The gasification of biomass and municipal solid waste (MSW) differ in many ways from the gasification of coal, petcoke, or conversion of natural gas to syngas. This section 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 straightforward, performance depends greatly on the unique characteristics of the biomass or MSW feedstock. 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.
|For more on this topic please see Benchmarking Biomass Gasification Technologies for Fuels, Chemicals and Hydrogen Production [PDF]|
Examples of Biomass and MSW Gasification Facilities
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, with many varying shapes, sizes, densities, 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 results is advantageous.
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. 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.
Co-gasification of Coal & Biomass
Co-gasification of coal and biomass blends is of considerable current interest, stemming from a number of benefits that can result from the approach relative to conventional gasification of straight coal:
Co-gasification also works to advantage by reducing the typical high tar content resulting from biomass gasification of straight biomass.
The basic operations involved in co-gasification of coal and biomass mixtures are illustrated in Figure 1.
Some of the complications arising from co-gasification are apparent from this figure. First, instead of a single feedstock preparation scheme, it is usually necessary to have separate preprocessing operations for coal and biomass. The typically high moisture-content biomass is usually not just dried, but also torrefied (which involves heating to temperatures typically ranging between 200 and 320°C in absence of oxygen, at which point the biomass undergoes a mild form of pyrolysis) and possibly compacted, which greatly improves the quality as a feedstock for either fuel use or gasification. Also, size reduction of both the coal and biomass to uniformly-sized particles is required for optimum gasification.
Co-gasification reactions and transformations share aspects of those for coal gasification and biomass gasification, but also include some synergistic effects that are not definitively described. However, in general the basic approach to co-gasification technology choice is the same as for conventional coal gasification, with the feedstock properties and the desired utilization of the syngas largely determining what type of gasifier to use. If the syngas is to be used for electricity generation, a downdraft fixed-bed gasifier is a good choice because it releases gas at high temperature with low impurities. Fluidized-bed gasifiers may not be the best choice for some co-gasification applications, because defluidization of the fluidized bed can occur due to agglomeration of low melting point ash present in the biomass, along with clogging of the downstream pipes due to excessive tar accumulation.
It has been observed that entrained flow gasifiers should be investigated for the co-gasification of coal and biomass, given their capacity to accept different types of feedstock, the uniform temperature profile inside the reaction zone, short reactor residence time, and high carbon conversions, all of which are of increased importance to addressing the issues associated with co-gasification.
Product gas compositions are influenced by both the type of biomass co-gasified, as well as its proportion in the feed mixture. Generally, higher H2 content results from greater biomass inclusion; in particular, lignin in woody biomass seems to boost H2 yield in syngas. A wide range of proportions of coal and biomass may be possible for given applications, but the optimum is a complex function of the type of coal used, type(s) of biomass, gasifier type and operating conditions, desired syngas composition, etc., not to mention the available quantities of the biomass which may be considerably less than the coal available.
Beside the gasifier, the type of gasifying agent is also important. The use of steam as a gasifying agent as opposed to air assists the water-gas shift reaction and produces H2-rich syngas. Also, the use of catalysts affects syngas production. An interesting example is a study of the co-gasification of Puertollano coal mixed with pine, petcoke, and polyethylene (PE). Findings were that the use of dolomite catalysts helped in increasing the gasification rate along with reducing hydrogen sulfide (H2S) generation and increasing sulfur and chlorine retention in the solid phase.
Syngas cleanup of co-gasification derived syngas includes the same operations needed for conventional coal gasification, including particulate removal, sulfur removal, etc., but may be more complicated than for coal gasification or biomass gasification alone, because both those species present in raw coal-derived syngas (sulfur and mercury) and those present in elevated amounts from biomass gasification (tars and alkalis) may need to be addressed.
In the future, co-gasification of coal and biomass holds promise as a way of substantially reducing the carbon intensity of gasification, to utilize low-cost opportunity biomass fuels such as wood waste and high-energy content, marginal land biomass crops such as switchgrass, and to enhance gasification processes by optimizing syngas quality and increasing throughput and output.