CA2156010A1 - Solid fuel products free of slag formation and method of producing same - Google Patents

Abstract

A biomass such as bark, wood shavings, peat moss, paper, cardboard, sludges, etc. sawdust has homogeneously blended therein at least 0.025 weight percent of an additive which is capable of raising the fusion temperature of slag forming substances that may be present in the biomass material, above the temperature in a combustion chamber when burning the biomass. The method of manufacturing same is also disclosed. The result is that no slag is formed in the combustion chamber and the impurities accumulated as friable ash can easily be discarded.

Description

~ - l 21 ~60~ ~
This invention relates to solid fuel products which are substantially exempt of slag formation during combustion. More specifically, the present invention relate~ to products such as pellet 5 f uel derived f rom wood wastes and which contain an additive which prevent the formation of a slag during combustion. The invention also relates to a method for manufacturing solid fuel products, such as pellets from bioma~s which comprises 10 homogeneously blending an additive into the biomass, which is capable of altering the melt point temperature of the slag forming ~mpurities which may be present in the bioma~s.
For the past years, the pellet fuels 15 industry has managed steady controlled growth, and little has changed in the makeup of the pellet fuel.
Some research work on additives for binding, BTU
enhancement by the sue of polystyrene, and water resistance using hemicellulose (natural components 20 within the wood) were carried out in the early 1980's, when energy alternatives were the hot topic of the day.
There is no doubt, mankind will one day run out of hydrocarbons and go back to a carbon based 25 economy, deriving most of its chemical base feed ~tocks as it did 150 years ago. In like manner, almost as a precursor to future energy needs, the pellet stove started out as a solution to an environmental need in the US Pacif ic Northwest Due 30 to air pollution from the emissions of older stick stoves, the States of Oregon enacted ~egislation (known today as the Clean Air Act across the USA) banning their use, and leading to the advent of the pellet burner, a clean, dependable non polluting 35 form of residential heating - 2 _ 21~6l~l 0 In the beginning, pellet fuel manufacturers did not have a problem with sawdust supplies. Few board and pulp and paper plants used the discards of the sawmill operations, and most of the waste 5 sawdust along with the bark was simply burnt or landfilled. As the pellet industry grew and demana for the raw materials increased from other industries, prices began to rise and materials became a valuable commodity.
Bark on the other hand is still plentiful, as there are few uses for this high energy, landfill, piled high, ground water polluting, renewable resource.
Engineered fuel derivea principally from 15 bark feed stock sources could enjoy many advantages over existing supplies of sawdust. In the Unitea States, the average pellet fuel producer is paying 10 . 00 to 25 . 00 dollars US per short ton of bone dry sawdust. In Canada the price varies according to 20 supply, demand factors related to geographical location and winter energy demands Prices vary from lO . 00 to 30 00 CDN per short ton delivered There are many more perceived benef its to pellet mill operations, such as decreased dependence 25 on non renewable and imported fossil fuels, the support of regional economic activities and jobs.
Employment opportunities are created in fuel handling and transportation, plant operations, ana supporting services. In regions, expenditures for 30 biomass fuels, and other goods and services, increase economic activity, and the use of lower cost fuels helps keep energy costg low There is thus a need to develop lower cost feed stock sources for the production of residential 35 pellet fuels, and to address the subject of landfill _ 3 _ ~15~
diversion of bark mill residues, which are known to leach and pollute ground waters, lakes and streams.
In recent years technological advances in board manufacturing, pulping processes and a need 5 for maximizing raw material usage, has led to greater competition for traditional pellet mill raw materials such as sawdust and planer shavings.
In 1994-95, the pellet industry produced over 675, 000 short tons of pellet fuel worth an 10estimated 100,000,000 dollar and 6,000 airect and indirect jobs.
The pellet fuels produced from over 70 operating plants throughout the United States and Canada are already experiencing higher prices for raw materials, as they compete with larger process industries .
The pellet fuels industry last year celebrated over 10 years of growth. ~very year in which 50 to 60, 000 pellet stoves are sold, leads to an approximate increase in demand of 150,000 to 180, 000 tons of wood pellets . There are over 30 manuf acturers of pellet 3toves in North America which produced in excess of 60, 000 units for 1993-94 and 70,000 units ~or 1994-95. Of these, over 90 percent of the units being produced are the top feed type designs, and these are considered to be the most ash sensitive types. They are sensitive to the oCcurrence of fusion of inorganic materials in the burn pot, which degrade the stove perf ormance, by restricting air flow to fuel As well, they were designed around less than 1% or less ash content as is the makeup of sawdust residues in the Pacific Northwest and require more attention to maintenance when greater percentages of ash in pellets are burnt, having been designed with small ash pans.

_ 4 _ 2~ o It is generally accepted in the industry, that the pellet stove of the future should be user friendly, and manufacturers have agreed that higher ash fuels in the 3 to 596 ash range wlll be a reality in the 5 future. It is therefore imperative to lmprove the pre-processlng technology of hlgh ash feed stocks by altering the lnorganlc content.
As the pellet stove ls based on the forge prlnclple, (hlgh alr to fuel ratlos) high 10 temperatures ln the burn pot area cause inorganlc content of the resldue to fuse, resultlng ln the formatlon of slag or cllnkers. If the melt polnt of the inorganlcs could be altered or elevated, then the lnorganic content of the fuel would simply 15 maintaln lts original form, fall through the burn pot grate and into the ash pan. On the other hand, bark ls a natural choice as an alternative pellet fuel feed stock, but due to the aforementioned clinker characteristics, it is not used ln the 20 majorlty of resldentlal pellet fuel stoves.
Fusion temperature lnf luences of a slngle element on a combination of elements are complex effects that can not be det~rm; n~fl with confldence.
Many factors play a dlrect role ln fuslon occurrence 25 and many more, though mlnor ln detall, can ultimately have a profound effect from one burn cycle to another In the case of lnorganlc materlals, present in bark and to a lesser extent ln sawdust, there ls a need to develop a successful 30 countermeasure.
A caref ul examlnation as to the makeup of the feed stock is necessary in order to assess whether or not addltlonal procedures could be lmplemented so as to lmprove bark quallty. A
35 careful examlnation regardlng the composltion (l.e.
bark or sawdust percentages), manipulatlon, h;~nfll in~

_ 5 _ 21$61~1 0 from source to pellet mill, seasonal harvesting practices, pellet appliance combustion conf iguration and delivery of pellet fuel in relation to combustion alr systems in combustion chambers, was 5 necessary in order to arrive at the ideal additive f ormulation .
As opposed to reducing the highest temperature the ash reaches, it has been decided that better combustion perfnr~n~ne~ could be 10 achieved by increasing the fusion point temperature of the inorganic material present in the feedstock, used in the manufacture of the pellet fuel.
A review of the prior art indicated that it is generally known to raise the fusing point of 15 materials that may otherwise form a slag when burning the products on which they may be present.
For example, in U.S. 2,016,821, Nelms discloses a process for treating coal by uniformly applying to the surface of the coal a liquid mixture 20 of alumina bearing materlal such as bauxite, the mixture having an alumina content of three to six percent o~ the weight of the ash of the coal.
On the other hand, in an article published by "Hazardous Iqaterials Control Resources Institute 25 (HMCRI)", Schofield et al generally disclose introducing chemical additives by feeding them along with materials to be heated, so as to increase the fusion temperature of a mixture of inorganic materials However, the general disclosure of 30 fusion temperature increase for slag forming materials is not suf f icient to Yolve the problem associated with these materials when present in fuel pellets .
It is an object of the present invention to 3 5 provide a method which will enable to use f uel - 6 - 215~Ql~
pellets even when the latter contain an excess of slag forming impurities It is another object of the present invention to provide pellet fuels which do not slag 5 during combustion, thereby enabling to operate a 6tove without interruption.
In accordance with the present invention, there i8 provided a method for manufacturing solid fuel products of high bulk density, which are 10 substantially exempt of slag formation following combustion in a combustion chamber. According to this method, there is provided a comminuted biomass material, the latter is treated to give a product of increased bulk density which is converted into 15 bonded shaped such as by extruding the product into pellet form having a bulk density of over 30 pounds per cubi foot At least about 0 . 25 weight percent of an additive capable of raising the fusion temperature of slag forming substances that may be 20 present in the biomass material, above the temperature in the combustion chamber when the latter is in operation, is substantially homogeneously blended into the biomass material bef ore converting the product into bonded shapes 25 such as by extruding the biomass into pellet form.
Although any type of biomass material may be used to produce pellets according to the invention, wood fibre is preferred, as well as bark, or mixtures of wood and bark, but may also include 30 refuse derived feedstocks such as cardboard, paper, sludges, switch grass and agriculture waste such as bagasse, shells or straws, etc In accordance with a preferred embodiment, the disintegrated biomass material is compressed 35 optionally in the presence of steam, until reaching a desirea increasea bulk density, for example the _ 7 _ 21 ~010 biomass may be contacted but not necessarily with steam to provide a mixture with a moisture content between about 12 and 18 weight percent, or the biomass may already have this percentage, in which 5 case no addition of steam is necessary.
In accordance with another preferrea embodiment, the additive may be selected among suitable metallic compounds, preferably metal oxides, such as aluminum, magnesium, manganese, 10 calcium, silicon or neodymium oxides. A mixture of oxides is preferred, and the latter will also include ammonium nitrate. A preferred additive is a mixture containing calcium oxide, manganese dioxide, magnesium oxide, aluminum oxide, barium oxide, iron 15 oxide, potassium oxide, silicates and ammonium nitrate, for example about 0 . Z to 2 weight percent calcium oxide, about 8 to 12 weight percent MnO2, about 30-40 weight percent Nl14NO3, about 8 to 12 weight percent MgO, about 0 . 5 to 2 weight percent 20 aluminum oxide, about 0.1 to 0.5 weight percent barium oxide, about 1 to 3 weight percent iron oxide, about 0 . 5 to 1 weight percent potassium oxide and about 30 to 50 weight percent SiO2.
In accordance with the invention, there is 25 also provided solid products of high bulk density for combustion in a combustion chamber. The products comprise a compressed biomass material in particulate form, and at least about 0 . 25 weight percent of an additive which is capable of ralsing 30 the fusion temperature of slag forming substances that may be present in the biomass material, above the temperature ln the combustion chamber when the latter is in operation. The additive is substantially homogeneously blended into the biomass 35 material, prior to forming bonded shapes such as a pelletized fuel - 8 - 21 a ~l O
.
Baseline evaluations were conducted on raw materials containing between 2596-10096 bark in the final densified form. Species of sawdust ana bark were derived from various soft and hardwood species, 5 including black and white spruce, red oak and maple.
No binding agents were used at any time in the manuf acture of the wood pellet samples tested .
Densified pellet fuels derived from bark residues conf orms to the Pellet Fuel Institute 10 standards classification in all categories. It identifies bark as a high ash fuel (over 1% ash content~ usually between 2 and 4% is common with this residue.
It should be noted that there is a direct 15 relationship between lower ash content pellet fuels and premium prices per ton FOB the mill.
It should also be noted that recently, many pellet appliance manufacturers in new models are designing larger ash pans to deal with higher ash 20 fuels of the future. In so doing, they are beginning to address the issue of ash volume, however, the problem of ash fusion phen~ ?n~ still exists. Increasing ash content of a pellet fuel is directly proportional to increasing the risk of clinkering or slagging in the burn pot from fusion.
Many examples can be sighted of low ash pellet manuf acturers having what is termed a bad batch of pellet fuel, sighting dirt getting into the f eedstock .
The combustion tests were carried out on 3 types of pellet burners, top feed, bottom feed and side feed systems. All units were placed in typical residential installation situations. Burn cycles were typical with low, medium and high draft and feed system settings Carryover effects from one test batch to another were minimi~ed during each 2l56ol~
g burn cycle by cleaning grates and ash pans at the completion of each test run.
Bark pellet f uels or mixtures of bark and sawdust have been produced for over 12 years.
The pellet fuel industry through the pellet fuels association has developed two pellet fuel grade classification, based on volume of residual ash. They are Premium, less than 1% ash content, and standard, between 1% and 3~6 ash content grades.
This ash classification standard for pellet fuels is based solely on ash quantities post combustion and its effect on current system design. More specifically it is based on the quantity of unburned inorganic content of carbon in each pellet type An additive consisting of about 0 . 2 to 2 weight percent calcium oxide, about 8 to 12 weight percent ~nO2, about 30 to 40 weight percent ammonium nitrate, about 8 to 12 weight percent aluminum oxide, about 0.1 to 0~5 weight percent barium oxide, about 1 to 3 weight percent iron oxide, about 0 . 5 to 1 weight percent potassium potassium oxide and about 30 to 40 weight percent SiO2 was initially blended with industrial grade bark pellets derived from black and white eastern spruce species. The pellets were manufactured at the Energex plant at Lac Megantic, Quebec. The additive wad applied to the surface area of the pellets at a rate varying from 5 to 20 grams per 18 kg of pellets, blended together then placed in an 18 kg hopper of an Enviro fire EFll top feed pellet stove. This particular type of combustion system was chosen, as it is considered to be one of the more ash sensitive top feed pellet burners on the market due to grate design.
The result of this pr~1 im;nilry study was to evaluate the effectiveness of the additive as an application onto, rather than into a finished - 10 - 2l~6olo product (pellet). Measured quantities of 5, 10, 15 and 20 gram portions of additive were mixed with 12 ( 2 per test burn ) test sample of 18 kg of wood pellets comprised of 100% bark.
~EST 1.
No ~ ;tiYe 100% ~rk r,c~ll~t The pellet samples containing no additive on high burn rate produced clinkers ~ solidif ication of inorganic elements ) . Clinkering or solidif ication and subsequent adherence to grate surfaces, of solidified inorganic materials, resulted in inadequate air to f uel ratios in the burn pot and a reduction in overall stove performance, i.e. reduced flame, reduced heat output, higher particulate matter out venting pipe, and bl~rk~n;ng of stove door glass. As pellet stove performance is directly related to a 3 0 to 1 air to f uel ratio, a reduction in air reaching the burn pot results in poor perf ormance .
In this example, the only course of action is higher maintenance by scraping the burner pot.
Steps required to improve stove performance are to turn the stove of f, open stove door and scrape the clinker build up off the grate surface. This had to be done repeatedly, in order to maintain some degree of stove performance. In addition to the obvious end user inconvenience, the negative health effects from indoor smoke spillage can cause serious health problems. Pellet stove owners purchase the appliance for the sake of convenience, and frequent attention of this sort is a frustrating experience and would result in a negative consumer attitude towards this type of quality pellet fuel.

- ll - 21 ~ 1 0 TEST Z.
Rnrk ~ellet~ wi~h 5 g~ - of :..lA~v~3 18 kg of Energex wood pellets were blenaed with 5 grasns of additive, using the same blending procedure as in test 1 The Envirofire top feed stove results post combustion were considered to be the same as in step 1, requiring frequent cleaning of clinker deposits by scraping the grate surface.
TEST 3.
0 R~rk PellQtS Wi~h 10 ~ ~ of ~ tive 18 kg of Energex wood pellets were blended with 10 grams of additive, using the same blending procedure as in previous tests . The Envirof ire top feed stove results post combustion were considered to differ from text 2. There was apparently less clinker on grate surf ace areas, but attention to grate surf ace areas was still accurate . Results were almost identical.
TEST 4 .
R;~rk ~ell~t~ with 15 of AA-Iitive 18 kg of Energex wood pellets were blended with 15 grams of additive, using the same blending procedure as in previous tests. The result was almost no clinker on grate surface areas at the end of the burn cycle. Most of the inorganic silicate (sand) simply fell through the grate and into the ash pan. There was no scraping of grate surface required, until the end of the combustion cycle although some accumulation was present It was concluded that there was a positive impact on clinker formation when a blend of 15 to 20 grams of additive is used on surface areas of bark wood pellets. The physical characteristics of the ash are significantly altered. Although surface applications gave rise to the fact that the additive has some effect on clinker formation, tests revealed - 12 - 215~10 varying end results. It is believed that this was probably the result of uneven distribution of the additive in the mix, static attachment of additive on metal and plastic surface areas or a combination 5 of circumstances involving a3h melting due to a hot spot near the point of air to pellet fuel combustion .
Surface applications of additive to a residential pellet fuel is not therefore desirable 10 due to varying end results. It has now been found that the additive needs to get into the pellet to be effective in providing greater exposure to inorganic silicate. A homogeneous mix is therefore necessary according to the invention and can thus only be 15 accomplished by blending the additive with the feedstock prior to pelletization. The optimum point of introduction of the additive in the industrial manufacturing process of wood pellets is therefore critical. A homogeneous blend of additive to wood 20 fiber achieved the desired result, as was demonstrated in further testing.
TE~ 5.
Samples of bark residues were obtained from the E3trie, Outaouais and Blainville areas of the 25 Province of Quebec, Canada. The samples were from process industries such as veneer manufacturers and sawmill operations. The bark samples varied in concentrations, as the bark had some sawdust present. Some of the samples were derived from 30 hardwood process industries while others were derived from softwoods (board manufacturers). Bark samples from, hardwoods appeared to contain more inorganic material than the softwoods, probably due to the uneven nature of the bark as compared to 35 softwood species. Quantities of sample material were purchased in truck load quantities, as this - 13 - 21~60 best represents the way in which a typical pelletizing operation would receive and process the same kind of material for further processing into pe 1 let f uel .
Typical moisture content for green sawmill residues varied, between 35% and 60% moisture content . The material was received and of f loaded into separate piles. The veneer bark (derived principally from hardwood species) was then dried down to a 10% to 12% moisture content with the use of a heil rotary drum dryer having an average per hr drying output of 8, 000, 000 BTU. The bark was then commuted through a 150 E~P hammer mill (one quarter inch screen mesh) 80 as to reduce the feedstock down to a homogeneous size prior to pelletization. It was at this point (hammer mill) that the additive was first introduced at a rate of approximately 525 grams per ton of material (bark ) throughput on an oven dry basis.
Approximately 12 tons of pellets were produced and random samples taken at alternating times throughout the production cycle. These pellets were then burnt in the Envirof ire II pellet s'cove at a rate of 18 kg per cycle.
The metering device was an agricultural based f ertilizer metering system . Attached to the hopper of the feeder was a small industrial vibrator ( so as to prevent caking of the additive ) . The suction action of the hammer mill and attached blowers drew the metered powder from the tube leading f rom the bottom edge of the hopper meter r-~h~sniqm (which controls the amount dispensed) and into the commuted wood f iber .
Post combustion results varied, with some 3 5 test samples clinkering and others not clinkering - 14 - 2~ o and it was concluded that the quantity of additive was insuf~icient to have an effect on the bark.
TEST 6.
Test 5 was repeated except that there were usea 750 to 850 grams of additive per metric ton of pellets produced. The pellets were burned in an Earth stove side feed and a Harmon underfeed pellet stove .
Each pellet burner was tested on low, medium and high burn settings for 10 burn cycles (189 kg of fuel each) which was more representative of a one week burn for an average household, in an average winter period. The burn levels were also more representative of a typical day and night cycle heat demand over a perlod of time, in this case 1 week with f luctuations in therm demand.
BTU output per stove was in the range of between 20,000 to 40,000 BTU per hour on the various settings .
There was virtually no fusion occurrence or accumulation on grate surfaces. Ash volume was high, due to an ash level of 2, 25% . But this quantity was manageable and required 3 cleanings (removal of ash from the ash pan) for the Envirofire EF II as opposed to 1 cleaning per week with less than . 5% ash for the highest grade premium type f uels .
The Earth and Harmon systems required in real terms 1.5 cleanings, as ash pan size and area are considerable in these types.
It has therefore been est~hl i ~ilG~ that when at least about 0 . û25 weight percellt of additive are homogeneously blended into the fuel pellets, no substantial slag formation takes place.
Although the tests were made with an additive based on MnOz MgO and NE~4NO3 f or - 15 - 215~
convenienGe, it is understood that any additive capable of raising the fusion points of impurities present in biomass material is within the spirit of the present invention.

Claims (30)

1. Method for manufacturing solid fuel products of high bulk density, which are substantially exempt of slag formation following combustion in a combustion chamber, which comprises providing a comminuted biomass material, treating said biomass material to give a product of increased bulk density and converting said product of increased bulk density into bonded shapes, wherein at least about 0.025 weight percent of an additive capable of raising the fusion temperature of slag forming substances that may be present in said biomass material, above the temperature in said combustion chamber when the latter is in operation, is substantially homogeneously blended into said biomass material before converting said product into said bonded shapes.

2. Method according to claim 1, wherein said biomass is selected from the group comprising wood shavings, wood flows, wood chips, wood bark, saw dust, peat moss, paper, cardboard, sludges, switch grass, agriculture waste, bagasse, shells, straws, or mixtures thereof.

3. Method according to claim 1, wherein said biomass comprises fibre and/or residues.

4. Method according to claim 1, wherein said biomass comprises sawdust.

5. Method according to claim 2, wherein said biomass comprises wood chips.

6. Method according to claim 2, wherein said biomass comprises wood shavings.

7. Method according to claim 2, wherein said biomass comprises wood bark.

8. Method according to claim 2, wherein said biomass comprises wood flour.

9. Method according to claim 1, wherein said biomass comprises a mixture of wood and bark.

10. Method according to claim 1, which comprises compressing said comminuted biomass material in the presence of steam, until reaching said increased bulk density.

11. Method according to claim 1, wherein said biomass is conditioned to provide a mixture with moisture content between about 5 and 18 weight percent.

12. Method according to claim 1, wherein said additive comprises metallic compounds.

13. Method according to claim 12, wherein said metallic compounds comprise metal oxides.

14. Method according to claim 1, wherein said metal oxides are selected from the group consisting of aluminum, magnesium, manganese, calcium, silicon, iron, potassium and barium oxides or mixtures thereof.

15. Method according to claim 11, which comprises mixtures of said oxides.

16. Method according to claim 12, wherein said mixtures of oxides also include ammonium nitrate.

17. Method according to claim 16, wherein said additive comprises a mixture of manganese dioxide, ammonium nitrate and magnesium oxide.

18. Method according to claim 16, wherein said additive comprises about 0.2 to 2 weight percent calcium oxide, about 8 to 12 weight percent MnO2, about 30 to 40 weight percent ammonium nitrate, about 8 to 12 percent MgO, about 0.5 to 2 weight percent aluminum oxide, about 0.1 to 0.5 weight barium oxide, about 1 to 3 weight percent iron oxide, about 0.5 to 1 weight percent potassium oxide and about 30 to 50 weight percent SiO2.

19. Solid fuel products of high bulk density for combustion in a combustion chamber comprising a compressed biomass material in bonded shapes, and at least about 0.025 weight percent of an additive which is capable of raising the fusion temperature of slag forming substances that may be present in said biomass material, above the temperature in said combustion chamber when the latter is in operation, said additive being substantially homogeneously blended into said biomass material.

20. Solid fuel products according to claim 19, wherein said biomass comprises wood waste.

21. Solid fuel products according to claim 19, wherein said biomass material comprises bark.

22. Solid fuel products according to claim 19, wherein said biomass comprises a mixture of wood and bark.

23. Solid fuel products according to claim 19, wherein said additive comprises metallic compounds.

24. Solid fuel products according to claim 23, wherein said metallic compounds comprise metal oxides.

25. Solid fuel products according to claim 24, wherein said metal oxides are selected from the group consisting of aluminum, magnesium, manganese, calcium, silicon, iron, potassium and barium oxides or mixtures thereof.

26. Solid fuel products according to claim 25, which comprises mixtures of said oxides.

27. Solid fuel products according to claim 26, wherein said metal oxides also comprise ammonium nitrate.

28. Solid fuel products according to claim 27, wherein said additive comprises a mixture of manganese dioxide, ammonium nitrate and magnesium oxide.

29. Solid fuel products according to claimm 27, wherein said additive comprises about 0.2 to 2 weight percent calcium oxide, about 8 to 12 weight percent MnO2, about 30 to 40 weight percent ammonium nitrate, about 8 to 12 weight percent aliminum oxide, about 0.1 to 0.5 weight percent barium oxide, about 1 to 3 weight percent iron oxide, about 0.5 to 1 weight percent potassium potassium oxide and about

30 to 40 weight percent SiO2.