Chemical Reaction Engineering Group

There are two main methods which cover a wide area of biomass conversion technologies, thermo chemical conversion and bio chemical conversion. To obtain the energy, the combustion factor is the key for both technologies. Hardware biomass conversion systems can be stationary or mobile. The hardware mobile systems are usually used in rural areas supplying power for a small number of homes, such as in a village, or for powering small to medium size countryside businesses. However, the principle for both stationary and mobile hardware combustion systems is similar.

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

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

There are three different types of boilers:

1. Pile Burners

2. Stationary or Travelling Grate Combustors

3. Fluidized-Bed Combustors

‘Direct Firing’ can be divided into four different methods. These methods come under the titles 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 all come under ‘thermo-chemical’ conversion process. On the other hand, anaerobic digestion and ethanol production come under ‘biochemical’ conversion process type. Biodiesel production comes under ‘chemical’ conversion process.

A number of uses can be made from biogas produced via anaerobic digestion or pyrolysis. These are:

1. Fuel for internal combustion engines

2. To produce heat for commercial and domestic needs

3. As a transport fuel

The following are three different methods for obtaining gases, as a source of energy, from biomass materials.

Gasification

Gasification is described as the process of converting the organic fraction of biomass at higher temperatures and with the presence of air, into a gas mixture with fuel value and more variation than the original solid biomass. This gas can be combusted to produce heat and steam, and can be used in internal combustion engines or gas turbines to produce electricity as well as mechanical energy. Reportedly, the production of electricity via gas turbines combined with steam cycles is the most effective and economical use of the gaseous product. Several biomass gasification processes have been developed (and/or under development) for electricity generation that offer advantages over direct burning, such as higher efficiency and cleaner emissions. Many of the gasification systems are currently at the demonstration stage, and the development of these efficient systems for electricity production is essential: BIGCC (Biomass Integrated Gasification and Combined Cycle) and BIG-STIG (Biogas Integrated Gasification Steam Injected Gas Turbine) plants can achieve efficiencies of 42-47%. Significant developments have been made over the past fifteen years in the field of biomass gasification, especially in the area of medium to large-scale electricity production. Gas cleaning to improve the quality of gas is a crucial issue in both combustion and gasification systems, and requires measures such as reduction of emissions and removing of particulates and tars.

Anaerobic digestion

Anaerobic digestion is the decomposition of wet and green biomass through bacterial action in the absence of air. Generally speaking, 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 output of methane (CH4) and carbon dioxide (CO2), called biogas. Landfill gas is the result of the anaerobic digestion of municipal solid waste buried in landfill sites. The methane gas produced in landfill sites 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 can be described under the environmental benefits, rather than on the technical or commercial side. Anaerobic digestion decreases methane emissions and can provide a good treatment system for organic waste and consequently can prevent groundwater contamination and reduce odour from the local environment associated with this waste.

‘The Government should review its current strategy for the anaerobic digestion sector. In doing so, we recommend that it considers practical and financial mechanisms for encouraging the expansion of the UK’s AD capacity, while ensuring that new AD systems deliver the optimal balance between production of biogas and prevention of uncontrolled methane emissions.’ (Biomass Task Force. 2005).

Pyrolysis

In a temperature ranging from 300 to 700 °C and with 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 eliminated completely.

The final outcome of the pyrolysis process is that the organic materials are transformed into gases and leave a solid residue (coke) made up from carbon and ash. Biomass gasification can also be integrated with fuel cells. Also, 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 at the demonstration stage. As the development is still in the early stages, like the rest of the bio-oil upgrading processes, there is still a need to neutralise negative aspects, such as corrosivity and low heating value. In conjunction with the existing systems, pyrolysis can be used for large scale electricity production.

This article is written by Najib Altawell with references from “Biomass Task Force (2005) Biomass task force report to the government. Department of environment, food and rural affairs (Defra) publications, London” and “USDE (2005) Energy efficiency and renewable energy. Biomass”. The article was adopted from http://EzineArticles.com/6256907.

Image source credited to: http://www.wtert.eu/default.asp?Menue=12&ShowDok=15