One of the more successful continuous high pressure steam pre-treatment processes for lignocellulosic biomass use has been the StakeTech reactor developed by Stake Technology, Ontario, Canada. The StakeTech reactor is primarily composed of a stainless steel horizontal pressure vessel which is designed to withstand operational pressures of 31 bar (i. e. 450 psig) [13]. Using an upstream screw conveyor the digester (i. e. the part of the vessel where the main stream treatment occurs) is fed continuously by the movement of the lignocellulosic biomass through the compression tube which also serves to build up the vessel pressure. The densifled biomass resulting from the feeder compression tube upon entering the reactor is transferred to a conical choke which facilitates a ‘break up’ of the dense biomass plug resulting in a scattering of undensified biomass materials onto the retention screw [13]. Once fed, the retention screw transports the biomass towards the discharge end in such a manner that the precise retention time is achieved to meet the required processing conditions. The post treated material is then conveyed to the discharge valve (which is made up of a rotary ball valve with its opening timed according to the desired production rate), using the discharge screw at the end of the digester [13]. The discharge valve further aids in providing an explosion effect on emptying the pre-treated samples while readying it for the subsequent processing stages. The StakeTech continuous technology has been widely used in research and has been reported to reach full commercialisation [13].

Apart from the batch and continuous pre-treatment processes previously outlined, various patents, that is [38-41] on the use and optimisation of the catalysed and auto-hydrolytic steam pre-treatment technologies, as well as improving the purity and accessibility of the desired process outputs from a wide variety of lignocellulosic biomass inputs have been granted and are continuously being researched.

3.2 Conclusion

In this chapter, different types of steam pre-treatment have been described, and examples of commercial implementations of steam pre-treatment have been given. It is clear that for biomass to play the future role of major supplier of carbon for energy and products, full utilisation of all parts of the biomass is necessary. This necessitates technologies that go beyond high quality, cellulosic feedstocks, targeting the more abundant yet significantly more complex lignocellulosic fractions. For this purpose, steam pre-treatment offers an efficient yet environmentally benign approach to opening the biomolecular structure, and preparing the substrate for subsequent separation or conversion. Uniqueness for steam pre-treatment is that it is able to address most of the limiting factors for subsequent digestion (conversion) and at the same time turn the feed into a pumpable slurry or paste, even without the addition of catalysts or other additives, which call for subsequent separation or effluent treatment. Being a thermal process, it can be possible to design the entire plant with a significant amount of heat integration, reducing the need for external supply of heat for steam generation.