Category Archives: Biomass Gasification and Pyrolysis
Use of catalysts in the thermochemical conversion of biomass may not be essential, but it can help under certain circumstances. Two main motivations for catalysts are:
* Removal of tar from the product gas, especially if the downstream application or the installed equipment cannot tolerate it (see Chapter 4 for more details).
* Reduction in methane content of the product gas, particularly when it is to be used as syngas (CO, H2 mixture).
The development of catalytic gasification is driven by the need for tar reforming. When the product gas passes over the catalyst particles, the tar or condensable hydrocarbon can be reformed on the catalyst surface with either steam or carbon dioxide, thus producing additional hydrogen and carbon monoxide. The reactions may be written in simple form as
CnHm + nH20 catalyst >(n + m/2) H2 + nCO (5.20)
Carbon dioxide (or dry) reforming reaction:
CnHm + nC02 catalyst > 2nCO + (m/2)H2 (5.21)
As we can see, instead of undesirable tar or soot, we get additional fuel gases through the catalytic tar-reforming reactions (Eq. 5.20). Both gas yield and the heating value of the product gas improve.
The other option for tar removal is thermal cracking, but it requires a high (>1100 °C) temperature and produces soot; thus, it cannot harness the lost energy in tar hydrocarbon.
The second motivation for catalytic gasification is removal of methane from the product gas. For this we can use either catalytic steam reforming or catalytic
carbon dioxide reforming of methane. Reforming is very important for the production of syngas, which cannot tolerate methane and requires a precise ratio of CO and H2 in the product gas. In steam reforming, methane reacts with steam in the temperature range of 700 to 1100 °C in the presence of a metal — based catalyst, and thus it is reformed into CO and H2 (Li et al., 2007):
CH4 + H2O catalyst > CO + 3H2 + 206 kJ/mol (_
— steam reforming of methane
This reaction is widely used in hydrogen production from methane, for which nickel-based catalysts are very effective.
The carbon dioxide reforming of methane is not as widely used commercially as steam reforming, but it has the special attraction of reducing two greenhouse gases (CO2 and CH4) in one reaction, and it can be a good option for removal of carbon dioxide from the product gas. The reaction is highly endothermic (Wang and Lu, 1996):
CH4 + CO2 catalyst > 2CO + 2H2 + 247 kJ/mol (_
— dry reforming of methane
Nickel-based catalysts are also effective for the dry-reforming reaction (Liu et al., 2008).
Catalysts for reforming reactions are to be chosen keeping in view their objective and practical use. Some important catalyst selection criteria for the removal of tar are as follows:
• Resistant to deactivation by carbon fouling and sintering
• Easily regenerated
• Strong and resistant to attrition
For methane removal, the following criteria are to be met in addition to those in the previous list:
• Capable of reforming methane
• Must provide the required CO/H2 ratio for the syngas process
Catalysts can work in in-situ and post-gasification reactions. The former may involve impregnating the catalyst in the biomass prior to gasification. It can be added directly in the reactor, as in a fluidized bed. Such application is effective in reducing the tar, but it is not effective in reducing methane (Sutton et al., 2001). In post-gasification, catalysts are placed in a secondary reactor downstream of the gasifier to convert the tar and methane formed. This has the additional advantage of being independent of the gasifier operating condition.
The second reactor can be operated at temperatures optimum for the reforming reaction.
The catalysts in biomass gasification are divided into three groups: earth metal, alkali metal, and nickel based.
Earth metal catalysts. Dolomite (CaCO3.MgCO3) is very effective for disposal of tar, and it is inexpensive and widely available, obviating the need for catalyst regeneration. It can be used as a primary catalyst by mixing with the biomass or as a secondary catalyst in a reformer downstream, which is also called a guard bed. Calcined dolomite is significantly more effective than raw dolomite (Sutton et al., 2001). Neither, however, is very useful for methane conversion. The rate of the reforming reaction is higher with carbon dioxide than with steam.
Alkali metal catalysts. Potassium carbonate and sodium carbonate are important in biomass gasification as primary catalysts. K2CO3 is more effective than Na2CO3. Unlike dolomite, they can reduce methane in the product gas through a reforming reaction. Many biomass types have inherent potassium in their ash, so they can benefit from the catalytic action of the potassium with reduced tar production. However, potassium is notorious for agglomerating in fluidized beds, which offsets its catalytic benefit. Ni-based catalyst. Nickel is highly effective as a reforming catalyst for reduction of tar as well as for adjustment of the CO/H2 ratio through methane conversion. It performs best when used downstream of the gasifier in a secondary bed, typically at 780 °C (Sutton et al., 2001). Deactivation of the catalyst with carbon deposits is an issue. Nickel is relatively inexpensive and commercially available though not as cheap as dolomite. Appropriate catalyst support is important for optimum performance.
Entrained-flow gasifiers have several advantages over other types:
• Low tar production
• A range of acceptable feed
• Ash produced as slag
• High-pressure, high-temperature operation
• Very high conversion of carbon
• Low methane content well suited for synthetic gas production
Supercritical water that exhibits complete miscibility with oxygen is a homogeneous reaction medium for the oxidation of organic molecules. This feature of SCW allows oxidation of harmful or toxic substances at low temperature in a process known as supercritical water oxidation (SCWO) or cold combustion. In a typical SCWO unit, the entire mixture (water, oxygen, and waste) remains as a single fluid phase with no interphase transport limitations. This allows very rapid and complete (>99.9%) oxidation of the organic wastes to harmless lower-molecular-weight compounds like H2O, N2, and CO2. Unlike thermal incineration, SCWO produces toxic by-products such as dioxin. This method of waste treatment is especially attractive for highly dilute toxic wastes in water.
One important shortcoming of this process is the production of highly corrosive liquid effluents because chlorine, sulfur, and phosphorous, if present in the waste, are converted into their corresponding acids (Serani et al., 2008). The destruction of polychlorinated biphenyls (PCBs) in supercritical water, producing carbon dioxide and hydrochloric acid, may be represented by the following simple reaction:
C12H10-mClm (PCB) + (19 + m)/2 O2 + (5 — m) H2O = 12CO2 + mHCl (7.4)
Conventional thermal incineration uses very high temperature to destroy byproducts like dioxin, which results in the production of another pollutant, NOx. This is not the case with SCWO owing to its low-temperature operation (450600 °C).
Based on the type of biomass, feeders can be divided into two broad groups: (1) those for harvested biomass and (2) those for nonharvested biomass.
Harvested fuels include long and slender plants like straw, grass, and bagasse, which carry considerable amounts of moisture. Examples of nonhar — vested fuels are wood chips, rice husk, shells, barks, and pruning. These fuels are not as long or as slender as harvested fuels, and some of them are actually granular in shape.
Harvested biomass, such as straw and nonharvested hay, is pressed into bales in the field, and sometimes the bales are left in the field to dry (Figure 8.14).
FIGURE 8.14 Tall grass is cut in the field, baled, and left in the field for drying in Nova Scotia. (Source: Photograph by the author.)
Baling facilitates transportation and handling (Figure 8.15). Cranes are used to load the bales at a certain rate depending on the rate of fuel consumption. The bales are brought to the boiler house from storage by chain conveyors.
Whole bales are fed into a bale shredder and a rotary cutter chopper to reduce the straw to sizes adequate for feeding into a fluidized-bed gasifier or combustor. In the final leg, the chopped straw is fed into the furnace by one of several feeder types. Figure 8.15 shows a ram feeder, which pushes the straw into the furnace. In some cases, the straw falls into a double-screw stoker, which presses it into the furnace through a water-cooled tunnel.
Wood and by-products from food-processing industries are generally granular in shape. Wood chips and bark may not be of the right size when delivered to
the plant, so they need to be shredded to the desired size in a chopper. However, fuels like rice husk and coffee beans are of a fixed granular size and so do not need further chopping. Rice husk, a widely used biomass, is flaky and 2 to 10 mm x 1 to 3 mm in size. As such, it can be fed as it comes from the source, but it can be easily entrained in a fluidized bed. For this reason one can press it into pellets using either heat or a nominal binder in a press.
Feeders for nonharvested fuels are similar to those for conventional fuels like coal. Speed-controlled feeders take the fuel from the silo and drop measured amounts of it into several conveyors. Each conveyor takes the fuel to an air-swept spout that feeds it into the furnace. If the moisture in the fuel is too high, augers are used to push the fuel into the furnace.
The feeding of biomass into a high-pressure (>22 MPa) reactor is a formidable challenge for an SCW gasifier. If the feed is a dilute stream of organics, the problem is not so severe, as pumps can handle light slurries. However, if it is fibrous solid granular biomass that needs to be pumped against high pressure, the problem is especially difficult for the reasons that follow:
In biomass, hemicellulose is like the cement in reinforced concrete, and cellulose is like the steel rods. The strands of microfibrils (cellulose) are supported by the hemicellulose. Decomposition of hemicellulose during torrefaction is like the melting away of the cement from the reinforced concrete. Thus, the size reduction of biomass consumes less energy after torrefaction.
During torrefaction the weight loss of biomass comes primarily from the decomposition of its hemicellulose constituents. Hemicellulose decomposes mostly within the temperature range 150 to 280 °C, which is the temperature window of torrefaction. As we can see from Figure 3.11, the hemicellulose component undergoes the greatest amount of degradation within the 200 to 300 °C temperature window. Lignin, the binder component of biomass, starts softening above its glass-softening temperature (~130 °C), which helps densi — fication (pelletization) of torrefied biomass. Unlike hemicellulose, cellulose
shows limited devolatilzation and carbonization and that too does not start below 250 °C.
Thus, hemicellulose decomposition is the primary mechanism of torrefaction. At lower temperatures (< 160 °C), as biomass dries it releases H2O and CO2. Water and carbon dioxide, which make no contribution to the energy in the product gas, constitute a dominant portion of the weight loss during torrefaction. Above 180 °C, the reaction becomes exothermic, releasing gas with small heating values. The initial stage (< 250 °C) involves hemicellulose depolymerization, leading to an altered and rearranged polysugar structures (Bergman et al., 2005a). At higher temperatures (250-300 °C) these form chars, CO, CO2, and H2O. The hygroscopic property of biomass is partly lost in tor — refaction because of the destruction of OH groups through dehydration, which prevents the formation of hydrogen bonds.
Computational fluid dynamics can have an important role in the modeling of a fluidized-bed gasifier. A CFD-based code involves a solution of conservation of mass, momentum, species, and energy over a defined domain or region. The equations can be written for an element, where the flux of the just-mentioned quantities moving in and out of the element is considered with suitable boundary conditions.
A CFD code for gasification typically includes a set of submodels for the sequence of operations such as the vaporization of a biomass particle, its pyrolysis (devolatilization), the secondary reaction in pyrolysis, and char oxidation (Di Blasi, 2008; Babu and Chaurasia, 2004). Further sophistications such as a subroutine for fragmentation of fuels during gasification and combustion are also developed (Syred et al., 2007). These subroutines can be coupled with the transport phenomenon, especially in the case of a fluidized — bed gasifier.
The hydrodynamic or transport phenomenon for a laminar flow situation is completely defined by the Navier-Stokes equation, but in the case of turbulent flow a solution becomes difficult. A complete time-dependent solution of the instantaneous Navier-Stokes equation is beyond today’s computation capabilities (Wang and Yan, 2008), so it is necessary to assume some models for the turbulence. The Reynolds-averaged Navier-Stokes (k-є) model or large eddy simulation filters are two means of accounting for turbulence in the flow.
For a fluidized bed, the flow is often modeled using the Eulerian-Lagrange concept. The discrete phase is applied to the particle flow; the continuous phase, to the gas. Overmann and associates (2008) used the Euler-Euler and Euler — Lagrange approaches to model wood gasification in a bubbling fluidized bed. Their preliminary results found both to have comparable agreement with experiments. If the flow is sufficiently dilute, the particle-particle interaction and the particle volume in the gas are neglected.
A two-fluid model is another computational fluid dynamics approach. Finite difference, finite element, and finite volume are three methods used for discretization. Commercial software such as ANSYS, ASPEN, Fluent, Phoenics, and CFD2000 are available for solution (Miao et al., 2008). A review and comparison of these codes is given in Xia and Sun (2002) and Norton et al. (2007).
Recent progress in numerical solution and modeling of complex gas-solid interactions has brought CFD much closer to real-life simulation. If successful, it will be a powerful tool for optimization and even design of thermochemical reactors like gasifiers (Wang and Yan, 2008). CFD models are most effective in modeling entrained-flow gasifiers, where the gas-solid flows are less complex than those in fluidized beds and the solid concentration is low.
Models developed by several investigators employ sophisticated reaction kinetics and complex particle-particle interaction. Most of them, however, must use some submodels, fitting parameters or major assumptions into areas where precise information is not available. Such weak links in the long array make the final result susceptible to the accuracy of those “weak links.” If the final results are known, we can use them to back-calculate the values of the unknown parameters or to refine the assumptions used.
The CFD model can thus predict the behavior of a given gasifier over a wider range of parameters using data for one situation, but this prediction might not be accurate if the code is used for a different gasifier with input parameters that are substantially different from the one for which experimental data are available.
A moving-bed gasifier may be designed on the basis of characteristic design parameters such as specific grate gasification rate, hearth load, and space velocity.
Specific grate gasification rate is the mass of fuel gasified per unit of crosssection area in unit time. The hearth load of a gasifier may be expressed in terms of the fuel gasified, the volume of gas that is produced, or the energy throughput.
2 4 Mass of fuel gasified Hearth load (kg/s • m2) =
Hearth cross-sectional area
TT, , W„T 3 Volumetric gas production rate
Hearth load (Nm3/s • m2) =
Hearth cross-sectional area
Heart, load (MW/m-)= Energy throughpu’ In product gas (6 28)
Hearth cross-sectional area
The hearth load in volume flow rate of gas per unit of cross-section area is also known as superficial gas velocity or space velocity, as it has the unit of velocity (at reference temperature and pressure).
The following section discusses type-specific design considerations.
Updraft gasifiers are one of the simplest and most common types of gasifier for biomass. The maximum temperature increases when the feed of air or oxygen increases. Thus, the amount of oxygen feed for the combustion reaction is carefully controlled such that the temperature of the combustion zone does not reach the slagging temperature of the ash, causing operational problems. The gasification temperature may be controlled by mixing steam and/or flue gas with the gasification medium.
The hearth load of an updraft gasifier is generally limited to 2.8 MW/m2 or 150 kg/m2/h for biomass (Overend, 2004). For coal it might be higher. In an oxygen-based coal gasifier, for example, the hearth load of a moving bed can be greater than 10 MW/m2. A higher hearth load increases the space velocity of gas through the hearth, fluidizing finer particles in the bed. Probstein and Hicks (2006) quote space velocities for coal on the order of 0.5 m/h for steam — air gasification and 5.0 m/h for steam-oxygen gasification. Excessive heat generation in such a tightly designed gasifier may cause slagging. Based on the characteristics of some commercial updraft coal gasifiers, Rao et al. (2004) suggest a specific grate gasification rate as 100 to 200 kg fuel/m2h for RDF pellets, with the gas-to-fuel ratio in the range 2.5 to 3.0. Carlos (2005) obtained a rate of 745 to 916 kg/m2h with air-steam and air preheat at temperatures of 350 and 830 °C, respectively.
For an updraft gasifier, the height of the moving bed is generally greater than its diameter. Usually, the height-to-diameter ratio is more than 3 : 1 (Chakraverty et al., 2003). If the diameter of a moving bed is too large, there may be a material flow problem, so it should be limited to 3 to 4 m in diameter (Overend, 2004).
The choice of catalyst influences reactor temperature, product distribution, and plugging potential. Section 7.4.2 discussed the catalysts used in SCW gasification. They are selected on the basis of the desired product. Catalyst deactivation is an issue assigned to most catalyzed reactions because the deactivated catalysts must be regenerated. If they are deactivated because of carbon deposits, as happens in a fluid catalytic cracker (FCC), they can be combusted by adding oxygen, preferably in a separate chamber. The combustion reaction reactivates the catalysts and can additionally provide enough heat for preheating the feed.
Consider a simple reactor receiving Wf of feed while producing Wp of product per unit of time. The product comprises a number of hydrocarbon components represented by species i. The total carbon in the product gas is its total in the individual gaseous hydrocarbons:
Total carbon production in the product gas = X WpCa kmol/s (7.9)
where aiT is the number of carbon atoms in component i in the gas product; Ci is mole fraction of i in the gas product; and Wp is the product gas flow rate (kmol/s). The amount of carbon in the feed is known from the feed rate, Wf (kg/s), and its carbon fraction, Fc. The carbon gasification yield, Y, is defined as the ratio of gasified carbon to the carbon in the feed:
where 12 is the carbon’s molecular weight (kg/kmol).
From Eq. (7.8) the reaction rate is given in terms of conversion as
ln (1 — Xc)
where t is the residence time in a reactor of volume V. For a continuous stirred-tank reactor,
Thus, for a known reaction rate, kg, and a desired conversion, Xc, we can estimate the reactor volume required for gasification.
As mentioned, syngas is an important source of valuable chemicals. These include
• Hydrogen, produced in refineries
• Diesel gasoline, using Fischer-Tropsch synthesis
• Fertilizer, through ammonia
• Methanol, for the chemical industry
• Electricity, generated through combustion
It should be noted that a major fraction of the ammonia used for fertilizer production comes from syngas and nitrogen (see Section 9.4.3).
Gasification is the preferred route for the production of syngas from coal or biomass. The low price of natural gas is currently encouraging syngas production from it, but the situation may change when the price rises. A steam reformation reaction is used to produce syngas from natural gas that is mainly CH4. This reaction is also the most widely used commercial method for bulk production of hydrogen, which is one of the two components of syngas.
In the steam reforming method, natural gas (CH4) reacts with steam at high temperatures (700-1100 °C) in the presence of a metal-based catalyst (nickel).
If hydrogen production is the main goal, the carbon monoxide produced is further subjected to a shift reaction (see Eq. 9.2), as described in the next section, to produce additional hydrogen and carbon dioxide.
The ratio of hydrogen and carbon monoxide in the gasification product gas is a critical parameter in the synthesis of the reactant gases into desired products such as gasoline, methanol, and methane. The product desired determines that ratio. For example, gasoline may need the H2/CO ratio to be 0.5 to 1.0, while methanol may need it to be ~2.0 (Probstein and Hicks, 2006, p. 124). In a commercial gasifier the H2/CO ratio of the product gas is typically less than 1.0, so the shift reaction is necessary to increase this ratio by increasing the hydrogen content at the expense of CO. The shift reaction often takes place in a separate reactor, as the temperature and other conditions in the main gasifier may not be conducive to it.