Fixed bed furnace

This work is aimed at exploration of pollutant emissions (e.g. CO, UHC) in a 40 MW fixed-bed furnace burning wet wood chips, using both experimental measurements and theoretical simulations. The following issues are emphasized (1) the influence of turbulent flow motion, namely the over-fire secondary air supply on CO formations and (2) the influence of char burnout on CO formations. 

Measurement provides only limited information about the combustion and pollutant emission process. To explore further details about the flow turbulence, the interaction of different processes and influencing factors, numerical calculations are performed. Different aspects need to be considered in the modeling of fixed-bed furnace combustion, including fixed-bed modeling, gas-phase and particle-phase simulation, as well as interaction between the different phases. 

As shown in Fig.5, the computed temperature is relatively low (< 1500 K), which is in agreement with the current measured data and data that have been reported in other studies of biomass combustion in fixed-bed furnace [14]. 

The low (ca 2%) yield of NO, the tendency to revert to N2 and O2 if the product stream is not quenched rapidly, the consumption of large (ca 60,000 kWh/1N2 fixed) amounts of electricity, and the concomitant expense to sustain the arc all led to the demise of this process. The related Wisconsin process for oxidising N2 at high temperatures in a pebble-bed furnace was developed in the 1950s (13). Although a plant that produced over 40 t/d of nitric acid was built, the product recovery costs were not economically competitive. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

Further down, ca 75 cm below the electrode tips, the mix is hot enough (2200—2500°C) to allow the lime to melt. The coke does not melt and the hquid lime percolates downward through the relatively fixed bed of coke forming calcium carbide, which is Hquid at this temperature. Both Hquids erode coke particles as they flow downward. The weak carbide first formed is converted to richer material by continued contact and reaction with coke particles. The carbon monoxide gas produced in this area must be released by flowing back up through the charge. The process continues down to the taphole level. Material in this area consists of soHd coke wetted in a pool of Hquid lime and Hquid calcium carbide at the furnace bottom. 

Fluidized Beds, Porous Media Fixed Bed Combustion, and Furnaces. 

The process essentially involves passing air through a bottom furnace distributor plate and a fixed bed of sand. As air flow rates increase, the fixed bed becomes more unstable and bubbles of air appear (minimum fluidized condition). Above this minimum level, higher air flow rates produce—depending on design—either bubbling fluidized beds or circulating fluidized beds, and the fuel is introduced onto these beds. 

The catalytic reforming of CH4 by CO2 was carried out in a conventional fixed bed reactor system. Flow rates of reactants were controlled by mass flow controllers [Bronkhorst HI-TEC Co.]. The reactor, with an inner diameter of 0.007 m, was heated in an electric furnace. The reaction temperatoe was controlled by a PID temperature controller and was monitored by a separated thermocouple placed in the catalyst bed. The effluent gases were analyzed by an online GC [Hewlett Packard Co., HP-6890 Series II] equipped with a thermal conductivity detector (TCD) and carbosphere column (0.0032 m O.D. and 2.5 m length, 80/100 meshes), and identified by a GC/MS [Hewlett Packard Co., 5890/5971] equipped with an HP-1 capillary column (0.0002 m O.D. and 50 m length). 

Reactors batch (B), continuous stirred tank (CST), fixed bed of catalyst (FB), fluidized bed of catalyst (FL), furnace (Furn.), multitubular (MT), semicontinuous stirred tank (SCST), tower (TO), tubular (TU). 

Cottrell, Frederick G. (1877-1948). American scientist, inventor of an electrostatic precipitator, now known as Cottrell Precipitator, for smoke, dust fumes. Among other inventions are the pebble bed furnace, boiling point apparatus the Cottrell-Daniels process for fixation of atmospheric nitrogen. Cottrell was Director of US Bureau of Mines Director of the Fixed Nitrogen Research Laboratory, and founder of the Research Corporation, a nonprofit organization... 

Industrial steam reformers are usually fixed-bed reactors. Their performance is strongly affected by the heat transfer from the furnace to the catalyst tubes. We will model both top-fired and side-fired configurations. 

The experiments were carried out in a fixed-bed tubular reactor, heated by an electric furnace divided into three heating zones. Prior to each experiment the catalyst was stripped with N2 at 482°C (reaction temperature) for 20 minutes. Then the reactant (which characteristics are given in Table II) was charged at the top of the reactor by a constant-rate positive-displacement pump. The catalyst to oil ratio with respect to the zeolite content was varied between 0.09 and 0.23 g.g 1 by changing gasoil feed (4.45-1.78 g) while keeping the weight of zeolite constant (0.40 g). 

Abbreviations reactors batch (B), continuous stirred tank (CST), fixed bed of catalyst (FB), fluidized bed of catalyst (FL), furnace (Fum.), monolith (M), multitubular (MT), semicontinuous stirred tank (SCST), tower (TO), tubular (TU). Phases liquid (L), gas (G), both (LG). Space velocities (hourly) gas (GHSV), liquid (LHSV), weight (WHSV). Not available, NA. To convert atm to kPa, multiply by 101.3. 

Conventional technology, which has been employed for over 25 years, uses three or four fixed bed reactors in series, these operating under adiabatic conditions. They are preceeded by heating furnaces that compensate for the overall endothermicity of the reaction. Catalyst performance was investigated separately in a pilot plant under isothermal conditions, employing ca. 300-400 g of catalyst. 

Many recommendations are common to all treatments (as well as to activation, examined below). The main recommendation is that in all these processes all the particles of catalyst be subjected statistically to exactly the same succession of conditions. A fixed bed does not ensure this uniformity. Only moving beds (fluid beds, rotating furnaces, circulating beds or spray drying) fulfil the above requirements. 

The experimental work has been conducted in horizontal furnace tube reactors containing fixed beds of sized (1/2 inch x 4 mesh) coke electrically heated and controlled at precise temperatures over the entire bed length. The coke is first calcined, following a temperature profile similar to that of commercial calcining. The calcining is immediately followed by desulfurization with pure or dilute sulfur gases metered from cylinders. 

Achieving the required degree of desulfurization dictated electrothermal heating. Fixed-bed electrothermal furnaces of the Acheson type were initially considered for use directly with granular coke but the test results were disappointing. The product of the Acheson furnace was not uniform in its sulfur content. Adaptation of the Acheson Process, which is a batch process, presented problems in materials handling that were considered very difficult to resolve at the 10,000 tons per year capacity determined to be the... 

There are many different reactor designs but the two most commonly used are fixed bed and batch slurry phase. For a fixed bed reactor a given volume of solid particulate or monolith supported catalyst is fixed in a heated tube located within a furnace and liquid and/or gaseous reactants flow through the bed. This type of process is commonly used for large continuous-volume production where the reactor is dedicated to making only one product such as a bulk chemical or petroleum product. 

Selectox Also called BSR/Selectox. A process for converting hydrogen sulfide in refinery gases to liquid elemental sulfur, without the need for a reaction furnace. The gases are passed over a fixed bed of a proprietary catalyst (Selectox 33) at 160 to 370°C. Claimed to be better than the Claus process in several respects. Often used in conjunction with the Beavon process. Developed by the Union Oil Company of California and the Ralph M. Parsons Company, and first operated in 1978. Twenty-one units had been installed by 2000. 

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