Gasification (in the context of this paper) is an energy process that produces a gas which can be used to substitute fossil fuels. It is a process which enhances biomass' usefulness as a renewable resource as the product gas (syngas) can easily be transported along existing gas pipelines, and can either be mixed with, or used as a replacement for natural gas combustion in gas turbines for power and heat generation. Syngas gas can also be used converted to various chemicals such as ethanol, hydrogen, methanol and Fischer-Tropsch fuels. There are currently various technologies for the gasification of biomass, and a review was conducted on the different technologies involved in the overall biomass gasification process.
Keywords: review; gasification; biomass; syngas; liquid fuel
1.BIOENERGY AND GASIFICATION
Current energy supplies in the world are dominated by fossil fuels, some 80% of the total use of over 400 EJ per year. Approximately 13% of the balance is provided by biomass fuels (Jared & John 2002), making energy from biomass by far the most important renewable energy source to date (Andre 2005). The USA has the potential to produce nearly 1.3 billion dry tons of biomass annually which could displace as much as one third of current transportation fuel demands ( Pradeep & Samir 2010). Biomass has a distinct advantage over the use of other renewables such as solar cells and wind power, which are restricted because of the intermittent power generation.
Biomass is biological material derived from living, or recently living organisms such as wood, agricultural residues, food and industrial waste. It excludes organic material such as fossil fuels. Biomass as a fuel is carbon neutral process, as the carbon dioxide released into the atmosphere during combustion is the same amount that had been captured by the plant during photosynthesis
Prior to the current use of fossil fuels, biomass was the primary source of energy for human consumption, used to generate heat via combustion. With the discovery of fossil fuels (in the form of petroleum, coal and natural gas), the human population has grown increasingly dependent on them to provide energy, so much so that currently there are multiple concerns about their usage - from the emissions of carbon dioxide where, in the EU alone in the period from 1990 to 2010, about 90% of CO2 will be attributable to transport (Kokossis & Yang 2010). There are increased concerns about their effect on global warming (Michael & Snorre 1996) and it is generally accepted that the Earth's fossil fuel reserves are being depleted (Shahriar & Erkan 2008b) with reserve estimates of coal, oil and gas at current production expected to last for 155, 41 and 65 years respectively (Shahriar & Erkan 2008a).
Gasification is the conversion of biomass to a gaseous fuel by heating in a gasification medium such as air, oxygen or steam (although strictly speaking, gasification also includes biochemical processes which involve microorganisms at ambient temperature under anaerobic conditions). It differs from combustion where oxidation is substantially complete in one process, gasification converts the intrinsic chemical energy of carbon in the biomass into a combustible gas in two stages, and the produced gas is easier and more versatile to use than the original biomass (Peter 2002). It is a clean and highly efficient conversion process that offers the possibility to convert various feedstock to wide range of applications, and as such it has been considered the enabling technology for modern biomass use (Arjan & Geert 2011).
With the need for alternative energy sources to fossil fuels over the past few decades, technologies have been developed to convert biomass into useful energy forms, usually electricity, or as a fuel for internal combustion engines, or it may be converted into another form such as liquid biofuel or...