1.8. Poly-generation of heat, power and biofuel

As described in section 2, the dual fluidized bed gasification technology integrates a circulating fluidized bed combustion unit and a bubbling fluidized bed steam gasifier to provide on-situ indirect heat supply for the endotermic gasification process. In this case thermal capacities of the both units have to be balanced, which is not easy to control, especially bearing in mind rather indefinite biomass properties. The only useful product of such a system is a producer gas.

Thunman et al. [19] proposed to retrofit existing CFB boilers integrating an indirect BFB gasification unit into the circulation loop of bed material of the boiler instead or in parallel with an external BFB particle cooler (see Figure 21).

Hot bed material entrained from the boiler is so transferred to the gasifier providing the heat for the production of a nearly nitrogen free product gas. Non-gasified char is returned together with the bed material into the boiler and converted. Biomass can be fed into both sections; the boiler and the gasifier. The gasification is separated from the boiler via two loop seals and a particle distributer, directing particles either back to the boiler or into the gasification section. For that reason the CFB boiler can be operated even after the retrofit independently, just like before, or in combined combustion/gasification mode. Such a retrofit is an easy way to extend the potential of a CFB boiler towards poly-generation (heat, power, biofuel) and enter new markets. Given that CFB boilers are of much larger thermal capacity (tens to hundreds MW) than gasifires, and the balance between heat supply and heat consumption in the intgrated gasifier is always fulfilled.

A characteristic feature of CFB combustors is their fuel flexibility. This means that an integrated poly-generating system can be used for efficient and reliable co-conversion of different fuels. For instance, a boiler fired with low grade coal or any other problematic fuel can supply heat to gasification of highly wet biomass, providing substitution of external steam for indirect biomass conversion and getting back a flow of absolutely dry highly reactive charcoal with re-circulating bed material from gasifier. Thus, the combustion process of a problem fuel becomes sustainable. Furthermore, a huge potential heat supply from the boiler stabilizes gasification of any problem biomass and waste.

The proposed poly-generation concept has been implemented at Chalmers University of Technology, Gothenburg, Sweden, where a 2-4 MW_th indirect gasification unit was integrated into the material circulation loop of the existing 8-12 MWth circulating fluidized bed boiler in 2007 [19, 20]. Figure 20 shows the Chalmers CFB boiler before (left) and after (right) being retrofitted.

Figure 20. Chalmers CFB boiler alone (left) and with integrated BFB gasifier (right) [18]

Figure 21 shows flow sheet of the integrated poly-generating system. A possibility is foreseen to use steam, N₂ or flue gases as fluidizing media in the gasifier in order to test different conditions of biomass conversion from pyrolysis to steam reforming.

Figure 21. Flow sheet of the Chalmers CFB boiler with integrated BFB biomass gasifier: the marked area comprises existing cyclone return leg and particle cooler 10, and newly installed particle distributer 9 and gasifier 11

So far producer gas from the gasifier is supplied into the boiler furnace as auxiliary fuel after having been analyzed. This is one of Europe’s (except from the Güssing gasifier in Austria) largest pilot plants for gasification of biomass. On this gasification plant a number of smaller subdevices are tested and sub streams extracted from different places. In this way the subprocesses can be analysed. This is a part of the preparation GoBiGas which uses the principles from the Chalmers gasifier on the first 20 MW plant to be built in Gothenburg for production synthesis natural gas from biomass residue.

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