Modelling advanced reforming of bio-compounds for hydrogen production

Abstract

In the efforts to decarbonise the energy system, there has been a great deal of interest in the potential of hydrogen (H2) as a versatile, low carbon energy vector. To support rising demand for hydrogen in existing and new applications, it will be necessary to find cost-effective routes for hydrogen production at scale. Recent research has identified new methods to optimise the steam reforming process as a means to achieve this. These include chemical looping, in which a metal oxide provides an unmixed source of oxygen directly into the reforming reactor, to enable autothermal reactor operation. Other work has considered sorption enhancement, in which solid CO2 sorbents provide in situ CO2 capture, enhancing product purity and improving process yields. Another branch of research considers the use of bioenergy feedstocks to reduce carbon intensity. One promising route is the fast pyrolysis of bioenergy feedstocks to produce bio-oils, followed by steam reforming. This route could combine the benefits of a flexible bio-based supply chain with those of the steam reforming process, including its thermal efficiency and cost-effectiveness. This thesis brings together these two branches of process development, to consider the feasibility and benefits of using bio-oil in advanced reforming processes, to produce hydrogen with low, or negative, carbon emissions. A thermodynamic evaluation is first presented, to determine the thermodynamic feasibility of different bio-oil reforming technologies, including conventional steam reforming (C-SR), sorption-enhanced steam reforming (SE-SR), chemical looping steam reforming (CLSR) and sorption-enhanced chemical looping steam reforming (SE-CLSR). When these benefits of chemical looping and sorption enhancement are combined, the resulting SE-CLSR process is autothermal, with reduced risk of carbon deposition, reduced H2 purification requirements, and the potential for readily separated CO2. A techno-economic evaluation is carried out on the viability of SE-CLSR and C-SR and with CO2 capture (C-SR-CCS), using heat and material balances derived from process models in Aspen Plus. C-SR-CCS and SE-CLSR produce hydrogen in a similar price range, of 3.8 to 4.6 $kgH2-1. Both processes have similar operating costs, but SE-CLSR has a lower capital cost, leading to a marginally lower hydrogen cost. SE-CLSR has certain other advantages, such as the elimination of fossil-based energy, and therefore increased potential for net negative emissions. Cost of carbon avoided, based on direct process emissions, including negative emissions, is estimated to be in the range of 95 to 105$ tCO2-1, similar to bioenergy with CCS (BECCS) in other industries. The analysis identified that a key process stage is simultaneous reduction-calcination, during which the reactor bed undergoes several important functions required to complete the SE-CLSR cycle. These include sorbent regeneration, reduction of the oxygen transfer material, and bed cooling. In Chapter 7, a dynamic packed bed reactor model is created in gPROMS Modelbuilder™ 4.1.0. This confirms that simultaneous reduction-calcination in a nickel-based system is feasible in principle. However, certain design and operating strategies will be required to manage the many complex and interacting factors in the system, including CO2 equilibrium pressure, CO2 product purity, and the relative speeds of reduction and calcination fronts. Future models of the entire SE-CLSR process will also require the derivation of bio-oil steam reforming kinetics. Chapter 7 details an experimental study on acetic acid, a major constituent of bio-oil that is commonly used as a model compound. A kinetic model is proposed, using a simplified reaction scheme comprised of acetic acid steam reforming, acetic acid decomposition to CO, and the water gas shift reaction. This model is subsequently used to compare steam methane reforming to bio-oil steam reforming in a low-pressure industrial-scale reactor bed. This identifies that the relatively slow kinetics of acetic acid steam reforming are another important aspect for consideration. Taken together, the above analyses provide a high-level assessment of the advanced reforming of bio-oils. In principle, SE-CLSR could offer certain technical and economic advantages compared to conventional steam reforming, and could offer a competitive route to hydrogen production with negative emissions. However, this is contingent upon several priority areas identified for process development