Waste-to-energy: Overview of the Gasification process

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This article introduces the main existing technologies to carry out a conversion line of a waste stream into an energy-usable gas mixture (syngas). The syngas also serves as an intermediate point in the synthesis processes of various high-added-value products.

Introduction

Faced with the problems of climate change and global warming, research and development have focused on the use of biomass as an alternative to fossil fuels, among other energy sources. The wide availability of waste has been widely recognized for its potential to provide greater amounts of useful energy with less environmental impact than fossil fuels.

Waste can be converted into a commercial product through biological or thermochemical processes. The biological conversion of biomass continues to face challenges related to low efficiency and cost-effectiveness. In the case of thermochemical processes, combustion, pyrolysis, and gasification are the three main conversion methods.

Biomass is traditionally burned to provide heat and electricity in industrial processes. The net efficiency for electricity generation from its combustion is usually very low, not exceeding 50%. Biomass combustion is usually limited to 10% of the total feedstock due to, among other things, carbon lock-in in existing feed systems. Pyrolysis, on the other hand, converts biomass in the absence of oxygen. Limited uses and the difficulty of downstream processing have restricted the wide application of biomass pyrolysis technology.

Finally, gasification converts biomass by partial oxidation into a gaseous mixture, with small amounts of carbon and condensable compounds. It is considered one of the most efficient ways to convert the energy stored in biomass and is becoming one of the best alternatives for the reuse of solid waste.

A process that is currently being developed in several parts of the world is the gasification of municipal solid waste (MSW), in which syngas is produced. After gasification, the syngas is treated to remove the main contaminants. Once the gas meets the necessary requirements, it can be used as an intermediate product for different processes, such as the production of SAF (sustainable aviation fuel) by Fischer Tropsch synthesis, or to produce DME and methanol, among other products of great interest on the world scene.

Block diagram of the process of Syngas production from waste gasification

1. Gasification

Gasification is a partial thermal oxidation resulting in a high proportion of gaseous products (CO2, H2O, CO, H2, and gaseous hydrocarbons), small amounts of carbon (solid product), ash, and condensable compounds (tars and oils).

Air, steam, oxygen, or a mixture of these is supplied to the reaction as the oxidizing agent. The gas produced can be standardized in quality and is easier and more versatile to use than the original biomass (e.g., it can be used to power gas engines and turbines or as a chemical feedstock to produce liquid fuels).

Gasification adds value to low-value feedstocks by converting them into marketable fuels and products.

1.1 Stages

In conventional biomass treatment plants, energy is obtained through incineration or gasification.

The biomass gasification process allows energy to be obtained in the form of heat or electricity, using the syngas (synthesis gas) to drive the shaft of a turbine, or burning it as fuel to drive an engine. This process typically uses coal as a feedstock.

The chemistry of biomass gasification is quite complex. In general terms, the gasification process consists of the following stages (drying, pyrolysis, gasification, partial combustion):

Gasification Stages

1.2 Gasification Reactor Design

Gasification reactor designs have been investigated for more than a century, resulting in the availability of several small and large-scale designs. They can be classified in several ways, although we will focus on classification by design.

Depending on the configuration, gasifiers are classified into three main types: fixed-bed, fluidized-bed, and entrained-flow.

These gasifiers can be divided into the categories shown in the figure. Fixed-bed gasifiers are ideal for small-scale biomass feedstocks.

Fluidized-bed gasifiers can be used to process biomass and refuse-derived fuel (RDF) from pre-treated waste feedstocks, which must meet size, composition, and moisture content specifications.

Entrained flow gasifiers are commonly used for coal because they can be fed in direct gasification mode, which makes feeding the solid fuel at high pressures economical. These gasifiers are characterized by short residence time, high temperatures, high pressures, and large capacities.

Classification of gasifiers and commercially available technologies by feedstock type.

1.3 Challenges and opportunities to improve gasification

The main problem arises in the heterogeneous nature of the reactor feed stream, the limited experience under commercial conditions, and the quality of the syngas obtained. Some of the problems related to waste heterogeneity are overcome by pre-treatment of the waste at the inlet of the gasifier. However, some energy is required and must be accounted for to make a proper balance.

The raw syngas may contain the following contaminants: tars, sulfur, nitrogen, and chlorine-containing gases (NH3, HCl, HCN, H2S, COS), fly ash, and particles containing K, Na, and traces of other elements that may influence catalyst performance.

2. Syngas

Synthesis gas or "syngas" is a mixture composed of carbon monoxide, carbon dioxide, hydrogen, and methane. It is produced by the gasification of a carbon-containing fuel to form a gaseous product that has a certain calorific value. Examples of synthesis gas production include gasification of carbon-rich compounds, gasification of various wastes, and steam reforming of coke.

It is a gas that is used as an intermediate product to synthesize other substances, which is why it is called synthesis gas. It is also an intermediate in the creation of synthetic oil for use as a lubricant or fuel.

2.1 Main contaminants in synthesis gas

Synthesis gas contaminants are composed of tars, nitrogen-based compounds (NH3, HCN, etc.), sulfur-based compounds (H2S, COS, etc.), hydrogen halides (HCl, HF, etc.), and trace metals (Na, K, etc.).

The presence of these contaminants in the synthesis gas poses several technical and operational problems ranging from corrosion (H2S) and fouling of equipment (tar), deactivation of catalysts (tar, H2S, NH3, HCl, and trace metals) or environmental pollution (NH3).

Most downstream applications of syngas have very stringent requirements in terms of compo