Two-phase-olive-mill-waste-was-used-to-produced-activated-hydrochars.

Although the growth of non-fossil fuel consumption is a reality, it seems that the dependence on fossil fuels for energy supply will be still very strong in the closest future. Therefore, there is still an urgent necessity to reduce the CO₂ emissions. Among all the technologies studied, CO₂ capture and geological storage is considered to be a major mitigation option for climate change. Of the different approaches available for capturing CO₂, adsorption with solids has been pointed to be one of the most promising strategies due to the low cost of their synthesis and the low energy penalties these technologies present when comparing with other processes [1].

Among all the solids adsorbents studied until now, carbonaceous adsorbents have shown several advantages towards other tested materials as zeolites, calcium oxides, hydrotalcites or metal-organic frameworks (MOFs) among others. Their higher resistance to moisture, higher thermal, mechanical and chemical stability and low energy requirements and price, make of these materials the most attractive adsorbents to capture CO₂ [1].

Moreover, when waste biomass is used as precursor of these sorbents, its attractiveness for the CO₂ capture process increases because of the relative ease and low cost of their synthesis [2]. Several have been the processes studied during the last years to convert this waste biomass on valuable carbon materials. Among all the possibilities to convert low-value biomass into these interesting sorbents, HTC (hydrothermal carbonization) has demonstrated several advantages such as lower energy consumption over pyrolysis. Feedstocks with a high moisture content yield low amounts of solid material after drying, which makes them insufficient sources for pyrolysis. Therefore, a greater variety of feedstocks could be considered for processing into hydrochar since drying the feedstock is not necessary for HTC. Another advantage is that HTC yields higher amounts of char and uses lower amounts of energy than pyrolysis [3].

As several authors have previously reported, the physical properties of the sorbent depend mainly on the nature of the precursor. A wide number of biomass precursors have been studied until the date: rice husks [4], vine shoots [5], almond shells [6], coconut shells [7], among others. In all the cases, the raw material is converted into biochar or hydrochar (through pyrolysis and hydrothermal carbonization, respectively) and therefore, activated either physically or chemically with the aim to get the higher quantity of ultra-micropores (pore size < 0.7 nm) in their structure. Previous authors [4,8] agreed that as highest was the amount of ultra-micropores in the studied activated carbons, highest was their CO₂ adsorption capacity. These works concluded that the high selectivity CO₂/N₂ and the excellent cyclability that these materials showed were more related to the big amount of ultra-micropores presented in their structures than with their specific surface area. This theory was more recently confirmed by Manyà et al. [5] who found that among all the sorbents studied in their work, the highest ultra-micropore volume was obtained for the activated carbon with a highest KOH/biochar ratio, which showed at the same time the major CO₂ uptake capacity at the experimental conditions studied (0 °C and 15 kPa).

In this attempt to find the adsorbents with the best capacities, a controversy regarding the role of the nitrogen-functional groups when incorporating to the porous carbons has come on. While some authors [[9], [10], [11]] advocate for the positive effect that N-doping has on the materials, improving their CO₂ capture capacity and heats of adsorption, others [[12], [13], [14]] have demonstrated that the presence of nitrogen in the porous structure does not influence on their CO₂ uptake.

Among these last, Adeniran et al. [13] demonstrated that after comparing some carbons doped with N with other undoped, all of them prepared with similar pore size distribution, no any beneficial regarding the CO₂ uptake was observed. Similar conclusions had been previously obtained by Sevilla et al. [12] and Kumar et al. [14]. While Sevilla et al. [12] found that the presence of N did not have any appreciable effect on CO₂ adsorption, the second [14] appreciated a marginal improvement of the N-doped adsorbents comparing with the undoped ones being the real effect observed on the CO₂/N₂ selectivity, which increased in the case of the nitrogen-enriched materials. Both works agree that the CO₂ uptake of these carbons depends, independently of the presence of N, on the pore distribution and more in particular on the proportion of ultra-micropores present in their structures.

However, on the other hand, there are some authors [[9], [10], [11]] who reported that the incorporation of nitrogen-functional groups into the carbon structure enhances the interaction with CO₂ of these materials. Among others works, Xu et al. [15] measured capacities up to 5.86 mmol g^–1 at 101.3 kPa and 0 °C for a nitrogen-doped carbon prepared from tree leaves and activated at 600 °C. Similar values were achieved by Yue et al. [9] who synthetized porous nitrogen-doped carbons by carbonization of coconut shell followed by urea modification and K₂CO₃ activation. In this last case, the adsorbents showed CO₂ uptakes of 3.71 mmol g⁻¹ at 101.3 kPa and 25 °C and 5.12 mmol g⁻¹ at 0 °C.

Therefore, one of the objectives of this work is to elucidate both, the effect of N-doping as well as the effect of the pore size distribution on the CO₂ capture capacity of these porous adsorbents.

The growth that the industry of oil production has suffered in the last decades, not only in the Mediterranean countries but also in other countries as Serbia and Montenegro, Macedonia, Cyprus, Turkey, Israel, Jordan, the USA, Australia, the Middle East and specially in China [17] has exacerbated the environmental problems due to the production of high amounts of by-products: olive pomace (OP) and olive mill wastewater (OMW) [18]. In Spain, the introduction of the two-phase centrifugation system for oil extraction during the decade of 90´s reduced the production of wastewater generation, still produced approximately four million tons per year of a solid olive-mill by-product called “alperujo” or two-phase olive mill waste (TPOMW). Characterization experiments of these samples have shown that this product has a high moisture content, slightly acidic pH values and a very high content of organic matter (lignin, hemicellulose and cellulose) [19]. Although several attempts to reuse the TPOMW disposal have been made as its use for the co-generation of electrical power, the necessity to find less expensive uses has led to research into new ways. One possibility is to use it for the preparation of soil organics fertilizers and amendments; however, its direct application to the soil has been demonstrated to effect negatively the soil structural stability, seed germination, plant growth and microbial activity [19]. Another potential application which is being under study is its use to develop sorbents for CO₂ capture. As it was introduced above, the use of biomass as a precursor of these sorbents presents several advantages and in this work, activated carbons using TPOMW as precursor have been prepared. Considering the already summarized advantages that hydrochar presents when comparing with biochar, hydrochars from this biomass were produced and then, activated following different processes. The final objective is to study the CO₂ capture capacity of these sorbents under postcombustion conditions. Both, physical activation with CO₂ and chemical activation with KOH processes were used to study their effect on the properties of the prepared hydrochars and its behaviour as CO₂ adsorbents. Besides, N-doped adsorbents have been prepared through a one-step hydrothermal carbonization to study the effect of nitrogen-doping in terms of CO₂ uptake. These doped carbons were also physically activated with the same method used with the other hydrochars.

In this work, the CO₂ adsorption capacities of the developed sorbents were measured at 0, 25 and 75 °C and at CO₂ pressure of 15 kPa (the normal value in postcombustion conditions). On the other side, the CO₂/N₂ selectivities were measured at 25 °C and 101.3 kPa in a thermogravimetric analyser under pure CO₂ or N₂ environment.