO ratio in order to match synthesis requirements. For example, Fischer-Tropsch transportation fuel synthesis using iron catalysts requires an H2/CO ratio of around 0.6, optimally, while for cobalt catalyst a ratio of 2 would be preferred. Methanol production would be favored with an H2/CO ratio of around 2 and for hydrogen production it should be as high as possible. If higher temperatures cannot be achieved inside the BFB gasifier, tar cracking might be required. Typically, though, this is not the case and therefore gas cleanup is somewhat minimal for synthesis applications. The study finds that BFB gasifiers are among the lowest capital cost options for biomass gasification and, all things considered, BFB gasifiers are quite suitable for fuels, chemicals, and hydrogen production.
- Circulating Fluidized-Bed (CFB) gasifiers, described in general here, have not been demonstrated to quite the extent of BFB. In fact, the literature surveyed showed very few tests at elevated pressure and all with temperatures below 1000 °C. While Bubbling Fluidized-Bed gasifiers have been tested (at the time of the article) up to 35 bar, CFBs have only been tested up to 19 bar. Like BFB gasification, particle sizes would need to be reduced and feedstock dried. Probably the biggest issue with CFB is the lack of demonstrations with pure oxygen and/or steam, which greatly limits the confidence in the technology for synthesis applications. From the information available, CO2 levels in the syngas are low, as are H2/CO ratios, because the lack of steam means the water-gas-shift reaction is suppressed.
- Fixed-Bed (FB) gasifiers, described here, have not been demonstrated over a large range with biomass. This gasifier design tends to produce large quantities of either tar or unconverted char and therefore have not been extensively pursued. However, they are able to handle heterogeneous feedstock like MSW and so have a use for waste-to-fuel or waste-to-power.
- Indirectly Heated gasifiers, which can be entrained, fluidized, or circulating-bed gasifiers, are at an early stage of development and have not been tested over a wide range for application suitability. In fact, as of June 2002, these units had only been tested at atmospheric pressure. They are more complicated (and with higher capital costs), owing to a separate combustion chamber, but are capable of producing a syngas with a very high heating value, which is important for power/heat applications. One advantage is that they do not require oxygen or air for gasification, which means no oxygen plant is needed (lower capital cost and efficiency losses) and no nitrogen dilution. These units tend to have higher methane and other hydrocarbon yields, which would be a problem for synthesis applications, but beneficial for heat/power generation. For fuels or chemicals synthesis, the hydrocarbons can be steam reformed or partially oxidized, usually through high steam addition rates which promote water-gas-shift activity. Primarily, though, these systems need to be studied further.
Gasifiers for Municipal Solid Waste
As noted above, FB gasifiers are able to handle heterogeneous feedstock like MSW. This is important because, as noted in the section on MSW characteristics, MSW can vary widely in composition (imagine the contents of a dumpster —shapes, sizes, density, and composition) and requires a flexible gasifier. Atmospheric pressure gasification reduces complexity compared to feeding a highly non-uniform feed at pressure. If possible, avoiding costly feed preparation systems like pulverization is valuable.
Plasma gasification, which uses an extremely hot electrical plasma arc to break down MSW into simple gases and leftover solids, is currently being considered for many large MSW gasification facilities (examples discussed in a section below). High voltage and current electricity produces a plasma arc between two electrodes. While this requires a substantial amount of energy, the syngas product can be used in a turbine to potentially generate more electrical power than required. The plasma arc can reach temperatures as high as 13,900 °C which can break down difficult feedstocks into simple constituent gas molecules and a solid slag byproduct.
Biomass and municipal solid waste can pose problems to gasification system designers. Both present issues for feed systems as these feedstocks are largely heterogeneous in their delivered state. Some biomass, such as sawdust from lumber mills, can be in a condition suitable for many existing feed systems, while others, like most MSW, would require extensive preparation or feed system customization. Biomass and MSW also may have characteristics like higher moisture content which may necessitate pre-gasification drying. Ash contents can also vary widely, meaning the gasifier must be able to handle potentially high levels of ash. Essentially, biomass and MSW gasification requires flexibility in design to handle non-uniform feeds.
Examples of Biomass and MSW Gasification Facilities