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Biomass as an energy resource

According to The European Commission, « Biomass means the biodegradable fraction of products, waste, and residues from biological origin from agriculture (including vegetal and animal substances), forestry and related industries including fisheries and aquaculture, as well as the biodegradable fraction of industrial and municipal waste.” [12] Biomass is considered renewable and sustainable because plants naturally capture carbon dioxide from the atmosphere during the photosynthesis process, which is released back into the atmosphere when used to generate energy. Thus, ideally, biomass is a CO2 neutral source of energy as it does not result in any net CO2 emission into the atmosphere. However, in real cases, when the emissions during transportation, storage, and drying of biomass are considered, its carbon neutrality may change; therefore, careful Life Cycle Analysis (LCA) needs to be done.
A typical composition of biomass comprises cellulose, hemicelluloses, lignin, extractives, lipids, proteins, simple sugars, starches, water, inorganics (ash), and other compounds. The cellulose and hemicellulose are the fiber of the biomass, whereas lignin acts as the glue of these fibers. Different biomass has different parameters, such as chemical composition, moisture content, ash, and inorganic substance content. The elemental composition of biomass mainly includes carbon, oxygen, hydrogen, nitrogen, calcium, potassium, silicon, magnesium, aluminum, sulfur, iron, chlorine, sodium, etc. [13] [14] Biomass has a relatively lower energy per unit mass, compared to natural gas. Therefore, biomass needs to be valorized in order to be used effectively.
Figure 3 shows the main routes for the conversion of the biomass into different forms of energy. The two main types of conversion routes are thermochemical conversion and biochemical conversion. In both types, absence or presence of oxygen makes the difference. For example, in thermochemical conversion, the presence of excess air results in combustion, and the absence of it results in pyrolysis. The thermochemical conversion is faster than the biochemical alternative. The thermochemical conversion route takes only up to a few minutes to get usable energy from biomass. Whereas, the biochemical conversion route can take up to a few days. The thermochemical route is more flexibility in terms of the type of intermediate energy carrier produced and end-user energy application. The thermochemical conversion paves the pathway for heat, electricity, and synthetic fuel applications. In the present project, gasification is the chosen conversion route for heat and power applications, which will be discussed in detail in the following section. [15] The technology is a promising way to valorize biomass and use it with high efficiency in combined heat and power plants, but also as for production of second-generation biofuels, and other synthetic chemicals via Fischer–Tropsch synthesis. [16]

Theory of Gasification

Gasification is the thermochemical conversion of an organic fuel like biomass in the presence of a limited supply of oxygen into the permanent gaseous products mainly comprising of carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2) and methane (CH4). The gas from the gasifier is called producer gas, which in addition to the main permanent gases, also includes higher hydrocarbons, tars, water vapor and even nitrogen, if air is used as the gasifying agent. Gasification is a partial oxidation process where the biomass undergoes a combination of drying, combustion, pyrolysis, and gasification reactions in a reactor called gasifier. [17] The oxygen supplied is only a fraction of the stoichiometric quantity needed for complete combustion. [6] If air is not used as a gasifying agent, the producer gas can be cleaned and conditioned to the desired composition of a synthesis gas, which mainly comprises of carbon monoxide and hydrogen. This synthesis gas can be used for the synthesis application for production of synfuels and chemicals.
Table 1 lists all significant reactions that take place during gasification, and Figure 4 shows how the heat and reactants interact with each other inside the gasifier. [13] [18] When the fuel enters the hot reactor, the combustion and partial oxidation reactions (1 and 2) provides the necessary heat energy for the rest of the reactions. Drying, pyrolysis (reaction 8), and main gasification reactions (3 and 4) are highly endothermic reactions. The methanation reaction (5) is a mildly exothermic reaction, which also provides some heat. The steam reforming reaction (6) is also a highly endothermic reaction that consumes heat. Figure 4 also shows how these reactions run parallelly and in sequence at the same time. The product of one reaction acts as the reactant of another, leading to a very complex reaction system. Reactions 1 and 2 are fast reactions, while reactions 3-5 are the slower and, therefore, rate limiting reactions. [19]
Tars are mainly produced during the pyrolysis in reaction (8), which are undesirable in the gasification process. According to International Energy Agency (IEA) Bioenergy Agreement, the US Department of Energy (DOE), and the DGXVII of the European Commission, all organic components of the product gas, having molecular weight higher than benzene, i.e., 78 g/mol, are classified as tar. [20] Tars are mainly aromatic hydrocarbons with dew point starting from 350°C, making them highly condensable. Higher production of tar may lead to choking, blocking, and fouling of downstream equipment. [21] Produced tars can be removed in two ways. Firstly, it can be decomposed within the gasification unit by using appropriate catalysts, or by altering the process conditions that favors tar reduction. Alternately, tar can be reduced downstream in a separate catalytic cracker and separated from the produced gas stream.
The gasifying agent has a significant impact on the produced gas composition, calorific value, and tar yield. Air, oxygen, steam, or a mixture of these are the most commonly used gasifying agents. Gasification using air produces gas with lowest calorific value as much of the gas is diluted with nitrogen. Gasification using steam and/or oxygen produce gas with higher calorific value. The Lower Heating Value (LHV) of produced gas using air as gasifying agent is between 4.5-6.5 MJ/m3, and is between 12.5-13 MJ/m3 for using a mixture of steam and oxygen. [22] These gasifying agents react with the carbon material and have a strong influence on the thermodynamics and kinetics of the reactions. In this study mixture of steam and oxygen is used as the gasifying agent. Equivalence ratio ( ) (Eq. 1) is the ratio between the actual oxygen supplied to the amount required for complete combustion. A complete combustion reaction has an equal to 1. The oxygen reacts in the partial oxidation reaction (2) and with the volatiles produced in the pyrolysis reaction (8), providing the heat for the rest of reactions (as shown in Figure 4) and simultaneously reducing the initial tar production significantly. [23]
A higher equivalence ratio leads to higher yield of produced gas with lower tar production. [6] However, higher significantly reduces the calorific value of the produced gas due to increased production of CO2. Therefore, the optimum value of is between 0.2-0.4. [23] For below 0.2, the oxygen will not be sufficient for the oxidation reaction (2) to provide sufficient heat and pyrolysis reaction (8) will dominate, producing excess tar and higher hydrocarbons. If the produced gas is to be used without cooling for combustion or other energy applications, gasifier can be operated starting from = 0.2 as tar will not be a problem and the calorific value will be higher. However, when gas application demands lower tar concentration, can be increased depending on the operating temperature. If the gasifier is operated at temperature lower than 850°C, tar yield will be higher due to insufficient thermal cracking, then gasifier can be operated with between 0.3 – 0.4 to compensate for such effect. [7] [24]
Another essential process parameter is the steam to fuel ratio. It is simply the ratio between the steam supplied and the dry fuel feeding rate. Increasing the steam to biomass ratio leads to improved calorific value due to higher yield of hydrogen mainly because of reactions 3, 6 and 7. [7] However, the effect of increasing the steam at higher pressure is unknown because at higher pressure the reaction will tend to move towards the side producing lesser number of molecules.

Types of Gasifier

Different gasification technologies are available, depending on scale, feedstock properties, and end user application. Gasifiers are designed based on three criteria, namely – mode of contact between the fuel and gasifying agent, method and rate of heat transfer, and residence time of the fuel in the reactor zone. [13] These different designs come with their own set of advantages and disadvantages. There are mainly three types of commercial gasifiers – fixed bed, fluidized bed, and entrained flow gasifier.
Updraft (counter-current) and downdraft (co-current) are the main designs based on fixed-bed reactor design. They have an excellent tolerance for coarse feed. In updraft gasifier, the gasifying agent is introduced from the bottom of the reactor and the fuel is fed from the top. There is a good contact between the solid material and the gasifying agent resulting in very high thermal efficiency. However, this leads to high tar content in the gas, because it passes through the low temperature pyrolysis and drying region at the top of reactor. This is a major disadvantage of this design as energy content of the tar is >20%, therefore, subsequent tar removal is needed. This problem is solved in the downdraft gasifier where both the gasifying agent and the fuel are fed from the top. Thus, all the gas, along with all the products from pyrolysis zone is forced to pass through the high temperature oxidation zone. This leads to thermal cracking of the volatiles, producing gas with much lower tar content. [25] However, as shown Figure 5, [26] downdraft gasifiers are only suitable for small scale applications.
The entrained flow gasifier is on the other hand good for very large-scale applications with <high carbon conversion and very low tar concentration due to its high operating temperature of over 1200°C. However, it can take in only very fine fuel feed with less than 15% moisture and a constant composition. [27] [28]
The fluidized bed gasifier is the medium range gasifier with its own set of advantages and disadvantages. Here the bed material which is in granular form is fluidized by the gasifying agent, which is forced from the bottom of the reactor, while the fuel is fed from the top. This constant mixing provides uniform bed conditions ensuring proper heat and mass transfer between the solid and the gas. The main advantage over fixed bed gasifier is the enhanced mixing, resulting in nearly isothermal conditions. [29] The operating temperature is highly dependent on the sintering temperature of the bed material which is often less than 900°C. This temperature is relatively low for achieving stable gasification. The short residence time is one of the reasons for not achieving chemical equilibrium. Therefore, catalytic bed materials are needed to push the reaction towards equilibrium. [14] Bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) are the two major types of fluidized bed gasifier. BFB is the simplest and one of the most cost-effective continuous feed gasifiers, which can handle a variety of biomass with various sizes. Although it provides a high rate of heat transfer and mixing between bed material and fuel, the slower diffusion rate of oxygen creates an oxidizing condition in the entire bed reducing the gasification efficiency. The CFC overcomes this limitation by providing longer residence time for the fuel, resulting in lower tar production. However, CFC operates at very high fluidization velocity making the process complex and relatively difficult to control. [7]
In the present study, bubbling fluidized bed (BFB) gasifier was used considering the scale of operation, lower footprint and ease of operation under pressurized conditions.

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Importance of Bed Material

Mechanical properties, chemical properties, and the cost determines which bed material is to be used in fluidized bed gasification. [30] The bed material is mainly responsible for the proper distribution of heat in the reaction zone. The heat produced from the exothermic reactions gets accumulated in the bed material and gets transferred to endothermic processes like drying, pyrolysis, gasification reactions due to intense mixing. This prevents the temperature peaks in the reactor, and nearly uniform temperature distribution can be achieved in the bed zone. However, fluidization conditions can cause mass loss due to attrition, producing fines which may enter the downstream processes. Therefore, good mechanical strength is also needed. [31] In principle, the bed material is expected to remain inert during the process. But then, if it possesses some catalytic properties, the choice of bed material can largely influence the produced gas composition in terms of hydrogen yield and tar content. In the absence of catalysts, methane and heavier hydrocarbons, including tar, are present in the gas, because the slow gasification reactions are unable to reach equilibrium at the operating conditions. [14]
The bed material can be a mineral material, such as silica sand, olivine or limestone material, but can also be made of metal or metal-oxide on inert oxide support. This kind of structure only gives the desired mechanical stability but also provide the large surface area for catalytic activities. Commonly, mineral-based material is preferred due to its lower cost. The biggest challenge comes when fuel with high ash content is used. Ash sintering and bed particle agglomeration may occur due to alkali metals and chlorine content of the fuel. These compounds form low melting eutectics with silica in the sand or the ash itself, causing the stickiness of the particles, eventually forming lumps called agglomerates. These lumps affect the hydrodynamics of the reactor negatively, ultimately leading to defluidization of the bed. Consequently, resistance to agglomeration is essential. In addition to these desired mechanical and chemical properties, the availability and cost of the bed material dictate its suitability for the process. Thus, naturally abundant materials are highly preferred. Good disposability of the bed material is also desired as disposal of degraded materials according to the environmental protection laws involves additional cost. Thus ideally, the bed material should have good resistance against agglomeration, have high mechanical stability with catalytic tar cracking abilities and low cost. [32]
Silica sand has excellent mechanical properties and is the most economical bed material. It is chosen for the reference test for being inert and non-catalytic. However, it is most susceptible to agglomeration. The potassium salts in the ash react with the silica in the sand and the ash itself to form different potassium silicates, which have a low eutectic temperature of 700°C. This behavior has been extensively studied under atmospheric pressure gasification, but not under pressurized conditions. Thus, it will be interesting to observe its performance. [32] Olivine has excellent resistance to agglomeration. It is a natural, inexpensive and magnesium containing disposable mineral with a lower level of silica compared to silica with an empirical formula ( 1− )2 4. Presence of iron in different oxidation states gives its catalytic properties towards the tar removal and the Water-Gas-Shift reaction. [33] Magnesite, on the other hand, is a mineral containing much lower content of silica and a much higher level of magnesium compared to olivine. In previous studies, it has been used both as an additive and as bed material. In addition to its tar destructing catalytic properties, it has excellent resistance to attrition. Ca- and Mg- containing minerals have good resistance to agglomeration at high temperatures. This is because they have a higher affinity towards potassium and silica, forming compounds with melting temperatures as high as 1100°C. [34] In one of the studies comparing different bed materials, magnesite was reported to have higher activity than olivine. [35]

Effect of Pressure and Temperature

More energy is expended in compressing a large volume of gas downstream of the gasifier than in compressing the reactants (solid biomass and gasifying medium) upstream of the gasifier. Therefore, pressurized gasification is very attractive from the process intensification point of view. [36] However, there are many challenges associated with high-pressure gasification. This is one of the reasons there is a lack of research performed in this area. Gasification by itself is a process with a lot of complex reactions, adding the effects of pressure to the thermodynamics will further complicate the matter.
In literature, both increase and decrease in the tar formation is reported. [6] According to Le Chatelier’s principle, an increase in system pressure will lead to a shift in the equilibrium to fewer molecules. This is in the opposite direction of the reforming reactions (6 and 7) and tar decomposition reactions. As a result, one may expect higher tar formation. However, high pressure also hampers the release of tars in the pyrolysis reaction. Therefore, it is difficult to predict the effect of pressure on total tar production. [16] This might have to do with other things that change when the operating pressure is increased. With an increase in pressure, the quantity of both oxygen and gasifying agent is increased. So, in order to keep the same steam-fuel ratio and equivalence ratio, feeding needs to be increased proportionally. [16] The size of the reactor then becomes the issue. Alternatively, one can dilute the system by feeding inert species like nitrogen, but that will reduce the quality of produced gas. In one study, it was observed that at high pressure, with an increase in air supply, the H2 and CO concentration decreased, while the level of CO2 and N2 increased. [37] In the present study, we will be using nitrogen to pressurize the system. As a result, the quality of gas produced will be very low. However, all our analysis and reporting of data will be N2-free to be able to compare to other studies.
The reactor temperature is one of the most important parameters of the gasification process. Based on thermodynamics and chemical kinetics, temperature influences the produced gas composition. While high temperatures favor the formation of products in an endothermic reaction, they shift the equilibrium to the reactant side in exothermic reactions. [7] Tar decomposition reactions are also strongly affected by the operating temperature. Higher temperature leads to better cracking and reforming of tars, resulting in improved gas yield.
[16] In case of steam gasification, studies have shown increase in H2 production due to better steam reforming. However, in case of air gasification, high temperature improves combustion of biomass, resulting in higher production of CO2 and N2. [6]
One of the most challenging aspect of high-pressure operations is continuous feeding into the reactor. Enough care should be taken not to have any leakage during the feeding. The air is more viscous when under pressure; thus, it changes the flowability of the fuel. Also, if the fuel particles are coarse and rough, they may get interlocked and stuck with each other under a pressurized atmosphere. This will lead to a problem in feeding, and the feeder calibrated under atmospheric conditions may not be feeding at the same rate under pressurized conditions.

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