Waste Gasification Technologies: Comparison
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The commercially available gasification technologies are classified according to various parameters, including the heat supply method, the gasifying agent used, and the reactor type. As for the design of the gasifier, they typically fall into three main categories, the fixed-bed (co-current, countercurrent, and cross current), fluidized bed (bubbling and circulating), and entrained fluidized bed, and, in addition to these, there are rotary kiln and plasma reactors. All these reactors have advantages and disadvantages, and the selection depends on the scale of operation, the characteristics of the feedstock, and the desired application of the produced gas.
  • gasifiers
  • thermochemical conversion
  • waste
  • waste-to-energy

1. Introduction

The growth of the world population and the increase in consumption levels project a generation of 3.4 bn Mt of waste by 2050 [1]. Considering waste production in 2018, the United States produced approximately 811 kg per capita, totaling 292.4 Mt. Landfilling was the treatment given to 50% of all these wastes, while 11.8% was combusted, 32% recycled or composted, and 6.2% was submitted to alternative treatments [2]. Regarding the European situation, according to Eurostat, 505 kg of municipal waste was produced per capita in 2020, led by Austria, Denmark, and Luxembourg (834, 814, and 790 kg, respectively). From all the municipal wastes generated by the EU in 2020, 48% were recycled (considering recycling and composting), while 26% was submitted to incineration, and approximately 26% was landfilled [3]. Regarding the generation of wastes by sector, construction contributes 37.1%, mining and quarrying 23.4%, manufacturing 10.9%, wastewater 10.7%, household 9.5%, services sector 4.5%, energy 2.3%, and agriculture, forestry and fishing 1.0% [3].
The destination of these wastes has changed in recent years, with the development of alternative recycling centers, composting, and waste-to-energy. Despite this, a considerable amount of the produced waste is still sent to landfills, where it accumulates, causing several environmental issues. To avoid landfilling, the European Environmental Agency set a target to reduce the discarding of wastes to 10% of the total by 2035, a compromise established through the EU Landfill Directive in 2018 [4]. With the adoption of this measure, the EU seeks to reduce the environmental problems associated with landfills, which include the contamination of groundwater resources through leachate, greenhouse gas emissions (GHG) from landfill gases (LFG), and soil contamination and usage [5]. Aiming to find alternatives to valorize the generated wastes, some techniques have been explored to produce biomaterials [6] (e.g., the fractionation of lignocellulosic biomass from forestry and agricultural activities [7]) and energy, such as incineration [8], anaerobic digestion [9], pyrolysis [10], and thermal gasification [11] using wastes as feedstock.
The utilization of waste to produce energy is usually referred to as waste-to-energy (WtE). It offers several advantages, such as the reduction in GHG emissions [12], sustainable production of electricity and heat [13], a more efficient alternative to waste treatment [14], and an enhanced social component through the creation of jobs in the waste management sector [15].
Among all the techniques adopted in WtE plants, thermal gasification has been used to valorize solid wastes, producing heat and syngas. Syngas is a gaseous product that can be used as fuel, mainly composed of carbon monoxide (CO), hydrogen (H2), nitrogen (N2), carbon dioxide (CO2), and light hydrocarbons (e.g., CH4, C2H4, C2H6). Syngas composition varies according to the type of gasifier, operational parameters, and feedstock composition [16,17][16][17]. At different times, gasification has used the most varied raw materials, such as coal and biomass, which are overall feedstocks with a homogeneous composition. Using valorizing wastes, such as municipal solid wastes (MSW), industrial wastes (IW), Refuse Derived Fuel (RDF), Solid Recovered Fuel (SRF), construction and demolition waste (CDW), or electronic waste (E-waste), in gasification can be challenging due to the heterogeneous composition of these wastes [18]. However, gasification still presents several advantages from an environmental aspect, offering an alternative to landfilling and helping to ensure energy security, currently aggravated in Europe by regional conflicts [19].

2. Thermal Gasification: General Principals

Thermal gasification can be defined as the process where a solid feedstock can be converted into a mixture of gases called syngas, containing mainly methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), and tars [20]. Syngas can later be used as fuel in internal combustion engines [21], as a heat source [22], to produce electricity [23], or it can be used in chemical synthesis (e.g., Fischer–Tropsch, higher alcohol synthesis) [24]. The composition of syngas is affected by residence time, gasifying, pressure, temperature, feedstock composition, and the type of gasifier [27,28,29,30,31][25][26][27][28][29]. Residence time has an essential role in thermal gasification. The increase in residence time positively affects H2 production, also increasing the efficiency of the process [32][30]. Regarding gasifying agents, air, steam, carbon dioxide, oxygen, or a mixture of these gases are usually used [30,33,34][28][31][32]. Despite using air as a gasifying agent, the syngas dilution due to the presence of nitrogen can reduce the resulting syngas’ high heating value (HHV) and the gasification process’ efficiency [35][33]. The utilization of O2 increases gasification costs despite preventing the problems associated with the presence of N2. As a cheaper and more convenient alternative to O2, steam has been studied as a gasification agent. It is cheaper than O2 and avoids the problems associated with the presence of N2. Moreover, using steam increases the H2 content in syngas due to the steam–methane reforming and steam–char reaction [25][34]. The utilization of CO2 also avoids the problems associated with N2 while allowing control of the H2/CO ratio and increasing synergetic effects. However, it can raise the plant’s operational costs [36,37][35][36]. Feedstock composition can affect not only the syngas composition but also the composition of gasification by-products (e.g., biochar, ash). Therefore, contaminants in biomass can contribute to slag formation, damaging reactors and pipes and increasing costs in the process [28,38,39][26][37][38]. To avoid these problems, some feedstock pretreatments can be applied, such as torrefaction, adding costs and new unit operations to the gasification process [38][37]. Despite these parameters having a strong influence on the gasification products, the technologies used in gasification can require different parameters, which are more efficient according to the feedstock composition. Thus, the next section will address the different gasifiers used to convert wastes into syngas.

3. Gasification Technologies Used in Waste Conversion

3.1. Fixed-Bed Reactors

Fixed-bed gasifiers are the simplest gasification technology. These gasifiers can use different gasifying agents and gasification takes place over time, from approximately 900–1800 s at high pressures [45][39]. Fixed-bed systems have a cylinder-shaped space where the raw material is introduced in the top of the reactor, while the gasifying agent is added at the bottom. The reactor operates at high pressures between 1 and 100 bar and with temperatures from approximately 500–1200 °C, resulting in a high carbon conversion [43][40]. Fixed-bed gasifiers reactors include updraft and downdraft configurations, respectively. These different configurations are related to the input of the feedstock and the gasifying agent. In an updraft gasifier, the feedstock is inserted into the top of the gasifier, while the gasifying agent is introduced into the side or bottom of the reactor. The production of syngas takes place along the reactor and the output of this gas takes place at the upper level of the reactor, while the ash is deposited at the bottom of the reactor. In the downdraft reactor, the raw material enters in the top of the reactor and the gasifying agent enters in the side or top of the gasifier; thus, the syngas output takes place at the bottom of the reactor. Several studies have recognized that fixed-bed reactors can be used with various types of wastes with a high carbon conversion rate and low ash emission. However, this type of gasifier is not normally used on a large scale due to the low moisture content required in the feedstock, which is one of the limitations for the use of MSW [41,43][40][41].

3.2. Fluidized Bed Reactors

In this type of gasifier, the feedstock is introduced into the reactor and the fluidization medium is injected along with sand, operating at temperatures from approximately 800–900 °C. These reactors can maintain a temperature range between 700 and 1000 °C [43][40]. Solid waste may take longer to react, resulting in increased heat transfer and leading to higher carbon conversion. Fluidized bed gasifiers have two main configurations, bubbling fluidized bed and circulating fluidized bed [46][42]. Bubbling fluidized bed gasifiers are designed to operate under low gas speed conditions between 1 and 3 m/s, operating at temperatures between 800 and 1000 °C. Particles are moved with the gas and are divided by a cyclone; hence, the raw syngas flows to the next stage, while the particles fall to the bottom of the reactor [47][43]. In a circulating fluidized bed gasifier, the gasification is conducted in two steps. First, there is a bubbling fluidized bed chamber that reacts with solid waste and generates syngas. In the second step, a higher gas speed is introduced, usually between 3 and 10 m/s, to drag the solid. Finally, the cyclone allows solid particles to separate and circulate in the fluidized bed chamber. Fluidized bed gasifiers are widely used for solid wastes on a large scale due to their good performance [43][40]. One of the most significant current applications in waste gasification is energy production. Energy efficiency is a good performance indicator and can identify the type of gasifier to be considered for a given type of waste. This indicator varies between gasification reactors and depends on several parameters, such as feedstock composition, gasification temperature, time of permanence of waste in the reactor, and the properties of the reactors. The energy content produced by a fluidized bed gasifier typically varies between 3.7 and 8.4 MJ/Nm3 for the bubbling fluidized bed and between 4.5 and 13 MJ/Nm3 for the circulating fluidized bed [47][43]. Since the gas from these reactors also have high volumes of carbon monoxide (25–30%) and hydrogen (35–40%), it is to be expected that these reactors are ideal for use in the fuel and hydrogen industries [43][40].

3.3. Entrained Flow Reactors

This technology is typically used for industrial-scale coal gasification because of its higher availability, higher throughput, and better product gas quality [48][44]. It is a deployable and mature technology for handling conventional feedstock, such as coal, lignite, and biomass [49][45]. In general, these reactors are operated at high temperatures (between 1200 and 1500 °C) and high pressures (between 20 and 80 bar); however, some particles remain in the bed during the residence time [43][40]. The main advantages of entrained flow gasifiers are the high fuel conversion and small heat losses due to their compact design [48][44].

3.4. Rotary Kilns

The rotary kiln reactor is widely used in commercial applications in waste incineration. This reactor contains a steel cylindrical-shaped chamber, which moves slowly and with operating temperatures from approximately 300–600 °C. The rotary kiln reactor operates slowly and with a downward inclination relative to the exit end; thus, the feedstock passes through the reactor for gasification. The feedstock is introduced at the top of the reactor, while the gasifying agent is injected into the bottom of the oven [43][40].

3.5. Plasma Reactors

Plasma gasification is a relatively new technology that uses electrically ionized gas at approximately 10,000 °C through plasma torches with pressures between 1 and 3 bar so that it is possible to break the feedstock into syngas [50][46]. In the plasma reactor, the feedstock is introduced at the top of the chamber, while the gasifying agent is inserted into the side of the reactor. The inorganic materials are transformed into inert and glazed slag, while the very high temperatures of the plasma torch can break down the organic materials, resulting in a clean syngas. In contrast, the need for high temperature can increase the operational cost of the reactor [43][40]. The plasma reactor system requires a significant amount of electricity, approximately between 1200 and 2500 MJ per ton of feedstock, which is a significant disadvantage from the technological commercialization point of view [51][47]. Nevertheless, plasma gasification has been deployed in several waste-to-energy pilot plants even while facing some economic and technical challenges [49][45].

3.6. Emerging Gasification Technologies

One of the most significant emerging technologies is gasification with supercritical water. This technology is especially studied in the context of carbon neutrality. This technology is a clean and efficient way to convert biomass into gases with a high content in H2 and CO2. This CO2 can be collected for hydrocarbon fuel production [52][48]. The advantages of gasification with supercritical water are its fast reaction rate, high gas rate, and the production of a very clean syngas [53][49]. It is reported that this technology is used for gasification of different materials, such as biomass, plastics, coal, and pig manure [52,53,54,55,56][48][49][50][51][52]. High-temperature steam gasification is another very promising technology. The process requires an external heat source and uses extremely high-temperature steam (approximately 1000 °C) as its gasifying agent in an oxygen-free environment to completely decompose the feedstock [57][53]. As expected, when using steam at the super-high temperature of 1000 °C, the composition of the syngas is dominated by hydrogen [57][53]. Lee et al. (2014), reported that it is possible to use this tecnology to gasify feedstocks such as wood, plastic, rubber, and MSW [57][53]. Furthermore, there is already ongoing research on the possibility to use this technique at a pilot-scale [58][54].

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