Hydrogen-rich gas production by continuous pyrolysis and in-line catalytic reforming of pine wood waste and HDPE mixtures

Highlights

• Plastic co-feeding improves the flexibility of biomass pyrolysis-reforming strategy.

• Hydrogen production is enhanced by increasing plastic content in the feed.

• The joint valorization of biomass and plastics attenuates catalyst deactivation.

• The amorphous coke derived from biomass is the main responsible for deactivation.

Abstract

The continuous pyrolysis-reforming of pine sawdust and high density polyethylene mixtures (25, 50 and 75 wt% HDPE) has been performed in a two-stage reaction system provided with a conical spouted bed reactor (CSBR) and a fluidized bed reactor. The influence HDPE co-feeding has on the conversion, yields and composition of the reforming outlet stream and catalyst deactivation has been studied at a reforming temperature of 700 °C, with a space time of 16.7 g_cat min g_feeding⁻¹ and a steam/(biomass + HDPE) mass ratio of 4, and a comparison has been made between these results and those recorded by feeding pine sawdust and HDPE separately. Co-feeding plastics enhances the hydrogen production, which increases from 10.9 g of H₂ per 100 g of feed (only pine sawdust in the feed) to 37.3 g of H₂ per 100 g of feed (only HDPE in the feed). Catalyst deactivation by coke is attenuated when HDPE is co-fed due to the lower content of oxygenated compounds in the reaction environment. The higher yield of hydrogen achieved with this two-step (pyrolysis-reforming) strategy, its ability to jointly valorise biomass and plastic mixtures and the lower temperatures required compared to gasification make this promising process for producing H₂ from renewable raw materials and wastes.

Introduction

The environmental awareness associated with the use of traditional resources (natural gas, petroleum and coal) has promoted the development of new routes for sustainable hydrogen production, whose demand is increasing because of its interest as an energy carrier and reactant in hydroprocessing units in refineries [1]. Within this scenario, biomass plays an important role as an alternative feedstock, given that it is a CO₂ neutral renewable source, and therefore allows producing sustainable chemicals and fuels [2].

Amongst the different thermochemical routes, direct steam gasification [3], [4], [5] and the indirect route of bio-oil reforming [6], [7], [8], [9] are the ones most studied for hydrogen production from biomass. The aim of gasification is to produce syngas, with tar formation being an issue for further industrial applications [10], [11]. The indirect route of bio-oil reforming has several problems related to its properties and its vaporization and re-polymerization [12], [13]. Accordingly, the two-stage strategy made up of pyrolysis and steam reforming has being gaining increasing attention in recent years because it avoids the need for condensing the pyrolysis outlet stream and re-vaporizing the bio-oil to be fed into the reforming reactor [14], [15], [16], [17], [18]. This process, in which each step is carried out in a different reactor, has its advantages over the single-step process of pyrolysis and in-situ reforming. On the one hand, the temperature in each step can be optimized in order to maximize the production of hydrogen [19] and, on the other hand, the catalyst is more effective for volatile transformation, i.e., the process is more versatile for establishing the desired catalyst/feed ratio. Therefore, a more uniform product stream will be obtained because the catalyst is more efficient at attenuating secondary reactions.

Nevertheless, the biomass feedstock’s low hydrogen content and high oxygen content are a drawback for high hydrogen production. Moreover, the catalyst undergoes a considerable deactivation by coke [16]. Consequently, the objective of this study is to increase H₂ production and attenuate catalyst deactivation by jointly valorising biomass and HDPE mixtures.

Several authors report hydrogen production increases by gasifying biomass with HDPE [20], [21], [22], [23]. Furthermore, co-feeding overcomes the seasonal limitations of biomass availability and helps to mitigate the environmental problems associated with waste plastic management. Although pyrolysis is considered a suitable route for the large-scale valorisation of waste plastics, and particularly polyolefins [24], [25], [26], [27], studies involving pyrolysis and in-line catalytic steam reforming of biomass-plastic mixtures are very scarce. Alvarez et al. [19] have studied the co-feeding of polypropylene in the pyrolysis-reforming of biomass in a batch laboratory scale reactor, obtaining higher gas yields and higher hydrogen productions than those with only biomass in the feed. In the same experimental unit, Kumagai et al. [28] performed the pyrolysis-reforming of a biomass/polypropylene mixture on a Ni-Mg-Al-Ca catalyst synthesized by co-precipitation, obtaining a maximum hydrogen production of 6.0 g of H₂ per 100 g of feed when the catalyst was calcined at 500 °C.

The aim of this study is to increase hydrogen production by co-feeding plastics into a continuous two-step process. Moreover, the influence of their joint valorisation on process performance and catalyst deactivation was analyzed. The novel strategy proposed combines the good performance of the CSBR for the pyrolysis of biomass [29] and plastics [30] with the suitability of the fluidized bed for the catalytic reforming process [31], [32]. The vigorous cyclic movement of the sawdust and sand particles coated with melted plastic in the CSBR minimizes segregation problems and avoids bed defluidization. Furthermore, the fluidized bed catalytic reactor allows controlling the temperature of the endothermic reforming reaction and delays the blocking of the bed by coke formation. This two-step configuration has been described in previous papers for the pyrolysis-reforming of biomass [16] and plastics [33], reporting a good performance of the process with high hydrogen yields and without operational problems.