Fuel gas from gasification, anaerobic digestion and pyrolysis

There are two main methods that cover a wide area of ​​biomass conversion technologies, thermochemical conversion and biochemical conversion. To get the energy, the combustion factor is the key for both technologies. Hardware biomass conversion systems can be stationary or mobile. Mobile hardware systems are typically used in rural areas to supply power to a small number of households, such as in a town, or to power small and medium-sized businesses in the countryside. However, the principle for stationary and mobile hardware combustion systems is similar.

Combustion can be carried out by means of a furnace or a boiler. A furnace (direct combustion) is one of the simplest methods used to obtain energy by burning biomass materials in a chamber to obtain heat in the form of hot gases released.

A biomass boiler can be used to transform heat into steam, this steam is used to turn the turbine to generate electricity.

There are three different types of boilers:

1. Burner Stack

2. Stationary or mobile grate combustors

3. Fluidized bed combustors

‘Direct firing’ can be divided into four different methods. These methods come under the headings of Pile Burner, Spreader Stoker, Fluidized Bed, and Suspension.

The other method is gasification, which can be divided into five different sub-branches, i.e. biological gasification, landfill gas, pyrolysis, thermal gasification and micro-scale biomass.

Direct combustion, gasification, pyrolysis and methanol production are under the ‘thermochemical’ conversion process. On the other hand, anaerobic digestion and ethanol production belong to the ‘biochemical’ type of conversion process. The production of biodiesel comes under the ‘chemical’ conversion process.

Various uses can be made of biogas produced by anaerobic digestion or pyrolysis. These are:

1. Fuel for internal combustion engines

2. To produce heat for commercial and domestic needs.

3. As transportation fuel

The following are three different methods to obtain gases, as an energy source, from biomass materials.

Gasification

Gasification is described as the process of converting the organic fraction of biomass at higher temperatures and in the presence of air, into a mixture of gases with fuel value and more variation than the original solid biomass. This gas can be burned to produce heat and steam, and can be used in internal combustion engines or gas turbines to produce electricity and mechanical power. Producing electricity through gas turbines combined with steam cycles is reportedly the most effective and economical use of the gaseous product. Several biomass gasification processes have been developed (and/or are in development) for electricity generation that offer advantages over direct burning, such as higher efficiency and cleaner emissions. Many of the gasification systems are currently in the demonstration stage, and the development of these efficient systems for electricity production is essential: the BIGCC (Biomass Integrated Gasification and Combined Cycle) and BIG-STIG (Biogas Integrated Gasification Steam Injected Gas Turbine) can achieve efficiencies of 42-47%. In the last fifteen years there have been important advances in the field of biomass gasification, especially in the field of medium and large-scale electricity production. Gas cleaning to improve gas quality is a crucial issue in both combustion and gasification systems, requiring measures such as emission reduction and particulate and tar removal.

anaerobic digestion

Anaerobic digestion is the decomposition of wet, green biomass through bacterial action in the absence of air. In general terms, the anaerobic digestion process is made up of four main biological and chemical stages:

1. Hydrolysis

2. Acidogenesis

3. Acetogenesis

4. Methanogenesis

It usually has a mixed gas outlet of methane (CH4) and carbon dioxide (CO2), called biogas. Landfill gas is the result of anaerobic digestion of municipal solid waste buried in landfills. The methane gas produced in landfills eventually escapes into the atmosphere. However, the gas can be extracted by inserting perforated pipes into the landfill.

There are a number of benefits related to anaerobic digestion; these may be described under the environmental benefits, rather than the technical or commercial aspect. Anaerobic digestion decreases methane emissions and can provide a good treatment system for organic waste and consequently can prevent groundwater pollution and reduce local environmental odor associated with these wastes.

‘The Government should review its current strategy for the anaerobic digestion sector. In doing so, we recommend that you consider practical and financial mechanisms to encourage the expansion of UK AD capacity, while ensuring that new AD systems provide the optimum balance between biogas production and the prevention of uncontrolled methane emissions. .’ (Working Group on Biomass. 2005).

pyrolysis

In a temperature range of 300 to 700 °C and in the absence of oxygen, the chemical decomposition of organic materials by heating is a process called pyrolysis. However, in most cases and in practical terms, the presence of oxygen cannot be completely eliminated.

The end result of the pyrolysis process is that the organic materials turn into gases and leave behind a solid residue (coke) made up of carbon and ash. Biomass gasification can also be integrated with fuel cells. Furthermore, using pyrolysis, a solid biomass can be liquefied ‘direct hydrothermal liquefaction’ (USDE, 2005). One of the main benefits of flash pyrolysis is that fuel production has been separated from power generation. This type of method is still in the demonstration stage. As development is still in the early stages, like all other bio-oil upgrading processes, there is still a need to neutralize negative aspects such as corrosivity and low calorific value. Along with existing systems, pyrolysis can be used for large-scale electricity production.

Najib Altawell