Lab-scale pyrolysis experiments have been performed to gain insight in the mechanisms influencing the metal behavior during carbonization of chromated copper arsenate (CCA) treated wood chips. To this end a fixed bed low temperature pyrolysis reactor and a thermogravimetric (TG) analyser have been used. Besides CCA treated wood chips, mixtures of metal and organic model compounds (As2O5, As2O3, CrAsO4, CrAsO4 + Ca(OH)2, As2O5 + glucose, As2O5 + lignin, As2O5 + activated carbon, As2O3 adsorbed on activated carbon, CrAsO4 + glucose, CrAsO4 + lignin, CrAsO4 + Ca(OH)2 + glucose, CrAsO4 + Ca(OH)2 + lignin) have been pyrolyzed in a controlled way. The aim of these model compound experiments is not to imitate the real CCA wood pyrolysis process, but to isolate some individual mechanisms (without being influenced by a variety of complex interactions) and to study the influence of different process parameters on these mechanisms. The parameters controlled during pyrolysis (and their corresponding range) are: temperature (330°C - 430°C), heating rate (5°C/min – 20°C/min), residence time (10 min – 40 min), reactor pressure (0 bar gage – 5 bar gage), heater gas flow rate (1000 Nl/h – 1800 Nl/h), oxygen concentration (1 vol% - 3 vol%) and steam concentration (5 wt% - 15 wt%).
Based on the experimental observations, rules of thumb are derived in order to identify an operating window that minimizes the volatilization of metals during thermochemical conversion of CCA treated wood, while ensuring a good quality of charcoal product.
The main observations related to metal and organic model compound experiments are:
1. The interactions between inorganic compounds (metals and minerals) and organic compounds play an important role during thermal decomposition. On the one hand the presence of organic compounds (wood, carbon, glucose, ...) promotes the reduction of As2O5 and subsequent volatilization of As2O3. On the other hand formation of complexes or agglomerates and adsorption effects may help to prevent arsenic loss below 400°C and may facilitate metal recuperation.
2. Arsenic adsorption on the solid residue may prevent the volatilization of As2O3, and may facilitate the formation of agglomerates of arsenic with wood minerals, giving rise to thermally more stable micro-structures. Furthermore, in a fixed bed or moving bed reactor, small fractions of As2O3 that volatilize at the bottom of the reactor can be re-adsorbed by above lying wood or partially pyrolyzed wood layers, giving rise to thermally more stable compounds.
3. Mixtures of metal model compounds and respectively glucose and lignin behave in a different way. Glucose, which is a representative organic model compound for tar formation and secondary char formation, shows foam formation during pyrolysis which hampers the release of arsenic trioxide. Elevated pressure has a pronounced effect on the foam density and the generation of pyrolysis gas, resulting in almost 5 wt% higher arsenic retentions at 5 bar gage compared to 0 bar gage in glucose + As2O3 and glucose + As2O5 samples. Lignin, which is a representative organic model compound for char production, shows no foam formation. The pyrolysis residue of lignin helps to stabilize arsenic trioxide.
4. Chromium arsenate forms thermally stable compounds with wood minerals consisting of Cr(III), As(V) and Ca(II), at temperatures below 310°C. Elevated pressure seems to have no significant influence on this process. Chromium arsenate, which had not formed stable compounds, decomposes to Cr2O3 (which may form volatile organo-chromium compounds) and As2O5, at temperatures below 310°C. Lignin helps to stabilize CrAsO4, and thus counteracts this decomposition.
5. Once As2O5 is formed, it is readily reduced to As2O3 (following a radicallary mechanism which is favored by elevated pressure) in the presence of reductive pyrolysis vapors. Once As2O3 is formed, mass transfer limitations and adsorption on the pyrolysis residue (which are both enhanced by elevated pressure) play an important role in retaining arsenic in the pyrolysis residue.
The CCA wood experiments show that:
1. Percentages of metals volatilized depend strongly on the operating conditions of the process. Therefore, arsenic release could be limited by choosing the operating conditions very carefully.
2. The effect of residence time on arsenic and chromium retentions is limited for the ranges considered.
3. Heating rate mainly affects chromium retentions, primarily through a higher thermal lag of the inner particle temperature at higher heating rates.
4. Reactor temperatures below 400°C result in a carbon-like product, having the same matrix as the original wood, only porosity has increased. This allows the minerals and metals to migrate during the thermal process, and to be liberated afterwards since the solid structure does not collapse (in contrast to conventional carbonization processes at 500°C).
5. Arsenic retentions decrease slightly with increasing temperature till 390°C, with a sharp decrease at higher temperatures, which may be attributed to accelerated desorption of arsenic trioxide at temperatures above 390°C. Chromium retentions show a less pronounced effect of temperature.
6. Arsenic and chromium retentions are affected in a different way by elevated pressure. Arsenic retentions increase about 5% at 5 bar gage compared to 0 bar gage, which is attributed to higher mass transfer resistances. In the case of chromium, the effect is less clear; the increased mass transfer resistance might be counteracted by enhanced heat transfer at the early stages of pyrolysis.
7. Since pressure increase does not only influence the mode of occurrence of arsenic, but also the char structure and extent of tar reactions (elevated pressure enhances secondary reactions and heat transport), the metal behavior and ease of metal recuperation depends on the reactor pressure.
8. Oxygen concentration mainly affects the reactor temperature (a change of inlet oxygen concentration from 1 vol% to 3 vol% increases the peak temperature from 420°C to 550°C at the bottom of the reactor, and from 390°C to 420°C at the top of the reactor), which results in a decrease in arsenic retention with increasing oxygen concentration.
9. A higher gas flow rate and the addition of steam mainly result in a more homogeneous reactor temperature profile. The net impact on metal retention is limited.
10. Scanning electron microscopy coupled to energy dispersive X-ray analysis (SEM-EDXA) shows that during pyrolysis the metal and mineral compounds form agglomerates, which suggests a relatively easy (mechanical) separation between metals/minerals and carbon. The mineral agglomerates (already existing in the initial wood before pyrolysis) may assist in the metal agglomeration process.
Although further research is needed to investigate the cross correlation between the various parameters, the experimental observations can be translated in following recommendations for reactor design and process conditions:
1. To ensure a good quality and brittle pyrolysis residue a residence time of at least 20 min and a process temperature of at least 370°C is necessary. However, the process temperature has to be limited to 390°C to limit metal release.
2. A more homogeneous wood input, with less fine particles, will result in a more homogeneous pyrolysis residue and reduced arsenic and chromium volatilization.
3. A longer residence time and lower temperature result in a similar wood conversion while reducing chromium and arsenic volatilization, however a lower plant capacity is obtained.
4. An operating pressure of 5 bar (or even higher) results in 5% higher arsenic retentions and mass retentions. Furthermore, elevated pressures allow to use lower heater gas temperatures to obtain the same wood conversion, resulting in even higher metal retentions.
5. The inlet oxygen concentration has to be strictly controlled below 1 vol% or even 0.5 vol% (at a heater gas flow rate of 1400 Nl/h and without steam addition), which increases both the arsenic retention and mass retention with 5% compared to an oxygen concentration in the range 1-3 vol%.
6. If oxygen control below 1 vol% is not feasible, the heater gas flow rate should be limited to 500 Nl/h to limit arsenic release. However, this could hamper the heating of the reactor bed. Nevertheless, heater gas flow rates should be lower than 1800 Nl/h to reach acceptable charcoal yields. Furthermore, small gas flow rates are important with respect to:
a. localizing the release of free and adsorbed water (drying) on the one hand and tar formation on the other hand in distinct phases
b. minimizing the dilution of combustible vapors
7. Tar problems can be avoided by:
a. preventing mixing of tar and water compounds (which would result in sticky tar compounds),
b. facilitating secondary reactions (such as cracking of tar); repeated cracking of organic compounds (stimulated by stratification of pressure and temperature in the reactor column, which is easier to be accomplished in a high wood column) eliminates the liquid phase.
8. Steam addition results in better reactor temperature control, and as a consequence increased metal and mass retentions. Moreover, thermal lag is reduced which is beneficial with respect to process time. However, significant amounts of steam are needed (40 wt% steam at an initial oxygen concentration of 2 vol% and 70 wt% steam at an initial oxygen concentration of 3 vol% to limit the reactor bed temperature to 390°C).
These recommendations still need to be tested in an industrial scale reactor.
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