Biomass gasification for large-scale electricity generation



Electrical power generation can make use of several biomass processes, including direct combustion, combustion of gas derived from fermentation or gasification, and combustion of pyrolysis fuel. Thermal gasification and pyrolysis are seen as the most promising technologies for the generation of electricity from biomass.

Biofuels are fuels produced directly or indirectly from organic material or biomass which includes plant materials and animal and human waste. Production of electrical energy using biomass as a fuel involves accessing the hydrocarbon portion of the biomass that can be converted into heat. Biofuels are considered renewable as they use energy from sunlight to recycle the carbon in the atmosphere in the form of carbon dioxide through a process, known as carbon fixation, that takes inorganic carbon (in the form of CO2) and converts it into organic compounds.

Much of the current biofuel production focuses on production of liquid fuels (by fermentation) for transportation. A comparative study of biomass electricity production and biomass ethanol production however, claims that it would be far more efficient to use the biofuels to generate electricity directly and power electric vehicles [1]. This study was conducted on biomass produced for energy generation and not just waste.

Fig. 1:  Gasification stages (Allpowerlabs).

With the move to electric vehicles on everyone’s agenda, it would seem that development effort in biofuels should be focused on electricity production rather than liquid fuels. This option offers a useful platform for agriculture to move to electric vehicles, as many farms already use small-scale gasifiers to produce electricity from agricultural waste.

Electricity from biomass

Electricity can be produced from biomass in three ways:

  • Direct combustion: Direct combustion of biomass produces steam used to drive steam turbines. This is common in the co-generation field in the sugar and forestry industries.
  • Thermal gasification: Thermal gasification of biomass produces syngas that is used to run either internal combustion (IC) gas engines or gas turbines. Small scale gasification of biomass is very common especially in agriculture, and conventional IC engines are commonly used to drive generators.
  • Fast pyrolysis: Fast pyrolysis produces syngas and liquid fuels with properties similar to diesel fuel. The liquid fuel has the advantage that it can be stored and used later. Pyrolysis fuels can be used in IC engine or gas turbine applications.

Most current biomass electricity production plants are based on direct combustion of waste biomass, such as bagasse and sawmill waste, although fermentation to produce biogas is in common use. Gasification and pyrolysis of crops grown specially for electricity production is a new approach that needs development.

Usage of biomass gasification and pyrolyis is still small compared to other techniques for exploiting biomass energy, but is growing. Advances in technology and the construction of large-scale gasification plants have not provided a sufficient boost to increase the level of implementation of gasification despite its advantages in aspects such as greater efficiency and the reduction in CO2 emissions, as there are numerous other methods of biomass energy conversion which provide stiff competition [3]. There is an increasing interest in pyrolyis because of the disconnect that is possible between the production and consumption, both in time and space, as the fuel produced can be stored and can be consumed at a site remote from the site of production.

Direct combustion

Direct combustion of biomass for electricity production is common in the forestry and timber industries. It has been estimated that 53% of the raw timber delivered to sawmills ends up as biomass in the form of woodchips, bark and sawdust [6].

Direct combustion of agricultural waste in co-generation plants has been used for many years in the sugar industries in South Africa, and co-firing of biomass with coal has been under investigation for some time. In the sugar industry the primary use of biomass is combustion for heating boilers to produce steam for sugar production process, and excess steam is used for electricity generation. The raw cane after crushing provides more biomass than is necessary to produce steam for sugar production, and this is used to produce steam for electricity generation, some of which is used in the sugar refinery. Co-generation plants often produce more energy than what is required and are able to feed surplus electricity into the grid. In some locations, such as Mauritius, the sugar production industry provides a significant proportion of the total electricity supply. Co-firing of raw biomass with coal has found limited application although it has been used successfully in some plants.

Fig. 2: Fixed bed gasifiers.

Gasification

Biomass gasification is a thermal process which converts organic carbonaceous materials (such as wood waste, shells, pellets, agricultural waste, energy crops) into a combustible gas comprised of carbon monoxide (CO), hydrogen (H) and carbon dioxide (CO2). This is achieved by reacting the material at high temperatures, without fully combusting it, using a controlled oxygen (O) inlet. The resulting gas mixture is called syngas. At temperatures of approximately 600 to 1000°C, solid biomass undergoes thermal decomposition to form gas-phase products which typically include CO, H, CH4, CO2, and H2O. In most cases, solid char plus tars that would be liquids under ambient conditions are also formed. The gas composition depends on many factors including the type of feedstock, gasification temperature and the reactor type.

The interest in small-scale gasifiers began over a century ago and has continued into the present. Small-scale gasification can be used to power conventional IC engines and is very popular in the farming community. Most plant uses wood as the feed stock, but the process can be used for any type of biomass. There are a large number of manufacturers offering small-scale gasification plants worldwide. The challenge is the use of large-scale gasifiers for grid scale electricity production.  Gasification of energy crops for electricity production is limited to small-scale plant using largely waste, and has not yet advanced to large-scale, which may be due to unavailability of large volumes of energy crops suitable for gasification. However biomass gasification is expected to become increasingly important as a carbon neutral means of electricity generation in future.

Large-scale is defined as including gasifiers capable of using several tons of biomass per day with thermal outputs of 10 to 20 MWth or more. These gasifiers would typically provide fuel for commercial power generation, a source of heat and/or power to meet major industrial needs, or gases for production of fuels and chemicals.

There are four stages in involved in gasification process (see Fig. 1):

  • Drying: In the drying zone, moisture in the feedstock is evaporated by the heat from the lower zones at a temperature of between 150 and 200°C. Vapours move down and mix with vapours originating in the oxidation zone. A part of the vapour is converted into oxygen with the remainder being retained in the producer gas.
  • Pyrolysis: This is the thermal decomposition of biomass in low oxygen conditions at temperatures ranging from 200 to 600°C. For gasification to take place, there must always be such a zone of relatively low temperatures where condensable hydrocarbons are generated. Pyrolysis results in the production of solid char, liquid tar, and a mixture of gases. The proportions of these components are influenced by the chemical composition of the biomass and operating conditions of the gasifier, of which reactor temperature is critical. It is generally understood that pyrolysis involves the breakdown of large molecules (such as cellulose, hemi‐cellulose, and lignin) into medium‐size molecules and carbon (char). If medium‐size molecules remain in the hot zone long enough, they will break down into smaller molecules and, along with the char, form molecules like CO, CO2, H2, CH4, etc.

Fig. 3: Fluidised bed biomass gasifiers.

  • Combustion: Oxidation occurs in the presence of a reactive gas (air or pure oxygen) which affect the calorific value of the gas leaving the gasifier. The use of air as reactive gas is the more common. Oxidation is the phase that provides heat for the phases of the gasification process. Produces carbon dioxide and water. An oxidation zone follows at which air is introduced at less than stoichiometric oxygen (O2) conditions. The principal reactions are highly exothermic and thus result in much higher temperatures, leading to the breakdown of medium‐size molecules, such as tars and oils generated in the pyrolysis zone, into smaller molecules including CO, H2, CH4, etc. Oxidation takes place at temperatures ranging from 700 to 1000°C. In addition to O2 and water vapor, ambient air contains large amounts of N2 and small amounts of other inert gases, all of which are considered to be non‐reactive with fuel constituents at relatively low pressures and temperatures.
  • Reduction: The products of the oxidation zone, hot gases and glowing char, move into the reduction zone. Since there is insufficient O2 in this high‐temperature zone for continued oxidation, a number of reduction reactions take place between the hot gases (CO, H2O, CO2, and H2) and char. The principal reduction reactions are a carbon dioxide reaction,a water gas reaction,and a water‐gas shift reaction.The char produced reacts with water vapour and carbon dioxide, thereby forming hydrogen and carbon monoxide, principal constituents of the combustible gas.

Types of gasifier

Gasifiers have been built and operated using a wide variety of configurations. The main types are fixed bed gasifiers and fluidised bed gasifiers.

Fixed bed gasifier (FBG)

This is the simplest type of gasifier consisting of usually a cylindrical space for a fuel feeding unit, an ash removal unit and a gas exit. The FBG system consists of a reactor/gasifier with an optional gas cooling and cleaning system. The FBG has a bed of solid fuel particles through which the gasifying media and gas move either up or down. The fuel bed moves slowly down the cylinder as gasification occurs. FBG gasifiers are of simple construction and generally operate with high carbon conversion, long solid residence time, low gas velocity and low ash carry over. In FBG, tar removal used to be a major problem, however recent progress in thermal and catalytic conversion of tar has given credible options.  FBG can be of two types , updraft and downdraft as shown in Fig. 2.

In a typical fixed bed (updraft) gasifier, fuel is fed from the top, while the gasifying agent is fed through a grid at the bottom. As the gasifying medium enters the bottom of the bed, it meets hot ash and unconverted chars descending from the top and complete combustion takes place, producing H2O and CO2 while generating heat. This heats up the upward moving gas as well as descending solids. The combustion reaction rapidly consumes most of the available oxygen, while further up partial oxidation occurs, releasing CO and moderate amounts of heat. The mixture of CO, CO2, and gasifying medium from the combustion zone, moves up into the gasification zone where the char from upper bed is gasified. The residual heat of the rising hot gas pyrolyses the dry biomass. The updraft gasifier is not appropriate for many advanced application, due to production of 10 to 20 wt% tar in the produced gas.

The reaction regions in downdraft gasifiers differ from the updraft gasifiers, as biomass fed from the top descends, while the gasifying agent is fed into a lower section of the reactor. The hot gas then moves downward over the remaining hot char, where the gasification happens. In the downdraft version air is supplied at the top or the middle of the gasifier and gas extracted at the bottom. Volatiles are cracked in the reduction stage and there is less tar in the output gas.

Fig. 4: Circulating fluidised bed gasifier.

In a FBG the biomass fuel is mixed with an inert bed material, and the bed is fluidised by the injection of oxidising gas, usually air or oxygen. Fuel at various stages of gasification bubbles in the gasifier bed of fine grained material into which air is introduced, fluidising the bed material and ensuring intimate mixing of the hot bed material, the hot combustion gas and the biomass feed. Two main types of FB gasifier are in use: Circulating fluidised bed and bubbling fluidised bed.

Bubbling FB gasifiers consist of a vessel with a grate at the bottom through which air is introduced (see Fig. 3). Above the grate is the bubbling bed of fine-grained material into which the prepared biomass feed is introduced. The biomass is pyrolysed in the hot bed to form a char with gaseous compounds, the high molecular weight compounds being cracked by contact with the hot bed material, giving a product gas with a low tar content.

In a circulating FBG, the bed material is circulated between the reaction vessel and a cyclone separator, where the ash is removed and the bed material and char returned to the reaction vessel (see Fig. 4). Circulating FB gasifiers are able to cope with high capacity throughputs and are used in the paper industry for the gasification of bark and other forestry residues. Gasifiers can be operated at elevated pressures, the advantage being for those end-use applications where the gas is required to be compressed afterwards, as in a gas turbine.

Fast pyrolysis (FP)

Fast pyrolysis is the rapid thermal decomposition of carbonaceous organic matter in the absence of oxygen. This process occurs at low pressure, moderate temperatures and in a very short amount of time. Fast pyrolysis produces three products: biochar, pyrolysis oil and non-condensable gases. Yields are dependent on many factors including process conditions (reactor temperature, pressure, residence time) and feedstock composition. Optimal biomass processing conditions include reaction temperatures around 500°C, high heating rates, and rapid cooling of the pyrolysis vapors after biochar has been sufficiently removed.

The main product of pyrolysis is fuel oil and the process is optimised to convert the maximum amount of carbon compound into oil. Yields of the order of 70% of the dry weight of the feedstock have been reported. Pyrolysis fuel oil has been successfully used with small gas turbines (1 MW) and with heavy fuel oil engines (diesel engines) for the production of energy. There is an increasing interest in pyrolysis because the fuel oil can be stored and transported, and does not need to be used at the same site as it is produced.

Generator plant

Biomass gas can be used for electricity generation in several ways:

  • Cofiring in a PCF plant: The gas is fired in the furnace together with the pulverised coal. Several plants are running on this principle the most notable the VAAS power station in Finland [5].
  • Internal combustion engines: Biomass gas has been used successfully as a fuel for diesel type engines of a wide range of sizes. This is the usual method for small biomass gasifiers, including DIY types and commercial product. Larger plant also uses diesel type engines.
  • Gas turbines: One of the problems with gasification is the tar content of the output gas. Diesel type engines are tolerant of this but gas turbines are not, as this causes damage to the blades, and gasifier product requires a scrubbing process before it is suitable for gas turbine operation, which increases the cost.
  • Indirect fired gas turbines: An emerging solution to this problem is the indirect fired gas turbine, where the combusted gasses do not pass through the turbine but through a heat exchanger, and only the heated compressed air passes through the turbine. This technique also allows smaller turbines to be used with biomass fuel. The hot exhaust air can be returned to the input of the gasifier, increasing the efficiency of the cycle.

There are no large power plants operating completely on gasified biomass, but several make use of co-firing of gas with coal. An example is the Vaasa coal fired plant in Finland. A circulating fluidised bed gasifier, produced by Valmet, with a capacity of 140 MW, using forestry and sawmill waste as a fuel, was added to the power station in 2013 [4]. The gasifier reduces coal consumption by up to 40%, and can use a variety of biofuels. Trial runs carried out in September 2014 proved that the boiler can be fueled solely with gas from gasifier. Since then, the boiler has been run purely on gas when the load is low during autumn and spring [5].

Direct combustion vs. gasification vs. pyrolysis for large plant

Research on the topic suggests that gasification has several advantages over direct combustion and that pyrolysis has advantages over both [3]. This depends largely on the size of the plant. In a direct comparison between direct combustion and steam generation, and gasification followed by gas turbine combine cycle plant, the GCC option shows a higher overall efficiency at all plant sizes from 5 to 50 MW in a study conducted in 2005.

References

[1] E Campbell, et al: “Greater transportation energy and GHG offsets from bioelectricity than ethanol”, Science, May 2009.
[2] M Blomberg: “Fuel conversion at the Vaasa power plant: large scale biomass gasification plant integrated to a coal fired boiler”.
[3] J Ruiz: “Biomass gasification for electricity generation: Review of current technology barriers”, Renewable and Sustainable Energy Reviews, Vol. 18, February 2013.
[4] A Bridgewater, et al: “An overview of fast pyrolysis of biomass”, Organic Geochemistry, Issue 30
[5] S Farzad: “A critical review on biomass gasification, co-gasification, and their environmental assessments”, Biofuel Research Journal, 2016.
[6] J Anderson: “Improving Energy Use in Sawmills: From Drying Kilns to National Impact”, Licentiate thesis: Lulea University of Technology.

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