Scholarly article on topic 'Pilot-Scale Investigation of an Innovative Process for Biogas Upgrading with CO2 Capture and Storage'

Pilot-Scale Investigation of an Innovative Process for Biogas Upgrading with CO2 Capture and Storage Academic research paper on "Chemical engineering"

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{"Biogas upgrading" / "CO2 capture and storage" / "Alkali absorption" / "Accelerated carbonation" / "Air pollution control residues"}

Abstract of research paper on Chemical engineering, author of scientific article — Renato Baciocchi, Ennio Carnevale, Giulia Costa, Lidia Lombardi, Tommaso Olivieri, et al.

Abstract In this work an innovative carbon dioxide removal method that, differently from the currently employed commercial techniques, allows also to capture and store the separated CO2 is investigated. This process, Alkali absorption with Regeneration (AwR), consists in a first step in which CO2 is separated from the biogas by chemical absorption with an alkali aqueous solution followed by a second step in which the spent absorption solution is regenerated for reuse in the first step and the captured CO2 is stored in a solid and thermodynamically stable form. The latter process is carried out contacting the spent absorption solution, rich in carbonate and bicarbonate ions, with a waste material characterized by a high content of calcium hydroxide and leads to the precipitation of calcium carbonate and to the regeneration of the alkali hydroxide content of the solution. The proposed processes were first investigated by preliminary laboratory and simulation analysis. On the basis of the results of these tests, air pollution control (APC) residues from Waste-to-Energy plants were selected as the material to use for the regeneration step and a pilot-scale regeneration plant was designed, built and installed in a landfill site downstream of the already existing absorption column. In this paper the sizing and design of the regeneration plant and the results of the preliminary AwR pilot- plant tests are reported. This study was carried out within the framework of the UPGAS-LOWCO2 (LIFE08/ENV/IT/000429) Life+ project.

Academic research paper on topic "Pilot-Scale Investigation of an Innovative Process for Biogas Upgrading with CO2 Capture and Storage"

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Energy Procedia 37 (2013) 6026- 6034

GHGT-11

Pilot-scale investigation of an innovative process for biogas upgrading with CO2 capture and storage

Renato Baciocchia*, EnnioCarnevaleb, GiuliaCostaa, Lidia Lombardib, Tommaso Olivierib, Alessandro Paradisib, Laura Zanchib, Daniela Zingarettia

aLaboratory of Environmental Engineering, Dept. of Civil Engineering and Computer Science Engineering, University of Rome

"Tor Vergata",via delPolitecnico 1, 00133 Rome, Italy b"Sergio Stecco" Department of Energy Engineering, University of Florence, via di Santa Marta 3, 5013 9 Florence, Italy

Abstract

In this work an innovative carbon dioxide removal method that, differently from the currently employed commercial techniques, allows also to capture and store the separated CO2 is investigated. This process, Alkali absorption with Regeneration (AwR), consists in a first step in which CO2 is separated from the biogas by chemical absorption with an alkali aqueous solution followed by a second step in which the spent absorption solution is regenerated for reuse in the first step and the captured CO2 is stored in a solid and thermodynamically stable form. The latter process is carried out contacting the spent absorption solution, rich in carbonate and bicarbonate ions, with a waste material characterized by a high content of calcium hydroxide and leads to the precipitation of calcium carbonate and to the regeneration of the alkali hydroxide content of the solution. The proposed processes were first investigated by preliminary laboratory and simulation analysis. On the basis of the results of these tests, air pollution control (APC) residues from Waste-to-Energy plants were selected as the material to use for the regeneration step and a pilot-scale regeneration plant was designed, built and installed in a landfill site downstream of the already existing absorption column. In this paper the sizing and design of the regeneration plant and the results of the preliminary AwR pilotplant tests are reported. This study was carried out within the framework of the UPGAS-LOWCO2 (LIFE08/ENV/IT/000429) Life+ project.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: Biogas upgrading, CO2 capture and storage, alkali absorption, accelerated carbonation, air pollution control residues.

1. Introduction

Alternative utilization strategies to heat and/or electricity production are currently being investigated in Europe for biogas generated in landfills or anaerobic digestion plants [1]. Some applications such as

* Corresponding author. Tel.: +39-06-72597022; fax: +39-06-72597021. E-mail address: baciocchi@ing.uniroma2.it.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.531

vehicle fuel use and injection in the gas grid require the pre-treatment of the gas in order to increase its methane content and remove undesired trace components. The first goal is achieved mainly by upgrading treatments which are aimed basically at separating CO2, the other major component of biogas besides CH4, from the gas. Several different commercial methods are available for biogas upgrading; however, all of these applications have the common feature of removing CO2 from biogas without focusing on the fate of the separated carbon dioxide, which is usually re-emitted into the atmosphere during the system regeneration phase. Since the CO2 content of biogas is of biogenic origin, the integration of biogas upgrading with a carbon capture and storage (CCS) step appears of great interest in view of attaining negative CO2 emissions with regard to the overall greenhouse gas emissions balance.

The development and pilot-scale investigation of innovative biogas upgrading treatments that include capture and storage of the separated CO2 was the focus of the LIFE08 ENV/IT/000429 project UPgrading of landfill gas for lowering CO2 emissions (UPGAS-LOWCO2). One of the processes investigated in this project, Alkali absorption with Regeneration (AwR), is based on CO2 chemical absorption by means of an alkali aqueous solution followed by regeneration of the spent solution by carbonation of a Ca(OH)2-rich material. In the first stage of the process CO2 is physically absorbed in the liquid solution where it reacts with the alkaline compound (NaOH or KOH) producing the respective carbonate and bicarbonate ions [2-5]. In Eq. 1 the reaction that takes place during the absorption step, assuming that the alkaline additive used for CO2 absorption is KOH, is reported. In the second stage of the process, the spent absorption solution is chemically regenerated by contacting it with calcium hydroxide. In this step, poorly soluble CaCO3 precipitation takes place (carbonation reaction), thus the CO2 separated from the biogas can be permanently stored in a chemically inert and thermodynamically stable form, whereas KOH (or NaOH) is recovered for the first step of the upgrading process. The regeneration reaction for a KOH-based solution is reported in Eq. 2. In order to increase the sustainability of the proposed regeneration process in terms of minimizing the use of primary raw materials it was decided to use alkaline industrial waste materials as Ca(OH)2-source in the regeneration process.

2 KOH + CO2 K2CO3 + H2O (1)

Ca(OH)2 + K2CO3 CaCO3 + 2KOH (2)

The proposed absorption and regeneration processes were first investigated separately by preliminary experimental and simulation analysis. In particular, the absorption step was examined by means of computer simulations and carrying out preliminary tests in a pilot-scale column installed at a landfill site and designed to treat 20 Nm3/h of biogas [5]. The regeneration step was instead investigated by laboratory testing in order to select the type of waste material to use as Ca(OH)2-source, the operating conditions and layout of the process and finally to design, size and build the pilot-scale regeneration unit to place downstream the existing absorption column.

Among the tested industrial residues, those collected from the Air Pollution Control (APC) section of a sanitary waste incineration plant were selected as the most suitable to use in the regeneration process due to their physicochemical properties and mineralogy [6-8]. Preliminary lab-scale carbonation experiments performed using potassium carbonate solutions prepared in the lab allowed to select the following optimal operating conditions for the regeneration reaction: T=55 °C, Ca/K molar ratio=1.2 and 1 h residence time [8]. In addition, a washing pre-treatment of the APC residues, aimed at removing soluble phases and chlorides in particular, proved necessary to increase the reaction yield; whereas, in order to improve the leaching behavior of the solid product of the process a final washing step was added. Based on these results, the scheme shown in Fig. 1 was selected for the integrated alkali absorption and regeneration pilot-scale plant.

MAKE UP

Fig. 1. Scheme of the absorption with regeneration biogas upgrading with CO2 storage process

This paper focuses specifically on the sizing and design of the pilot-scale regeneration unit and presents the first results of AwR pilot-scale tests. In order to verify the operating conditions selected from the preliminary lab-scale tests and to define the conditions to adopt in the pilot-scale regeneration plant, for each unit operation, a new series of lab-scale tests were performed. In particular, for the regeneration step, carbonation experiments were carried out using the spent solution samples collected at the outlet of the already operating pilot-scale absorption column.

2. Materials and methods

2.1. Absorption tests

Absorption tests were carried out in the pilot plant installed in a landfill site located in Central Italy. The plant consists of a packed column in which an aqueous solution of KOH or NaOH is fed counter-currently to landfill gas extracted from a collection station in the landfill and fed to the column by means of a side channel blower. The column, built in stainless steel and characterized by an overall height of 990 mm and a diameter of 80 mm, is packed with Sulzer laboratory DX packing. The pilot plant, suitable to process about 20-25 Nm3/h of landfill gas and 40-60 l/h of absorbing solution, was built within a previous LIFE+ project [4] and modified in the current UPGAS-LOWCO2 project, as described in [9].

Each test was started by flowing the landfill gas through the column. Once the inlet and outlet flow rates were stable, the absorption solution, stored in a tank, was fed to the column. The duration of each test, constrained by the liquid flow rate and the volume of the storage tank, was typically of 10 minutes. The spent solution was collected at the outlet of the column during steady-state operation, which was typically achieved after the first two minutes of operation. For the entire duration of the test, temperature, pressure, flow rate and composition of the inflow and outflow gas were continuously measured and recorded. The collected spent absorption solution was analyzed by titration with a 10 eq/l HNO3 solution

so to allow to quantify the different species in the solution based on its buffering capacity at specific pH ranges: KOH for pH = 14, K2CO3 for 8^pH^11 (carbonates) and for 5^pH^7 (bicarbonates).

2.2. Lab scale tests for the definition ofprocess specifications for the regeneration pilot-scale plant

The APC residues used for these tests were sampled from the baghouse section of the same plant, a hospital waste incineration facility located in central Italy, from which the residues used in the preliminary lab-scale experiments had been collected [7-8]. Before the testing phase, the residues were characterized in order to establish the amount to use in each regeneration experiment. The residues presented a similar composition to the ones used in the previous tests with Ca and Cl being the main compounds (over 55% wt.) and portlandite (Ca(OH)2), calcium hydroxychloride (CaOHCl), halite (NaCl) and calcite (CaCO3) the main mineralogical phases detected by XRD analysis.

Regarding the washing pre-treatment (see Fig. 1), lab-scale experiments were carried out in order to select the liquid to solid ratio (L/S) to apply in the pilot-scale unit. The pre-treatment consisted in washing the APC residues for 15 minutes with distilled water at a L/S ratio of 10 or 5 l/kg and at ambient temperature or at 55 °C applying a stirring rate of 250 rpm. The efficiency of the treatment was evaluated by analyzing the chloride content of the wastewater so to determine the amount removed from the ash. Since for the pilot-scale plant drying and milling of the solid product of each stage of the regeneration process were considered unfeasible, the new regeneration experiments were carried out using the washed APC residues directly after liquid-solid separation.

For the regeneration/carbonation step (see Fig. 1), lab-scale experiments were carried out using spent solution samples produced from pilot-scale absorption tests with KOH solutions and washed APC residues. Different tests were carried out to evaluate the effect of the amount of residues used to regenerate a specific solution sample, i.e. the molar ratio of Ca as Ca(OH)2 to K2CO3 (R), on the achieved regeneration efficiencies. Lab-scale batch regeneration experiments were carried out in a 600 ml beaker that was placed in a thermostatic water bath for temperature control. At the beginning of each experiment the selected amount of washed APC residues was placed into the reactor and contacted with 200 ml of spent absorption solution. The experiments were carried out at 55 °C for 1 hour (the conditions selected during the preliminary evaluations phase) and for the entire duration of the experiment the suspended solution was stirred with a paddle-type impeller at a constant rate of 250 rpm. The solutions produced from the regeneration/carbonation experiments were analyzed by titration with the same method employed for the spent absorption solution. Also after the regeneration/carbonation reaction, the residues separated from the solution were directly fed to the final washing treatment and the amount of added water was calculated taking into account the humidity of the solid material in order to obtain a L/S of 5 l/kg. The operating conditions applied were the same as those used for the washing pre-treatment.

Vacuum filtration was selected as the liquid-solid separation method to use after each treatment stage; for the lab-scale experiments a Büchner filtration system was adopted. A SEFAR propylene fabric presenting a mesh opening of 14 m and a thickness of 200 m was selected as filter media.

3. Definition of process specifications for the pilot-scale regeneration plant

In this paragraph the main findings of the lab-scale tests carried out to select the operating conditions to apply in each step of the regeneration process (pre-washing treatment, regeneration and final washing treatment) are reported.

For the washing pre-treatment, the results of the lab-scale experiments allowed to select a L/S ratio of 5 l/kg and ambient temperature as the operating conditions to use in the pilot-scale unit. Under these conditions in fact the chloride removal efficiency resulted higher than 90%. After the washing pre-

treatment a significant weight loss (around 40%) of the solid material was measured. The composition of the washed residues was analyzed to calculate the amount of residues to use in the regeneration experiments. From this analysis the Ca content as Ca(OH)2 of the material was estimated as 37% wt.

Regarding the results of the carbonation/regeneration tests carried out with solutions produced from the absorption column, Fig. 2 reports the composition of a 2.35 eq/l absorption solution before and after the regeneration reaction carried out under optimal operating conditions, i.e. R=1.2 eq Ca/eq K2CO3. The spent solution contained only potassium carbonates and bicarbonates (2.35 eq/l), indicating that complete conversion was achieved in the absorption column, whereas, after the regeneration step, the buffering capacity of the solution showed that just potassium hydroxide was present. From these results hence it may be observed that also the regeneration reaction was almost complete. However, as shown in Fig. 2, the normality of the obtained KOH solution amounted to only 1.95 eq/l, against the 2.35 eq/l of the K2CO3 initial solution, due to the dilution effect caused by the use of humid residues. As a consequence, the KOH regeneration efficiency achieved was equal to 77%. Accordingly, the solid product was mainly made up by CaCO3 with a concentration of 73.5% as measured by calcimetry analysis, corresponding to a CO2 uptake of 37% wt. and a 40 % wt. increase of the material.

Regarding the final washing treatment, the solid product presented a similar mineralogical composition to the carbonated residues (i.e. predominance of calcite) but a lower content of halite and portlandite. As for the effects of this treatment on the leaching behavior of the final product of the treatment, similar results to those obtained in the preliminary tests were achieved, i.e. a significant decrease of the release of Cl, Pb, Cu, Zn and Cr compared to the untreated residues [8].

The humidity values (52-58%) of the solid products obtained after each treatment step in the lab-scale vacuum filtration tests proved similar if not lower than the values that are typically achieved with commercial separation devices such as filter presses (55-60% wt.).

0 0.5 1 1.5 2 2.5

Conc. (eq/l)

Fig. 2. Comparison of the acid titration curves obtained for a 2.35 eq/l KOH absorption solution before and after the regeneration

reaction in the lab-scale test

4. Sizing and design of the pilot-scale regeneration plant

On the basis of the results of the lab-scale tests, it was decided to carry out in sequence in the same reactor each stage of the regeneration process (washing pre-treatment, regeneration/carbonation reaction and final washing treatment) followed by a liquid/solid separation step. Regarding the size of the pilot-

scale unit, it was designed in order to treat a maximum spent absorption solution volume of 10 l, i.e. the amount that can be stored in the tank of the existing pilot-scale absorption column, and the maximum concentration of carbonate phases measured for the spent absorption solutions produced by the pilot-scale plant (3.15 eq/l).

The following criteria were selected to carry out the final design of the pilot scale regeneration reactor: volume greater than the maximum value required by each of the three treatment stages, i.e.: 20.6 l (volume required for the washing pre-treatment stage) and height chosen to grant a height/diameter (H/D) ratio of 2, which was considered as a compromise condition for optimizing mixing requirements (high H/D value) and filtration demands (low H/D value). Hence considering a diameter of 30 cm, a 60 cm reactor height was selected. In Fig. 3 a 3-D drawing of the pilot-scale regeneration unit is shown. The pilot-scale regeneration unit is composed by the following main elements: The reactor, manufactured in stainless steel, heated with electrical coils and equipped at the bottom with a filtering system that can be opened to remove the solid cake material; The vacuum filtration system, composed by a vacuum pump and a stainless steel tank; The frame, made in stainless steel, designed to support and move all equipment.

Fig. 3. Front view of the designed pilot-scale regeneration unit

5. Results of pilot-scale AwR tests

In the following paragraphs the main results of an absorption with regeneration experiment carried out in the designed pilot-scale unit using a 2.35 eq/l KOH absorption solution are described and compared to the results achieved in the lab-scale tests. The detailed report and discussion of the results of the different AwR tests carried out in the pilot-scale reactor also in view of achieving high purity (>90%) biomethane is presented in [9], while the environmental properties of the solid product of the regeneration treatment are reported in [10].

5.1. Absorption stage

The operating conditions applied for the test during the absorption stage and the obtained results are reported in Table 1. The composition of the spent solution is reported in Fig. 4a. As can be observed, the

spent solution obtained presented a 2.33 eq/l K2CO3 hence showing, similarly to the solution used in the lab scale tests (see Fig. 2) a complete conversion from KOH to K2CO3, while the CO2 removal efficiency was of 25%. It should be noted that these tests were not aimed at achieving the target of biomethane composition (>90% CH4) but more at analyzing the effect of varying the type and composition of alkali reagent and at producing solutions to use in the regeneration experiments. In fact, as can be noted, in this case the CH4 concentration in the outlet gas was slightly over 60%.

Table 1. Operating conditions and results of a 2.35 eq/l KOH absorption test

Solution flow rate 60.0 l/h

KOH concentration 2.33 eq/l

CO2 IN 42.7 ± 3 Vol. %

CO2 OUT 37.1 ± 3 Vol. %

CH4 IN 56.3 ± 3 Vol. %

CH4 OUT 60.6 ± 3 Vol. %

Gas flow rate IN 19.8 Nm3/h

Gas flow rate OUT 17 Nm3/h

Temperature gas IN 12.4 °C

Temperature gas OUT 19.6 °C

Temperature solution IN 30.4 °C

Temperature solution OUT 27.8 °C

CO32- concentration spent solution 2.33 eq/l

CO2 removal efficiency 25.2 %

5.2. Regeneration stage

The regeneration stage of the test was carried out in the designed pilot-scale plant, adopting a very similar operating procedure as that used in the laboratory tests. First, the washing step was carried out contacting 2.55 kg APC residues with 12.8 l deionized water for 15 min. Then the liquid was separated from the solid by vacuum filtration and 7.6 l of spent absorption solution were poured into the reactor. The reactor was heated during the regeneration step for which a slightly longer reaction time was used compared to the lab-scale tests (1.5 h). Then the regenerated solution (7.1 l) was separated from the solid, and 7.8 l of deionized water was added in the reactor for the final washing treatment which lasted 15 min.

The main results of the regeneration test carried out in the pilot-scale plant with the spent solution sample obtained from the absorption step, showed to be quite similar to those of the lab-scale tests. As can be observed in Fig. 4a in fact, the predominance of a pH buffering plateau at pH 14 demonstrates that an almost complete (90%) conversion of K2CO3 to KOH was achieved. However, the total buffering capacity of the solution (around 1.7 eq./l) was slightly lower than that obtained in the lab-scale test (1.85 eq/l), probably owing to a higher dilution effect or lower effectiveness of the washing pre-treatment. The solid product of the pilot-scale process, shown in Fig. 4b, exhibited a similar CaCO3 content (72.3% wt.) to that obtained from the lab-scale tests, but a corresponding lower CO2 uptake (21.5% wt.), due to the higher initial calcite content resulting for the washed residues used in the test.

6. Conclusions

An innovative method for removing carbon dioxide from landfill gas with the final aim of upgrading its quality to that of natural gas was proposed and investigated. With respect to commercial methods for biogas upgrading, the proposed process presents two additional environmental benefits: i) the carbon

dioxide separated from the methane is permanently stored by accelerated carbonation of an alkaline waste material (air pollution control residues from waste incineration flue gas treatment), ii) the series of treatments applied during the regeneration process has shown to improve the leaching behavior of the residues. This process, named Alkali absorption with Regeneration (AwR), consists in a first stage in which CO2 is separated from the biogas by chemical absorption with an alkali aqueous solution followed by a second stage in which the spent absorption solution is regenerated for reuse in the first stage of the upgrading process and the captured CO2 is stored in a solid and thermodynamically stable form.

In this paper the activities carried out during the second phase of the UPGAS-LOWCO2 project in \size and design the pilot-scale plant were reported. The results of the preliminary tests carried out in the pilot-scale AwR plant were quite similar to those obtained in the lab-scale tests and hence confirmed the feasibility of the proposed process also at pilot-scale.

Fig. 4. (a) Comparison of the acid titration curves obtained for a 2.35 eq/l KOH absorption solution before and after the regeneration reaction in the pilot-scale test; (b) picture of the solid product of the carbonation/regeneration step after liquid separation

Acknowledgements

The authors wish to acknowledge the Life+ Programme and European Commission for co-funding the activities of the UPGAS-LOWCO2 project (LIFE08/ENV/IT/000429) www.upgas.eu.

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