Summary

There is an increasing demand for solid biofuels. Since wood for energetic utilization becomes scarce recently, straw is recognized as an alternative green fuel. However, its application for combustion still suffers from several drawbacks. There are several characteristics that have to be considered if straw is to be used as solid biofuel.

• Energy density and transportation properties: The heating value of straw is in the same range as for woody fuels (about 15 MJ/kg). However, straw has much lower densities than wood (beech ~750 kg/m³, straw bulk 65-80 kg/m³) and thus poor transportation properties. For that reason, straw is mostly baled (density 110-200 kg/m³), or in some cases is pelletised (density 400-600 kg/m³). Doing so, energy density is significantly increased though remains lower than for fuel alternatives (straw (bulk) 750-1050 MJ/m³, straw baled 1500-2400 MJ/m³, beech 11000-15000 MJ/m³, brown coal 18000 MJ/m³, heating oil 36000 MJ/m³). Thus larger storing capacities are required.
• Input: The morphology demands adapted input technologies that can handle the stalks. Alternatively, an adequate cutting technology has to be applied.
• Ash content, ash melting characteristics: Compared to wood, agricultural plants show some unfavourable fuel properties. Herbaceous biomass has a significantly higher ash content. Thus furnaces and boilers have to be equipped with appropriate ash discharge components. Adequate ash disposal has to be considered. Furthermore, straw ash mostly has lower melting points increasing the risk of slagging. For that reason, straw combustion technologies comprise advanced control units to control the fuel bed temperature by limiting fuel and air supply and/or cooling of the grate.
• Emission: Another critical parameter during straw combustion is the emission of small particles and harmful pollutants (carbon monoxide (CO), nitrogen oxides (NO_x), volatile organic compounds VOCs, dioxins, furanes). Small particle emission is usually met by using filters and cyclones. Emission of other pollutant can be reduced by appropriate process control (e.g. preheating, air excess rate, temperature, flue gas recirculation, post-combustion).

In Germany rather strict regulations are implemented for biomass combustion (1. BImschV and 4. BImschV). Straw can be used as fuel only for furnaces and boilers >15 kWth. Other European countries have less strict regulations. In Denmark fairly loose regulations are implemented to promote straw combustion technologies. Consequently, the utilization of straw for commercial and district heating is the most advanced in Denmark.