Low temperature circulating fluidized bed gasification and co-gasification of municipal sewage sludge. Part 1: Process performance and gas product characterization

Review of inventory data for the thermal treatment of sewage sludge

The thermal treatment of sewage sludge has gained much interest in recent years, as exemplified by the 269 papers found in the scientific literature for the period 2010-2021. We identified 140 datasets in 57 papers presenting inventory data related to mass flows, energy and emissions for the incineration, gasification and pyrolysis of sewage sludge. Sewage sludge incineration (excess oxygen, 850-950 ℃) is an established technology; however, data on flue gas cleaning and air emissions are scarce. The recovery of energy is close to the amount of energy used for incinerating dried sludge (0.2 kWh/kg TS), while dewatered sludge incineration uses more energy (1-2 kWh/kg TS) than what can be recovered. Sewage sludge gasification (limited oxygen, 650-950 ℃) is an experimental technology with four outputs (kg/kg sludge TS): char 0.43, tar 0.02, fly ash 0.06 and syngas 0.53. The data vary significantly in this regard, suggesting than many factors affect the performance of the gasification process. Sewage sludge pyrolysis (no oxygen, 400-800 ℃) is an experimental technology with five outputs (kg/kg sludge TS): char 0.53, tar 0.21, water < 0.05, fly ash set to zero and syngas 0.21. The values are somewhat different for digested sludge. Energy consumption for the pyrolysis of sewage sludge cannot be estimated from the literature. The current literature provides useful data on the main flows of thermal technologies, although large variations are in evidence. However, data are limited on energy consumption and recovery in general, and they are scarce on direct emissions to the air from incineration.

Energy Recovery from Waste Paper and Deinking Sludge to Support the Demand of the Paper Industry: A Numerical Analysis

The recovery of fibres from waste paper (WP) and deinking sludge (DIS) reduces the stress on nature compared to the collection of virgin pulp for paper production. Moreover, if not recycled, WP and DIS are mainly landfilled and incinerated, being thus responsible for the release of greenhouse gases (GHGs) into the atmosphere. In this context, energy recovery from WP and DIS would contribute to increasing energy independence and improving waste management in the pulp industry. From a broader perspective, it would increase renewable energy generation, supporting the paper industry in reducing fossil fuel consumption and GHGs emissions, in line with the goals of the European Union (EU) Green Deal 2021. For these reasons, in the present study, the combined heat and power generation potentiality of WP–DIS blends through gasification in combination with an internal combustion engine is numerically assessed for the first time. The air gasification process is simulated by applying a restricted chemical equilibrium approach to identify the optimum operating temperature (850 °C) and equivalence ratio (0.2). Electrical and thermal energy generation potentiality, considering WP and DIS production in the EU in 2019, is estimated to be in the ranges of 32,950–35,700 GWh and 52,190–56,100 GWh, respectively. Thus, it can support between 25 and 28% of the electrical and 44–48% of the thermal energy demand of the paper manufacturing sector, reducing the CO2 emission in the range of 24.8–28.9 Gt.

Design of a 1 MWth Pilot Plant for Chemical Looping Gasification of Biogenic Residues

Chemical looping gasification (CLG) is a promising process for the thermochemical solid to liquid conversion route using lattice oxygen, provided by a solid oxygen carrier material, to produce a nitrogen free synthesis gas. Recent advances in lab-scale experiments show that CLG with biomass has the possibility to produce a carbon neutral synthesis gas. However, all experiments have been conducted in externally heated units, not enabling autothermal operation. In this study, the modification of an existing pilot plant for demonstrating autothermal operation of CLG is described. Energy and mass balances are calculated using a validated chemical looping combustion process model extended for biomass gasification. Based on six operational cases, adaptations of the pilot plant are designed and changes discussed. A reactor configuration using two circulating fluidized bed reactors with internal solid circulation in the air reactor is proposed and a suitable operating strategy devised. The resulting experimental unit enables a reasonable range of operational parameters within restrictions imposed from autothermal operation.

Thermal treatment of sewage sludge in Germany: A review

Sewage sludge from municipal wastewater treatment plants in Germany is currently disposed of via thermal treatment and agricultural utilization or used for landscaping. Increasing focus on hygiene, soil protection and most recently on phosphorus recovery combined with the associated legal changes leads to an increased relevance of thermal sewage sludge treatment processes. This article reviews existing technologies for thermal treatment of sewage sludge with a view to the situation in Germany. Thermal sewage sludge treatment can be divided into different processes: drying reduces high water contents of mechanically dewatered sewage sludge and often precedes subsequent treatment processes. Today, most of the sewage sludge in Germany is incinerated, about half in mono-incineration, mostly in stationary fluidized beds, and the other half in co-incineration, in particular in coal-fired power plants, cement kilns or, to a lesser extent, waste incineration plants. Some alternative thermal processes, mainly pyrolysis and gasification, but also metallurgical approaches, are tested in bench or pilot scale. Recent amendments to the German Sewage Sludge Ordinance will restrict the disposal route of co-incineration in future. Consequently, a significant increase in mono-incineration capacity is expected. These processes should enable the combination of environmentally friendly disposal and phosphorus recovery.

Characterization of herb residue and high ash-containing paper sludge blends from fixed bed pyrolysis

High ash-containing paper sludge which is rich in various metal oxides is employed in herb residue pyrolysis to enhance the yield of fuel gas and reduce tar yield in a drop tube fixed bed reactor. Effects of heat treatment temperature and blending ratio of paper sludge on the yields and composition of pyrolysis products (gas, tar and char) were investigated. Results indicate that paper sludge shows a significantly catalytic effect during the pyrolysis processes of herb residue, accelerating the pyrolysis reactions. The catalytic effect resulted in an increase in gas yield but a decrease in tar yield. The catalytic effect degree is affected by the paper sludge proportions, and the strongest catalytic effect of paper sludge is noted at its blending ratio of 50%. At temperature lower than 900 °C, the catalytic effect of paper sludge in the pyrolysis of herb residue promotes the formation of H₂ and CO₂, inhibits the formation of CH₄, but shows slight influence on the formations of CO, while the formation of the four gas components was all promoted at 900 °C. SEM results of residue char show that ash particles from paper sludge adhere to the surface of the herb residue char after pyrolysis, which may promote the pyrolysis process of herb residue for more gas releasing. FT-IR results indicate that most functional groups disappear after pyrolysis. The addition of paper sludge promotes deoxidisation and aromatization reactions of hetero atoms tars, forming heavier polycyclic aromatic hydrocarbons and leading to tar yield decrease.

IR and kinetic study of sewage sludge combustion at different oxygen concentrations

Thermal degradation of sewage sludge disposal is attracting more attention due to the increase in municipal wastewater treatment. In this work the performance of the thermochemical processes of sewage sludge at different oxygen concentrations was investigated by thermogravimetric (TG) and Fourier transform infrared analysis (FTIR) study. The oxygen concentrations were varied systematically from 0 to 20%, representing heating process from pyrolysis to full combustion. The evolutions of surface functional groups in these processes were also investigated by in situ diffuse reflectance infrared Fourier transform spectra (DRIFT), which helped to understand the reaction mechanism during the thermal degradation, especially when the reaction conditions were different. The heating process was divided into four stages, dehydration (below 200 °C), devolatilization (200-400 °C), char combustion (above 400 °C), and secondary devolatilization (above 650 °C). Reaction mechanism and kinetic model was proposed based on the stages of heating process. Oxygen concentration was presented explicitly in the reactions and kinetic equations. The model was then developed for the heating processes at different oxygen concentrations, followed by fittings of kinetic parameters. Some of the parameters in the model were fixed as constants to minimize the number of variations. The fitted model agreed well with the TG curves at different oxygen concentrations and could illustrate the evolution of intermediates and products during the heating process. The developed kinetic model could be further applied for the modeling of sewage sludge pellets combustion considering oxygen diffusion process.

Low temperature circulating fluidized bed gasification and co-gasification of municipal sewage sludge. Part 2: Evaluation of ash materials as phosphorus fertilizer

The study is part 2 of 2 in an investigation of gasification and co-gasification of municipal sewage sludge in low temperature gasifiers. In this work, solid residuals from thermal gasification and co-gasification of municipal sewage sludge were investigated for their potential use as fertilizer. Ashes from five different low temperature circulating fluidized bed (LT-CFB) gasification campaigns including two mono-sludge campaigns, two sludge/straw mixed fuels campaigns and a straw reference campaign were compared. Experiments were conducted on two different LT-CFBs with thermal capacities of 100kW and 6MW, respectively. The assessment included: (i) Elemental composition and recovery of key elements and heavy metals; (ii) content of total carbon (C) and total nitrogen (N); (iii) pH; (iv) water extractability of phosphorus after incubation in soil; and (v) plant phosphorus response measured in a pot experiment with the most promising ash material. Co-gasification of straw and sludge in LT-CFB gasifiers produced ashes with a high content of recalcitrant C, phosphorus (P) and potassium (K), a low content of heavy metals (especially cadmium) and an improved plant P availability compared to the mono-sludge ashes, thereby showing the best fertilizer qualities among all assessed materials. It was also found that bottom ashes from the char reactor contained even less heavy metals than cyclone ashes. It is concluded that LT-CFB gasification and co-gasification is a highly effective way to purify and sanitize sewage sludge for subsequent use in agricultural systems.