Fuels and chemicals from rice straw in Egypt

Abstract

In Egypt rice cultivation in the river Nile Delta produces large amounts of rice straw as residue. Assuming that about 20% were used for other purposes about 2.8 Mt were left on the fields for burning within a period of 30 days to get quickly rid of leftover debris. The resulting emissions give a significant contribution to the air pollution called the Black Cloud. A suitable technology for the use of rice straw is an important contribution to reduce air pollution and gives a significant contribution to use biomass as renewable energy in Egypt. Based on that fact rice straw was studied as possible feedstock for the BIOLIQ process. The BIOLIQ process which is under development at the Forschungszentrum Karlsruhe (Germany) is a two-step process for the production of chemicals, fuels or electricity from lignocellulosic biomass with high ash content. In the first step the biomass is liquefied by fast pyrolysis. The pyrolysis char and the pyrolysis liquid are mixed to form a slurry. In the second step the pumpable slurry is used to produce syngas in a pressurised entrained flow gasifier. After gas cleaning and conditioning the syngas is used for the production of fuels, chemicals and electricity as by-product. In this work rice straw is studied as a possible feedstock for fast pyrolysis and slurry preparation. First the Egyptian rice straw was chemically characterised. Ultimate analysis of rice straw showed an elemental composition of 47,8% C, 6,0 %H and 45,3% O, (daf). According to the elemental composition the HHV was calculated to be about 18,1 MJ/Kg (Dulong) up to 20,5 MJ/kg (Ebeling). Based on the experimental HHV 15,3 MJ/kg (rice straw dry, 18% ash) the HHV on a moisture and ash free basis was calculated to be 18,7 MJ/kg. According to the literature the lignin, hemicellulose and cellulose contents of rice straw were about 12%, 28% and 60%, respectively. The rice straw was chopped and sieved (2 mm sieve) and most of the straw dust was separated before use. The pyrolysis process was studied on laboratory and PDU (process demonstration unit) scale. During pyrolysis lignin gave the main contribution to the char formation. TGA pyrolysis experiments showed about 60% of volatiles, 20% fixed carbon and 20% of ash on the average (dry). These results were very close to the results of fast pyrolysis experiments in a PDU with a twin-screw reactor. The chlorine was focussed in the pyrolysis char up to a temperature of ∼500°C. If the pyrolysis temperature exceeded 500°C the chlorine was released and its concentration in the chars decreased. The outside of the rice straw stems was completely covered with silica forming a close protection layer. SEM-studies show that the pyrolysis took place only at the inner sides of the stems. Therefore rice straw had to be chopped very carefully in order to brake up short stem pieces and to split them up. Additionally the SEM pictures confirmed the volatility of chlorine at temperatures above 500°C. Condensate viscosity, char porosity and the particle size spectrum as well as slurry production techniques play an essential role for slurry preparation and result in practicable condensate/char-weight ratio variations between about 3 to 1.3 for worse and unfavourable situations. With a special colloid mixer, a robust tool well known for the preparation of a very homogeneous cement grout it was possible to prepare a pumpable char/tar mixture with a high mixing ratio of almost 50% by weight.