Combined steam/dry reforming of bio-oil for H₂/CO syngas production with blast furnace slag as heat carrier

Highlights

• Combined steam/dry reforming for syngas production and further methanation.

• BF slag as heat carrier supplies all the heat for the bio-oil reforming process.

• Low consumptions of feedstock and energy were pursued.

Abstract

Combined steam/dry reforming of bio-oil with blast furnace slag as heat carrier for the syngas production with the H₂/CO ratio of 3:1 for further methanation, was investigated. The increase of H₂O addition can increase the total yield of H₂ and CO, but also increase the critical temperature at which 3:1-H₂/CO syngas was obtained, while the increase of CO₂ addition can decrease the critical temperature, but the syngas yield was also decreased. When the steam/carbon (S/C) ratio was 3.0 and the CO₂/carbon (CO₂/C) ratio was 0.5, the critical temperature decreased to 804 °C, with the potential H₂ yield of over 90%. Although the addition of slag and how much slag to be added had almost no any thermodynamic effect on the combined reforming of bio-oil under the condition where higher potential H₂ yield can be obtained, the slag as heat carrier could supply all heat for the combined reforming process. When the added slag mass was 3.99 times bio-oil mass, the combined reforming at the S/C ratio of 3.0 and the CO₂/C ratio of 0.5 can occur spontaneously for the production of 3:1-H₂/CO syngas. The present study could offer important guidance toward utilization of this novel process for further methanation.

Introduction

Nowadays, fossil fuels, primarily coal and natural gas, are still the major sources of energy worldwide. As energy demand is increasing across the globe and the environment protection is being focused more, natural gas having high calorific value and being cleaner than coal, is more demanded [1]. In spite of having an abundant coal reserve, China carried out a “coal-to-gas switch” project nationally in the few past years, to alleviate haze weather which was considered to be mainly caused by direct use of coal (such as coal combustion). But, the project has been proved failure, because of the “lean gas, rich coal” energy structure in China. So, the ever increasing demand of natural gas with high price has led or is leading researchers to focus on the alternate methods of natural gas generation. Converting coal to natural gas could satisfy the demand for natural gas, especially in the nations with abundant coal resources, such as China and United States, and the main conversion way was coal gasification (steam-oxygen gasification, hydrogasification, or catalytic steam gasification), followed by methanation which is an exothermic catalytic conversion of synthesis gas (CO and H₂) to methane (CH₄, the major component of natural gas) (Eq. (1)) [2]. This kind of artificially produced natural gas was normally called “synthetic natural gas” (SNG), which can diversify energy options and reduce natural gas imports, and can also be helpful to stabilize fuel prices [3]. But, a severe problem with SNG produced from coal is the additional CO₂ created via the process. To reduce emission from the coal-to-SNG process to atmosphere, the gas cleaning was generally added, to separate and capture CO₂, as well as H₂S also co-produced in coal gasification process to syngas [4]. Bassani et al. [5] purposed an innovative process converting CO₂ and H₂S to syngas, and found that a coal feed with a high sulfur content allowed higher reductions of carbon dioxide emissions. Compared to fossil fuel, carbon-containing renewable biomass with abundant reserves and neutral CO₂ emission, is recognized as one of the promising environmentally options. The first process step of SNG generation from biomass is also a thermo-chemical gasification, primarily steam gasification [6]. Energy Research Center (Netherlands), Paul-Scherrer Institute (Switzerland), Center for Solar Energy and Hydrogen Research (Germany) had done lots of works on the methanation from biomass gasification, and Germany planned to achieve the goal of replacing 10 billion Nm³ imported natural gas with bio-SNG per year until 2030 [2,7].

CO+3H₂↔CH₄+H₂O

However, compared to coal, biomass distributes rather dispersedly, mainly in the vast countryside, while the sites that produce and need natural gas are mainly in the urban district or the suburb near the urban district. So, it is not feasible to produce SNG directly from biomass feedstock in any of the two places, due to the expensive collection and transportation of biomass with low energy density and the security problems for the transportation of the inflammable and explosive gas. The mature fast pyrolysis technique of biomass to produce fast pyrolysis oil (bio-oil) with flexible operability was considered a good approach to improve the energy density, with the yield of bio-oil more than 60% and the energy density of bio-oil about 10 times of that of original biomass, resulting in convenient storage and transportability [8,9]. So, the fast pyrolysis of biomass (in the biomass-rich place) followed by converting the produced bio-oil to H₂/CO syngas (in the SNG-need place), is known as one of the most promising and economically viable methods for the productions of H₂-rich syngas and then SNG [10].

Bio-oil reforming for the production of H₂/CO syngas has rapidly developed in recent years, with the CO₂ reforming (dry reforming) [[11], [12], [13], [14], [15]] and steam reforming [[16], [17], [18], [19], [20], [21], [22]] focused widely. The priority of CO₂ reforming is a good approach to utilize CO₂ as well as bio-oil, but the H₂/CO ratio in the obtained syngas is very lower because of the low hydrogen content in the feed-stock, which seriously limits the development of CO₂ reforming technology. For the methanation of bio-syngas, the optimal H₂/CO ratio in the syngas should be controlled at 3:1, according to Eq. (1). On the other hand, steam reforming of bio-oil is considered to be capable of producing higher H₂/CO-ratio syngas effectively, mainly because the H₂ in the syngas was not only from the feed-stock, but also from the steam [23]. Therefore, developing the combination of steam and dry reforming to produce a syngas with the adjustable H₂/CO ratio is an urgent matter at present. Although the pure steam reforming and dry reforming had been investigated widely, it has not achieved any progress in the combined reforming of bio-oil. For the combined reforming of bio-oil as well as any one pure reforming, the reaction processes were strongly endothermic processes. Among these investigations on the pure reforming of bio-oil, most focused on reaction characteristics for bio-oil conversion, syngas production and the developments of high-performance reforming catalysts, but few concerned on the heat demand of the reforming process.

With the increasingly serious energy and environmental crisis, energy saving had become a key point to improve the further industrial development. The steel industry, also located in the urban district or the suburb, is a main big energy consumer. As one of main by-products during the ironmaking process, over 2.10 million tons blast furnace (BF) slag are discharged in China every year, with the discharged temperature about 1450–1550 °C [24]. However, such huge amounts of high-temperature heat energy aren’t recycled, and most BF slag currently is directly disposed via water quenching (also called wet granulation) to glassy granules mainly as feedstock of cement, yet causing the huge amount of sensible heat wasted [25]. Therefore, researchers performed lots of works to recycle the BF slag sensible heat, with physical method and chemical method. The physical method was mainly used to recycle waste heat of solid slag granules (∼1200 °C) after dry granulation, for which the rotary-cup granulation method has been extensively studied because of high treatment capacity and continuous working [[25], [26], [27], [28], [29], [30], [31], [32]]. Liu et al. [25] found that compared to packed bed, moving bed and fluidized bed, a gravity bed boiler exploited by his team was more suitable for the waste heat recovery of the solid slag granules, due to the production of high-temperature steam and the heat recovery rate as much as 91%. On the other hand, the chemical method by which the physical sensible heat of BF slag was converted to chemical energy, had great advantages compared to the physical method, such as integration of multiple sectors and production of high-value syngas [33], thus being widely concerned. Kasai et al. [34] first proposed the molten slag as the heat carrier of methane reforming, and then Li et al. [35] and Duan et al. [36,37] used it into coal gasification, while now the solid slag as heat carrier was widely investigated, mainly in coal pyrolysis [38] and gasification [39,40], biomass pyrolysis [41] and gasification [42], waste tire pyrolysis [43], municipal solid waste gasification [44], sludge pyrolysis [45] and gasification [46], etc. Sun et al. [47] performed the combined CO₂/H₂O gasification of biomass using the waste heat of the slag, and the result indicated that the introduction of slag remarkably increased the syngas yields. According to the above, in this paper the BF solid slag was to be used as heat carrier for the combined steam/dry reforming process of bio-oil for the production of H₂/CO syngas, especially the syngas with the H₂/CO ratio of 3:1 for methanation.

In this work, the combined reforming process of bio-oil with BF slag as heat carrier was investigated based on thermodynamic analysis, aimed to prove the theoretical feasibility of this process and provide a pre-insight for further research. Firstly, the effects of the CO₂/C ratio (the mole ratio of CO₂ to carbon in the bio-oil), S/C ratio (the mole ratio of steam to carbon in the bio-oil) and FS/B ratio (the mass ratio of BF slag to bio-oil) on the equilibrium compositions of bio-oil reforming process were discussed, with the optimal reforming conditions for the production of the 3:1-H₂/CO syngas to be achieved. Then, the energy balance of the combined reforming process was discussed, with the least consumptions of bio-oil and slag to be achieved in the premise of the high H₂/CO syngas yield.