Biomass is currently used for heat and/or power generation in the UK. There are four main approaches for utilising biomass, namely i) combustion in dedicated boilers, ii) pyrolysis, iii) gasification and iv) co-firing with fossil fuels, where dedicated combustion and co-firing as the two most common approaches.
1) Combustion
Combustion is the oldest biomass thermal conversion treatment that involves incineration, direct firing and burning with air (Klass, 1998). This process leads to the formation of carbon dioxide and water vapour. Stages that involve in combustion of biomass are displayed in Figure 1.15 and already explained in Section 1.11.1. The flame temperature can exceed 2000°C, depending on the heating value and moisture content of the fuel, the amount of air to burn the fuel and the construction of the furnace (Brown, 2003). A combustor is the device that is used to convert the chemical energy of fuels into high temperature exhaust gases, where the heat from the gases can be utilised for power generation (Brown, 2003). Combustors include grate-fired systems, suspension burners and fluidised beds. Fluidised bed combustors are the recent innovation and they are developed since 1980s especially for industrial applications. An incomplete combustion can however, lead to large emissions of
pollutants, formation of tars and raise environmental concerns (Williams et al., 2012). This is a consequence of biomass having high volatile matter (Murphy et al., 1996). Moreover, Brown (2003) stated that burning high-moisture fuels can affect the performance of combustion for two reasons. Firstly, the energy imbalance to recover the energy required to evaporate the moisture with the energy to cool the water vapour in the exhaust gases. Secondly, this type of fuels does not combust well because the process of fuel drying suppresses fuel temperatures to below those required for ignition. With that, biomass that contains 30% moisture is unacceptable in most boilers. Another problem that can affect combustion is serious slagging and fouling due to the presence of high alkali metals and silica contents in the fuel. Alkali metals exist as oxides and the vapours combine with sulphur and silica, forming compounds that have low melting points.
2) Pyrolysis
This process involves a decomposition of biomass at high temperatures (500-900°C) under inert atmosphere to produce a solid char, liquid and non-condensable gases (for example, CO2, H2O, CO, C2H2, C2H4). The liquid product (known as tar) is the main of interest in pyrolysis. It contains up to 20% water and consists mainly of homologous phenolic compounds. Basu (2013) stated that the product of pyrolysis depends on the design of the pyrolyser, composition of biomass and the following parameters, that is, heating rate, final temperature and residence time. Based on the heating rates, here are two types of pyrolysis. If it is a slow pyrolysis, which operates at 200-800°C and long residence times, more char yield will be produced. Fast pyrolysis at low temperatures (below 650°C) yields more vapours and condense to liquids while that at high temperatures (up to 1000°C) yields more gases (Murphy et al., 1996; Brown, 2003). Based on the biomass composition, the individual constituents that make up lignocellulose have different temperature ranges for initiation of pyrolysis. They respond differently as well. Cellulose is a primary source of condensable vapours while hemicellulose yields more non-condensable gases and less tar. While lignin degrades slowly, making a contribution to the char yield. Based on temperature, the amount of non-condensable gases increases and the composition varies with increase in temperature.
3) Gasification
Gasification is defined as an endothermic process that uses high temperatures (750-850°C) to convert solid carbonaceous fuels into flammable gas mixtures (Brown, 2003). In combustion, the main products are carbon dioxide and water vapour but for gasification, the gas mixtures,
which are also known as producer gases, consist of carbon monoxide, hydrogen, methane, small amounts of nitrogen, carbon dioxide and higher hydrocarbons. Biomass has a very high volatile content and high reactivity char, which makes it suitable as an ideal gasification fuel (Brown, 2003). Low temperatures and high pressures favour the formation of methane, while high temperatures and low pressures favour the formation of hydrogen and carbon monoxide (Brown, 2003).
4) Co-firing biomass in coal-fired boilers
Co-firing is a process where biomass is burned together with coal and this approach is receiving attention globally. It is an alternative to completely replace coal with biomass fuel in a boiler (Brown, 2003). Co-firing has been carried out in the UK since the introduction of Renewable Obligations in April 2002 (Drax, 2011). Most of the coal-fired power stations practice direct co-firing with biomass. By substituting part of coal with biomass, a significant amount of carbon dioxide emissions can be reduced, provided that the biomass is produced sustainably. Industries that generate biomass wastes can also use co-firing instead of landfilling. Furthermore, co-firing can help to reduce sulphur emissions from boilers since biomass has a low sulphur content. With that, ash-fouling can also be reduced.
The following are six basic options available for the direct co-firing of biomass at coal-fired stations as suggested by Livingston (2012) in Figure 1.18.
Option (1): Milling biomass in coal mills
The first option involves milling biomass in coal mills, where these mills have to be operated in cold primary air to avoid combustion. There are a few plants around Europe that have converted their coal firing plants to 100% wood pellet firing such as in Vasthamnsverket in Helsingborg, Sweden, Unit 9 at Amer Centrale in the Netherlands, Hasselby Heat and power plant, Sweden and very soon, Drax Power station, UK. One drawback of this first option is the fact that biomass has a high volatile matter content and release combustible volatiles at temperatures above 180°C, which can lead to explosion and mill fires. Therefore, it is important to control the flow rate and temperature of the primary air.
Option (2): Co-firing by pre-mixing and co-milling
Livingston (2012) reported that this second option of pre-mixing biomass with coal and further processing these mixtures to coal mills has been the preferred approach for coal- power stations that are doing this for the first time. In Britain, there are several mills used for example, ball mills, tube mills, roller mills, and vertical spindle ball and ring. One disadvantage is that biomass tends to accumulate in the mill so it takes longer time, and this means more energy is required for biomass to clear from the mill. Biomass is well-known for its high moisture content, so wet biomass may have impact on the mill heat balance.
Options (3) – (5): The direct injection of pre-milled biomass
There are three basic direct injection co-firing options listed by Livingston (2012):
i. Into the pulverised coal pipework (3), in which this option is only applicable to limited biomass materials and power plants,
ii. Into modified coal burners or directly into the furnace with no combustion air (4), in which this encompasses the full conversion of existing coal-fired power stations by taking coal out of the energy mix and delivers a cost effective form of renewable power to burn biomass only and
iii. Through new, dedicated biomass burners (5), where these are plants that are dedicated to burn solely biomass. Since they are newly built, they are considered as a more expensive option compared to the first two (Drax, 2011).
Option (6): Gasification of biomass
This is an indirect option for co-firing. Figure 1.19 a) shows a schematic diagram of the gasification co-firing. It involves the installation of separate biomass gasifier and boiler but it
is too expensive and complex to be implemented in UK (Livingston, 2012). Basu (2013) stated that the gasifier does not interfere with the operation of the coal-firing system, therefore, this approach offers a high degree of fuel flexibility. One disadvantage is the evaporation of alkali in the biomass that can cause fouling and corrosion of boiler tubes (Basu, 2013).
The other option is parallel co-firing, which involves the installation of a completely separate biomass-fired boiler to produce steam (Figure 1.19 b)). Basu (2013) described that this option uses low temperature and pressure from the biomass boiler instead of using the high pressure steam from the main boiler. This approach avoids fouling and corrosion but this operation is also expensive.
Figure 1.19. Options for co-firing in a coal-fired boiler, where a) represents indirect co-firing
and b) parallel co-firing (Basu, 2013).