Substitute Natural Gas from Waste: Technical Assessment and Industrial Applications of Biochemical and Thermochemical Processes

By Pier Ugo Foscolo

Substitute Natural Gas from Waste: Technical Assessment and Industrial Applications of Biochemical and Thermochemical Processes provides an overview of the science and technology of anaerobic digestion and thermal gasification for the treatment of biomass and unrecyclable waste residues. The book provides both the theoretical and practical basis for the clean and high-efficiency utilization of waste and biomass to produce Bio-Substitute Natural Gas (SNG). It examines different routes to produce bio-SNG from waste feedstocks, detailing solutions to unique problems, such as scale up issues and process integration. Final sections review waste sourcing and processing.

This book is an ideal and practical reference for those developing, designing, scaling and managing bio-SNG production and utilization systems. Engineering students will find this to be a comprehensive resource on the application of fundamental concepts of bio-SNG production that are illustrated through innovative, recent case studies.

• Presents detailed scientific and technical information
• Describes up-to-date concepts, processes and plants for efficient anaerobic digestion and gasification of wastes and syngas utilization
• Compares gasification with anaerobic digestion for different situations
• Proposes alternative strategies to increase efficiency and overcome energy balance limitations
• Includes benchmarking data and industrial real-life examples to demonstrate the main process features and implementation pathways of bio-SNG systems from dry and wet waste, both in developed and developing countries

• Language: English
• Publisher: Academic Press
• Release date: May 1, 2019
• ISBN: 9780128156445

Book preview

Kingdom

Preface

Massimiliano Materazzi, Department of Chemical Engineering, University College London, London, United Kingdom

Pier Ugo Foscolo, Department of Industrial Engineering, University of L'Aquila, L'Aquila, Italy

The development of alternative, renewable sources to reduce or replace our dependence on fossil fuels, and the reduction of increasing amounts of waste being incinerated or sent to landfill, have emerged as two paramount priorities today. An essential component of the quest for energy independence and more sustainable waste management is to develop renewable, environmentally friendly sources of energy via the conversion of waste (municipal and industrial) and low-value biomass to biofuels.

Renewable gas is increasingly considered to be one of the most attractive heat and transport fuel vectors that will accelerate the transition towards greener energy systems. It is a clean and relatively low-carbon-intensity fuel, and can be utilized efficiently in existing infrastructure and demand-side technologies (gas boilers for heating and an increasingly wide range of available vehicles). Biomethane, in particular, retains all the attributes of methane from natural gas, with the crucial advantage that the fuel is renewable, offering substantial CO2 savings. Very few other renewable vectors are as fungible, with so few demand-side constraints!

Biomethane, or bio-SNG (the acronym stands for substitute natural gas) can, and is being produced via the upgrading of biogas obtained mostly via two processing routes: anaerobic digestion and gasification routes (this is commonly termed ‘bio-SNG’ as synthetic natural gas, although, for simplicity, we will not make this distinction in this book). Whilst both technically feasible, the waste-approach, as opposed to conventional biomass, is less mature. Transition from aspiration to widespread operating facilities and infrastructure requires a detailed understanding of the technical and commercial attributes of the full chain from feedstock supply through to delivery of grid-quality gas, in both cases.

This book provides a critical appraisal of the opportunity afforded by bio-SNG, building on a review of the issues associated with waste sourcing and processing, a detailed analysis of the technology options and description of industrial cases. We engaged with the highest expertise and practitioners in the field to give this book a strong scientific, yet practical, imprint.

Part I (Chapters 1–3) introduces the subject, with particular attention to natural gas and bio-SNG framework, feedstock definition, and best sorting practices. The following discussion reviews and compares the two main production routes for bio-SNG, that is, the biological and thermochemical routes, in sequence. Part II (Chapters 4 and 5) presents a general overview of feedstock preparation and anaerobic processes for the production of biogas and its upgrading to bio-SNG. Part III (Chapters 6–9) is structured to give a comprehensive description of the fundamental research focused on thermochemical treatment, including waste gasification, gas cleaning and subsequent transformation to synthetic natural gas.

In contrast to Parts II and III, which have a more fundamental character, the final parts are more practically oriented. Part IV (Chapters 10–13) provides an overview of bio-SNG utilization in developed and developing countries, upgrading technologies for final use and a sustainability assessment of the two production routes examined. Part V (Chapters 14–19) draws on benchmarking data and industrial examples spread around the world, some of which are very recent and innovative, to demonstrate the main process features and implementation pathways.

The level of discussion throughout is at the postgraduate/professional level, but practising engineers and scientists concerned with biofuels and new waste-management practices will, it is hoped, find this book interesting and useful.

We would like to thank all distinguished authors for their efforts in writing the chapters and meeting the deadlines, and staff at Elsevier (in particular Raquel Zanol, Jennifer Pierce, Peter Adamson, and Vijay Bharath Rajan) who helped to finalize this book in a very professional way. We are also grateful to several publishers who permitted free use of their material for this book.

1

The role of waste and renewable gas to decarbonize the energy sector

Massimiliano Materazzi¹ and Pier Ugo Foscolo²,    ¹Department of Chemical Engineering, University College London, London, United Kingdom,    ²Department of Industrial Engineering, University of L’Aquila, L’Aquila, Italy

Abstract

With a continuous population increase and economies expansion, global energy consumption is increasing fast, and so is waste generation, whereas cheap fossil fuels as non-renewable sources are rapidly depleting. A significant transition to renewable sources, along with improved technologies and better waste management solutions, are some of the proposed solutions that can soon reduce the dependency on traditional fossil sources, leading to a slow but consistent decarbonisation of the world energy sector. In this context, natural gas is seen as bridging fuel that will help to meet short-term emission targets for power generation, heating and transport while restructuring the electricity mix towards renewables. Renewable gas or Green Natural Gas (GNG) can, and is being produced via the upgrading of biogas from Anaerobic Digestion of many types of waste feedstock. However, in order to achieve a step change in production capacity, alternative approaches such as via thermochemical routes are also necessary. Depending on the technology improvements, future cost reductions and support schemes, the volume of GNG production will dramatically increase and this increase will facilitate rapid growth of its use to fully decarbonise the heating and transport sectors.

Keywords

Global energy consumption; waste treatment; natural gas; renewable gas; decarbonization pathways; green natural gas (GNG)

Chapter Outline

Introduction 1

Decarbonization pathways 3

The role of waste in decarbonizing the energy sector 5

Natural gas and renewable gas 7

Overview of waste treatment technologies for the production of green natural gas 11

Biochemical conversion 11

Thermochemical conversion 13

Other green gas technologies 14

Conclusions 16

References 17

Introduction

Today’s global community faces a challenge of epochal proportions. On a planet which will soon house 8 billion people, the greatest challenge is to meet every day the growing demand for energy at sustainable cost, while avoiding the creation of disruptive environmental imbalances. It is no secret that over the past several decades, the world has dramatically changed, largely thanks to the contribution of fossil fuels (e.g., coal, oil, and natural gas) and the massive contamination of the environment by wastes. Fossil fuels have provided us with cheap and convenient energy which we use for heating and electric power generation, and have been widely used as transportation fuels and also for chemical production. With a continuous population increase and expansion of economies, global energy consumption is increasing rapidly, and so is waste generation, whereas cheap fossil fuels as nonrenewable sources are rapidly depleting. Moreover, their massive utilization has also caused many problems such as environmental damage (e.g., ozone depletion, global warming) associated with various emissions. Greenhouse gas (GHG) emissions resulting from the provision of energy services have contributed significantly to the historic increase in atmospheric GHG concentrations. Recent data confirm that consumption of fossil fuels accounts for the majority of global anthropogenic GHG emissions (International Energy Outlook, 2018). Therefore, finding effective, sustainable solutions to combat the effects of anthropogenic global warming is the greatest challenge faced by the global community in the 21st century. To this end, a massive change in the energy supply structure is required in the future to meet the growing demand for energy, while reducing the impact on the environment. In this regard, several scenarios have been forecast by different institutions based on different perspectives and techniques (Energy and Climate Change Committee, 2016).

According to the International Energy Outlook (IEO) 2017 published by the International Energy Agency (IEA), world energy consumption will increase by 28% between 2015 and 2040, with more than half of the increase attributed to non-Organization for Economic Cooperation and Development (OECD) countries (i.e., those countries outside the OECD, including China and India), where strong economic growth drives increasing demand for energy (International Energy Outlook, 2018). Fig. 1.1 presents world marketed energy consumption from different fuel sources over the 2015–40 projection period. Although renewable energy and nuclear power are the world’s fastest-growing forms of energy, fossil fuels are expected to continue to meet much of world’s energy demand, sharing more than 80% of world marketed energy consumption. Among them, liquid fuels remain the world’s largest source of energy due to their importance in the transportation and industrial end-use sectors, whereas their share is predicted to decrease from 33% in 2015 to 31% in 2040, as the supply is projected to be driven by high and fluctuating world oil prices.

Figure 1.1 World energy consumption by fuel type, 1990–2040 (quadrillion Btu) (International Energy Outlook 2018 - IEO 2018).

Natural gas is the world’s fastest-growing fossil fuel, increasing by 1.4%/year, compared with liquid gas at 0.7%/year growth and virtually no growth in coal use (0.1%/year). The study shows that coal is increasingly replaced by natural gas, renewables, and nuclear power in electricity generation, even in countries such as China, where coal has always been the primary source of energy.

Similarly, natural gas continues to be an attractive fuel for the electric power and industrial sectors in many countries. These two uses account for almost 75% of the projected increase in total consumption between 2015 and 2040. This is mostly because of low capital costs, favorable heat rates, and relatively low fuel costs. Furthermore, the new limits for sulfur content of marine fuels will lead to a greater use of liquefied natural gas (LNG) as a bunkering fuel compared to traditional liquid fuels (Kumar et al., 2011).

Along with nuclear, the other fastest-growing source of world energy is renewable power. Renewables are generally defined as those energy resources which are naturally replenished on a human timescale such as sunlight, wind, biomass, tides, waves, geothermal heat, etc., and do not directly contribute to GHG accumulation in the atmosphere. In the reference case, the renewables share of total energy use rises from 12% in 2015 to 17% in 2040. A combination of these two fast-growing markets (i.e., natural gas and renewables), along with improved technologies and better waste management solutions, are some of the proposed solutions that could soon reduce the dependency on traditional fossil sources, leading to a slow but consistent decarbonization of the world energy sector.

Decarbonization pathways

Sustainable development is currently considered the best approach to address the complex and interrelated threats that the world is facing today (Waas et al., 2010), including both a secure energy supply and environmental pressure (Tagliaferri, 2016). The energy transition toward a new energy model based on a CO2-emission-free society is one of the most critical challenges for our global community. It implies the integral transformation of productive processes at all levels such as energy production (shifting rapidly from fossil to renewable energy sources), energy vectors (heat, electricity, gas, etc.), and final utilization (industry, transportation, heating, etc.). Such transformation must include new advance technologies to make possible a transition from an energy system sustained by centralized energy generation toward a distributed one, in which the clear difference between energy producers and consumers becomes less evident (Abánades, 2018).

The emissions from the energy sector strongly depend on the sources and technologies used to supply the energy. Therefore, energy policies must play a major part in the modification of the energy mix in order to globally and locally decrease the impact of this sector. This is deemed high priority for a balanced development of the energy supply, as specified by the energy trilemma. This concept is most usually used to describe a balance between energy security, social impact, and environmental sensitivity, which are often presented as conflicting aspects of energy production (World Energy Council, 2016) (Fig. 1.2).

Figure 1.2 The energy trilemma.

For example, focusing on GHG reduction may impede energy security and access, while focusing on increasing affordability may impact energy security and environmental sustainability (Tagliaferri, 2016). In order to maintain the delivery of a balanced energy system in line with the trilemma, each country has still to face a number of substantial challenges for the future:

• The population worldwide is continue increasing, with a growth of more than 25% by 2050 [more than 9.8 billion people are expected to be living in the world in 2050 (World Bank, 2013)]. Hence, the challenge to deliver an affordable energy supply is self-evident.

• The energy infrastructure is rapidly ageing. In Europe, for example, many old coal power plants will be forced to close and will be replaced by lower-carbon and more efficient energy sources and technologies. Energy efficiency will of course be a very important part of the overall strategy, whatever route is followed; in particular it can help reduce investment costs. But energy efficiency on its own (i.e., without fuel switching) will not be enough to meet the emissions reduction targets.

• Currently most developed countries still strongly rely on solid and liquid fossil fuels for energy production (69% of the electricity production is based on coal and oil). International, European, and local agreements are pushing towards a drastic cut in GHG emissions and towards energy savings.

• Natural gas is seen as a reliable, flexible, and clean fuel as it can reduce GHG emissions when compared to coal and can provide a competitive energy price for a number of purposes (UK Houses of Parliament, 2017). According to a study by BP, globally, for every increase of 1% in the consumption of gas replacing coal or oil, emissions fall by an amount that would be achieved with the growth of renewables of over 10% (BP Energy Economics, 2018). Therefore, switching to gas is deemed important to meet short-term emission targets for power generation, heating, and transport while restructuring the electricity mix towards renewables.

Natural gas may play a very important role in this energy transition, also because it is one of the biggest single sources of heat, and available gas resources are a long way from being exhausted. The lower GHG emission rate in comparison with coal and oil is forcing the use of natural gas in most of the energy transition strategies to achieve environmental targets in the long term. Nevertheless, switching massively from coal or oil to natural gas is not enough to comply with the demands of the energy sector, and its utilization in a low-carbon economy implies the development of additional measures to reduce emissions in the short–medium term. Three main options are normally considered:

• Carbon capture and storage (CCS) is a family of technologies and techniques that enable the capture of CO2 from natural gas combustion or industrial processes, the transport of CO2 via ships or pipelines, and its storage underground, in depleted oil and gas fields (L’Orange Seigo et al., 2014).

• Carbon capture and utilization (CCU) converts the CO2 into commercially viable products such as chemicals, fertilizers, and fuels. These could replace fossil-fuel-based products further reducing GHG emissions and reducing waste materials, adding at the same time economic value to the process. However, it must be noted that the demand for CO2 is quite limited compared to the vast amount of CO2 that needs to be removed from the atmosphere, in order to reduce the detrimental environmental impacts of climate change. Within this context, it is more practical to consider a carbon capture, utilization, and storage (CCUS) alternative which combines the two options above (Norhasyima and Mahlia, 2018).

• Direct decarbonization decomposes the methane molecule in natural gas into hydrogen and carbon in a solid form, which is relatively easy to be stored/reused. This can be obtained by sorption enhanced reforming (Grasa et al., 2017; Di Giuliano et al., 2018) or cracking techniques (Amin et al., 2011). Hydrogen is used as a replacement for natural gas in the heating and power sectors and produces no emissions at the point of use.

Even though the overall costs are decreasing, these technologies are still very expensive. As a result, their widespread application has been hindered around the world and they have made limited progress. Furthermore, many of these options are still in the research and development phase, that is, not yet commercialized on a large scale. At the same time, there is the need to introduce a renewable element to the energy mix, in order to meet the energy security and sustainability of the energy trilemma in the long term.

Looking to an increasingly sustainable future, gas is itself becoming a renewable source thanks to the growth of biomethane (the green gas produced by urban and agricultural waste), hydrogen, and synthetic methane. Renewable gas is a programmable and flexible source that can take advantage of existing transport and storage infrastructure for natural gas. Furthermore, the costs of storage, transportation, and distribution of gas are much lower than those of electricity (transporting and storing gas costs 20- to 40-times less than electricity, respectively), thus allowing a more efficient decarbonization of the energy system (Seadi et al., 2008).

While the rate of growth will be influenced by government policy, ultimate potential growth in biogas production may be limited by the availability of sustainable and affordable feedstock (Keay, 2018). In fact, renewable gas from biomass can be controversial where clean water and fertile lands are taken up with large-scale production of energy crops or where feedstock needs to be transported over a long-distance to reach a production facility. Furthermore, in assessing the total feedstock available for biogas production, this clearly cannot be considered in isolation from other applications of biomass (e.g., direct burning of biomass for power generation).

The role of waste in decarbonizing the energy sector

Municipal solid waste (MSW) is considered as a renewable resource, no less than biomass or wind (Bosmans et al., 2013). In reality, general waste is only partially renewable due to the presence of fossil-based carbon (mostly plastics) in the waste, and only the energy contribution from the biogenic portion is counted towards renewable energy targets. However, according to a study from the World Bank, the average global MSW composition has 46% of organics, 17% of papers, 10% of plastics, 5% of glass, 4% of metals and 18% of other materials (Bhata-Tata and Hornwed, 2008). Therefore, at least 60% of the MSW is composed of biodegradable materials (biomass), which are renewable sources of energy. In low-income countries, this share could be above 70% for the total composition of the waste. Furthermore, even if nonnegligible fractions of waste plastics are present, the material is still considered a low-carbon and relatively green feedstock, as it offers the opportunity to feed back into the circular economy a material that would have otherwise been lost in the environment.

In the United Kingdom, the lowest cost opportunity for production of renewable waste lies with household waste, as the annual average production of household waste per person amounts to around 415 kg (DEFRA, 2016). In other EU countries, municipal waste generation totals vary considerably, ranging from 777 kg per capita in Denmark (in 2016) to 261 kg per capita in Romania (Eurostat, 2017). The variations reflect differences in consumption patterns and economic wealth, but also depend on how municipal waste is collected and managed. There are differences between countries regarding the degree to which waste from commerce, trade and administration is collected and managed together with waste from households.

The modern waste management strategies promote minimization, recycling, and reuse of waste, with disposal to landfill being considered as the least desirable option to prevent pollution of surface water, groundwater, soil and air, and to reduce GHG emissions from the landfill site. According to this principle, while there is an obvious need to minimize the generation of wastes and to reuse and recycle them, the technologies for recovery of energy from wastes can play a vital role in mitigating the problems. Besides recovery of substantial energy, these technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal, which can be better managed for safe disposal in a controlled manner while meeting the pollution control standards. In particular, where there is residual waste (i.e., remaining waste that cannot be economically or practically reused or recycled), the aim would be to get the most value from it via energy recovery, where doing so is the best overall environmental option. This can contribute to our renewable energy targets, and help with the move towards a more secure fuel supply.

A host of technologies are available for realizing the potential of waste as an energy source, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste. These include both thermochemical (e.g., incineration, gasification, pyrolysis, hydrothermal, etc.) and biological routes [anaerobic digestion (AD), fermentation, composting, etc.]. Some technologies can cope with a broad range of calorific values and water content of their waste fuel; others require much more specific levels to operate efficiently. The availability and general composition of waste also affects the technologies that are suitable to deliver environmental benefits. For example, renewable gas produced by AD of crops, agricultural residues, and waste has grown strongly over the last years in Europe and worldwide. However, the potential of conventional AD is limited by the availability of suitable feedstocks. New technologies that can process a wider range of materials, including lignocellulose and plastics polymers, are required for renewable gases to make a meaningful contribution. To this end, thermochemical routes offer a promising expansion in the coming years.

Thermochemical technologies have historically been used to produce heat and electricity (power plants) via incineration of the waste feedstock, alone or together with other fuels. More recent developments, which fall into the category of advanced thermal treatments (ATTs), convert the waste into other energy vectors, including fuels, renewable gas, hydrogen, or chemicals, utilizing catalysts.

The conventional way to generate electricity from waste is through direct combustion, with the heat used to produce steam to drive a turbine. Modern plants have an overall energy efficiency close to 30%, and the electricity generated is rapidly transmitted to the grid to meet the energy demand.

Conversely, in the production of fuels route, gas in particular, the energy can easily be stored, used when needed, and ultimately released directly to the final user, which can be a heating appliance, a vehicle engine, or a new-generation power generator (e.g., fuel cells). Biofuels are therefore potentially a more efficient use of the energy in the waste, provided creating the fuel itself is not too costly and energy demanding.

Within the context of this book, renewable gas produced from wastes perfectly exemplifies the circular economy concept for two important reasons: it utilizes waste and refuse from conventional production processes to obtain further products, thereby avoiding wastage of materials and energy with much lower impact to the environment. An economy based on this approach is the best example of a sustainable economy capable of getting the most out of waste, abandoning the produce–consume–discard philosophy and reducing drastically carbon emissions. For this reason, renewable gas from waste is now considered an ally in the decarbonization of the economy, particularly in the heating and transport sectors.

Natural gas and renewable gas

Natural gas has already been described as the cleanest fossil fuel, producing less CO2 per joule delivered than either coal or oil and far fewer pollutants than other hydrocarbon fuels. However, in absolute terms, it does contribute substantially to global carbon emissions, and this contribution is projected to grow. In the United States and China, increases in natural gas production between 2015 and 2040 are projected to mainly come from the development of shale resources (US-EPA, 2016). Shale resource development accounted for 50% of US natural gas production in 2015, and this is expected to increase to almost 70% in 2040, as the country leverages advances in horizontal drilling and hydraulic fracturing techniques and taps into newly discovered technically recoverable reserves.

In addition, natural gas itself is a GHG more potent than CO2. Natural gas is composed mainly of methane, which has a greenhouse potential 20-times greater than CO2. Recent estimates by the Environmental Protection Agency (EPA) place direct emissions of methane at 16% of all global anthropogenic GHG emissions in 2014 (EPA, 2010). The substitution of natural gas by a renewable equivalent is, therefore, an interesting option to reduce the use of fossil fuels and the associated GHG emissions, as well as from the point of view of security of supply.

The renewable alternative for natural gas is the so-called green natural gas (GNG), that is, gaseous energy carriers produced from biomass or waste comprising both biogas/biomethane and bio-synthetic natural gas (bio-SNG), as shown in Fig. 1.3. For clarity of definitions of the gas terminology, we refer to Table 1.1 from now on.

Figure 1.3 The natural gas pyramid for sustainable heat and gas production in the future.

Table 1.1

Biogas (biomethane) and bio-SNG are versatile low-carbon sources of energy produced by physical and chemical transformation of biogenic substrates. They both contain large fractions (from 60% to 90% vol.) of methane and they are therefore used to substitute natural gas. Few other renewable vectors are as fungible, with so few demanding side constraints (Cozens and Manson-Whitton, 2010).

Due to the very wide range of input materials, a significant amount of energy can be produced with these low-carbon gases. A recent report released by Ecofys estimated that it is possible to produce at least 98 billion cubic meters (bcm) of biomethane in Europe, that is, 1072 TWh of energy annually by 2050 (Timme van Melle et al., 2018). This figure could double if bio-SNG from the thermochemical production of synthetic natural gas is added. It is very clear, therefore, that the renewable GNG potential available just at the European level could make a sizable contribution to the energy supply of the future.

In addition to the large volume, the application of GNG as an energy source is remarkable. In fact, GNG can be used in all applications designed for natural gas. The main difference between the two fuels is that, in addition to methane, natural gas contains a variety of other hydrocarbons, such as butane, ethane, and propane, which give it a higher calorific value than pure methane.

Biogas or syngas from gasification are commonly burned in internal combustion engines to generate electricity. Small-scale internal combustion engines, with a rated capacity of less than 200 W, have an electrical conversion efficiency of up to 25%. Larger internal combustion engines (up to 1.5 MW) have much higher electrical conversion efficiencies, of 30%–35% (DEFRA, 2014). When gas is used to produce electricity, there is the added potential for heating water (cogeneration) from the engine’s exhaust and cooling systems to generate low-pressure steam (and hot water) for local appliances. Cogeneration systems have an overall conversion efficiency as high as 65%–85%, and represent the most used application of biogas in Europe at present (Deublein and Steinhauser, 2010). A promising near-future application for electricity generation from GNG is the use of gas turbines. For larger-scale systems, combined-cycle power stations are made up of gas turbines, steam turbines, and waste heat recovery boilers that function together to produce electricity. Modern gas turbine plants tend to be small, extremely efficient, environmentally friendly, and visually unobtrusive. Gas turbines allow a greater fraction of waste heat to be recovered as steam, a critical commodity for many industries, so overall efficiency levels for gas turbines can be up to 75%. Future alternatives also include the use of fuel cells for conversion of biogas or syngas into renewable electricity. In particular, high-temperature fuel cells, such as solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs) offer the advantage of very high energy efficiencies (close to 50%), high tolerance to contaminants, and effective heat integration with no need to remove traces of carbon oxides, which are normally detrimental for the more common low-temperature alternatives (Bocci et al., 2014). For all these applications, the comprehensive and effective use of the heat generated in a biogas or bio-SNG plant, both at the front-end or at the final use, is a great incentive to the use of GNG. For example, residue and waste-recycling plants have a potential for the sustainable supply of heat in rural areas that, to date, has not been sufficiently recognized.

Unlike wind and solar power, renewable electricity and heat produced by biogas or syngas is available at any time and can supply base load demand. Peak requirements can also be met if sufficient quantities are stored, or if the GNG is directly available from the grid. Not only can green gas be easily be fed to the existing natural gas network, but also the costs of storage, transportation, and distribution of gas are much lower than those of electricity, thus allowing a more efficient decarbonization of the energy system. This represents an important milestone in the development of new distribution networks, which were originally designed to run on a steady electricity supply from centralized power plants but which encounter problems when supply fluctuates, due to the difficulty in energy storage.

Within this context, the production and use of hydrogen as a possible energy storage source has long been investigated, although largely abandoned due to the lack of adequate H2 storage infrastructure. Renewable gas would largely obviate the issue. This is due to the fact that methane can be produced from direct biomass/waste processing, but also synthetized from excess production of electricity (see Power to gas section) where this is available at low cost and near a source of CO2 (e.g., power plants, CCS, etc.), providing a valid alternative to electrical batteries.

In addition, biomethane and bio-SNG could also be used as fuel for powering vehicles available in the market today, such as heavy good vehicles (HGVs), or liquefied to LNG for the naval industry. Noise levels generated by methane-powered engines are considerably lower than those of diesel engines, a plus in congested urban environments. Exhaust fume emissions are also much lower than those from diesel engines, and nitrogen oxide emissions are very low (Abdelaal and Hegab, 2012).

For all these reasons, GNG will certainly have an important function in a future sustainable energy system. How fast and at what cost this function can be performed depends largely on the general political and legal situations. Furthermore, the allocation of more research funding within the sector is necessary to provide adequate support for the development and implementation of new and relevant concepts and strategies. Funding should cover not only research into efficiency gains and cost reduction, but also the actual construction of such installations to prove the efficacy at different scales and address the cost differentials between natural gas and low-carbon alternatives.

Overview of waste treatment technologies for the production of green natural gas

For many years, sewage sludge and agricultural manures were the principal inputs for green gas production, making up over 80% of the total (Seadi et al., 2008). More recently, though, producers have been experimenting with biogas-specific agricultural crops, including rapeseed and maize. Both the crop itself and the resultant fodder (silage) are used. Due to the space issues associated with biofuel-specific crops, the future of this second generation of feedstocks is questionable. For them to reach anything like critical mass, the amount of land and clean water that would be needed, especially in small- to medium-sized countries, would soon become both impractical and politically unacceptable. Thus, the GNG industry, much like the biodiesel and ethanol industries, will need to find alternative feedstock if it is to flourish and become a force in the energy sector. Therefore, the use of waste as renewable feedstock for green gas generation (either as bio-SNG or biomethane) is an important (perhaps the only) way to help meet the targets on renewable energy and climate change while addressing energy security at the same time.

The most widespread technologies for waste treatment and gas production fall into two categories: biochemical (AD, composting, and landfill) and thermochemical conversions (gasification and pyrolysis). In the following sections the main disposal and treatment techniques are reviewed succinctly, leaving the wider discussion to the following chapters.

Biochemical conversion

The biochemical conversion processes, which generally include AD in a controlled or spontaneous way, is preferred for wastes having high percentage of organic biodegradable (putrescible) matter and high moisture content. Organic waste from various sources is composted in oxygen-free conditions circumstances resulting in the production of biogas which can be used to produce both electricity and heat, or upgraded to biomethane. AD also results in a dry residue called digestate which can be used as a soil conditioner, or as a fuel itself for thermochemical applications, due to the significant energy content. The same process can be deployed with two different approaches, that is, engineered landfills and AD plants.

Engineered landfills

The term landfill is used to refer to a wide range of facilities ranging from open dumps to highly engineered facilities as bioreactor landfills, flushing-bioreactor landfills, and semiaerobic landfills. In landfills, waste material degrades and yields landfill gas (LFG, almost 50% methane), the leachate, and the residual material which has not degraded. The composition of LFG produced by organic matter deposit in a municipal landfill varies significantly, both during the operation phase (acceptance of waste by the landfill) and after landfill closure. The intensity of gas production varies too, depending on the time elapsed since the deposition of waste in the landfill.

In a conventional landfill, measures to enhance the waste degradation are not taken, but measures are implemented to recover part of the gas produced and to manage the leachate generated. Conversely, modern engineered landfills have a range of landfill gas utilization and control systems that reduce the emissions of methane and, when possible, include recovery of energy. Active measures such as leachate recirculation, water addition, and air injection, to enhance the waste degradation process and to improve efficiency, are usually adopted (Deublein and Steinhauser, 2010). This allows for greater gas recovery, and significantly lower losses of GHGs to the environment.

By way of example, the 280 kilotonnes of methane collected from landfills, on average, in Canada each year reduce GHG emissions by around 6 million tonnes, the equivalent of taking around 1.5 million cars out of circulation. Moreover, the energy content in this volume of LFG is close to that of 3 million barrels of oil—more than sufficient to heat over 150,000 average-sized homes. This is despite the fact that landfill gas has an energy value inferior to that of natural gas: the heat value of natural gas, at close to 40 MJ/m³, is around twice that of LFG, which varies between 19 and 22 MJ/m³, depending on the proportion of biological material in the landfill. LFG’s lower energy value is partly due to the fact that the vast majority of landfills are not designed or managed to achieve optimal gas production.

Anaerobic digestion

AD underpins all biological production methods, which are essentially different ways of processing and extracting the resultant biogas. AD systems are used for a variety of purposes, including manure stabilization, sludge volume reduction, and industrial and organic waste treatment (Ford, 2007). The digestate is usually a dry-matter product, rich in plant nutrients. It can be recycled as high-quality organic soil fertilizer provided the presence of heavy metals and organic pollutants is not too high. During the AD process, the bacteria decompose the organic matter in order to produce the energy necessary for their metabolism; methane is a by-product.

The resulting biogas is a renewable fuel, used mainly to produce electricity and heat. It consists of methane (CH4) and CO2, as well as minor quantities (less than 1% of total gas volume) of nitrogen, hydrogen, ammonia, and hydrogen sulfide. Typical biogas composition is reported in Table 1.2. For reference, landfill gas and North Sea gas are also shown.

Table 1.2

The widespread, natural occurrence of methane-creating bacteria demonstrates that anaerobic degradation can take place over a wide temperature range, from 10°C to over 100°C, and at a variety of moisture contents, from 60% to 99% (Ford, 2007). Due to this high-temperature tolerance level, AD can also be applied to the decomposition of dry solids, such as MSW or diluted industrial wastewaters. In these instances, however, the digester design needs to be optimized for each type of feedstock. Currently, no digester system is able to process all types of waste efficiently.

The energy recovered from biogas is either used on the production site or sent to the national energy network. Biogas can be directly used to produce heat and power or cleaned and upgraded to produce biomethane. This is usually injected into the grid as grid-quality methane.

AD is certainly the most developed and commercially widespread way to produce renewable gas at the moment. In 2016, the number of biogas production plants in Europe had tripled, with more than 18,000 plants operating across the EU countries (EurObserver, 2012).

In common with all biotechnological processes, the AD process has certain disadvantages. The most serious is its inability to degrade lignin, a major component of wood. Despite this, producers and researchers have successfully used crops including aquatic plants, marine plants, and woody biomass as AD feedstocks. The AD process also functions less well in low-temperature environments, where the anaerobic bacteria thrive less well, or not at all. In hotter, tropical climates, by contrast, the process faces fewer problems (Ford, 2007). As a result, producers in colder climates need to heat their tanks, adding costs to operation of the plant.

Thermochemical conversion

Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower-moisture feedstock and is generally more flexible and robust than biological processes (Materazzi, 2017). Thermochemical conversion includes incineration, pyrolysis, and gasification. The incineration technology is the controlled combustion of waste with the recovery of heat to produce steam which, in turn, produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine, or for further cleaning and upgrading to biofuels. Those technologies are reported to have a number of advantages, such as reduced level of air emissions, including GHGs, when compared to landfill and AD; and exploitation of rich, renewable energy streams while recycling valuable materials including metals (Bosmans et al., 2013).

When utilized for GNG production, combustion and incineration are excluded from the assessment for obvious reasons, leaving gasification and pyrolysis as the two processes for biomass/waste degradation. Pyrolysis involves the purely thermal degradation of the organic matter into oil, char, and gas. Only part of this can be recovered as renewable methane, which is usually less than 5%–10% of the total hydrocarbon fraction. For this reason, pyrolysis is rarely applied on its own for gas production, whilst its deployment for oil and char recovery offers many more opportunities (Qu et al., 2011).

For bio-SNG generation, gasification is the first step to produce the raw syngas (or producer gas) from the solid feedstock; the producer gas is then cleaned to generate a high-standard syngas which is then converted into methane via catalytic processes (also known as methanation) (Materazzi et al., 2018). Typically, the gas generated from gasification will have a net calorific value (NCV) of 4–10 MJ/Nm³ (Materazzi, 2017). The other main product of gasification is a solid residue of noncombustible materials (ash) which contains a relatively low level of carbon and other inorganic contaminants (e.g., chlorides, sulfites, heavy metals, etc.). The gases from gasification need to be purified and cleaned of acid contaminants before being used as a feedstock for biofuel synthesis via catalytic stages (e.g., Fischer Tropsche, methanation, etc.).

The thermochemical approach accommodates a wider range of input feedstocks. It also converts the full calorific value rather than only part of the biodegradable fraction. This also means that for bio-SNG, the majority of the mass and energy flow goes to the out-turn product (gas), whilst in AD, the majority of the mass flow is to the residual digestate. For these reasons the bio-SNG approach can be executed on a more substantial scale. Whilst technically feasible, this approach is less mature than AD. Transition from aspiration to widespread operating facilities and infrastructure requires a detailed understanding of the technical and commercial attributes of the full chain from feedstock supply through to delivery of grid-quality gas, as well as the development of the first crucial operating facility which provides the tangible proof of concept for roll out.

However, it is estimated that thermal gasification will have reached full commercial maturity in the next 5–10 years (Lane, 2004). If this is real, the possibility of using a lower grade feedstock and at much larger scale, will make bio-SNG cheaper than biomethane from AD. This is also thanks to biomass-to-biomethane conversion efficiency which is in favor of the thermochemical route, with 0.36 m³ of biomethane per kg of feedstock for biomethane from AD versus the 0.55 m³/kg for biomethane from gasification (Timme van Melle et al., 2018).

Other green gas technologies

Another set of technologies are currently being developed to facilitate the transition into renewable gas, while adding important benefits to the decarbonization of the energy sector.

Power to gas

Bio-SNG is not only an attractive, versatile energy carrier for bioenergy, but can also be used for storage of surplus power from renewable sources (e.g., hydroelectric, solar, wind). This is the so-called power-to-gas (PtG) concept, in which low-cost renewable power is used to produce H2 via electrolysis of water. This hydrogen can then be used to convert residual carbon from an existing plant (that would be normally released to atmosphere) into bio-SNG (via methanation) (Tichler and Bauer, 2016).

CO2 from various sources is normally reacted with H2 on a Ni-Ru catalyst, to exploit the Sabatier reaction (Müller et al., 2013). Power to gas is thus a form of CCU. CO2 can be gained from concentrated sources such as biogas plants or cement plants as well as from conventional power plants. It is also possible to extract CO2 from the air but this is still very energy-intensive at present.

It is also possible to combine different paths for SNG synthesis. For example, electrolytic hydrogen can be fed into a biogas plant to increase the methane yield by converting the surplus CO2 to further methane. The same applies to bio-SNG plants where a significant amount of CO2 is produced in the water gas shift stage. The latter can be bypassed if excess H2 is present, enhancing the methane output (per inlet carbon atom in the feedstock) and the plant flexibility.

Biological methanation

Biological methanation (or microbiological methanation) is a conversion process to generate methane by means of highly specialized microorganisms (Archaea) within a technical system. This process can be applied in a PtG system to produce biomethane and is appreciated as an important storage technology for variable renewable energy in the context of energy transition (Lecker et al., 2017).

This technology was successfully implemented at a first PtG plant of that kind in 2015. Compared to catalytic methanation, biological methanation has a lower reaction rate due to a lower temperature and a lower volumetric mass transfer coefficient and a high tolerance of impurities of the input gas (Seifert et al., 2013). Advantages arise from the synergy effect of a combination of biological methanation with the biogas process. In fact, after CO2 reduction through H2 injection into the process, biogas has a much higher calorific value due to the conversion of CO2 to methane. Therefore, biological methanation could lower the costs for biogas upgrading to natural gas quality. Today, however, the process is used mostly on a pilot scale and more research is needed to enable scaling up.

Biohydrogen

Many long-term scenarios for the energy sector are based around the so-called hydrogen economy. Hydrogen is an energy carrier rather than a primary energy source in itself, such as electricity. In this sense, renewable electricity and hydrogen are similar and somewhat competitive technologies. This is due to the fact they both produce no emissions at the point of use, and they both rely (mostly) on the use of other renewable sources, such as wind or solar power. There are reasons why hydrogen might, none the less, have a significant role in a low-carbon economy, including the ability to be stored (although with many more technical difficulties compared to methane), and its flexibility in use. Hydrogen can be combusted for heating or in internal combustion engines for transport. However, it can also be used to power fuel cells, or mixed with natural gas in the existing grid for heating and cooking appliances. Renewable hydrogen can be produced from a variety of sources, including renewable methane [via steam reforming (Zech et al., 2015)] or separation from biomass-derived syngas (Moneti et al., 2016). In these cases, the final product is also called biohydrogen, to distinguish it from electrolytic hydrogen. Electricity of course can itself be used to produce hydrogen by electrolysis of water, effectively converting electricity to storable form. While this is only a minor source of hydrogen today (around 5% of the total) its importance could increase in future with the growth of intermittent renewables, as for PtG