• Energy Research

The CO2 adsorption capacities (AC) of biochars obtained at 350, 550, and 750 °C from the main organic (cellulose, lignin, and protein) and inorganic (CaCO3) macro-components of biogenic waste, as well as from co-digested manure (CDM), have been determined for different CO2 concentrations (2–83 vol%) at 25 °C and atmospheric pressure. CO2 adsorption isotherms have been determined using two different experimental methodologies: thermogravimetric and fixed-bed dynamic adsorption tests, yielding similar results. The composition effect has been analyzed by comparing the adsorption performance of the chars derived from individual macro-components and the potential interactions occurring during their co-pyrolysis. Lignin and cellulose-derived chars showed higher CO2 retention (≈77 mg gbiochar−1) than those produced from protein (≈40 mg gbiochar−1). Pyrolyzed CaCO3 exhibited negligible CO2 adsorption. For surrogate_CDM chars, prepared at pyrolysis temperatures high enough to decompose CaCO3 in the organic matrix, experimental results showed a synergistic effect, with AC between 14 % and 47 % higher than theoretical predictions. This decomposition promoted the reverse Boudouard reaction and enhanced char microporosity. However, the improvement was insufficient to offset the dilution effect caused by the high CaCO3 content. AC results have been discussed based on the biochar textural and chemical properties, with ultramicroporosity being the key factor determining adsorption capacity. The AC of CDM-derived sorbents is similar to that of cellulose-derived, expressed per gram of waste (7–13 mg gwaste−1). Furthermore, the biochars retained at least 80 % of their initial AC after 3 adsorption-desorption cycles, indicating their potential for stable CO2 capture.

Phosphorus was recovered from the ash obtained after combustion at different temperatures (600 °C, 750 °C, 900 °C) and gasification (obtained at 820 °C using a mixture of air and steam as fluidizing agent) of char from sewage sludge fast pyrolysis carried out at 530 ºC. Depending on the extraction conditions (type of acid, extraction time and acid concentration) phosphorus yields of 90% to 100% are reached. Char ash seems to be a renewable resource of phosphorus. It could be an alternative to rock phosphate for fertilizers production. The combination of these processes contributes to the integral exploitation of sewage sludge. Proceedings of the 21st European Biomass Conference and Exhibition, 3-7 June 2013, Copenhagen, Denmark, pp. 500-505

A selective online condensation system, including a scrubber and an electrostatic precipitator, has been tested in a lab-scale fluidized bed pyrolysis plant with the aim of reducing the water content of the pyrolysis liquid obtained from sewage sludge. Water and triethylene glycol at different temperatures were tested as washing liquids in the scrubber. The pyrolysis liquids collected with this liquid recovery system showed lower water contents (13−30 wt %) than those collected with a previous system (48 wt %) consisting of two condensers and an electrostatic precipitator. In spite of these significant reductions in the water contents, the liquids obtained still separated into three phases (light organic, heavy organic, and aqueous). The properties and yields of these phases were also affected by the operational conditions used in the scrubber. The aqueous phase was the most affected because the compounds present in this phase are those with greater affinity for the polar washing liquid used in the scrubber. The properties of the organic phases were less affected by the condensation system, although it is of interest that when using triethylene glycol at 80 °C as washing liquid, the yield to the light organic phase was increased by about 41% compared to the previous system without affecting its good properties as fuel.

An experimental work was carried out to investigate the viability of energy recovery from the air–steam gasification of sewage sludge. The relative influence of different factors, as well as the effect of their possible interactions, has been determined by means of analysis of variance. Temperature was found to be the most influential factor for most of the variables analyzed. Solid yield (35–41 wt.%) and tar content (11–45 g/m3 STP) were largely reduced with temperature, whereas gas production (0:89 -- 1:32 m3 STP/kg sewage sludge dry and ash free), carbon yield to gas phase (62–90 wt.%), gasification efficiency (39–66%), and H2 and CO yields (20–52 and 137–414 g/kg sewage sludge dry and ash free, respectively) were improved at high temperature. Other important parameters for the end-use of the gas such as its heating value (4.12–6.20 MJ/m3 STP) and its H2/CO molar ratio (1.46–3.25) were greatly influenced by the composition of the gasification medium, since the increase in the steam to oxygen ratio was favorable for both. The comparison of experimental and theoretical results highlights that equilibrium was not reached during the experimental runs

Abstract This work examines the influence of sewage sludge pre-treatment by torrefaction on the distribution and properties of the products obtained from its pyrolysis followed by catalytic post-treatment of the hot pyrolysis vapors. Sewage sludge was first torrefied in an auger reactor under two different conditions: temperature of 250 °C and 275°C and average solid residence times of 13 min and 24 min, respectively. Pyrolysis was conducted at 530 °C in a fluidized bed reactor. The catalytic post-treatment of the hot pyrolysis vapors was carried out in a fixed bed reactor using gamma-alumina (γ-Al 2 O 3 ) as a catalyst. The experimental results showed that the combination of the torrefaction pre-treatment with the catalytic post-treatment of the hot pyrolysis vapors did not noticeably improved the properties of the liquid organic phases for use as a fuel, in terms of O/C molar ratio and nitrogen content, compared to pyrolysis of sewage sludge with the same catalytic post-treatment of the hot pyrolysis vapors but without the torrefaction pre-treatment. Generally, the torrefaction did not have effect on the type of chemical families identified in the liquid phases and had little effect on the proportion of these families. The advantages of the combination of treatments were, on the one hand, the catalyst saving resulting from the lower amount of vapors generated by the pyrolysis of torrefied sewage sludge and, on the other hand, the slight decrease in the coke yield.

Abstract The high output of sewage sludge, which is increasing during recent years, and the limitations of the existing means of disposing sewage sludge highlight the need to find alternative routes to manage this waste. Biomass and residues like sewage sludge are the only renewable energy sources that can provide C and H, thus it is interesting to process them by means of treatments that enable to obtain chemically valuable products like fuels and not only heat and power; pyrolysis can be one of these treatments. The main objective of this review is to provide an account of the state of the art of sewage sludge pyrolysis for liquid production, which is under study during recent years. This process yields around 50 wt% (daf) of liquid. Typically, this liquid is heterogeneous and it usually separates into two or three phases. Some of these organic phases have very high gross heating values, even similar to those of petroleum-based fuels. The only industrial sewage sludge pyrolysis plant operated to date is currently closed due to some technical challenges and problems of economic viability.

Abstract New alternative processes such as low temperature gasification are currently being developed in order to use of black liquors from pulp and paper mills with energy proposes. The development of these new processes makes necessary to study the behavior of black liquors during pyrolysis and gasification paying special attention on their thermoplastic properties, which can cause important operational problems due to its swelling when heated. Present work is focused to the study of pyrolysis of alkaline black liquors from pulping of straw. The influence of an oxidation stage at low temperature previous to pyrolysis, on specific surface area of the char and on the black liquor swelling, is studied. For that, two main variables are analyzed: time of preoxidation and final temperature of pyrolysis. Dry black liquors from alkaline pulping of straw were used as material. The resulting chars obtained showed a clear decrease in swelling level of the black liquors, as well as an increase on their specific surface area, within the pyrolysis temperature and the preoxidation time. Materials with specific surface areas varying from 2 up to nearly 500 m 2 g −1 can be produced with the correct choice of experimental conditions. From the results, further research on the influence of a preoxidation stage on black liquors may lead to a better understanding of new alternative processes, which are nowadays being developed.

A lab-scale auger reactor was used for the study of dry sewage sludge torrefaction. The influence of the torrefaction temperature (between 250 and 300 °C) and the solid residence time (between 13 and 35 min) on the product distribution and properties was investigated. The results have shown that both parameters affect dry sewage sludge torrefaction products to a similar extent within the ranges of study. The yield of torrefied sewage sludge decreases when increasing the torrefaction temperature and the solid residence time, while the yields of liquid and noncondensable gases show the opposite trend. Carbon dioxide and hydrogen sulfide are the major noncondensable products. The yield of water is higher than the initial moisture content of sewage sludge. Organic compounds are also released during torrefaction, especially under severe conditions. Torrefaction liquid separates into an organic phase and an aqueous phase. The former is rich in oxygen-containing aliphatic compounds and steroids and their derivat...

Abstract The pyrolysis of dry and of torrefied sewage sludge in a lab-scale fluidized bed reactor has been studied in order to determine whether torrefaction pre-treatment could enhance the properties of the liquid product obtained after pyrolysis. The aim of this work is to evaluate the influence of the torrefaction temperature (220–320 °C) and average solid residence time (3.6–10.2 min) on the product distribution and the properties of the pyrolysis products. Pyrolysis was conducted at 530 °C with an average solid residence time of 5.7 min and a nitrogen volumetric flow per reactor area of 0.074 m 3 (STP) m −2 s −1 (measured at 0 °C and 1.01·10 5 Pa). The experimental results show that torrefaction pre-treatment affects the pyrolysis liquid product, although it does not improve the homogeneity of the liquid. Specifically, it reduces the yields of water and the liquid aqueous phase obtained in the pyrolysis step, especially after torrefaction under the most severe conditions, but it does not have a great effect on the properties of the liquid organic phases obtained. The cumulative yields of gas and organic compounds from the two-step process are not different from the yields obtained from one-step pyrolysis.