Removal of phosphate and nitrate over a modified carbon residue from biomass gasification
Graphical abstract
Introduction
Energy generation from biomass has become more common and popular in recent years. This has led to a considerable increase in the amount of solid residues which need to be utilised techno-economically. According to the Finnish strategy on waste materials, in accordance with the corresponding European strategy, all kinds of waste must be utilised primarily as material (reuse and recycling), secondary as energy and if neither of those utilisation methods are possible, they can be disposed via ecologically beneficial methods. The different options available for dealing with waste can be described by a waste hierarchy which is derived from five categories; i.e. prevention, reuse and preparation for reuse, recycling, recovery (e.g. as energy) and disposal (Directive, 2008/98/EC of the European parliament and of the council).
Biomass gasification is used for generating energy (heat and power) from different types of organic materials. It is a process that converts carbonaceous materials, such as biomass, to gaseous compounds. Gasification is one of the most effective energy conversion technologies for the utilisation of biomass. Biomass can be converted either at high temperature (1200–1400 °C) to syngas, which mainly contains CO and H2, or at low temperature (800–1000 °C) into a product gas that contains CO, H2, CH4, and other hydrocarbons (CxHy) (Knoef, 2005). Syngas produced via biomass gasification can be used directly as fuel for an internal combustion engine or as a chemical feedstock to produce liquid fuels by the Fischer-Tropsch method (McKendry, 2002). In exothermic Fischer-Tropsch reaction, carbon monoxide and hydrogen react and form hydrocarbons. Reaction can be catalysed, e.g. on iron or cobalt (Knoef, 2005). Climate change is one of the most serious environmental problems in the world today and the use of biomass as an energy source will only increase in the future (Berndes et al., 2003). Therefore it can be assumed that gasification will become more popular in the future and that the amount of solid residues formed in the gasification process will also increase. The amount of formed carbon residue in the biomass gasification is dependent on e.g. the type of gasifier, gasification temperature and used fuel (biomass). According to Knoef (2005), the amount varied widely from 0.1% to 15%.
Activated carbon is a versatile adsorbent due to its good adsorption properties and due to its important role on filter material for the removal of odour, colours and tastes from liquids and gases. Activated carbon could also be used as an adsorbent in water purification. The preparation of carbon adsorbents is generally energy consuming and consequently, commercial activated carbon is fairly expensive so other adsorbents must be investigated. Selecting an alternative adsorbent is undertaken based on the following criteria (Bart and von Gemmingen, 2005):
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Capacity of the adsorbent
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Product purity and selectivity
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Regeneration method
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Length of unused bed (for fixed-bed systems)
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Price
Activated carbon can be produced from a variety of fossil or biomass based raw materials such as coal and coconut shell (Ahmadpour and Do, 1997). There are also large numbers of studies regarding the preparation of activated carbon from different waste materials, e.g. peanut husk and cherry stones (Lussier et al., 1994, Ricordel et al., 2001). Basically, there are two different processes for the preparation of activated carbon: physical and chemical activation. Physical activation can be performed for example with CO2 whereas chemical activation can use different chemical activating agents. These are typically alkali and alkaline earth metals containing substances such as KOH, NaOH and ZnCl2 or some acids such as H3PO4 (Ahmadpour and Do, 1996, Marsh and Rodríquez-Reinoso, 2006). An activating agent can act as a dehydrating agent (e.g. ZnCl2) which is commonly used for the activation of lignocellulosic materials (Srinivasakannan and Zailani, 2004). In this study, the following chemicals HCl, H2SO4, ZnCl2, KOH and HNO3 were used as chemical activating agents. It is known that chemical activation could modify the adsorbent's surface in two ways towards anion adsorption: firstly, an increasing surfaces positively charge or secondly providing new functional groups having a higher affinity for adsorbate. Studied adsorbates were anions and therefore acids (HCl, HNO3 and H2SO4) were chosen to chemical activating agent. Impregnation of metals or metal oxides onto the adsorbent surface has a similar effect as surface protonation and they also can increase the surface area and pore volume of adsorbent which were the reasons why ZnCl2 and KOH were selected to this study (Loganathan et al., 2013).
Knowledge of different variables during the activation process is very important to develop the porosity of carbon while the surface area of activated carbon is strongly dependent on, for example, the amount of activating agent. Chemical activation can improve the pore size distribution and increase the surface area of adsorbents. Used temperatures are lower in chemical activation compared with physical activation, and therefore development of a porous structure is better and energy costs are reduced (Ahmadpour and Do, 1996, Lillo-Ródenas et al., 2001, Lillo-Ródenas et al., 2003). Other advantages for chemical activation compared with physical ones are, for example, a shorter process time during the preparation step and higher yield of the final product (Lillo-Ródenas et al., 2001, 2003). Disadvantage for the use of chemical activation process is the pollution generated by the activation chemical. In addition, chemical consumption and price must also be considered.
The high cost of activated carbon limits its use as an adsorbent and there is a growing need to derive activated carbon from cheaper and locally available waste materials (Ahmadpour and Do, 1996). Carbon residues from the gasification process have typically lower specific surface area compared with commercial activated carbon but its carbon content is very high (Kilpimaa et al., 2013). In an adsorption process the specific surface area of an adsorbent must be large (Afkhami et al., 2007) and therefore, the adsorption properties of carbon residue would be better suited if their specific surface area was also larger. Therefore, chemical or physical activation would be required to increase the specific surface area of carbon residue.
Eutrophication is the term given to the enrichment of water bodies with nutrients, such as nitrates and phosphates. Phosphate in wastewater is one of the main factors which leads to the eutrophication, such as blue and green algae growth, and deterioration of water bodies, which leads to short- and long-term environmental and aesthetic problems. Eutrophication may occur when the concentration of phosphate is higher than 0.02 mg L−1. Adsorption is a popular method to remove nutrients, especially phosphorus, from effluent by using porous materials such as zeolites. Precipitation is another method for phosphate removal (Bolan et al., 2004, Chen et al., 2006, Huang et al., 2009, Zeng et al., 2004, Zhao and Sengupta, 1998). In addition to phosphate, nitrate can also cause several environmental problems. Nitrate is an emerging pollutant in agricultural, municipal, industrial and mining wastewaters (Majumdar and Gupta, 2000). Nitrate stimulates eutrophication and it has been linked to the outbreak of infectious diseases. Excess nitrate in drinking water may cause methemoglobinaemia, also called a blue baby disease, in newborn infants (Feleke and Sakakibara, 2002). Also nitrate can be removed in many ways such as by adsorption (Katal et al., 2012). Wastewaters possess harmful levels of phosphates and nitrates, and the removal of it before discharge is paramount. Therefore, the novel adsorbents for the removal of these anions are important to develop.
The overall aim of this work is to modify an industrial carbonaceous by-product by chemical activation to enhance its adsorption properties and therefore, use it as an adsorbent for wastewater purification. Chemical activation is typically used in commercial activated carbon production. To achieve this goal, this study investigates the effects of different parameters on the chemical activation of the carbon residue. The parameters to be investigated are: the chemical agent, liquid-to-solid ratio to the carbon residue and chemical activating agent, the concentration of impregnation solution and contact time. A chemically activated carbon residue is used as an adsorbent for phosphate and nitrate removal whilst the influence of adsorption time, initial phosphate or nitrate concentration and pH is also investigated. Results are fitted to pseudo-first-order and pseudo-second-order kinetic models to identify the rate of adsorption of phosphates and nitrates on the adsorbent's surface. Langmuir and Freundlich adsorption isotherm models were also studied. Commercial activated carbon and carbon residue without any pre-treatment process will be used as reference samples.
Section snippets
Raw material
The carbon residue obtained from the biomass gasification process that was used in the laboratory experiments was acquired from a biogasification pilot plant (Sievi, Finland) which involved a 150 kW air-blown downdraft gasifier operating at a temperature of 1000 °C. Finnish wood chips (pine and spruce) were used as fuel for the gasifier at a feeding rate of 50 kg h−1. There was no separate carbon residue collector in the gasifier. The gas produced in the gasifier was washed by a wet scrubber and
Effect of chemical activation on the adsorbent properties
Specific surface areas of chemically activated carbon residues were determined in order to evaluate the success of the activation with different activation parameters. Based on the surface area measurements, the highest specific surface area was obtained by using 5 M ZnCl2 as a chemical activating agent; results are shown in Table presented in supporting information section. There are many experimental variables which have influence on the porosity of the material and therefore, the next step
Conclusions
The experimental results indicated that the chemically activated carbon residue may be used as a low-cost adsorbent for the removal of phosphates and nitrates, for example, in wastewater treatment. Actually, it was observed that in the case of phosphates, modified carbon residue had notable higher adsorption capacity compared with commercial activated carbon. Adsorption capacity is also substantially better for activated carbon residue compared with carbon residue and therefore, the adsorption
Acknowledgements
This study has been performed with the financial support of the EU/InterregNord, within the project HighBio2 (Biomass to fuels and chemicals, 304-8455-10). The authors would like to thank Mr. Jaakko Pulkkinen for his assistant in phosphate and nitrate analysis whilst our gratitude also goes to M.Sc. Henrik Romar for his assistant in BET, pore size and pore volume measurements. The authors would also like to thank Mrs. Kaija Aura-Miettilä for elementary analysis and Mr. Jarno Karvonen for
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