ReviewDrying of biomass for second generation synfuel production
Introduction
In the planning of biorefineries and production of liquid biofuels for transport via synthesis gas route, several biomass materials, such as wood, forest residues, bark, straw, energy crops, peat, and agricultural residues, are used. In addition to conversion, the pre-treatment of feedstocks is important including transfer, storage, chipping, crushing and drying. For this chain there are many different techniques with variable cost structures. The pre-treatment depends on the feedstock and the concept. In all cases drying will probably be the most challenging step.
Important issues in drying are energy efficiency, emissions, heat integration and dryer performance. In synthesis gas production the feedstocks must be dried to below the 30 wt% moisture content, preferably to about 15 wt%, and in pyrolysis to below 10 wt%. Drying to low moisture contents is problematic and has not been optimised for biomass conversion processes. At this stage for the upcoming demonstration plants it is of utmost importance to survey the current drying options and develop effective, preferably low-cost concepts.
Many types of dryer or drying process are encountered in industry. The dryers applied for biofuels have been presented e.g. in references [1], [2], [3], [4]. The reader is referred to the literature covering drying theory and practice, e.g. [5]. There are different direct or indirect drying techniques utilising air, flue gas, or steam as drying medium. Forced evaporative drying requires large amounts of energy as well as the provision of often expensive equipment. The impact of drying operation on overall plant efficiency can however be reduced by integration, making use of surplus energy streams within the process.
The present survey aims to define and evaluate the most promising 2–3 optimised concepts for biomass pre-treatment/upgrading scheme in the production of second generation synfuels for transport. The concepts will be defined for two different cases including 1) large-scale wood-based gasification synfuel production, and 2) small-scale pyrolysis process based on wood chips and miscanthus bundles. The most promising commercially available drying processes will be reviewed considering the technologies where water is removed by evaporation. Emphasis will be put on the applications, operational factors and emissions of dryers. The biomass considered is derived either from forestry or agricultural residues, or from dedicated plantations of short rotation coppice (SRC) or herbaceous crops, usually comminuted to the required size.
Section snippets
Material physical characteristics
The biomass material in question will usually have moisture content on delivery to the plant in the range 30–60 wt%, depending on type, location, time of harvest and period of storage after harvest. Particle size requirements are dictated largely by the bio-energy process, but the biomass at the point of delivery to the drying process is likely to be in large particulate form – e.g. chips or chunks with a largest dimension in the range 10–80 mm. Rotary dryers may accept large and variable
Dryer types for biomass feedstocks
The dryers for biofuels can be classified, for example, according to the drying medium (e.g. flue gas dryers and superheated steam dryers), or to the heat exchange used (conductive/convective or indirect/direct dryers, respectively). The most common types of flue gas dryers are rotary and flash dryers. The commercial scale steam dryer types are tubular dryer, fluidized bed dryers and pneumatic conveying dryers.
Dryer integration
It is usually foreseen that production of synfuels will be integrated to existing pulp and paper mills or to biomass power and CHP plants. The key issue when selecting the drying process to a gasification or pyrolysis plant is the integration of the drying process with the energy infrastructure of the main process. This enables the utilisation of available steam supply or low value surplus energy streams like low pressure steam and hot water. Also, depending on the drying process, residual
Conclusions
Biomass feedstocks need often to be dried prior to the conversion process, such as pellet production, pyrolysis and synthesis gas production. A number of different dryer types may be suited for the purpose, and the final choice should be made after careful consideration of operational and economic factors specific to the application.
At small scales costs are likely to dictate either a batch perforated-floor technology using heated air, or a simple band conveyor using exhaust gas or heated air.
Acknowledgments
The study was carried out within the Network of Excellence “Overcoming Barriers to Bioenergy” (Bioenergy NOE), sponsored by the EC DG Research (SES6-CT-2003-502788) [34].
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