Waste-to-energy conversion technologies in the UK: Processes and barriers – A review

Highlights

• Lack of manure treatment in the UK results in 28% of all ammonia emissions.

• NO_X emitted from agriculture accounts for 3% of the total NO_X emissions in the UK.

• Manure and slurry contribute about 50% of anthropogenic UK's methane emissions.

• Drops in tariff rates result in fall in number of anaerobic digestion plants in UK.

• AD could produce 1.615 TWh electricity, i.e. 0.45% of total UK's annual demand.

Abstract

This paper reviews the sector of waste-to-energy looking at the main processes and feedstock involved. Within this, incineration, gasification, pyrolysis, anaerobic digestion and hydrothermal liquefaction are named and discussed. Through the discussions and scrutiny, manure is highlighted as a significant source of ammonia, methane, and nitrogen oxides emission, estimated to be 40%, 22.5% and 28% respectively of the total UK's anthropogenic emissions. Manure, and indeed the pollution it poses, are shown to remain largely ignored. In waste to energy processing, manure is capable of providing biogas for a number of pathways including electricity generation. Anaerobic digestion is highlighted as a suitable process with the crucial capability of drastically reducing the pollution potential of manure and slurry compared to no processing, with up to 90% reduction in methane and 50% reduction in nitrogen oxide emissions. If the majority of the 90 million tonnes of manure and slurry in the UK were to be processed through biogas harvesting, this could have the potential of producing more than 1.615 TWh of electricity. As such, the economics and legislation surrounding the implementation of anaerobic digestion for manure and slurry are discussed. In the end, restraining factors that limit the implementation of anaerobic digesters on farms in the UK are discussed. These are found to be mainly capital costs, lack of grants, insufficiently high tariff systems, rather than low gas yields from manure and slurry.

Introduction

The need to become more sustainable through the threat of global climate change and resource depletion is ever more prominent. Coupled with an ever-increasing population, rapid industrialisation, depleting fossil fuel resources present significant biowaste disposal and energy demand problems. In the UK, around 7.4 million tonnes of biodegradable municipal waste were sent to landfill in 2017 [1]. This waste could otherwise have been processed and recycled. The environmental impact of biodegradable waste extends beyond increasing greenhouse gasses due to the decomposition process. Untreated biodegradable waste release unpleasant odours due to decomposition and attracts scavenger animals and pests [2]. This has an impact on general public health and changes the biodiversity in the surrounding areas. Leaching from landfills not only contaminates the groundwater but can also affect the adjacent soil quality. In EU legislation, it is stipulated that biodegradable waste ending up at landfill must be reduced by 35% by 2020 compared to 1995 levels. This is one example of the driving forces behind waste to energy (WtE) processing, focused on reducing the volume of waste, recovering valuable products and producing electricity.

The term ‘waste-to-energy’ can be used interchangeably and encompass a variety of processes and technologies. The conversion of waste into energy will be analysed in this paper by the following processes: incineration, gasification, pyrolysis, anaerobic digestion, and hydrothermal liquefaction. The schematics of waste to energy processes are shown in Fig. 1.

Incineration is known as the complete oxidation within a waste stream of combustible materials and operates as temperatures above 850 °C. All feedstocks of waste addressed in this paper can be incinerated. This is one of the key advantages of incineration, the ability to deal with a diversity of wastes. Gasification in many sectors has been operating worldwide on a large basis for more than 80 years. During high temperatures (500–1800 °C), partial oxidation is accomplished by reducing the access to oxygen. The gases produced known as 'syngas' do not burn but can be gathered and processed for subsequent use. Pyrolysis operates similarly to gasification where partial oxidation is used to maintain thermal conditions. While this development is not new, a widespread deployment has not yet been accomplished. The process operates at about 300–700 °C. Anaerobic Digestion (AD) is an established process for the treatment of organic waste within the waste to energy sector. In 2007 the Department for Environment, Food and Rural Affairs recommended companies in England and Wales to use AD to better achieve electricity goals. Interest decreased because of concerns about economic viability. AD is still considered a key process for achieving a circular economy, increasing resource-efficiency and for the bioenergy-economy. Hydrothermal liquefaction is the thermochemical conversion of biomass into biocrude oil that can then be refined into petroleum derived fuels. The process is conducted in a 4–22 MPa pressurized environment at temperatures 250–374 °C. With promising biomass yields this process can become more widespread in the future in the waste-to-energy sector.

The rise in WtE has contributed to energy recovery increases in the UK with tonnage of processed wastes up to 7.3 million in 2018, nearly 4 times that of 2014 at 1.9 million [1]. The estimated range of total biological waste in the UK in 2020, including forestry residue and sewage sludge waste streams, amounts to 406.86 PJ, as shown in Table 1.

Large amounts of waste are now processed at facilities capable of energy production. On top of this, wastes once discarded into landfills through enhanced landfill mining, can be dealt with past and present, altering previous perceptions of what a landfill is, considering them simply as ‘‘temporary storage awaiting further processing’’ [7], with vast amounts of valuable materials and heavy metals that can be recovered. The waste generated worldwide is losing its potential contribution to sustainable living. Therefore, this paper looks to review the different wastes and the processes involved in WtE and assessing process capabilities and waste streams that can be incorporated. It also looks at the question on what more can be done and what if any significant waste streams remained untapped or not utilized to their full potential, how this can cause significant environmental and sustainable problems.

This paper also emphasizes on manure that has great potential to be used as energy source in anaerobic digesters if implemented on small scales at local farms. A global concern is poor production and utilisation of nitrogen (N), phosphorus (P), and potassium (K) from livestock [8]. Organic matter and nutrients recycled in manure are essential for agricultural soil structure and nutrient content [9]. Manure has a natural nitrogen and phosphorus content so if it is not utilized as a fertiliser on agriculture, natural nutrient cycles are disrupted, possibly that nutrient leaching, so artificial fertiliser needs are generated. Nitrogen fertiliser processing requires extensive usage of natural gas and produces pollution that lead to global warming [10]. In addition, it is stated that existing usage of small phosphate supplies for phosphorus fertiliser is unsustainable [11]. Therefore, some issues may be mitigated by rising the use of artificial fertiliser by reusing manure.

On the other hand, the vast quantities of excreta produced in localized areas will add to the nutrient excess at the regional level [12]. Excessive use of manure as an organic fertiliser can contribute to soil and water eutrophication, pathogen transmission, air contamination, and greenhouse gas emissions [13]. Sustainable processing of these large units of output is only possible if manure is reused properly. Composting is a potential stabilising procedure. A significant drawback, though, is the strong nitrogen depletion. This phenomenon decreases the fertiliser benefit and may cause odour disturbance and present a serious environmental threat [14]. An option to eco-friendly treatment is anaerobic digestion (AD), which provides added advantage to restore the caloric content by biogas production. Unfortunately, manure ‘s strong nitrogen content is prohibitive to successful AD. Organic Nitrogen is transformed to ammonia through microbial degradation. Ammonia exerts a strong inhibitory influence on microbiological conversion at high concentrations. Non-dissociated free ammonia triggers the toxicity [15,16]. This compound diffuses into cells, causing a proton imbalance or interfering with microorganisms' metabolic enzymes [17]. Overcoming ammonia inhibition is essential to effective manure AD.

To make this implementation feasible and sustainable, we have highlighted the need for further processing and changing application methods of slurry and muck to land as a requirement to reduce ammonia, methane and NOx emissions. The paper also discusses the barriers in the form of inadequate high banding tariff and systems, planning, high capital costs, lack of government subsidies and low biogas yields. It has been suggested that a lower high-paying tariff banding system needs to be introduced to increase anaerobic digestion plants on farms. It is required addition of a gate fee payment to reduce the high energy crops use as supplements for biogas yield, and to increase the amount of slurry and muck that are digested. The paper also discusses the bespoke nature of anaerobic digesters on farms and the scales of anaerobic digestion plants. The value of this paper is that it has reviewed different challenges and aspects of implementation of anaerobic digestion systems on farms within a framework of waste-to-energy conversion.

In addition to technological and environmental prospects of WtE, previous studies also tried to understand social acceptance of wate to energy and renewable energy technology. Shackley et al. [18] performed work on carbon dioxide absorption and storage in Europe and found that most of the respondents accepted this issue under the regional CO₂ mitigation plan. Wolsink [19] points out that including local citizens in the policymaking phase would help strengthen the policies on social acceptance and that without societal recognition it is difficult to accomplish both waste-to - energy and sustainability targets. Social tolerance also has to be taken into consideration through decision formation. The three reasons for popular resistance to renewable energy technology were stated by Rogers et al. [20]: inadequate growth size, unreasonable cost-to-public benefit ratio and the lack of proper connexion between the local people and their views. Wang et al. [21] analysed the waste management engagement in China, as well as how waste processing, sorting, collection, cost, age and education impact waste sorting satisfaction. They also examined the impact of satisfaction on participation in terms of enthusiasm, social contact and active involvement between region and gender by using systemic equation analysis from multiple communities.

To summarize what was mentioned above, we want to emphasize that this paper is a first attempt to look at the waste-to-energy that reviews the status of different WtE technologies in the UK, including the incineration, gasification, pyrolysis, anaerobic digestion and hydrothermal liquefaction. The reviews [[1], [2], [3], [4], [5], [6]] mentioned above highlighted the expected amount of different types of waste in the UK that would be available by 2020 but did not specify the processes to treat these types of waste. The reviews [[8], [9], [10], [11]] discussed the importance of using manure as an organic fertiliser and also the importance of pre-treatment of manure by using AD to avoid environmental impacts associated with soil and water eutrophication, pathogen transmission, air contamination, greenhouse gas emissions and overcoming the ammonia inhibition of AD processes [[12], [13], [14], [15], [16], [17]]. However, these reviews did not discuss the potential barriers associated with the economic aspects of AD such as tariffs, incentives and implementation of AD in farms. Therefore, the aim of this review is to cover the current status of WtE in the UK, understand its limitations, advantages, environmental effects, identify challenges in regards to the implementation of the waste, and assess what can be done to further utilize waste to energy in the effort to reduce pollution, resolve waste disposal issues and address energy needs.