Integrated microwave drying, pyrolysis and gasification for valorisation of organic wastes to syngas

The development of integrated platforms is desirable in order to improve the efficiency of biorefineries. This study deals with an innovative all-in-one thermochemical process (drying, pyrolysis and gasification) based on the use of microwave energy, which could eliminate the need to pre-dry municipal solid waste in order to produce a high quantity of synthesis gas. Depending on the initial moisture content and pyrolysis time, it is possible to obtain different syngas compositions but only during the first 20 min. Beyond this time the syngas composition remains almost constant. In addition, the moisture content was found to improve the volumetric gas production by almost 50%. The energy consumption of this all-in-one process is discussed by comparing it with the scarce information available in the literature concerning the scaling-up of microwaveassisted processes, which may be more cost-effective at a higher scale. Keywords Microwave pyrolysis; Drying; Municipal Solid Waste; Syngas; Energy Efficiency Final	  version	  published	  in	  Fuel,	  2014,	  132	  ,	  20-­‐26 2 1. Introduction The concept “biorefinery” refers to a processing plant where biomass feedstock is converted into valuable products by means of a combination of biological, thermal and chemical processes. The resulting high-value products, such as chemicals and new materials, derived-fuel and the additional power and heat production may well help to lower energy costs and reduce greenhouse gas emissions produced by traditional power facilities. Nonetheless, there remain a number of barriers related to the implementation and commercialization of the biorefinery that need to be overcome. Apart from nontechnical barriers, such as restrictions on use of land or the environmental effects of large areas of feedstock monoculture, reliable equipment designs for the thermochemical conversion of organic feedstock need to be extensively demonstrated on a larger scale [1]. To this end, integrated process platforms appear the most promising way forward. In general, the drying of biomass materials before their introduction into the conversion units is desirable, since high moisture contents are known to produce fuel gas of lower effective heating value and lead to higher transportation costs, although a large amount of energy is consumed to remove the moisture. Dryers are designed taking into consideration that the wet material must be heated from the inlet temperature up to the wet bulb temperature in order to produce a driving force for surface water to be removed from the material. Afterwards, the material must be heated to drive water from the inside of the biomass to the surface so that it can evaporate. Nevertheless, the Final	  version	  published	  in	  Fuel,	  2014,	  132	  ,	  20-­‐26 3 benefits provided by the drying of organic feedstock must be balanced against the increased operating and capital cost [2]. Pyrolysis and gasification are the main thermochemical conversion processes that have been developed within the biorefinery concept to produce synthesis gas (or syngas). Both technologies need an external source of heating to yield gaseous and oily products, whose characteristics depend on feedstock composition, temperature, reactor configuration and gasifying agent, etc. Pyrolysis may be defined as the regime in which the organic matter is degraded by thermal reactions in the absence of oxidizing agents. A gasification process, however, begins with a pyrolysis step followed by the gasification of char and reforming reactions of tar and light hydrocarbons through the addition of a gasifying component such as steam. As a result, gasification usually yields higher amounts of gas fraction than pyrolysis [3]. In order to develop an integrated platform of these processes in a biorefinery, microwave heating should also be considered as an attractive alternative. This method is very tolerant of water compared to conventional pyrolysis and is suitable for most biomass types without pre-drying [4]. Owing to its volumetric nature and high heating rates, a large quantity of steam is generated from the moisture content at the temperatures at which gasification and reforming reactions take place. As a consequence, drying, pyrolysis and gasification processes of wet organic feedstock could all be integrated together in a single microwave induced process (MIP). This method has already been demonstrated by our research group to be a feasible option [5]. In that particular case, it was used to demonstrate a new alternative for wet sewage sludge management which resulted in a gas fraction rich in CO and H2, making this approach interesting for further production of valuable chemicals [6]. In the previous study, conventional and microwave heating Final	  version	  published	  in	  Fuel,	  2014,	  132	  ,	  20-­‐26 4 techniques were compared, it being concluded that higher gas yields could be obtained with conventional heating, though the gas from microwave heating was richer in syngas. It should be pointed out however, that the gas yields were expressed in terms of weight percentage of the initial sewage sludge mass, which could be misleading for the evaluation of gas production. For instance, a gas fraction richer in hydrogen would result in a lower mass of gas due to the low molecular weight of hydrogen, which does not mean a lower volumetric gas production. An alternative more accurate way to calculate gas production might be based on the volume of gas produced per unit of feedstock mass. Results from the literature show that the production of syngas from biowaste microwave pyrolysis by means of this alternative approach is far higher than that from conventional heating [7]. The influence of the moisture content, and the need for feedstock pre-drying, on the gas characteristics from pyrolysis and gasification has been evaluated in several studies, even though further research is still needed. Xiong et al. performed the pyrolysis of different sewage sludges by conventional heating [8]. The steam-rich atmosphere generated by the moisture content of the sludge was found to favour the steam reforming reactions of volatile compounds as well as the gasification of solid char, which in turn improved the gas weight yields and the H2 content. Xie et al. reported that an increase in the moisture content in sewage sludge improved the contents of CO2, CH4 and H2 and reduced the tar yield under air gasification, but they provided no results on gas production [9]. Niu et al. simulated the gasification of municipal solid waste with different moisture contents. A higher heat input to the gasifier for turning liquid water into superheated steam and a lower gasification efficiency were the effects of the high moisture contents, suggesting that the input feedstock stream should be predried to Final	  version	  published	  in	  Fuel,	  2014,	  132	  ,	  20-­‐26 5 ensure an optimal gasification performance [10]. Moreover, the H2 concentration increased slightly in spite of a noticeable drop in CO content. Domínguez et al. observed that the influence of sewage sludge moisture on the gas products from microwave pyrolysis was negligible, but only a wet and a dried samples were used [11]. However, they attributed this to the fact that water was eliminated very quickly from the reactor and did not have to time to participate in gasification reactions. This problem could have been resolved by using a better reactor design, to enhance the extent of these reactions. Since microwave heating is driven volumetrically, heterogeneous reactions between the water released and the organic waste are much more favored than under conventional heating, which may lead to an increase in the yield of valuable products. Wang et al. recently conducted sewage sludge microwave pyrolysis at 850 oC in a single mode reactor and observed an increase in the H2 and CO concentrations at higher moisture contents while the CO2 and CH4 concentrations decreased [12]. Interestingly, CO production seemed to increase with moisture if microwave heating was applied instead of conventional heating. The results obtained were substantially higher than under conventional pyrolysis. Syngas production by means of microwave heating in the presence moisture has been widely discussed. The question that still remains to be answered is whether the presence of moisture improves gas production from the microwave pyrolysis of organic residues. If so, this introduces the possibility of creating a new efficient integrated platform for the green disposal of municipal solid wastes through the integration of the drying step into the pyrolysis reactor. The aim of this work is to demonstrate the feasibility of this