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WtE: Behaviour of variables in a bubbling fluidized bed gasifier

Waste to Energy

In industry, gasification is usually carried out auto-thermally. In this way, less than the stoichiometric amount of oxygen is reacted with the carbon-rich feedstock, in this case, the waste. The ranges of this operation are usually between 700 and 1300ºC, depending on the technology chosen.

There is a wide and varied range of technologies available for gasification. Many of them are shown in the figure below, as mentioned in the previuos article.

Classification of gasifiers and commercially available technologies by feedstock type.

The main objective of this technology is to convert fossil fuels, such as natural gas or oil, into usable synthesis gas. Subsequently, to produce hydrogen, ammonia, or Fischer-Tropsch liquids.

One of the most promising and booming products is synthetic jet fuel (SPK). Biomass and waste are the first materials to be used today for their production, due to their renewable nature and low environmental impact. However, they present problems of variability in their composition and sometimes low energy density.

1.1 Detailed description

Fluidized bed gasification can be either a circulating or bubbling bed.

Bubbling and circulating fluidized bed gasifier

There are several commercially available circulating fluidized bed (CFB) gasification technologies.

Initially for coal gasification, and later for biomass gasification, the U-Gas technology was designed. This gasification technology consists of a single-stage circulating fluidized bed, which was designed to process all ranges of coal and provide a medium-low calorific value syngas.

For feedstocks from waste, bubbling fluidized bed technologies have been developed, and it is this technology that is the focus of this article.

The bed is heated by direct injection of oxygen and steam. Thus, the gasifier receives the feed (biomass) and gasifies it using oxygen and superheated steam. In addition, it is heated using a high-temperature flue gas passing through tubes inserted in the gasifier, thus indirectly providing heat to the process. This provides better temperature control. It also improves the control of the syngas composition.

This indirect heating is not implemented by all bubbling fluidized bed gasification technologies, but they simply inject oxygen and steam into the bed.

1.2 Feeding

Biomass and waste are potential fuel sources for gasification. However, their heterogeneous nature means that their composition is characterized as containing both combustible and non-combustible materials.

Therefore, for many gasification technologies, waste must be pre-treated to form a fuel to specification.

Pretreatment generally involves the removal of components such as glass, metals, or concrete. To reduce operating costs, the moisture content must be reduced, and the waste homogenized.

2. Behaviour of the main compounds involved

2.1 Pressure

The effects of pressure change are not as important as other parameters.

Methanization and methane reforming are the only reactions that are significantly affected by an increase in pressure. This makes the product gas richer in methane. Tar formation is also favoured by increased pressure. On the other hand, H2 and CO concentrations are reduced.

While the objective of combustion is to produce as much heat as possible by oxidizing all the combustible material, the objective of gasification is to convert most of the combustible solids into gases such as hydrogen, carbon monoxide, and methane.

2.2 Temperature

In general, higher temperatures enhance gasification reactions.

There are different temperature ranges for different compounds:

CH4 concentration increases when the gasification temperature varies from 660 to 800 °C, while a decrease in its concentration is observed from 800 °C onwards. The initial increase in temperature can be explained by the fact that temperature favours the dealkylation of tars and the cracking of hydrocarbons to methane.

With increasing temperature, methane shifts towards H2 and CO due to the exothermic nature of the methanation reaction.

With a constant oxygen stream, the H2 content increases throughout the logical range. The H2/CO ratio undergoes a strong variation at low temperatures.

CO and CO2 concentrations show opposite trends. In the range from about 600 to 750 °C, the CO2 concentration increases slightly. In this range, CO increases rapidly.

2.3 Oxygen.

Oxygen, and thus the equivalent ratio (ER), has a clear effect on the temperature and, to a lesser extent, on the syngas yield.

The gas yield increases linearly in the common range of gasification.

In contrast, tar and coal yields decrease with increasing ER. An increase in ER implies a greater amount of available oxygen and thus a greater extent of partial oxidation reactions.

The hydrogen content starts to decrease as the equivalence ratio increases above certain amounts. In this case, the H2 concentration decreases as the amount of O2 increases above a certain limit.

Similarly, the concentration of CH4 decreases under the same conditions.

2.4 Evolution of HCN, NH3, and H2S in the synthesis gas (vs. T)

The contaminants considered for a general analysis are NH3, HCN, and H2S.

The main component at low temperatures is NH3. The effect of gasification temperature (from 600°C to 800°C) shows the decrease of NH3 and H2S, while the concentration of HCN increases to a lesser extent.

These compounds have been selected to follow the evolution of nitrogen in the gas phase, as they have been identified as the main gaseous nitrogen compounds in the pyrolysis and gasification processes.

Feed nitrogen is converted to ammonia (NH3), hydrogen cyanide (HCN), and nitrogen oxides (NO, NO2, N2O, and other NOx), as well as to its stable form of N2.

NH3 is the most abundant and depends on the nitrogen content of the feedstock. As the temperature increases without additional oxygen supply, the ammonia concentration decreases and N2 becomes the predominant compound.

In the whole gasification range, HCN appears in a much lower concentration compared to NH3. The HCN concentration remains at an almost stable level, while the amount of NH3 decreases in the same temperature range.

H2S is identified as the main sulphur compound in the syngas, although it also appears to a lesser extent in the form of COS and CS2.

Under gasification conditions, the release of sulphur in the synthesis gas then occurs as H2S, while the other sulphur compounds, including also thiophenes, appear in concentrations about 20 times lower.