Is it worth generating energy with garbage? Defining a carbon tax to encourage waste-to-energy cycles

Highlights

• Carbon taxes affect the prices of fossil energy and stimulates renewable sources.

• A carbon tax was modeled for hybrid- and incineration cycles burning solid wastes.

• Destroyed exergy is taken into account in the Carbon Exergy Tax model.

• The Carbon Exergy Tax encourages the desirable destruction of solid wastes.

Abstract

The rising of municipal solid waste generation is a serious current problem, whose incorrect disposal leads to major impacts to human health and the environment. Waste-to-Energy systems have been some of the adopted solutions but they carry the burden of the emission of pollutant gases. To control such emissions, international agreements have introduced the concept of the carbon tax, i.e. a levy on carbon dioxide emissions imposed on energy systems. Considering the specificity of municipal solid wastes for their inclusion in the energy mix of the countries, this study aims to apply a carbon tax model in Waste-to-Energy plants to encourage the utilization of this particular waste. The proposed carbon tax utilizes the concept of exergy destruction from the second law of thermodynamics and is based on the cycle’s efficiency and the type of fuel to be burned. Hybrid-cycles associating gas turbines burning natural gas and an incinerator burning municipal solid wastes were compared to the conventional incineration steam cycle. One finding of this study is that the incineration cycle is less penalized in terms of CO₂ emissions compared to the rates applied to hybrid-cycles in the same exergetic base, considering its attribution to the environmental liability destruction represented by the municipal solid waste. The sensitivity revealed that the municipal solid waste price significantly affects the incineration cycle and hybrid-cycles with the lowest gas turbine capacities and natural gas price variations influence large capacity hybrid-cycles.

Introduction

On December 11th, 1997, the Kyoto Protocol was signed, aiming mainly at reducing greenhouse gas (GHG) emissions by 5% in comparison to data from 1990. Twelve years have passed since its validation, and it is now in its second commitment period [1]. Such agreement was accountable for introducing two of the main GHG reduction mechanisms, i.e. the International Emissions Trading and the Clean Development Mechanism. In the International Emissions Trading, countries committed to achieving GHG emissions reduction were able to negotiate target surplus with each other. The Clean Development Mechanism allowed developed countries to implement projects aimed at reducing emissions in developing countries in exchange for certified emission reductions [1].

By anticipating the end of the Kyoto protocol in 2020, the United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement was signed on December 12th, 2015, aiming to mitigate the effects of climate change, and focused on maintaining the average global temperature increase below 2 °C by the end of this century in comparison to pre-industrial ranges. In this context, the investment in innovative technologies and renewable fuels, e.g. biomass, increases. It is also important to discuss how to optimally deal with environmental problems, especially those concerning gaseous pollutants.

In the last decades, municipal solid waste (MSW) production has increased considerably, and it tends to increase even further in the upcoming years, which leads to the forecasting of the overall global urban MSW potential for Waste-to-Energy (WtE) that is estimated to increase from 1.9 EJ in 2013 to 11 EJ in 2050 [2]. Although WtE plants are less efficient than natural gas (NG) power plants, their implementation may add a significant amount of renewable energy in the energy mix of the countries (e.g., 2.5 GW in the Brazilian energy mix [3]), although some anthropogenic CO₂emissions may be present (20%, according to [4]).

Currently, there are multiple WtE technologies, such as incineration, anaerobic digestion, gasification, and pyrolysis. However, the predominant technology still consists on large-scale incineration on a moving grate and superheated steam generation that feeds a steam turbine operating in an incineration cycle [5]. Unlike other thermal plants, WtE plants achieve relatively low electricity conversion efficiency, i.e. ranging from 9% for old ones to 25% for new WtE plants [6]. Another factor that impairs the efficiency of these thermal power plants is that the temperature of flue gases in the stack must not be lower than ± 200 °C to reduce the risk of condensation of flue gas’ aggressive compounds.

The landfill gas is considered one of the major sources of anthropogenic methane emissions [7] and, as a consequence, an incineration facility is environmentally better than landfilling, although not financially advantageous in the same comparison [4]. The appropriateness of landfilling and incineration was previously and comparatively discussed in the literature [8], [9], [10] and is not the objective of this work.

Incineration is one of the found alternatives to the final disposition of non-recyclable materials, and countries like Japan landfill 3%, recycle 17% and incinerate 74% of produced MSW [11]. Incineration is one of the most traditional technologies for MSW processing to generate steam and, consequently, electricity, and can be used to efficiently reduce waste volume (80–90%) and mass (70–80%) [12]; therefore, the need to use landfills is reduced. Another aspect that is worth mentioning is that thermal incineration plants can be located near large centers, i.e. where the most substantial amount of waste is generated, cutting down transportation costs.

As aforementioned, there are advantages in municipal solid waste incineration, although not implemented or not often used due to some factors. Waste incineration is not a novel idea and it has been applied in Europe since the end of the 19th century. There has been great public pressure against its use, mainly because the process of waste burning occurred without any concern for the kind of gases it generated, often associated to pollutants that are harmful to public health. Currently, there are constraints stipulated by governmental agencies regarding the level of pollutants emitted by thermal plants, and a considerable advance in technology to treat gases generated from solid waste combustion (e.g. to mitigate highly harmful compounds such as dioxin, furan, and NOx) is available.

Pavlas et al. [13] demonstrated that WtE plants may provide clean and reliable energy in cogeneration mode, contributing to primary energy savings in conventional utility systems. Eight compositions of proven technologies (reciprocating engine cogeneration, biomass-fired steam plant, biomass-fired organic Rankine power plant, and cogeneration-only plants) were compared to a 100 kt/year WtE plant with backpressure turbine. The environmental impact of WTE was quantified reveling that just for SOx emissions, WtE plant has comparatively higher values. In spite of this, the general public demonstrates difficulties in the acceptance of using such technology due to the known “Not In My Back Yard” (NIMBY) syndrome [14], [15]. A very recent and holistic analysis of WtE incineration that contemplates all these questions is presented by Makarichi et al. [16]. The involvement of communities for sharing common responsibilities, the harmonization of WtE plants in the landscape through innovative architectural structures, the opening of WtE installations to the public, and the dissemination of scientific information for changing the public's perception of waste burning are relevant efforts for improving WtE acceptance.

Gas turbine integration with gasification [17] and with incineration [18], [19] of MSW have been searched aiming at the increase of WtE efficiency. The system with a gas turbine integrated to an MSW incinerator is also known as ‘hybrid-cycles’, and it is the object of the present work. According to Korobitsyn et al. [19], the low efficiency of WtE plants is due to some precautions that must be taken during their operation and that results from the aggressiveness of the flue gas generated during MSW incineration, such as the boiler rearrangement to avoid corrosion, the erosion and the slag formation. These precautions keep the generated superheated steam temperature at values that range from 400 to 450 °C. The Zabalgarbi thermal power plant in Bilbao, Spain, is a commercial initiative of this sort [20].

The use of MSW to generate electricity in WtE thermal power plants has been proposed as an interesting alternative for its adequate disposal, whilst avoiding the use of fossil fuels [21] and consequent carbon dioxide emissions. Even with results like the ones from Bidart et al. [22], who identified an economic potential with a factor of two for WtE relatively to landfill gas-to-energy system in Chile, high investment and operating costs are incurred in MSW power generation. These factors lead countries, especially underdeveloped and developing countries, to prefer other waste disposal alternatives.

WtE may prove to be a cost-effective technology by assessing the levy of environmental taxes for power generation cycles by means of carbon taxes. The carbon tax is a form of a Pigouvian tax used as a charge imposed on the fuel costs related to the carbon content share of the fossil fuels used for energy production. The implementation of carbon taxes impacts the prices of fossil energy by raising them and, as a consequence, renewable energy alternatives to fossil fuels are promoted [23]. A possible way to stimulate alternative energies through the imposition of a carbon tax on fossil fuels is through the application of subsidies funded by these taxes, considering that the increase in the price of fossil fuel alone will not necessarily promote greater use of renewable energy.

WtE technologies may be benefited in a carbon trade market, in which the internalization of CO₂ emissions may represent earnings for the MSW entrepreneurs. Different approaches, in which Life Cycle Analysis (LCA) models [24], [25] assume more and more significant relevance, were proposed for environmental impact evaluation. The introduction of a tax on incineration processes in Sweden in 2006 was discussed to encourage material recycling [26]. More recently, new changes in legislation on waste management were reviewed in the different EU Member States in the context of the circular economy by considering incineration taxes and sanitary landfills [27].

Frangopoulos and Caralis [28] observed that there is no single and universally acknowledged method to physically and economically assess social and environmental impacts exerted by the construction and operation of energy systems and internalize environmental costs. Two decades after this publication, values of CO₂ emission taxes for various countries obtained from the World Bank's recent survey reveal large numerical disparities among applied environmental taxes [29]. Values varied from 0.01 to 126.26 USD/tonne of CO₂ (in Ukraine and Sweden, respectively), with several European countries setting their taxes in the range of 7 to 30 USD/tonne of CO₂ and applied linearly for all the energy mix power generation technologies.

Academic efforts to better understand the introduction of environmental taxes have been made and the realization that they can hardly ever impact the energy system operation without penalizing the inefficiency of power generation plants started being acknowledged [30], [31]. MSW is an environmental liability, considering that it must be necessarily disposed of in landfills or incinerated due to health issues. For reducing the amount of the anthropogenic CO₂ generated from the hydrocarbons combustion process for generating energy, many countries have adopted carbon taxes based on political considerations (i.e., with the political influence of groups interested in the continued use of fossil fuels) and defined by considering the cost to repair the damages caused in society by such emission [32].

Therefore, it is important to consider adequate conceptual frameworks for carbon taxes definition based on technical and environmental points of view (and reducing the political weight in the final decision), as well as better explaining how the proposed value is positioned in a very ample range of taxes under consideration nowadays. Models proposed for such analysis must adequately consider losses that occur in the power generation processes and that fuels must only be penalized for their anthropogenic carbon content.

At the beginning of the 21st century, in a world context of changing from coal to natural gas as the primary source of thermal power plants, a carbon exergy tax (CET) was proposed [33]. The original CET model considered the concept of destroyed exergy inside the system and the exergy rejected into the biosphere with the plant wastes for assigning a cost to the inefficient energy transformations activities. The application of the exergy destruction concept is relevant for the technological enhancement of equipment and processes by determining the location, the magnitude, and the source of thermodynamic inefficiencies in a thermal system [34]. The concept of destroyed exergy was recently applied to the technical and environmental evaluation of WtE systems [35], [36].

The carbon exergy tax (CET) model [30] is based on the exergetic concept; however, it does not adequately encompasses the burning of renewable fuels in its original form. The CET model was then applied to renewable thermal power plants burning biomass (sugarcane bagasse) in comparison to a combined cycle gas turbine (CCGT) burning natural gas [37]. It was demonstrated that a modification of the original model adequately suited the environmental exergy tax on thermal cycles with CO₂ emissions, with neutral emissions and with CO₂ capture and storage, which were gradually and respectively reduced.

This work presents an analytical procedure for carbon tax assessment to encourage municipal with energy generation-associated solid waste destruction based on the exergy concept. The original CET model is capable of defining an individual cost for CO₂ emissions based on the exergetic efficiency of power plants in order to penalize the most inefficient according to their destroyed exergy. Therefore, the original CET model is not adequate to calculate a carbon tax for the MSW burning, considering it was conceived to penalize fossil fuel burning, and MSW is intended to be destroyed. A modified CET model is presented to adequately consider WtE cycles burning MSW, to comply with environmental requirements for MSW elimination and, at the same time, to compute their corresponding carbon taxes, considering that, unlike fossil fuels, their incineration and subsequent elimination should be encouraged.