Abstract:
The energy output of a power plant combustion chamber that combusts fuel comprising biomass as all or part of the fuel can be increased by feeding oxygen into the combustion chamber so that said fuel is in contact with gaseous oxidant whose oxygen content exceeds that of air by up to 5 vol. % above that of air.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation application of and claims priority from U.S. patent application Ser. Number 13/285,654 filed Oct. 31, 2011, which claims priority from U.S. Provisional Application Ser. No. 61/412,119 filed Nov. 10, 2010. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to combustion of biomass, especially in power plants that generate steam. 
       BACKGROUND OF THE INVENTION 
       [0003]    Growing demand for electrical power obtained from fuel-fired power plants, combined with growing interest in using biomass as fuel for such plants, has increased interest in finding efficient methods for combusting biomass in power plants. The moisture content of biomass is typically very high. For example green wood typically contains 40 to 60% moisture. This increased moisture content, and its low energy density, are among the primary issues with firing biomass in boilers and especially boilers that were designed for other fuels such as coal. For example, converting a coal-fired boiler to fire biomass typically cause the boiler to be derated by 30-50%. 
         [0004]    Many boilers are ‘flue gas limited’ and can only handle up to a specific amount of flue gas. This flue gas limitation may be due to the capacity of fans if present for impelling flow of flue gas, or may be based on design limits. For example, boiler design considerations, such as the maximum allowable velocity in the convective section, can limit flue gas volume. Since the flue gas volume per unit heat input, or “specific flue gas volume”, increases dramatically when a fuel such as coal is replaced with biomass, it causes a large impact on the distribution of heat absorption in the furnace. A boiler is typically designed for a relatively narrow range of specific flue gas volume. Within this range the boiler is designed for a specific amount of heat absorption in the furnace, or radiant section, and the convective section. When the specific flue gas volume is increased more heat is ‘pushed’ from the radiant section into the convective section. This increase in heat transfer in the convective section often requires the use of water sprays into the steam flow to maintain the desired steam temperature, which may decrease overall efficiency. This shifting of heat transfer from the radiative furnace section to the convective section of the boiler further reduces or derates the boiler capacity. 
         [0005]    Conversion of an existing boiler to biomass firing can also significantly degrade the combustion performance of the unit. The reduction in combustion performance is due to both changes in the fuel characteristics and the firing system. The high moisture content the fuel makes it more difficult to ignite and burn. This problem is compounded by the fact that grate firing systems often suffer from uneven fuel distribution over the grate and non-uniform mixing between the air and the fuel—leading to incomplete combustion on parts of the grate and high CO emissions in flue gas. To overcome both of these problems boiler operators typically operate the boiler at increased stoichiometric ratios (defined as the ratio of air supplied to that required to burn the fuel). The stoichiometric ratio is often measured as the amount of oxygen left in the flue gas at the end of the combustion process. For example, a typical coal-fired boiler will operate with 3% “excess oxygen”. This means the flue gas contains 3% oxygen (by volume, wet basis). In contrast the flue gas from a biomass-fired boiler typically contains at least 4.5% O 2  (vol, wet basis) to control CO emissions within regulatory limits. 
         [0006]    The extra air further increases the flue gas volume and impacts both the thermal efficiency of the boiler, and the auxiliary power required for the boiler. In the first case the extra air volume carries heat out the stack, increasing the sensible heat loss. The extra air also increases the power required by both the blower that pushes combustion air into the boiler (typically called the forced draft, or FD, fan), and the blower used to draw the flue gas from the boiler (typically called the induced draft, or ID, fan). Therefore the overall effect of the excess air is to increase the specific flue gas volume, which is the gas volume per units of energy output (further limiting the amount of fuel that can be fired), reduce the thermal efficiency (allowing less of the fuel that is fired to be used to raise steam), and increase the auxiliary power (reducing the net power available 
         [0007]    The present invention provides an improved method for combustion of biomass in boilers. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    One aspect of the present invention is a method of combustion, comprising 
         [0009]    (A) providing apparatus that includes a combustion chamber in which fuel having a given moisture content and a given specific energy content fed into the combustion chamber at a given mass feed rate can be combusted in air to produce heat energy at a given rate, 
         [0010]    (B) feeding into said combustion chamber fuel that contains biomass and that has a specific energy content lower than said given specific energy content, so that combustion in air of said fuel fed at said given mass fed rate in said combustion chamber in air produces heat energy at a rate lower than said given rate, while feeding oxygen into said combustion chamber so that said fuel is in contact with gaseous oxidant whose oxygen content exceeds that of air by up to 5 vol. % above that of air, and 
         [0011]    (C) combusting the fuel with said gaseous oxidant in said combustion chamber. 
         [0012]    Another aspect of the invention is a method of increasing fuel combustion rate in a combustion chamber with a convective heat transfer zone in which fuel that contains biomass is combusted with combustion air in said combustion chamber to produce flue gas containing a specific oxygen concentration between 3 vol. % and 8 vol. % at a given maximum fuel feed rate limited by the capacity of an FD fan if present for feeding said combustion air, the capacity of an ID fan if present to evacuate flue gas from said combustion chamber, the flue gas velocity in said convective heat transfer zone, or the carbon monoxide concentration in said flue gas, feeding into said combustion chamber additional fuel containing biomass and additional oxidant containing at least 50 vol. % O 2 , reducing said combustion air flow rate by the amount that reduces said oxygen concentration in said flue gas by 0.1 to 5.0 vol. % and combusting said additional fuel without exceeding said FD fan capacity, said ID fan capacity, said flue gas velocity, nor said carbon monoxide concentration. 
         [0013]    Yet another aspect of the invention is a method of increasing fuel combustion rate in a combustion chamber with a grate for combustion of fuel with a convective heat transfer zone in which fuel that contains biomass is combusted with combustion air in said combustion chamber to produce flue gas containing a specific oxygen concentration between 3 vol. % and 8 vol. % at a given maximum fuel feed rate limited by the carbon monoxide concentration in said flue gas, feeding into said combustion chamber additional fuel containing biomass and additional oxidant containing at least 50 vol. % O 2  to one or more oxygen deficient areas on said grate to maintain or reduce said carbon monoxide. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross-sectional view of one embodiment of combustion apparatus in which the present invention can be practiced. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention is an improvement in the combustion of fuel comprising biomass in a combustion chamber. “Biomass,” for the purposes of the present invention, means any material not derived from fossil resources and comprising at least carbon, hydrogen, and oxygen. Biomass includes, for example, plant and plant-derived material, vegetation, agricultural waste, forestry waste, wood, wood waste, paper waste, animal-derived waste, poultry-derived waste, and municipal solid waste. Other exemplary feedstocks include cellulose, hydrocarbons, carbohydrates or derivates thereof, and charcoal. Typically biomass can include one or more materials selected from: timber harvesting residues, softwood chips, hardwood chips, tree branches, tree stumps, leaves, bark, sawdust, off-spec paper pulp, corn, corn stover, wheat straw, rice straw, sugarcane bagasse, switchgrass, miscanthus, animal manure, municipal garbage, municipal sewage, commercial waste, grape pumice, almond shells, pecan shells, coconut shells, coffee grounds, grass pellets, hay pellets, wood pellets, cardboard, paper, plastic, and cloth. The present invention can also be used for fuels that also comprise carbon-containing feedstocks other than biomass, such as a fossil fuel (e.g., coal or petroleum coke), i.e. mixtures of biomass and fossil fuels. 
         [0016]    The present invention is especially applicable to combustion of biomass in a combustion chamber that is part of a system that includes, in addition to a combustion chamber, heat exchangers that absorb heat of combustion into, for instance, water. Preferred systems include power generation boilers, especially in which heat exchange to boiler feed water is achieved by radiant heat transfer and by convective heat transfer. The heat exchange produces steam, superheated steam, and/or supercritical steam, which can be used to generate electric power. 
         [0017]    The present invention is especially applicable to combustion of biomass in a combustion chamber including a grate on which fuel rests as it is being combusted. However, the present invention can be practiced in systems wherein the fuel is combusted in the combustion chamber by grate firing, suspension firing, or a combination of grate firing and suspension firing, or by firing in a bubbling fluidized bed or in a circulating fluidized bed. 
         [0018]    The following description refers to  FIG. 1 , and illustrates practice of the invention in one embodiment in which grate firing is employed. 
         [0019]    Combustion chamber  1  includes grate  2  on which fuel can rest after the fuel is fed into combustion chamber  1 , for instance as fuel stream  3 . Grate  2  is solid and includes a plurality of openings through which gas can flow, including primary air which is fed as primary air stream  4 . Optionally, overfire air stream  5  can also be fed into the combustion chamber  1 . 
         [0020]    Combustion of fuel in combustion chamber  1  produces heat of combustion, and flue gas which exits combustion chamber  1  as stream  7 . The heat of combustion can be transferred to feed water flowing through boiler tubes in the walls of combustion chamber  1 , to heat the feed water. Heat of combustion can also be transferred from flue gas by indirect heat exchange to feed water, or to steam, in heat section  6  which generally includes a region (the “radiant section”) in which heat transfer occurs predominantly by radiative heat transfer, and a region (the “convective section”) in which heat transfer occurs predominantly by convective heat transfer. 
         [0021]    In the present invention, oxygen is fed in small amounts into the region below grate  2 , into the region of the fuel on grate  2 , or into both regions. Sufficient oxygen is fed so that the gaseous atmosphere in contact with the fuel has an oxygen content higher than that of air, i.e. at least 21 vol. %, up to 5 vol. % higher than that of air and preferably not more than 1 vol. % higher than that of air. 
         [0022]    The oxygen can be fed into the region below grate  2  in any of numerous ways, such as by mixing it with primary air that is fed as stream  4 , or inserting a lance  8  into the region below grate  2  and feeding the oxygen through the lance into the region below grate  2  where it then can mix with primary air. 
         [0023]    The oxygen can be fed into the region above grate  2  in any of numerous ways, such as by inserting a lance  9  into the region above grate  2  so that oxygen emerging from the lance  9  can contact fuel present on the grate, and feeding oxygen through the lance  9 . 
         [0024]    The oxygen that is fed below or above the grate  2  is preferably fed as a stream comprising at least 50 vol. % oxygen preferably 90 vol. % oxygen. Streams having such oxygen content are readily available from commercial sources. Alternatively, streams having such oxygen content can be formed in apparatus located near the combustion chamber such as VPSA units that separate oxygen from air. 
         [0025]    The practice of the present invention provides numerous advantages in its own right, and especially compared to prior practice relating to combustion of biomass. 
         [0026]    The moisture content of fuel comprising biomass is typically very high. This increased moisture content, and its low energy density, are among the primary issues with firing biomass in boilers and especially boilers that were designed for other fuels. For example, converting a 50MW net  coal-fired boiler (heat rate of 11,500 Btu/kWh net ) to fire biomass would be expected to cause the boiler to be derated by 20-45% just to account for the moisture in the fuel. The shift in boiler heat transfer balance and the increased excess air requirement increase the required derate to 30-50% for many boilers. The present invention permits efficient combustion of biomass fuels, even in boilers that were designed for combustion of fuels having lower water contents, and/or higher energy density, than biomass. The invention is useful when the fuel containing the biomass has a water content of at least 25 wt. %, or when the fuel containing the biomass has an energy content less than 7500 BTU/lb or even less than 5000 BTU/lb. 
         [0027]    In the present invention, the addition of only a small amount of oxygen enhances and controls combustion both on and above the grate as a means to recover lost generating capacity. The enhanced combustion, in turn, enhances flame stability and ensures more complete burnout. Oxygen injection over the grate can also stabilize and improve the combustion process. In general, by using oxygen in the combustion environment according to the present invention, it is possible to reduce the excess air flow, and thereby reduce the specific flue gas volume. The lower specific flue gas volume allows the boiler operator to increase the firing rate to regain some of the generating capacity lost when the boiler was converted to biomass firing. Even small reductions in excess air can allow boiler capacity lost during the conversion to biomass to be recovered (reducing the required boiler derate). 
         [0028]    Another operational benefit of oxygen injection according to the present invention is that less heat will be ‘pushed’ into the convective section due to both the reduced specific flue gas volume and the increased temperature near the fuel bed on the grate. Both of these effects lead to increased heat absorption in the radiative part of the boiler—reducing the need to spray in cooling water to control superheat and reheat temperatures in the convective section. 
         [0029]    In the present invention oxygen could be added by combination of being directly injected or mixed with combustion air (enrichment). For example, one might enrich the undergrate air to ensure there are no “hot spots” nor “cold spots” on the grate, while using high momentum lances to inject oxygen above the grate to promote good mixing and volatiles/CO burnout. The over-bed oxygen lances can also be used to move heat (by influencing mixing) into different parts of the grate. For example, some of the heat from the volatile combustion zone of the grate can be moved into the drying portion of the grate to facilitate drying. Overfire air  5  (air supplied through ports located at one or more elevation from the grate) can also be enriched to enhance volatile combustion. Alternatively oxygen enrichment under the grate may be increased through the use of a lance to target areas where the grate is known to be ‘cold’, or combustion is poor. The amount of oxygen required to recover capacity by enabling reduced excess oxygen operation is much less than that estimated for a simple direct replacement of combustion air. For example, the stoichiometric oxygen requirement for a typical dry ash-free wood is about 2,000 SCF (123 lb) per 1,000,000 Btu and produces about 3,200 SCF of flue gas. Conversely, 1 lb of oxygen can combust about 8130 Btu of fuel and produces 26 SCF of flue gas. In order to maintain the original flue gas volume and burn additional fuel a portion of the original combustion air volume must be reduced and replaced with additional oxygen. The oxygen requirement to increase the capacity (or fuel firing rate) by 10% under the condition of constant flue gas volume flow rate was calculated for both dry and wet wood with 45% moisture content at two different excess oxygen levels (3 and 4.5% by volume in wet flue gas) and summarized in Table 1. The amount of oxygen required ranges from 2850 to 3410 SCF per MMBtu of additional fuel input at the constant excess O 2  in flue gas. By reducing the excess oxygen level by 1 vol. %, the amount of oxygen required is reduced to less than half, in a range from 1140 to 1510 SCF per MMBtu of additional fuel input. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Oxygen required 
                   
               
               
                   
                 (SCF/MMBtu): 
               
             
          
           
               
                   
                   
                 Constant 
                 1% reduction 
               
               
                   
                 Biomass 
                 excess O2 
                 in excess O2 
               
               
                   
                   
               
               
                   
                 Dry wood, 3% Excess O2 
                 2850 
                 1260 
               
               
                   
                 Dry wood, 4.5% Excess O2 
                 2870 
                 1140 
               
               
                   
                 Wet wood, 3% Excess O2 
                 3410 
                 1510 
               
               
                   
                 Wet wood, 4.5% Excess O2 
                 3410 
                 1330 
               
               
                   
                   
               
             
          
         
       
     
         [0030]    The current invention has several additional advantages. First, by using only enough oxygen enrichment to achieve flame stability on and above the grate, gross changes to furnace operation can be avoided. For example, many furnaces are designed for a specific heat absorption pattern. In a steam boiler for power generation the balance between heat transfer in the radiant (furnace) section is often carefully balanced with that in the convective section by the boiler designer. Variations in heat transfer pattern from the design point can cause significant upsets in boiler operation. When high oxygen enrichment levels, such as those presented in the prior art (&gt;25%) are used, the heat transfer to the radiant section is often dramatically increased. For a utility boiler this means the steaming rate (rate of steam production) is increased, but there is insufficient heat available to superheat the steam to the desired turbine inlet temperature. In the current invention the transition to a high moisture fuel often leads to off-design furnace operation where heat transfer to the radiant section is reduced compared to the design case. By using a small amount of oxygen enrichment and thereby reducing the excess air requirement the radiative/convective heat transfer balance can be restored, at least in part, without increasing the radiative heat transfer past the design limits. 
         [0031]    The present invention also does not require exhaust gas recirculation for over-grate mixing. This leads to a much lower capital requirement (EGR fans, ducts, and the like) and reduced operating cost. 
         [0032]    Additionally, by using the oxygen addition of the present invention only to support combustion and thereby reduce the specific flue gas volume through excess air reduction, the volume reduction compared to oxygen use is much higher than in the prior art. This enhanced effectiveness of oxygen addition for flue gas reduction leads to much lower oxygen requirements. 
         [0033]    A significant advantage of the current invention over the prior art is related to the use of oxygen enrichment only to support combustion and thereby reduce the specific flue gas volume through excess air reduction, the flue gas volume reduction compared to the simple replacement of a portion of combustion air with oxygen is much higher than in the prior art. This enhanced effectiveness of oxygen addition for flue gas reduction leads to much lower oxygen requirements. An example for converting a 20 MW net  coal-fired boiler to fire biomass is shown in Table 2. For these calculations the flue gas volume was held constant, consistent with a flue gas limited boiler. The baseline generating capacity was defined as that after the boiler was converted to biomass firing (using a 32% moisture fuel) and was 14.7 MWnet in this example. The increased generating capacity was first estimated assuming the oxygen concentration in the flue gas was held constant at 4.5% (vol, wet) and combustion air was replaced with increasing levels of oxygen. This condition is the conventional ‘volume reduction’ strategy where the nitrogen in the combustion air is simply removed by using oxygen in place of a portion of the air. As can be seen in Table 2, the generating capacity can be increased significantly, but the oxygen requirements are high enough that oxygen use may not be economically justified. In the case of the current invention, kinetic data was used to estimate the increase in firing rate from oxygen enrichment. The air injection rate was reduced by the amount of oxygen injected and the firing rate increased—resulting in a lower oxygen concentration in the flue. With injection targeted to particular locations in the combustion chamber, such as described below, the oxygen consumption may be even lower. The data in Table 2 show that the oxygen use is dramatically lower for a given increase in capacity for the current invention. Using oxygen in this way can be economically viable. 
         [0000]                                                        TABLE 2                   Increase in generation   Oxygen required (SCF/MW baseline):                (% of baseline)   Volume reduction   Present invention                    2%   710   70       10%   3500   340       22%   7420   1020                    
“Volume reduction” means operating such that the reduction in specific flue gas volume is attained only by the replacement of air with an equal amount of oxygen.
 
“Present invention” means operating such that the reduction in specific flue gas volume is attained in part by reduction in the amount of excess air.
 
The optimal embodiment of the current invention uses small amounts of oxygen to support the various stages of biomass combustion. These stages include:
       Preheating/drying,   Volatile release,   Volatile combustion,   Char combustion.       
 
         [0038]    In a grate fired-combustor, such as that shown in  FIG. 1 , these steps can occur in-flight or on the grate, depending on the fuel characteristics (size) and fuel spreader/boiler design. For example, fine particulate are likely suspended as they are ‘thrown’ into the furnace. Therefore for the fine materials the entire combustion process occurs in flight. For the largest particles they may dry slightly as they exit the fuel spreader but land on the grate before drying is complete. Therefore, for these particles the combustion process occurs primarily on the grate. Combustion problems can occur when the fuel and air distribution are not matched across the grate and overfire air. For example, if too much fuel is deposited on a specific portion of the grate the combustion air may be insufficient to burn the material. Although optimal overfire air designs promote good mixing above the grate, there may still be regions where the oxygen levels are too low to complete combustion (and other areas where the excess air is much higher than required for combustion). Further, the heat release pattern from the volatile combustion may not match that required to promote drying/devolatilization of materials that have landed on the grate—causing material on portions of the grate to ‘smolder’ instead of burn. 
         [0039]    It is known that high levels of oxygen enrichment can enhance combustion and overcome problems associated with air/fuel distribution and heat release mismatches. However, the objective of the current invention is to use the least amount of oxygen to enable the excess air to be reduced (and thereby enable an increase in boiler firing rate). Therefore the optimal embodiment is to use a lance, or lances, above the grate to inject oxygen into oxygen-deficient areas above the grate. Often the oxygen deficient area looks darker than the rest of the grate as the local temperature is colder. Such area can be detected by in-furnace video camera, by an optical pyrometer or by visual observation. Other methods of detecting the oxygen deficient area include gas analysis using a gas sampling probe and by an optical gas species measurement device. With careful lance design mixing can be controlled between the injected oxygen and the oxygen deficient (and likely high CO) flue gas. Further, by targeting the injected oxygen jet tragectory high oxygen containing flue gas ‘pockets’ in the furnace atmosphere can be drawn into the oxygen deficient area. The combination of aerodynamic effects from the lance design and the kinetic effect of high oxygen concentrations enhance volatiles and CO combustion. The over-grate lances can also be used to ‘move’ volatile combustion to add heat to cooler portions of the grate to support the combustion process on the grate. 
         [0040]    In addition to the over-grate lances the optimal embodiment can also use directed oxygen enrichment under the grate to enhance combustion on specific regions of the grate. For example, if the windbox under the grate has partitions to divide the airflow to different parts of the grate, different levels of oxygen enrichment could be used in the different partitioned areas (through use of oxygen distributors in the air supply duct for each partition). Alternately a carefully designed oxygen injection lance could be installed either below the grate or immediately above the grate to enrich the combustion air in the immediate vicinity of a known ‘cold spot’, or oxygen deficient areas.