Life-cycle performance of indirect biomass gasification as a green alternative to steam methane reforming for hydrogen production

Highlights

• Life cycle assessment of hydrogen from indirect gasification of poplar biomass.

• Process simulation as the main source of inventory data.

• Identification of the main sources of environmental impact.

• Calculation of the life-cycle energy balance of the system.

• Comparison with hydrogen from conventional steam methane reforming.

Abstract

The environmental performance of hydrogen production via indirect gasification of poplar biomass was evaluated following a Life Cycle Assessment approach. Foreground data for the study were provided mainly from process simulation. The main subsystems and processes that contribute to the environmental impacts were identified. Thus, poplar production and direct emissions to air from the processing plant were found to be the main sources of environmental impact. Furthermore, a favourable (positive) life-cycle energy balance was estimated for the gasification-based system.

The environmental profile of hydrogen from poplar gasification was compared with that calculated for hydrogen from conventional steam methane reforming according to inventory data also obtained through process simulation. Gasification-derived biohydrogen was generally found to be a promising hydrogen fuel, with reduced greenhouse gas emissions and a low non-renewable energy demand. However, its suitability depends on the evaluated impact categories. Moreover, the biomass demand should be minimized to enhance the efficiency of the system.

Introduction

The shortage of fossil fuels and the growing demand of energy have led to a global context of increasing energy prices [1]. This situation, along with the fact that the combustion of fossil fuels produces a large amount of greenhouse gas emissions (mainly CO₂), has derived in a growing interest in the development of clean, renewable energy systems. In this context, hydrogen is often considered as one of the most promising alternatives to fossil fuels since the only product from hydrogen combustion is steam. However, the environmental performance of hydrogen-production systems highly depends on the type of primary energy and conversion technology used [2].

Conventional technologies for hydrogen production are currently based on fossil fuels. In fact, hydrogen is mainly produced from natural gas via steam methane reforming (SMR) [3]. However, the use of fossil fuels poses serious dangers not only to the environment (e.g., climate change), but also to the health of all living species [4]. Therefore, other alternatives for hydrogen production are being studied. Among the numerous methods for green hydrogen production categorized by Dincer [5], those driven by electrical and/or thermal energy have attracted much attention. For instance, water electrolysis is a common electricity-driven technology to produce hydrogen, but it requires the use of renewable electricity to be considered a green method. Within the group of thermal energy-driven methods, thermochemical processes are being widely studied, focussing mainly on thermochemical water-splitting cycles, biomass gasification, and biofuel reforming [5], [6].

In particular, hydrogen production through lignocellulosic biomass gasification is seen as a system with a promising performance in terms of global warming impact and energy security [7]. Lignocellulosic biomass gasification involves the thermochemical conversion of biomass at elevated temperature in a gasification medium such as air, oxygen and/or steam. The product obtained is a gaseous fuel called (bio)syngas, which consists of carbon monoxide, hydrogen, carbon dioxide, methane, other light hydrocarbons, water, and trace amounts of other compounds such as char and tars. When pure steam is used as the gasifying agent, a syngas with high hydrogen content is produced. This process requires an external energy source to satisfy the heat demand, which is often done through char combustion in a separate reactor. This is called indirect gasification. Due to the high hydrogen content of the produced syngas and the renewable nature of biomass, energy systems based on this process could be appropriate candidates for the sustainable production of hydrogen.

However, even though lignocellulosic biomass is a renewable resource, its use does not guarantee an appropriate environmental performance [8]. Therefore, comprehensive analyses are needed to evaluate the suitability of this type of energy systems. Life Cycle Assessment (LCA) is a well-defined methodology to assess the environmental aspects and potential impacts associated with a product by compiling an inventory of relevant inputs and outputs of the product system, evaluating the potential environmental impacts associated with those inputs and outputs, and interpreting the results of the inventory analysis and impact assessment phases in relation to the objectives of the study [9], [10]. The present work addresses the LCA of hydrogen produced via indirect gasification of lignocellulosic biomass, in comparison with hydrogen from conventional SMR. LCA results are expected to be useful not only for product development and improvement, but also to orientate decision and policy making towards environmental sustainability [9].