Comparing direct and indirect fluidized bed gasification: Effect of redox cycle on olivine activity

Abstract

Fluidized bed gasification processes are generally considered a good choice for biomass and waste because of its fuel flexibility. Furthermore, it is a relatively low‐temperature highly efficient process operating at 700–900°C compared with, for example, coal‐based entrained flow processes that mostly operate at 1400–1600°C. Indirect fluidized bed gasification is becoming increasingly popular for some applications due to the possibility of producing a N2‐free gas without the need for an air separation unit, as well as complete conversion of the fuel. ECN has developed MILENA indirect gasification, in which gasification and combustion are physically separated, but both reactors are placed in the same vessel. This article aims to compare the performance of olivine as bed material in direct‐ and indirect (MILENA) gasification. With this purpose, oxidation/reduction cycles have been simulated in a direct gasifier to determine the effect of olivine preoxidation in terms of tar destruction and oxygen transport. Results show that olivine preoxidation mainly enhances the capacity of oxygen transport of the bed material, which adds up to the catalytic effect of iron in olivine towards tar destruction. Oxygen transport capacity of olivine has been quantified as ER = 0.25 at the maximum initial CO2 peak, and it has been estimated that 20–25% wt of iron in olivine is able to transfer oxygen during the first 10 min operation. On the other hand, it has been found that MILENA operating conditions are equivalent to the point of initial maximum peak of CO2 in the devolatilization stage. This means that oxygen transport capacity of olivine is kept at its maximum due to the continuous combustion/gasification cycles, and olivine is kept activated by cyclical migration of iron into the surface and subsequent fast reduction. The effect of oxygen transport on the overall heat balance of the MILENA gasifier is equivalent to oxidation of additional fuel in the combustor. © 2014 American Institute of Chemical Engineers Environ Prog, 33: 711–720, 2014

Characteristics of olivine as a bed material in an indirect biomass gasifier

“…Considering the previous studies, in which the activity of the olivine was connected to the formation of an iron layer and calcium layer on the surface of the particles [20,21,25], the observed result after bed replacement is anticipated due to the dilution of active bed material in the system. Considering the ability of olivine to transfer oxygen, a side-effect of the dilution with silica-sand is the increase in CO content and the decrease in CO 2 content as the oxygen transport decreases proportionally with increasing dilution.…”

“…The changes in activity of olivine is commonly attributed to Fe migration in the particle, which has been shown to be a result of calcination [3], and to the CaO layer that is formed during the interaction with biomass ash [20,21,25,26]. Besides calcium, the fuel contains varying amounts of potassium (K), silicon, and sulfur (S), which may also have effects on the chemistry within the system.…”

“…For this reason, it is used as the active bed material in the plant located at Guessing (Austria), which is operated commercially [20]. One drawback of olivine is its ability to transfer oxygen between the combustion side and the gasification side of the indirect gasification system [16,21], which lowers the lower heating value (LHV) of the product gas. Furthermore, traces of heavy metals in olivine are environmentally problematic, making disposal more difficult.…”

Connecting lignin gasification with syngas fermentation

“…The H 2 /CO ratio was 1.2, which is a typical value for a woody biomass gasification test [28]. The use of olivine as bed material, results in enhanced H 2 production and decreased CO and CH 4 content compared to silica sand, due to its catalytic effect on the reforming of hydrocarbons and tar and the promotion of the water-gas shift (WGS) reaction [29,30,31]. At position B, downstream OLGA, H 2 and CO 2 concentration appear slightly reduced, while CO concentration is increased.…”

Performance of large-scale biomass gasifiers in a biorefinery, a state-of-the-art reference

“…Olivine is a natural magnesium‐iron‐silicate ore that is commonly used as the bed material in DFB gasifiers because of its ability to reduce the yield of tar and its tendency not to agglomerate at these process temperature levels . However, to achieve the desired catalytic behaviour, olivine needs to be activated.…”