Tham khảo tài liệu 'biofuels, solar and wind as renewable energy systems_benefits and risks episode 2 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 16 Developing Energy Crops for Thermal Applications 415 high-ash grass pellets with high silica contents can also produce a low-density ash that retains the shape of the former pellet. As an example consider that the bulk density of reed canary grass ash has been assessed to be half that of wood ash Paulrud 2004 . Thus the residual ash leftover after burning grass pellets in the 3-5 ash range can take up to 10-20 times the volume of the ash from burning ash wood pellets. To burn 3-5 ash grass pellets ash pans will need to be modified in smaller appliances to create larger ash collecting areas. Combustion units burning high-ash grass pellets will require more frequent cleaning and may experience increased operational problems such as automatic shutdown of the combustion appliance if the ash builds up into the combustion chamber. Conversely silica is generally not a problematic element for commercial combustion boilers. Paulrud et al. 2001 working with reed canary grass found that the relative content of K and Ca in the ash was more important for agglomeration and clinker formation than the silica content. High-ash agro-pellets approximately 5 ash with low to moderate levels of aerosol forming compounds are readily burned in most coal boiler technologies and greenhouse producers in Canada are now installing multifuel boilers capable of burning both coal and agro-pellets. A comprehensive strategy will be required to reduce the silica content of grasses to make them more convenient for combustion applications and to improve their energy content. The understanding of silica uptake into the plant is improving amongst agronomists and plant breeders. The main cultural factors which appear to have potential to reduce the silica content are soil type production region photosynthetic cycle of the biomass crop and the choice of grass species and variety. The main breeding strategies to reduce silica content include increasing the stem to leaf ratio of the species and .