Utilization of energy from residual biomass of rice straw in Egypt

Resumen

The utilization of biomass as a renewable source of energy is important from the energetic as well as from the environmental point of view. Egypt produces a considerable amount of biomass from agricultural residues, municipal solid wastes, animal wastes, and sewage sludge, with a total theoretical energy content of 416.9×1015 J. The amount of dry biomass produced from agricultural crop residues is about 12.33 million tons/year, and 63.75% of this amount is produced from rice straw. In consequence, rice straw could be used as a renewable fuel for heat and power generation, contributing to: (i) the replacement of fossil fuels; (ii) the reduction of CO2 emissions according to the renewable energy framework in Egypt; and (iii) the prevention of the pollution caused by open burning of straw. In relation to the thermal conversion process of rice straw, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have showed that it could cause some operating problems such as those caused by the presence of high contents of potassium, chlorine and silicon in the raw material. This composition produces high quantity of ashes with important sintering and slagging tendencies that negatively affect to thermal conversion systems. In order to reduce these problems and improve the combustion behavior, samples of rice straw were washed with water as a pretreatment, which resulted in a reduction of undesirable inorganic compounds related to ash problems. The use of rice straw as a fuel implies high costs associated with handling, storage and transportation because of the low value of its bulk density. Pelletisation process consists of a mass densification that reduces these problems, increases the traditionally low thermal efficiency of biomass, and provides homogeneous size, which facilitates an automatic or semi-automatic treatment. In general, the quality of the pellets depends on the properties of the feedstock and quality management of the manufacturing process. Therefore, in order to determine the best conditions to produce high-quality pellets, the pelleting of rice straw was performed under different operating conditions related to: (i) the moisture content of feeding material (12, 15, and 17%); (ii) the use of starch as an additive with percentages of 1% and 2%; (iii) the operating temperature (<50°C and >50ºC); and (iv) three flat dies with different diameter/compression length (6/20, 6/24 and 8/32 values in mm). The quality properties of the pellets manufactured such as durability, hardness, moisture content, dimensions, and pellet and bulk density were determined according to the norm UNE-EN ISO 17225-6:2014 about Solid biofuels - Fuel specifications and classes - Part 6: Graded non-woody pellets. The results showed that the moisture content of the feeding material, the die hole size, the starch additive content, and the operating temperature, represented important factors for the improvement of pellet qualities in the majority of the cases. The pellet durability and bulk density were the most indicative parameters for pellet quality and were significantly affected by most of the different factors. Finally, in relation to the use of rice straw to produce liquid biofuels (especially ethanol) it can be considered as a highly feasible option if there is access to proper low-cost conversion and fermentation technologies. In consequence, the pelleting process could reduce the costs related to handling, storage and transportation, but it could also affect the fermentation process. The effect of pelleting on hydrolysis and saccharification of rice straw as preliminary steps for bioethanol production was studied by using Streptomyces consortium in microbial hydrolysis. Results have showed that the saccharification rate reached up to 40% and the Streptomyces consortium increased the efficiency of saccharification by 55% in the case of pellets with starch additive. Moreover, the applied consortium was efficient and had the ability of penetrating and disintegrating the interior layers of rice straw.