The use of demineralisation and torrefaction to improve the properties of biomass intended as a feedstock for fast pyrolysis

Abstract

Pre-treatments of biomass were investigated to reduce its undesirable properties which may affect the quality of fast pyrolysis bio-oil. A pre-treatment sequence was developed in this study to incorporate both biomass demineralisation and torrefaction. Demineralisation was performed by dilute acid leaching, primarily to reduce the inorganic concentration in raw biomass, whereas torrefaction targeted a reduction of the carboxyl, moisture and oxygen content. The liquid produced during torrefaction was recycled back as the leaching reagent for demineralisation. This solution contained dilute organic acids; therefore, the viability of leaching with organic acids (acetic and formic acid) compared to commonly used mineral acids (sulphuric, nitric and hydrochloric acid) was validated. Synthetic leaching solutions reduced the inorganic content in raw biomass from 0.41 wt% to 0.14 wt% when leached with 1% formic acid and to 0.16 wt% when leached with 1% acetic acid, which was comparable to leaching with the mineral acids. Recycled torrefaction liquid that contained other acidic compounds in small quantities reduced the inorganic content to 0.14 wt%, suggesting it is effective to use the recycled torrefaction liquid as the leaching solution. From the experimental results, the optimal conditions for biomass torrefaction were 260 °C for 20 min to minimise the char formation during pyrolysis, based on the increase in the acid-insoluble fraction of the biomass. However, the torrefaction temperature may be increased to 280 °C if further reductions in acetyl and oxygen content are required. Higher temperatures are associated with severe biomass loss and the initiation of hydrogen loss. It should be noted that even at 280 °C, the oxygen reduction is minimal. If oxygen reduction is the principal target when pre-treating biomass, it is suggested that torrefaction alone is not a suitable method to obtain bio-oil with a low oxygen content due to the low pyrolysis yields obtainable. This study demonstrated that the combined use of demineralisation and torrefaction as biomass pre-treatments has the ability to decrease the inorganic, acetyl and moisture content of biomass, which reduces undesirable catalytic reactions during fast pyrolysis to improve the quality of bio-oil produced.

Effects of Pretreatments of Napier Grass with Deionized Water, Sulfuric Acid and Sodium Hydroxide on Pyrolysis Oil Characteristics

“…The corresponding removal efficiency recorded was 90% and 74% respectively. This suggests that considerable amount of the Na and K in the NGS are present in form of water soluble salts which are generally salt of chlorides, nitrates, carbonates and phosphates[50][51][52]. For other elements, slight declines were observed in calcium (Ca), aluminum (Al), iron (Fe) and silicon (Si) content after the treatment.…”

“…For other elements, slight declines were observed in calcium (Ca), aluminum (Al), iron (Fe) and silicon (Si) content after the treatment. The Ca, Al and Fe minerals can be said to be in cation form bound to reactive sites of the NGS biomass which can be removed substantially through ion exchange[50][51][52]. The small reduction in Si content suggest that NGS biomass is made up of two forms of silica, the amorphous silica which is normally deposited on the outer walls of epidermal cells and dissolved in water as Si(OH) 4 .…”

Advanced biofuels production by upgrading of pyrolysis bio‐oil

“…The pyrolysis of torrefied biomass generally yields higher biochar and gas amounts, mainly to the detriment of bio‐oil, although with an improvement in the quality of the latter. Likewise, Wigley et al demonstrated that the combined use of demineralization and torrefaction for biomass pretreatments has the ability to decrease the inorganic, acetyl, and moisture content of biomass, reducing undesirable catalytic reactions during pyrolysis and improving the bio‐oil quality in terms of higher organics but lower water, acids, and oxygen content.…”

Characteristic, Thermochemical Behaviors and Kinetic of Demineralized and Torrefied Empty Fruit Bunches (EFB)

“…From previous research, feasible pre-treatments on EFB have been investigated such as demineralization and torrefaction in order to enhance the properties of biomass prior to thermal conversion [11,12,13]. Demineralization is an efficient pretreatment that can be carried out using water, acid or alkaline solution to eliminate the inorganic compounds that assist the secondary reaction during the pyrolysis.…”

“…This process is a mild version of slow pyrolysis with the aim of reducing the carbonyl, carboxyl, hydroxyl group compounds, and lowering moisture and oxygen contents. In addition, the grindability and hydrophobicity are improved by breaking down the cell wall and fiber structures of the biomass [11]. Washing, drying and size reduction on biomass is a simple pre-treatment that need to be prepared prior to pyrolysis [18].…”