Biomass is claimed to be the fourth largest energy resource in the world after oil, coal and gas and biomass power plants are becoming one of the most promising alternatives to the established power generation technologies based on fossil fuels.
Biomass is defined as any organic material derived from plants, available on a sustainable basis. Generally identified as feedstocks, these materials include: wood, from forestry trimmings or wood processing systems; energy crops, which are crops grown exclusively for energy purposes; agricultural residues; municipal waste such as waste paper, cardboard or food waste; and also animal waste from farms and animal processing industries.
The production of electricity from biomass is described as a carbon neutral technology because the carbon dioxide released into the atmosphere when plant material is burnt or decomposed during the electricity generation process, is then absorbed again by new growing plant material. This process maintains the atmospheric CO2 levels and is known as carbon cycle.
There are currently two main processes used in power plants for the production of electricity from biomass: and they are direct combustion and gasification [2].
Direct combustion is essentially the incineration of dry biomass in the presence of air to produce heat.
Gasification is the thermo-chemical transformation of biomass into a combustible gas which is called syngas (synthesis gas) and is a combination of principally carbon monoxide and hydrogen. This process occurs at high temperature (700°C to 1000°C) in the presence of a limited amount of oxygen [3].
The heat produced by direct biomass combustion can be used to generate electricity using a steam turbine in the same way as in a coal-fired power plant. The biomass material is collected, taken to the power station and then burnt in the boiler. The heat from burning the biomass is used to boil water which generates steam that rotates the turbines. The turbines are connected to generators where the mechanical energy is converted to electrical energy.
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Plants designed for working just with biomass are called dedicated biomass power plants. However, biomass combustion can be combined with coal combustion in existing coal-fired power plants. This process is called co-firing and is one of the most profitable ways of transforming biomass into electricity because it makes use of the infrastructure of the coal plant and therefore it reduces the total investment.
Co-firing power plants can be designed under three concepts: direct, where biomass and coal are mixed and burnt in the same boiler; indirect, where biomass is first gasified before the combustion with coal in the boiler; or parallel, where biomass and coal are burnt in separate boilers and the processes are connected on the steam side [4].
In efficiency calculations, the scale of operation is a very important factor. In systems producing from 10MW to 50 MW, the efficiency is in the range of 18% to 33% [5]. The maximum efficiencies could reach about 45% in large scale plants producing more than 100 MW [6][7]. In co-firing plants, efficiency of 39% can be reached [8].
Biomass gasification process can be couple with a conventional combined-cycle gas turbine (CCGT) power plant to produce electricity. Biomass feedstock is first dried and then injected into the gasifier. The resulting biogas is purified in a cleaning system and from there the procedure is the same as in a natural gas power plant [9].
To produce from 10 kW to 10 MW, biogas produced in the gasification process can also be used in combustion engines with efficiency of 30% - 35% [10]. At larger scales (>20 MW), where gasification-based systems are coupled with combined cycle gas turbines the efficiency increases up to about 45% [11].
There are two others processes which can be used to produce electricity from biomass but they are not commercially developed: pyrolysis and anaerobic digestion.
Pyrolysis is a thermo-chemical decomposition of organic material at high temperatures (325°C to 500°C) similar to gasification but in this case there is no presence of oxygen. This process generates combustible gas and liquid products that could be used in power generation units or upgraded to transport fuel [12]. A carbon-rich residue called biochar is also produced from pyrolysis, and one of the important aspects of biochar is that it is a natural fertiliser that can be used to improve soils quality, which can potentially increase energy crop productivity.
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The second one, anaerobic digestion, is a natural biochemical process in which the biomass material is broken down by microorganisms in a environment with no air, to produce biogas, which is mainly a mixture of around 60% methane and 40% carbon dioxide. This process can be applied to wet biomass, municipal or animal waste to produce power generation on site [13].
There are a number of technologies that support the different methods for converting biomass material into electricity, which include: drying, pelletisation, briquetting, cleaning and lately carbon capture and storage.
Drying is fundamental because in order to increase the energy density of biomass feedstocks, their moisture content needs to be reduced (< 20%). It is also significant to reduce transport costs and improve the efficiency of the combustion process. After harvesting, feedstocks can be left dying naturally on the collection site, but in humid climates, or regions with heavy snow fall, feedstocks need to be covered with waterproof sheets. Biomass material can also be actively dried using waste heat [14].
Pelletisation and briquetting are technologies to compact feedstocks mechanically, very useful for their transportation and management. Pelletisation is used for example to compress low quality wood, and agricultural residues are compacted through briquetting process [14].
The gas obtained from gasification contains impurities and particulates that need to be removed before using it in power plants, because these contaminants can cause erosion and corrosion in the gas turbine components, and decrease the strength of the system. Conventional methods for removing contaminants from biogas are typically based on physical cleaning processes at low temperatures (< 40°C), where gas is condensed and the contaminants are removed. But this process is not effective for biomass power plants applications because the gas has to be reheated before entering the gas turbine. There is another technology, relatively new, known as 'hot gas cleanup' where the cleaning process is carried out at high temperature, so that the efficiency loss is reduced and the commercial viability of the plant is improved. However the cost of the process is high and the operational experience is limited.
Combining biomass power plants with carbon capture and storage processes (BECCS) to provide "negatives emissions" [15] is a new approach. When the CO2 emitted during biomass electricity generation is captured and stored, new growing plant material will absorb CO2 from the atmosphere reducing the current high level concentrations.
The key advantage of power generation from biomass is that it is based on a CO2 neutral process and it can be a clean and reliable power source if sufficient feedstocks are available. It is also a way to utilise waste materials that otherwise would represent environmental risks. Biomass electricity deployment has also a significant social and economic impact because it can create employment in the agricultural and forestry sector, benefiting rural communities and in general developing countries which economies are based on agriculture [16].
The expansion of biomass power generation faces several challenges such as high costs, low conversion efficiency and availability of biomass material [17]. As any new technology, biomass power generation currently requires financial support which make it less commercially competitive compared to fossil fuel based electricity. Biomass electricity production will depend also on technology improvements in order to increase efficiencies at small and large scale. Major concerns are associated to biomass production (intensive farming, biodiversity conservation) and competition for land with food production.
Energy from biomass has been used since fire was discovered from the combustion of wood, and before the industrial revolution wood was used for all of our energy needs. However In 1890 coal began to displace wood used in steam power generation. During the 1980s decades, high prices of oil and the instability caused by the dependence on foreign fossil fuels created new interest in biomass energy in several countries, especially in North America. A large biomass power industry rapidly developed in California, who had 850 MW of installed biomass power capacity by 1985. Due to concerns about greenhouse gases emissions and global warming, governments took a greater interest in using biomass as a renewable and clean alternative to produce electricity.
Currently most biomass electricity generation is based on direct combustion in dedicated and co-firing steam power plants. Electricity supply from biomass has augmented gradually since 2000, and in 2010 biomass provided 1.5% of world electricity production approximately. Although biomass power generation is still stronger in developed countries, China and Brazil are also becoming important electricity producers in particular from agricultural residues thanks to support programmes. The models established in these China and Brazil could become a viable way to encourage electricity generation from biomass in other developing countries with similar conditions [18].
According to the International Energy Agency [19], world electricity generation from biomass will multiply by more than 10 times from around 280 TWh in 2010 to 3100 TWh in 2050 and could provide around 7.5% of world electricity generation. China will become the major producer of bioenergy electricity with 920 TWh, above OECD Americas (520 TWh) and OEDC Europe (370 TWh) which will also increase their generation levels.
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