Gasification. Steps away

Biomass, MSW and gasifier technology

Gasification of biomass and municipal solid waste (MSW) differ in many ways from the conversion of natural gas to syngas¹. This post looks at differences, approaches used to gasify municipal solid waste biomass and MSW and provides 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 natural gas. In addition, the non-uniformity of biomass and MSW and the variability of their specific compositions over time require flexible and robust gasifiers.

Gasifiers for biomass and MSW

National Energy and Technology Labs (NETL) in 2002 published information about innovative (at the time) and operational biomass/msw 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. Key findings of their study are summarised below.

**Bubbling Fluidised-Bed** (BFB) gasifiers, a gasifier characterised by larger cross-section (relative to) shorter height, lower fluidisation velocities and denser beds, are the most demonstrated of the biomass gasification technologies reviewed by NETL.

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, pulverised 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 clean-up 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 Fluidised-Bed (CFB) gasifiers, generally characterised by smaller cross-section versus taller height profiles - and higher fluidisation velocities - have not been demonstrated with biomass to the extent of BFB.

In fact, literature reviews 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, carbon dioxide (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 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, fluidised, or circulating fluidised 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.

Indirectly heated gasifiers are more complicated (and have 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.

[Disclaimer: The contents of this post were based on articles published by NETL. The illustration and the shortening of text is courtesy Environment Tobago].