Climate changes are the most pressing environmental challenge the world faces today, and there is an urgent need to promote the use of renewable energy sources in order to ensure a sustainable future [1, 2]. The industrial revolution, along with the economic growth and the rising global population that have taken place in the past few centuries, have driven the energy demand upwards [3]. The increasing energy requirements needed to meet the modern way of life have resulted in a rapid increase in the global greenhouse gas (GHG) emissions. Consequently, the Earth’s average surface temperature has experienced a sharp rise that causes a set of worrying changes to the Earth’s climate [1]. The average surface temperature rose with roughly 1ᵒC during the period from 1880 to 2020, and of this 0.7ᵒC from 1980 to 2020. Thus, two-third of the global warming occurred in the last 40 years, meaning that the rate of temperature increase has nearly quintupled during these years. Without mitigating policies, the global average temperature is predicted to rise by 2ᵒC - 6ᵒC compared to pre-industrial levels by the end of 21st century [4]. Figure 1-1 shows the deviation in the Earth’s average surface temperature in the period from 1880 to 2020. The temperature anomalies are calculated based on the average temperatures from 1951 to 1980 [4].
Figure 1-1. Global land-ocean temperature index [4].
___
4
The world emits around 50 billion tonnes CO2 equivalents of GHG every year [5]. CO2
comprises for 76% of the global GHG emissions, methane, nitrous oxides and hydrofluorocarbons contributes to 16%, 6% and 2%, respectively [6]. The primary emission source is the conversion of energy, which make up nearly three-quarters of the annual global GHG emissions. Within the energy sector, heat and electricity represents about 31% of the 2016 global GHG emissions, followed by the transport sector that stands for 16% [5]. Figure 1-2 shows the breakdown of global greenhouse gas emissions by sector in 2016.
Figure 1-2. Global manmade Greenhouse Gas Emissions by sector in 2016. Based on data from [5].
Fighting the climate changes requires global action, and the importance of gathering global consensus and cooperation to tackle the ongoing crisis is essential. National and international climate policy guidelines include a global strategy to prevent the man-made climate changes by reducing the emissions and stabilizing the levels of GHG in the atmosphere. The global climate change mitigation is governed by commitments through the Paris Agreement, which aims to limit the average global surface temperature rise to well below 2ᵒC above the pre-industrial levels by the end of the 21st century. In the long term, the goal is even further below 1.5ᵒC [7, 8].
Although the climate change is a global issue, each country must play its part by drawing up comprehensive national climate action plans. Norway aims to be a driving force in the international climate work. The Norwegian government’s goal is for Norway to become climate-neutral by 2030, and a low-emission society by 2050. One of the priority
___
5
areas for actions is to reduce the emissions from the transport sector. The transport sector stands for 19% of the annual GHG emission in Norway, where the road transport is by far the biggest emitter accounting for more than 12% of the GHG emissions in 2016 [5]. One strategy is to reduce the sources of these gases by speeding up the introduction of low-emission alternative transport fuels, such as liquid transport biofuels [7, 9].
Liquid transport fuels are currently mainly produced from fossil fuels, which are non-renewable resources such as petroleum, natural gas and coal [2, 10]. The challenges with fossil fuels are, not only that the use of fossil fuels emits substantial amounts of GHG, but the stocks are finite and the availability of these resources is limited. The massive expansion in the transport sectors worldwide, and the rising fear over the effect of climate changes, have brought to life the search for a climate-friendly alternative to fossil fuels [11]. Biomass has become one of the key resources to reduce the dependence on fossil fuels in the transport sector, and at the same time provide energy in a more sustainable and climate-neutral manner [12]. Biomass refers to a broad variety of feedstock including harvested wood, forestry residues, energy crops, agricultural crops and residues as well as urban waste from commercial industry [13]. Unlike underground fossil reserves, biomass is abundantly available. It is considered a renewable energy source based on the concept that the plant material used can be replaced through re-growth. Biomass energy does not generate any net additional CO2
into the atmosphere since the CO2 emitted is already part of the biogenic carbon cycle.
Thereby biomass offer immediate reductions in the greenhouse gas emission.
New and efficient technologies that make it possible to produce transport fuels from renewable sources, such as biomass, have lately become more popular. Fluidized bed gasification is a promising energy conversion technology, which converts the biomass into a high-quality syngas in presence of heat and a gasifying agent [14]. The syngas consists of mainly H2 and CO, and can be processed into any gaseous and liquid transport fuels, as well as several other convenient chemical products [15]. However, processing biomass in fluidized bed is challenging due to the complex high-temperature chemistry of the biomass ash. The fluidized beds suffer from operational problems due to molten
___
6
biomass ash that interacts with the bed material. The key to unlocking gasification as a viable route for biomass to transport biofuels is therefore by solving the ash related problems.
This PhD-work is part of project 280892 FLASH (Prediction of FLow behaviour of ASH mixtures for transport biofuels in the circular economy). The research is funded by the Research Council of Norway, program for Energy Research (EnergiX). The main objective of the FLASH-project is to accelerate the implementation of biomass to biofuels via gasification. The strategy is to mitigate the ash-related challenges, which still are the main barrier for a commercial breakthrough of thermo-chemical conversion of biomass.
An important aspect is to discover the underlying ash mechanisms (ash behaviour and ash chemistry) that currently separates the two dominating gasification technologies (entrained flow and fluidized beds). The FLASH-project is divided into three work packages, WP1, WP2 and WP3, where this PhD-work is part of WP3. The main objective of WP1 is to increase the fundamental understanding of ash properties and ash behaviour in thermal systems, and particularly thermal systems under reducing conditions. The work package covers measured ash melting behaviour in correlation with ash viscosity, compared with calculated thermodynamic predictions of ash behaviour and viscosity (for ash speciation and phase distribution). WP2 proposes the development of methods and models for predicting ash behaviour through experimental investigation of ash viscosity. The viscosity data obtained are implemented to suggest and develop new methods for ash viscosity measurement.
WP3 defines and tests strategies to mitigate ash-related challenges based on theoretical and experimental studies of ash melt in bubbling fluidized bed reactors. The FLASH-project group consists of partners from the University of South-Eastern Norway, SINTEF Energy Research, University of Natural Resources and Life Sciences, Austria and Aalto University, Finland.