H₂ enriched fuels from co-pyrolysis of crude glycerol with biomass

Abstract

Pyrolysis of glycerol has been identified as a possible route for producing high added value fuels like renewable hydrogen (H₂). Crude glycerol (CG) is the main byproduct of biodiesel industry and without purification it is a low added value material due to the presence of impurities. Co-pyrolysis of CG with biomass may improve the efficiency of the process and as a primary step of gasification give important information concerning the maximization of H₂ concentration in the produced gas. Moreover, the thermochemical treatment of crude glycerol–biomass mixtures may offer several economic and environmental advantages in biodiesel industry and reduce the cost of biodiesel production. A mixture of CG with olive kernel (OK) was used as pyrolysis feed material. Pyrolysis of a 25 wt% mixture of CG with OK at high temperature (T = 720 °C) seemed to promote steam reforming reactions leading to an increase of H₂ concentration of 11.6 vv% in the pyrolysis gas in comparison to H₂ in gas obtained by low temperature pyrolysis (T = 520 °C).

Introduction

Worldwide scientific and technological concern focuses nowadays on establishing innovative and environmental-friendly energy production processes, due to the challenge of the depletion of fossil energy reserves. In parallel, excessive emission of carbon dioxide and other harmful pollutants to the atmosphere, due mainly to the development of the transport and fossil energy production sectors, seems to threaten the environment. New EU energy production policies and stricter legislation concerning environmental protection, by means of CO₂ emissions reduction, fluctuation and considerable increase of oil prices, as well as considerable difficulties in accessing the fossil fuel sources were some of the key factors enabling the growth of renewable biofuels production in Europe. Towards that direction biofuels, and especially biodiesel, have an outstanding position among other renewable fuels and present significant benefits when compared to fossil ones. Biodiesel is considered as a ‘green fuel’ produced by the trans-esterification reaction of vegetable oils with alcohols, usually methanol (CH₃OH), in the presence of a catalyst (e.g. KOH). In parallel the by-product of low grade (crude) glycerol (C₃H₈O₃) is produced in a ratio of 1:10 per weight [1]. Moreover, worldwide increase of biodiesel production has inevitably led to a parallel increase in CG production resulting in stocking of significant amounts [2].

On the other hand, purified glycerol is utilized as a feeding material in many sectors of the chemical industry; however, CG's purification towards a high grade glycerol (purified) is an expensive practice. Purified glycerol (PG) is a high added value by-product useful in chemical industry for further producing various chemicals like feedstock for food, pharmacy, cosmetics and tobacco industry [3], as along with its contribution to many other industrial sectors. CG refining for PG production greatly affects the production cost of biodiesel and depends also on the availability of a high efficient purification facility in the biodiesel plant. Even though, large scale biodiesel producers refine their CG and sell it to other industries [4], the practice seems to be quite expensive for small and medium scale biodiesel companies which are the most common case of Greece. Small and medium biodiesel plants do not generally afford the extra cost of CG distillation and purification. Selling of CG in refining companies leads to low profit benefits of 0.025–0.05 €/kg for a high calorific value by-product like crude glycerol [5].

In contrast to pure glycerol, crude glycerol (CG) has a lower added value due to impurities. CG is usually burned in furnaces for heat production [6]; however, such practice raises concerns over the particulates and harmful emissions release in atmosphere. According to Bohon et al. [6] increased particulate emissions due to residual catalysts content of CG are likely to be an issue during combustion process; a properly designed combustor with the ability to provide a suitable environment for efficient combustion of glycerol is therefore needed. Moreover, the recent research and development interest is focused in the alternative and environmental acceptable ways of CG valorization for high added value chemicals, energy and H₂ production in the concept of an integrated biorefinery.

Greece has assumed a position in the world biofuels energy production scenario and during the latest decade has promoted the implementation of the biodiesel production policy, according to the 2003/30/EC Directive. As a consequence of the practice of gradual mixing biodiesel with regular diesel up to 7 vv%, the problem of the exploitation and storage of the produced CG can be easily predicted. The increased biodiesel production in Greece, during the last decade, led to an increase in CG stocks. As a result, a reduction of CG prices due to storage and purification related barriers was noticed. The business interest started to focus in finding alternative exploitation ways of CG for moreover ensuring the profitability of biodiesel production and the establishment of the positive environmental footprint.

The production of CG in Greece back in 2009 was estimated at 77,000 tonnes [7], [8]. Taking in account that there is a potential of producing considerable amounts of biodiesel from indigenous oil crops in Greece [9], such as cotton, sunflower and corn seeds, the perspectives of alternative exploitation of produced CG amounts are quite promising. Although, chemicals production could be more beneficial, this seems to be a long term perspective for small biodiesel plants.

The present study concerns the investigation of solutions for valorization of crude glycerol produced annually in Greece in biodiesel plants and viability of utilization of CG for energy production by mixing it with local produced biomass. The ultimate goal of the study was to indicate a possible solution for small and medium scale biodiesel plants with in a sustainable way. Present experimental results consist part of a project aiming at using glycerol as co-fuel in dedicated bioenergy plants based on fluidized bed gasification. Moreover, previous investigations of the same group of researchers [10] indicated that high temperature pyrolysis could simulate the first step of gasification in a pilot fluidized bed reactor, which is running actually for olive kernel energetic valorization [11]. Therefore co-gasification of crude glycerol with biomass in the existing biomass dedicated pilot plant [11] is the ultimate goal, pyrolysis being the precursor process and laboratory scale experiments enforce the knowledge of the global process. Aim is to obtain enhanced in H₂ gas that after purification could be converted to energy in an integrated scheme with high efficiency.

In the present study, the researchers’ specific interest was to deepen in pyrolysis mechanism and to assess pyrolysis gas product quality towards hydrogen maximization. The evaluation of the pyrolysis gas product is important in order to integrate the pyrolysis process, in an efficient scheme, and exploit all the product streams or even to obtain useful information and further move towards performing gasification. Therefore, pyrolysis of crude glycerol with olive kernel (CG–OK mixtures) is the process studied.

Pyrolysis is a CO₂ neutral process that also consist a significant step in the gasification process, while its products are classified into three categories: (a) a non-condensable mixture of stable gases, (b) liquids (tars and/or bio-oil) and (c) a char solid containing mainly carbon and ash that could be all used as biofuels. Pyrolysis liquids especially show the highest heating value among all pyrolysis products. Moreover, the potential exploitation of these liquids for energy production purposes is widely recognized, as well as their ability towards the direction of replacing liquid fossil fuels in internal combustion engines, a field where the research on the pyrolysis liquids production focuses [12]. However, the complexity of the design and the relative infancy of such technology make it currently unsuitable for stationary and constant power applications [12]. In addition, the gas stream represents a significant proportion of the pyrolysis products, depending on the process conditions, with heating value comparable to those of some fuel gases.

An optimization of the pyrolysis process would thus result in gases that could be used to enhance the energy balance of the process and even place emphasis on the maximization of the bio-gas product. Furthermore, results of fast pyrolysis used in the present study aim at investigating the pyrolysis mechanism, which is the first step of gasification process.

The specific pyrolysis process, used for the experiments in the present study, is characterized by high heating rates and high temperature. Therefore it was considered useful to simulate the first step of gasification, especially in fluidized bed reactors since in both processes increased heat exchange rates are obtained. The results indicated the production of a hydrogen rich fuel gas from fast pyrolysis at elevated temperatures. However, pure hydrogen production is not the aim of the specific study but a hydrogen enhanced fuel gas with attractive heating value that could be exploited in an integrated scheme towards energy production.