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[en] Highlights: ► Hydrothermal liquefaction (HTL) at acidic, neutral and alkaline conditions. ► Bio-oil compositions varied with acidic, neutral and alkaline conditions. ► Reaction mechanisms varied with acidic, neutral and alkaline conditions. ► HTL should be classified to acidic, neutral and alkaline processes. -- Abstract: Hydrothermal liquefaction (HTL) of biomass to bio-oil under alkaline or neutral conditions has been widely reported in literature. However, there has been limited data available in literature on comparing HTL of biomass to bio-oil under acidic, neutral, and alkaline in terms of chemical compositions and yields by using the same reaction conditions and reactor. Using cellulose as a feedstock we conducted the comparative studies for pH = 3, 7 and 14 at temperatures of 275–320 °C with reaction residence times of 0–30 min. Results showed that the chemical compositions of the bio-oils were different for acidic, neutral and alkaline conditions. Under acidic and neutral conditions, the main composition of HTL bio-oil was 5-(Hydroxymethyl)furfural (HMF). Under alkaline conditions, the main compounds became C2–5 carboxylic acids. For bio-oil yields, it was observed that high temperatures and long residence times had negative effects, regardless of the pH levels. However, the corresponding reaction mechanisms are different. Under acidic conditions, the decrease in the bio-oil yields was mainly caused by polymerization of 5-HMF to solids. Under neutral conditions, the bio-oil yields decreased because 5-HMF was converted to both solid and gaseous products. Under alkaline conditions, the bio-oil decomposed to gases through the formation of short chain acids and aldehydes. Therefore, although they were all referred to as HTL bio-oil in literature, they were formed by different reaction pathways and had different properties due to their different chemical compositions. Given these differences, different strategies are recommended in this study to further improve HTL of biomass to bio-oil.
[en] Effective removal of nitrogen oxides (NOx) from flue gas allows more fossil fuels to be produced and utilized with less negative impact on the environment. It would be more cost-effective, however, if nitric oxide (NO) is oxidized to soluble nitrate and nitrite and then removed from the air by existing desulfurization wet scrubbers. This paper compares the effectiveness of three different oxidants for this purpose, namely, ethylenediaminetetraacetic acid; iron (2+) (Fe(II)–EDTA), hexamminecobalt(II) chloride ([Co(NH3)6]Cl2), and hydrogen peroxide (H2O2). Experimental results using column reactors showed that [Co(NH3)6]Cl2 was more effective over the same period of time. The best initial NO removal efficiency of about 96.45% was measured at the inlet flow rate of 500 ml/min; the temperature of approximately 19 °C; the pH value of around 10.5; and the concentrations of [Co(NH3)6]Cl2 , NO and O2 of 0.06 mol/L, 500 ppm and 5.0%, respectively.
[en] A new process based on aqueous-phase dehydration/hydrogenation (APD/H) has been developed to directly produce liquid alkanes (C7–9), which are the main components of fossil gasoline, from cellulose in one single batch reactor without the consumption of external hydrogen (H2). In this new process, part of the cellulose is first converted to in situ H2 by steam reforming (SR) in the steam gas phase mainly; and, in the liquid water phase, cellulose is converted to an alkane precursor, such as 5-(hydroxymethyl)furfural (HMF). In the final reaction step, in situ H2 reacts with HMF to form liquid alkanes through APD/H. Accordingly, this new process has been named SR(H2)-APD/H. Experimental results show that the volumetric ratio of the reactor headspace to the reactor (H/R) and an initial weakly alkaline condition are the two key parameters for SR(H2)-APD/H. With proper H/R ratios (e.g., 0.84) and initial weakly alkaline conditions (e.g., pH = 7.5), liquid alkanes are directly formed from the SR(H2)-APD/H of cellulose using in situ H2 instead of external H2. In this study, compared with pyrolysis and hydrothermal liquefaction of cellulose at the same temperatures with same retetion time, SR(H2)-APD/H greatly increased the liquid alkane yields, by approximately 700 times and 35 times, respectively. Based on this process, direct formation of fossil gasoline from renewable biomass resources without using external H2 becomes possible. -- Highlights: ► A process of producing gasoline alkanes from cellulose was proposed and studied. ► Alkane precursors and in situ H2 were formed simultaneously in a single reactor. ► Alkanes subsequently formed by reactions between in situ H2 and alkane precursors. ► The yields were 700 and 35 times higher than pyrolysis and hydrothermal conversion.