Scalable dual fluidised bed system for fast pyrolysis of woody biomass

Abstract

Pyrolysis of biomass is the thermochemical conversion process whereby the long lignocellulosic polymers in biomass are cracked into several higher-value products such as bio-oil, bio-char and combustible non-condensable gases (NCG). Fast pyrolysis in particular is aimed at maximising the yield of crude liquid bio-oil, with the production of bio-char and NCG as co-products. Since a large quantity of under-utilised biomass is produced in the forestry sector annually, as by-product from harvesting, this sector has shown particular interest in this process. Furthermore, the continuing drive for renewable and sustainable energy production, particularly of drop-in liquid biofuels, has urged the development of such technology on a commercial scale. The main purpose of this investigation was to evaluate the technical feasibility and performance of the scalable dual fluidised bed (DFB) reactor system designed and constructed at the University of Pretoria by Swart in 2012. The sub-objectives of this study were as follows: • Biomass pre-treatment equipment was implemented to ensure that the physical characteristics of the biomass feedstock meet the pyrolysis process requirements. • The scalable DFB reactor system, including all sub-systems and ancillary equipment, was commissioned to ensure satisfactory operation of the complete system. • Continuous, steady-state experimental runs were conducted to produce fast pyrolysis products in the scalable DFB reactor system. • The fast pyrolysis products were quantified and characterised to evaluate the technical feasibility of the DFB reactor system. • A material and energy balance was conducted over the pyrolysis fluidised bed (PFB) reactor to quantify its performance. Eucalyptus grandis raw material, as received from Sappi Southern Africa’s Ngodwana mill, was successfully converted to bio-oil, bio-char and NCG in the scalable DFB reactor system. Fast pyrolysis was conducted at a pyrolysis temperature of 500 °C, a vapour residence time of 4 s and a sawdust feed rate of 2.0 kg/h. The PFB reactor temperature could be controlled easily, at the desired setpoint (500 °C), by continuously circulating hot solids between the two bubbling fluidised beds. The excellent temperature control of the PFB reactor makes the DFB system a suitable reactor system for the fast pyrolysis of biomass on a commercial scale. At these PFB reactor conditions the yield of fast pyrolysis products, on a dry feedstock basis, was determined as 36.3, 14.0 and 49.7 weight % for bio-oil, NCG and bio-char respectively. High-value process heat, in the form of hot flue gas (450–500 °C), was produced in the combustion fluidised bed Although the crude liquid bio-oil contained highly oxygenated compounds (including organic acids, water, alcohols, esters, sugars, aldehydes, ketones, furans, pyrans and phenolics) it may be utilised for heat generation when co-fired with conventional fossil fuels, including heavy furnace oil. However, the scalable DFB reactor system allows for integrated catalytic fast pyrolysis, which would enable catalytic cracking of the biomass feedstock, and the subsequent pyrolysis vapours, to selectively produce deoxygenated bio-oil compounds, compatible with conventional refinery streams. The DFB reactor system allowed easy separation of bio-char from the pyrolysis vapours by means of the bio-char cyclone. The bio-char had a high heating value of only 17.0 MJ/kg because of an unexpectedly high inorganic content of 54.4 weight % on a dry basis. However, 77.0 weight % of the inorganics were identified as entrained silica sand fines. Notwithstanding the entrained silica fines, the bio-char carbon content was determined as approximately 55 weight % on a dry basis, which would result in a high heating value of approximately 29 MJ/kg. Combustible NCG (including carbon monoxide, methane, ethane, ethylene, acetylene and propene) were produced as co-product from the fast pyrolysis of E. grandis sawdust in the DFB reactor system. The high heating value of the NCG was estimated at 7.3 MJ/kg or 8.3 MJ/Nm3 . Furthermore, it was demonstrated that both the solid bio-char residue and NCG could be combusted in the CFB reactor to supplement its energy demand. At the sawdust feed rate of 2 000 g/h and silica sand circulation rate of 50 kg/h, the production rate of pyrolysis products was estimated at 687.8, 265.2 and 940.0 g/h for bio-oil, NCG and bio-char respectively. However, only 13.0 g/h of bio-char was collected from the bio-char cyclone, with the balance (i.e. 927.0 g/h) understood to have been transferred to the CFB with the silica sand heat carrier. The recycle rate of the NCG was determined as 7 689.7 g/h. The total energy input from the feedstock and recycled NCG was determined as 150 W, while the energy supplied to the PFB by means of the hot silica sand was determined as 3 889 W. The pyrolysis reaction energy demand, at the feed rate of 2 000 g/h, was determined as 1 000 W. The pyrolysis reactor freeboard temperature was found to be much lower than the fluidised bed temperature (± 195 °C vs. ± 500 °C) as a result of heat loss. Therefore, the energy output from the pyrolysis products was determined as only 344 W. The overall heat loss from the PFB reactor was estimated at a very high 2 696 W, which implies that approximately 69% of the total energy supplied to this reactor by means of the hot silica sand was dissipated to the surrounding atmosphere. From a heat loss evaluation, it was concluded that the biomass throughput could be increased by as much as five to ten times by mitigating the heat loss.