Improving the quality of bio-oil by fast pyrolysis of acid leached and torrefied Pinus radiata.
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Fast pyrolysis is a thermochemical process used to convert biomass into a liquid fuel termed bio-oil. The high organic acid, water, oxygen, and inorganic content in bio-oil make it unstable and cause corrosion issues. Consequently, it is widely accepted that bio-oil must be upgraded to be considered a realistic candidate as a large scale transport fuel. Upgrading bio-oil through catalytic cracking and hydrotreating has been extensively researched, but there are high costs and low yields associated with both techniques due to the difficulty in upgrading such a diverse mixture of compounds. Pretreating biomass prior to pyrolysis was investigated in this thesis to improve the bio-oil quality to simplify current upgrading techniques or for direct use as a marine fuel. Three catalytic compounds naturally inherent in biomass were identified to cause undesirable reactions during pyrolysis; these were inorganics, organic acids (acetyl compounds), and water. A pretreatment sequence incorporating both acid leaching and torrefaction was developed to reduce/remove these compounds from biomass prior to pyrolysis. A fast pyrolysis reactor with a maximum capacity of 1 kgh-1 of feed biomass was designed, constructed, commissioned, and used for pyrolysis. The pyrolysis reactor was a fluidised bed with nitrogen as the fluidising gas and silica sand as the fluidising medium. Char was separated in a high efficiency cyclone and bio-oil vapours were condensed in a series of three shell and tube condensers. Remaining aerosols in the vapour stream were collected in an electrostatic precipitator and filter. Pyrolysis at 500 °C of Pinus radiata yielded 46.9±0.5 wt% (dry basis) bio-oil, which contained 3.5±0.4 wt% acetic acid and 24.0±1.2 wt% water. Acid leaching targeted the biomass‟s inorganic content. The acidic liquor produced during torrefaction was rich in acetic and formic acid; the viability of leaching biomass with this solution was demonstrated by comparing the leaching efficiency of acetic and formic acid to nitric, sulfuric, and hydrochloric acid. Similar leaching efficiencies were achieved for the organic and mineral acids. Therefore, the optimal leaching conditions were summarised as leaching at 30 °C with 1% acetic acid for 4 h. This reduced the biomass‟s inorganic content from 0.41±0.04 to 0.16±0.02 wt% without altering its structural composition. Pyrolysis of biomass leached at the optimal conditions yielded 54.6 wt% (dry basis) bio-oil, which contained 1.9±0.1 wt% acetic acid and 17.1±1.3 wt% water. This indicates that secondary reactions during pyrolysis were reduced in the inorganic limited environment, which was confirmed by the higher levoglucosan yield of 7.83% for bio-oil from leached biomass, opposed to 2.30% for bio-oil from raw biomass. Torrefaction targeted the biomass‟s moisture and acetyl content, and was optimised between 220 and 290 °C. For torrefaction at 290 °C for 20 min, the acetyl content in the biomass was reduced from 1.51 to 0.43 wt% and the oxygen content decreased from 43.1 to 35.7%. However, the mass loss during torrefaction was significant at 38.5 wt%. Therefore, the optimal torrefaction temperature was 270 °C for 20 min; pyrolysis of the torrefied biomass yielded 46.1 wt% bio-oil, which was equivalent to an overall yield of 38.9 wt% bio-oil, taking the into account the mass loss during torrefaction. The bio-oil contained 0.6±0.4 wt% acetic acid and 6.1±0.3 wt% water. It was slightly enriched in levoglucosan (3.64%) and aromatics compared to bio-oil from raw biomass. Torrefaction increased the char yield at the expense of biooil due to stable carbon-carbon crosslinks formed by dehydration of biomass polymers during torrefaction. Acid leaching and torrefaction were integrated and optimised. The optimal pretreatment sequence was summarised as 1% acetic acid leaching at 30 °C for 4 h, followed by torrefaction at 270 °C for 20 min. Next, main pyrolysis operating conditions (pyrolysis temperature, cyclone temperature, and silica sand loading in the fluidised bed) were optimised for raw and pretreated biomass in terms of the pyrolysis yield and operability. The pyrolysis temperature was optimal at 450 °C for both raw and pretreated biomass. Pyrolysis of pretreated biomass required 75 g sand in the fluidised bed to prevent char agglomeration, while pyrolysis of raw biomass only required 25 g. The cyclone was optimal at 400 and 425 °C for pyrolysis of raw and pretreated biomass respectively. Pyrolysis of raw biomass yielded 55.3±2.5 wt% bio-oil, 25.0±1.0 wt%, char, and 12.5±1.2 wt% non-condensable gas, while the corresponding yields from pyrolysis of pretreated biomass were 57.8±1.7 wt% bio-oil, 23.7±2.6 wt%, char, and 11.5±0.7 wt% non-condensable gas. Mass balances gave a 7.20±0.27 and 6.17±0.49% discrepancy for pyrolysis of raw and pretreated biomass respectively. The discrepancy was due to limitations in measuring the pyrolysis products. Integrating acid leaching and torrefaction as biomass pretreatments significantly reduced undesirable heterogeneous and homogeneous reactions during pyrolysis. This improved the bio-oil‟s quality in terms of the organic acids (2.46±0.13 to 0.16±0.05%), water content (16.8±1.6 to 3.6±0.3 wt%), aldehydes (1.58±0.04 to 0.50±0.10%), high molecular weight compounds (10.2±4.6 to 4.2±0.4%), inorganics (0.162±0.056 to 0.091±0.030 wt%), and stability. The oxygen content of the bio-oil was only reduced on a wet basis as torrefaction reduced the bio-oil‟s oxygen content, but acid leaching increased it. An economic analysis indicated 10.39 MW of bio-oil production from raw biomass required a capital investment of $NZ 53,800,000 and could produce bio-oil at $NZ 29.87 per GJ. To produce 11.44 MW of bio-oil from pretreated biomass required a capital investment of $NZ 46,300,000 and could produce bio-oil at $NZ 29.67 per GJ. Both systems were competitive with price of No. 6 heavy fuel oil at $NZ 30.92 per GJ in the second quarter of 2015, but more expensive than Brent crude oil and WTI crude oil at $NZ 13.78 and 12.78 per GJ respectively. Energy balances indicated that non-condensable gases and a portion of the char would be sufficient to supply the heating requirements during pyrolysis, with an energy surplus of 2.69 and 4.47 MW for pyrolysis of raw and pretreated biomass respectively. It is recommended to investigate the use of calcium oxide in the fluidised bed to reduce the bio-oil‟s oxygen content, to further improve its stability and energy density. This study indicated that both acid leaching and torrefaction of biomass were required to limit homogeneous and heterogeneous reactions of pyrolysis vapours, which are catalysed by water, organic acids, and inorganics. Leaching is required as inorganics are highly catalytic during pyrolysis, but leaching is expensive; torrefaction reduces the cost of acid leaching by providing the leaching reagent and eliminating the need for biomass rinsing after leaching. Torrefaction also reduces the biomass grinding costs, which is required to offset the additional process costs for pretreating biomass. Pyrolysis of solely torrefied biomass is constrained by the high torrefaction temperatures required for significant bio-oil improvements, leading to low yields due to the mass loss during torrefaction and increased char formation during pyrolysis. Finally, the reduced thermal conductivity of dry torrefied biomass increases the time for secondary reactions with inorganics during pyrolysis, which become concentrated in torrefied biomass. Therefore, the integration of both pretreatments is required to produce a high quality crude bio-oil cost effectively.