Bed material for bubbling fluidised bed combustion

Abstract

The invention is in the technical field of bubbling fluidized bed combustion and relates to the use of ilmenite particles with an average particle size <dp> between 0.1 mm and 1.8 mm as bed material for a bubbling fluidized bed (BFB) boiler with an excess air ratio () below 1.3 and to a method for operating a bubbling fluidized bed (BFB) boiler, comprising carrying out the combustion process with a bubbling fluidized bed comprising ilmenite particles as defined in any one of claims 1 and 4-5; and setting the excess air ratio () to a value below 1.3.

Claims

1. A method, comprising: i) providing an amount of ilmenite particles with an average particle size between 0.1 mm and 1.8 mm as bed material in a furnace with a bubbling fluidized bed boiler, wherein the amount of ilmenite particles as a proportion of total bed mass is at least 20% by weight, and ii) operating the bubbling fluidized bed boiler for combustion of biomass fuel or waste-based fuel with an excess air ratio , below 1.3, wherein in a single bubbling fluidized bed boiler operated under continuous conditions, the ilmenite particles exhibit a reducing-oxidizing effect utilized during the combustion.

2. The method of claim 1, wherein , is 1.25 or less.

3. The method of claim 1, wherein , for the combustion of waste-based fuel is 1.23 or less.

4. The method of claim 1, wherein i) the ilmenite particles have an average particle size that is at least 0.2 mm; and that is not more than 1.8 mm; and/or ii) the ilmenite particles have a particle size in the range from 0.1 mm to 1.8 mm.

5. The method according to claim 1, characterized in that the ilmenite is crushed rock ilmenite.

6. A method for operating a bubbling fluidized bed boiler, comprising: a) carrying out a biomass fuel or waste-based fuel combustion process in a furnace with a bubbling fluidized bed comprising ilmenite particles having an average particle size between 0.1 mm and 1.8 mm wherein the amount of ilmenite particles as a proportion of total bed mass is at least 20% by weight, wherein in a single bubbling fluidized bed boiler operated under continuous conditions, the ilmenite particles exhibit a reducing-oxidizing effect utilized during the combustion process; and b) setting an excess air ratio , in the furnace to a value below 1.3.

7. The method of claim 6, wherein , is set to 1.25 or less.

8. The method of claim 6, wherein , for the combustion of waste-based fuel is set to 1.23 or less.

9. The method of claim 6, wherein the bubbling fluidized bed further comprises silica sand particles, wherein i) the silica sand particles have a particle size in the range from 0.25 mm to 2.0 mm; and/or ii) the silica sand particles have an average particle size between 0.6 mm and 0.8 mm.

10. The method of claim 6, wherein the amount of ilmenite particles as a proportion of the total bed mass is at least 30% by weight.

11. The method of claim 6, characterized by setting the fluidizing gas velocity to at least 0.03 m/s.

12. The method of claim 6, characterized by providing at least 50% of the combustion air as primary fluidizing air.

13. The method of claim 6, characterized by a sooting interval of at least 2 days.

14. The method of claim 6, wherein fuel and/or the ilmenite particles are continuously supplied to the bubbling fluidized bed boiler.

15. The method of claim 6, wherein the ilmenite particles are supplied to the bubbling fluidized bed boiler at a rate of less than 3 kg/MWh thermal output when biomass fuel is used; and at a rate of less than 6 kg/MWh thermal output when waste-based fuel is used.

Description

(1) It is shown in

(2) FIG. 1: a BFB boiler with a bubbling fluidized bed comprising ilmenite particles;

(3) FIG. 2: fluidizing properties of silica sand and ilmenite in a bubbling fluidized bed;

(4) FIG. 3: a schematic drawing of the boiler and gasifier system used for BFB experiments;

(5) FIG. 4: CO and CO.sub.2 concentration versus fluidization velocity for BFB combustion with ilmenite and silica sand as bed material;

(6) FIG. 5: CO and CO.sub.2 concentration versus fuel load for BFB combustion with ilmenite and silica sand as bed material.

EXAMPLE 1

(7) FIG. 1 shows a BFB boiler (1), with primary air supplies (2) and an air distributor (3) at the bottom of the furnace (4) and secondary air ports (5) and tertiary air ports (6) in the freeboard of the furnace (4). Heat exchangers (7) and the flue gas cleaning line (8) are also shown. The fuel is fed, preferably continuously, through fuel ports (9) and is combusted in a bubbling fluidized bed (10) comprising ilmenite particles. Preferably, the bed material consists of ilmenite particles with a particle size dp in the range from 0.3 mm to 1.0 mm and an average particle size <dp> between 0.4 mm and 0.6 mm. The ilmenite particles can be crushed rock ilmenite, which, before carrying out the combustion process, has been screened to exclude particles with a particle size too large to be fluidized and too small to be retained in the system by sieving off particles which are too large or too small.

(8) The use of the ilmenite particles allows to operate the boiler closer to stoichiometric combustion. In particular, the boiler (1) is operated with an excess air ratio () below 1.3, for example with 1.05<<1.23 for waste fuel and with 1.05<<1.19 for biomass fuel. Preferably is set to a value between 1.05 and 1.1 for both types of fuel. The majority (>50%) of the combustion air is provided as primary air via the primary air supplies (2) and preferably all of the combustion air is provided as primary air. The boiler is operated with a fluidizing gas velocity between 0.3 and 1.5 m/s.

(9) The use of the ilmenite particles in bed (10) leads to a better balanced oxygen distribution which enables a more complete combustion and reduces CO, No.sub.x and unburned carbon emissions in the flue gas line (8).

(10) The ilmenite particles in the bed can absorb alkali and are therefore less prone to agglomeration when compared with silica sand bed material. This allows to extend the exchange rate for the bed material. The ilmenite particles are supplied to the boiler at a rate of 1.5 kg/MWh thermal output or less when biomass fuel is used and at a rate of 3 kg/MWh thermal output when waste based fuel is used.

(11) Alternatively, boiler (1) is operated with a mixture of ilmenite particles and silica sand particles as bed material with particle ratios disclosed in the general part of the description. In this case, it is preferred that the silica sand particles have a particle size dp in the range from 0.5 mm to 1.2 mm and that the average particle size <dp> of the silica sand particles is between 0.6 mm and 0.8 mm.

EXAMPLE 2

(12) Particle Size of Bed Material in a BFB Boiler

(13) The particle size (dp) in a fluidized bed application should be determined to suit the purpose of the application. The particle size affects the fluid dynamics and also the amount of fluidizing media needed. The recommended average particle size of sand in a BFB-boiler is between 0.6-0.8 mm. The sand particle size distribution can be within the interval of 0.5 1.2 mm. Additional parameters that affect the fluid dynamics in a boiler are e.g.: solids density (.sub.s), the sphericity (.sub.s) of the particles and the voidage () created between the particles in the bed. It is possible to estimate the behavior for fluid dynamics of different bed materials, and one parameter which is commonly used is the minimum fluidization velocity (u.sub.mf). This velocity gives information about when the bed material starts to fluidize. There are three major paths to determine the u.sub.mf, 1), experimentally, 2), theoretical calculations or 3), semi-empiric calculations. Here, a semi-empirical calculation route has been used. The calculation is based on the Ergun equation (1) (Kunii D., Levenspiel O., Fluidization Engineering, second edition, Butterworth-Heinemann, 1991):

(14) $\begin{array}{cc}\frac{1.75}{{\mathrm{.Math.}}_{\mathrm{mf}}^3.Math.{}_s}.Math.{\mathrm{Re}}_{,\mathrm{mf}}^2+\frac{150.Math.\left(1-{\mathrm{.Math.}}_{\mathrm{mf}}\right)}{{\mathrm{.Math.}}_{\mathrm{mf}}^3.Math.{}_s^2}.Math.{\mathrm{Re}}_{,\mathrm{mf}}^2=\mathrm{AR}& \left(1\right)\end{array}$
Where Re.sub.mf is calculated according to Eq. 2, where .sub.f is the density of the fluid and is the kinematic viscosity of the gas.

(15) $\begin{array}{cc}{\mathrm{Re}}_{\mathrm{mf}}=\frac{\mathrm{dp}.Math.u_{\mathrm{mf}}.Math.{}_f}{v}& \left(2\right)\end{array}$

(16) The Archimedes number (AR) is calculated according to Eq. 3, where g is the gravimetric constant.

(17) $\begin{array}{cc}\mathrm{AR}=\frac{{\mathrm{dp}}^3.Math.{}_f\left({}_s-{}_f\right).Math.g}{v^2}& \left(3\right)\end{array}$
The .sub.s of the particles have been received but not the .sub.mf number. The .sub.mf number is here calculated via a semi-empiric correlation presented by Wen and Yu (Wen C. Y., Yu Y. H., A generalized method for predicting minimum fluidization velocity, American Institute of Chemical Engineers, Vol. 12, Issue 3, May 1966, pages 610-622) according to Eq. 4

(18) $\begin{array}{cc}14=\frac{1}{{}_s.Math.{\mathrm{.Math.}}_{\mathrm{mf}}^3}& \left(4\right)\end{array}$

(19) Two different ilmenites have been considered, 1), one Norwegian rock ilmenite with a .sub.s of 0.7 on the fresh particles (which has been determined at the University of Chalmers), 2), one round ilmenite with a .sub.s of 0.86 which corresponds to the .sub.s of silica-sand. The results from the calculations are presented in FIG. 2 as u.sub.mf versus dp for the two ilmenites and the ordinary silica-sand. If the recommendation for silica-sand is considered to be the basis for the average particle size, and if the comparison is built on fulfilling the same u.sub.mf, then the corresponding average ilmenit dp is given in the blue shaded area of the plot. If keeping the same u.sub.mf as the basis for the heavier ilmenite then the same volume flow of fluidization media can be considered to be valid. There are two obvious advantages with this choice, 1), ordinary gas flows or even lower can be considered, which is positive as the volume flow of gases and fan capacity in a combustion system usually is a restriction, 2), a smaller particle size promotes the oxygen-carrying effects as more surface area is initiated and a better gas/solid contact can be expected in the splash zone above the bed.

EXAMPLE 3

(20) 1) Setup Used for BFB Experiments

(21) A 2-4 MW.sub.th gasifier system at Chalmers University of Technology was used for BFB combustion experiments with ilmenite. It is of the type indirect gasification. In this technique, the actual gasification reactions are separated from the combustion reactions and the heat needed for the endothermic gasification reactions is supplied by a hot circulating bed material. The bubbling fluidized bed gasifier is connected to the 12 MW.sub.th circulating fluidized bed boiler and the two reactors are communicating via the bed material, see FIG. 3. Fuel is fed on top of the bed in the gasifier and the gasifier is fluidized with pure steam. Usually the system is operated with silica-sand and the gasifier is operated in the temperature interval of 750-830 C. FIG. 2 shows the boiler and gasifier setup, wherein the reference numerals indicate: 10 furnace 11 fuel feeding (furnace) 12 wind box 13 cyclone 14 convection path 15 secondary cyclone 16 textile filter 17 fluegas fan 18 particle distributor 19 particle cooler 20 gasifier 21 particle seal 1 22 particle seal 2 23 fuel feeding (gasifier) 24 fuel hopper (gasifier) 25 hopper 26 fuel hopper 1 27 fuel hopper 2 28 fuel hopper 3 29 sludge pump 30 hopper 31 ash removal 32 measurement ports
2) Ilmenite Operation in the Gasifier
Variations in Fluidization Velocity at Constant Fuel Feed

(22) With the aim of investigating gas/solid contact between the volatiles and the bed material, the gasifier was operated with 100 wt. % of ilmenite with an average particle size of 0.14 mm as bed material for a few days. The first experiment was conducted at four different steam flows yielding a variety in gas velocities: 0.13, 0.19, 0.25 and 0.28 m/s, which corresponds to 5, 7, 9 and 11 times the minimum fluidization velocity of the ilmenite fraction. During this experiment the gasifier was continuously fed with 300 kg of fuel (wood-pellets) per hour and the bed temperature was kept at 820-830 C. FIG. 4 shows the analyzed gas components CO.sub.2 and CO in the outlet of the gasifier during ilmenite operation. Data for ordinary silica-sand during normal gasification conditions (Ref, sand, marker color red) has been added in the figure for comparison with the ilmenite. As can be seen in FIG. 4, the CO concentration is clearly decreased and the CO.sub.2 concentration is increased by almost a factor 4 when ilmenite is used in comparison to the silica-sand operation. As the gasifier is fluidized with pure steam all the extra oxygen supplied for the increased oxidation of hydrocarbons and CO is coupled to the oxygen-carrying properties of the ilmenite. This further shows the oxygen buffering effects that ilmenit possesses and the ability to transport oxygen from oxygen rich to oxygen depleted zones during fuel conversion. The fluidization conditions and gas solid contact in the gasifier can be compared to the conditions in a BFB-boiler and it is therefore likely that ilmenite will contribute with increasing oxygen transport also in a BFB boiler.

(23) Variation in Fuel Feed During Constant Fluidization Velocity

(24) The second experiment was conducted during a constant steam flow of 200 kg/h (yielding a gas velocity of 0.19 m/s, corresponding to 7 times the minimum fluidization velocity) and a variation in fuel feed: 200, 300 and 400 kg.sub.fuel/hour (wood pellets). FIG. 5 shows the measured gas concentrations of CO and CO.sub.2 in the outlet of the gasifier. The trend is very similar to the one in FIG. 4, a clearly decreasing CO concentration as a function of the oxygen transport via the ilmenite. The CO.sub.2 concentration also reveals that hydrocarbons are combusted and not only CO is oxidized. This result shows that even though the fuel feed is increased from 200 to 400 kg/h there is still oxygen enough to support the oxidation of CO and hydrocarbons.

(25) During combustion in a fluidized bed boiler, air is usually supplied both as primary air via nozzles below the bed and as secondary air in the freeboard of the furnace. The experiments in the gasifier show that a high fuel conversion can be achieved via the buffered oxygen in the ilmenite bed, i.e. without any addition of air at all. This means that a high degree of oxidation of the volatiles is conducted already in/or close to the bed and suggests the operation of a BFB boiler with less or no secondary air.

(26) The preliminary tests indicate that an excess air ratio of 1.23 or less can be achieved for waste. It is suggested that an excess air ratio of 1.19 or less can be achieved for biomass fuel.