Gasification and Pyrolysis

Process configuration is more important and more complex for gasification and pyrolysis than combustion because the products are used for down­stream upgrading as well as power generation. Process configuration has many options [6]: (a) gasify both coal and biomass together in the same gas­ifier and have a unified downstream operation, (b) gasify both coal and bio­mass in separate gasifiers and then combine the product gas for a unified downstream operation, (c) have separate syngas cleanup steps but combine syngas coming from coal and biomass at some downstream step, (d) use different forms of feed (dry or liquid) for biomass, or (e) share a common or separate air separation unit to produce oxygen for the biomass and coal gasifiers. All these options lead to numerous process configurations. Ratafia — Brown et al. [6] considered six possible configurations and their advantages and disadvantages. The analysis of Ratafia-Brown et al. [6] is briefly sum­marized below. This analysis will allow others to consider other possible configurations.

Configuration 1: Co-feeding coal and biomass to the gasifier as a mix­ture, either in dry or slurry form.

Configuration 2: Co-feeding biomass and coal to the gasifier using sep­arate gasifier feed systems, either in dry or slurry form.

Configuration 3: Pyrolyzing as-received biomass followed by co-feed­ing pyrolysis char and coal to the gasifier and separately feeding pyrolysis gas to the syngas cleanup system.

Configuration 4: Biomass and coal are co-processed in separate gasifi­ers followed by combined syngas cleanup.

Configuration 5: Biomass and coal are co-processed in separate gasifi­ers followed by separate syngas cleanup trains, and the syngas feeds are combined prior to sulfur and CO2 removal unit operations.

Configuration 6: Same as 4 and 5 but share common air separation unit (ASU) for oxygen feed to the separate gasifiers.

Ratafia-Brown et al. [6] have given an extensive assessment of these six process options. The following paragraphs briefly summarize their assess­ments of each option.

According to Ratafia-Brown et al. [6], the advantages of the first two con­figurations are: (a) they take advantage of economy of scale; gasification can proceed with or without biomass; (b) in Configuration 1, a separate biomass feed system is not required and it reduces gasifier complexity via the use of a single feedstock injection; (c) in Configuration 2, a separate biomass system can be designed without affecting the coal system; and (d) both Configurations 1 and 2 depend on a single syngas processing system. The disadvantages of the first two configurations are: (a) gasifier design should avoid tar production or the syngas system should treat tars, (b) the common gasifier will need to be larger than a single coal processing gasifier because of the low energy content of biomass, (c) control of the syngas composition (H2/CO ratio) for subsequent FT processes may become an issue with a sin­gle gasifier processing both coal and biomass, and (d) the value of biomass ash which contains phosphate and potash for fertilizer purposes will be reduced due to the addition of coal ash.

As shown by Ratafia-Brown et al. [6],the advantages of Configuration 3 are: (a) it is thermodynamically more efficient than off-site biomass process­ing, (b) the pyrolysis gas can be used as a feed to syngas, (c) and the pyroly­sis process can handle biomass with different properties and char can be directly injected to the gasifier with coal, although this will depend on the gasifier type and design and properties of char. The disadvantage of this configuration is that it may not be cost-effective depending on the scale of the gasification facility and level of biomass consumed.

According to Ratafia-Brown et al. [6], the advantages of Configurations 4, 5, and 6 are: (a) they allow maximum flexibility in coal and biomass properties and an independent control of biomass gasification for syngas production, (b) they take advantage of the economy of scale of syngas processing, par­ticularly in Configuration 4, (c) depending on the type of gasifier employed, they allow separate recovery of biomass ash, and (d) for Configurations 5 and 6, independent control of two gasification processes but taking advan­tage of economy of scale for the air separation unit.

The disadvantages of Configurations 4, 5, and 6 are: (a) they do not take care of coal gasification economy of scale; (b) parallel biomass gasification increases investment cost, design, and operational complexity; (c) a tar and particulate removal system may be required depending on the type of bio­mass gasifier used; and (d) more land area and parallel ash handling and storage systems for coal and biomass are required.

Although the above discussion is mainly focused on a mixed coal and bio­mass feedstock, the same thought process can be applied to the different types of mixed feedstock (including different types of biomass) with other variable properties. Earlier in Section 7.3, various process configurations and feed preparation options for biomass with different types of properties as identified by Ratafia-Brown et al. [6] were briefly outlined. These options are also valid for the mixed (coal and biomass, coal and waste, etc.) feedstock.