Vinasse a Potential Biomass Cofiring in a Fluidised Bed

Transcription

1 Vinasse a Potential Biomass Cofiring in a Fluidised Bed M. Akram 1, CK. Tan 2, SM. Thai 2, R. Garwood 2 1 University of Sheffield 2 University of South Wales 10 th ECCRIA conference September 2014 University of Hull

2 2 Synopsis What is vinasse and issues with it? Problem statement Pre-assessment of materials (Agglomeration indices, muffle furnace tests) Fluidised bed Experimental setup SEM analysis Conclusions

3 3 Vinasse? Molasses from the sugar manufacturing process is used to produce alcohol and the remaining liquor after alcohol extraction is termed as vinasse In UK sugar is made from sugar beet, process is different to cane sugar Distillery feedstock Characteristics Cane molasses Cane juice Cassava Organic matter Total nitrogen Sulphate (SO 4 ) Calcium oxide (CaO) Phosphorus pentoxide (P 2 O 5 ) Magnesium oxide (MgO) Potassium oxide (K 2 O)

4 Issues: 4 Distilleries produce huge amounts of vinasse with a medium sized distillery processing 110, ,000 tonnes of molasses per year producing about 70,000 tonnes of concentrated (60 Brix) vinasse per year. Vinasse constitutes a high volume, high strength acidic waste so that disposal of untreated vinasse to natural waterways is not feasible. Vinasse is probably the strongest of all industrial wastes in terms of its polluting load specially Biological Oxygen Demand (B.O.D). Average B.O.D of vinasse from a molasses distillery is about 35000ppm. A small distillery discharging only 50,000 gallons of vinasse per day can contribute a B.O.D load equivalent to the sewage of a town with 100,000 inhabitants. To maintain about 3ppm dissolved oxygen in the stream receiving the waste minimum dilution required is times that of the waste or about cusec (cubic feet per second) of water. Moreover, the waste poses a significant threat to fish.

5 Possible uses of vinasse 5 Possible outlets for this by-product could be composting, fodder, filler in paper, cattle feed, methane production by anaerobic fermentation, fertilizer (high potassium content), as a binder in briquetting and combustion to recover useful energy. Vinasse can be applied to land as a fertilizer to increase its ph because high Ca and Mg improves physical and chemical properties, mineral and water containing characteristics and fertility of soil For economic disposal of vinasse as a fertilizer a large land area adjacent to the distillery is required and should be in a low-medium rainfall region which is highly unlikely in the UK. Moreover there are huge costs of piping network deep well disposal of vinasse as a cheaper alternative. However limited underground storage and specific geological formation preclude a wide scale stillage disposal by this method In UK 1280 m 3 of vinasse per hectare per year is required. The process is not economical if odour is a series issue Anaerobic digestion: economics associated with technical problems act as a limiting factor for methane gas production by anaerobic fermentation of vinasse There are few studies on combustion of vinasse but aftre concentrating it

6 6 Problem statement: It is essential to use process industry byproducts in environmentally friendly way To produce onsite power OR in other heat sources e.g. Drying

7 Characteristics: 7 Thoresby coal Vinasse Proximate analysis Moisture (%) Ash (%) Volatiles (%) Fixed carbon (%) Ultimate analysis Carbon (%) Hydrogen (%) Oxygen (%) Nitrogen (%) Sulphur (%) Caloric values Gross CV (MJ/kg) Net CV (MJ/kg) Alkali oxides Na 2 O (%) K 2 O (%) CaO (%) MgO (%)

8 8 Pre- assessment

9 Agglomeration indices: 9 Alkali index (A) is the ratio of the sum of K 2 O and Na 2 O to higher heating value of the fuel. Values of alkali index higher than 0.17 kg/gj are considered to cause possible problems. Values exceeding 0.34 kg/gj almost certainly cause agglomeration and slagging problems. [1] Alkali index represented by I is the mass ratio of alkaline earth oxides to the alkaline oxides. The I values higher than about 2 should not present risk of sintering. [2] Another determination of agglomeration propensity which includes S and Cl, known as agglomeration indicator AI. [3]

10 10 Agglomeration potentials of Vinasse: Alkali index A for vinasse is very high showing that there are considerable chances of agglomeration during firing of this material. Alkali index I for vinasse (0.0035) is considerably lower than safe limit of above 2, indicating higher chances of sintering. The value of AI is also above the threshold value of 1 so that the likelihood of agglomeration during vinasse combustion is high. Agglomeration Indices Calculated values Alkali index A (kg/gj) 7.7 Alkali index I Agglomeration indicator AI 2.1

11 11 Muffle furnace tests: During these tests vinasse was mixed with sand in different ratios and kept at a controlled set temperature for a specific period of time. In the first set of these tests the percentage of vinasse in sand was varied from zero to 50% to make total sample weight for each test equal to 40 g. The samples were kept at 800 and 850 C in muffle furnace for 60 minutes. In order to simulate continuous combustion operation, it was also decided to perform top up tests, where certain amount of vinasse was added to the sample left in the previous test. The sample was kept in muffle furnace controlled at 850 C for another 20 minutes during each top up test. The amount of vinasse added, to 30 g of sand, for each top up was 12 g, is equivalent to around 30% vinasse content in the sample. The sand was fresh when used for the first top-up test but for the subsequent tests, vinasse was added to the sand sample left from the previous test.

12 12 Muffle furnace tests: Samples of Vinasse and sand mixtures after 1 hour at 850 C in a muffle furnace Samples of Vinasse and sand mixtures after 1 hour at 800 C in a muffle furnace

13 Top up tests: 13 Muffle furnace top-up tests with vinasse

14 The Fluidised bed: 14 Nominal input based on coal, 25kW Bed diameter, 168mm Preheating with gas Water cooled distributor plate Sandsize : BS 14/25 (0.6 to 1.18 mm) Fluidising velocity 1-4 m/s

15 Coal feeding system: 15 The feed system was calibrated for coal by means of a series of 5 independent tests at each particular motor speed and the measured feed rate was repeatable to within ± 6%. The singles coal (with a top size of 25 mm) used in the tests was broken up and subsequently sieved to provide a particle size ranging from 10 to 14 mm.

16 Liquid feeding system: 16 6 mm diameter stainless steel feed tube Injection point 51 mm above the stand pipes The maximum capacity of the pump is 185 l/min (tube can be changed). Compressed air is used to prevent the feed pipe from being blocked by the bed material and also to atomise the liquid biomass so that it is sprayed into the bed. Cooling water is used to cool the feed pipe to prevent the biomass from being coked in the hot feed pipe.

17 17 Air and Gas flows Measurements: Coal (screw feeder) and Vinasse (peristaltic pump) flow rates Combustion chamber temperature Bed temperature and pressure Freeboard temperature (restricted by the height of the tube, 2m) Flue gas analysis (NO x, SO x, O 2, CO, CO 2 ) sampling line was water cooled to remove water

18 18 Data logging: Testo 350XL flue gas analyser with ceramic probe

19 19 Experimental conditions of coal and vinasse cofiring test Coal flow (kg/h) 2.1 Coal flow (kw) 17.5 Vinasse flow (l/h) 1.65 Vinasse flow (kg/h), Density = 1320 kg/m Coal to vinasse feed ratio (kg/h)/(kg/h) 0.95 Coal (%) 48.8 Vinasse (%) 51.2 Coal only feed time (hrs) 1.27 (Coal + Vinasse) feed time (hrs) 0.61 Total coal feed time (hrs) 1.88 Total coal feed (kg) 3.95 Total vinasse feed (kg) 1.34 Coal to vinasse mass ratio (kg/kg)* 2.95 Sand to vinasse mass ratio (kg/kg)* 5.6 *Based on the total fuel mass input into the bed

20 Bed behaviour: 20 Sudden changes in bed and freeboard temperatures and bed pressure Bed temperature and freeboard temperature both went up at the start of agglomeration Soon after the onset of agglomeration, freeboard temperature was considerably higher than bed temperature and was so for some time after stopping both the coal and vinasse feeds Temperauer ( C) Bed Temperature (C) Freeboard Temperature (C) Bed Pressure (mmwg) Time (hrs) Bed Pressure (mmwg) Temperauer (ᵒC) Bed Temperature (C) Freeboard Temperature (C) Bed Pressure (mmwg) Time (hrs) Temperature and Pressure observations during coal-vinasse agglomeration test Temperature and Pressure observations during vinasse feeding Bed Pressure (mmwg)

21 21 Agglomerated sand: Sticky deposits on feed pipe In-situ agglomerated material

22 22 SEM analysis of Coal and Vinasse cofiring test Cofiring test After 3 days Site of interest 1 Site of interest 2 Coal only Element Weight (%) Weight (%) O F Na Mg Al Si P S K Ca Fe

23 Bed material analysis: 23 During the cofiring test almost 3.9kg of coal and 1.34kg of vinasse was fed into the bed. So the total amount of sodium (as Na 2 O) added to the bed during this test was about 0.03 kg. During three days of coal only testing around 35kg of coal was fed which equates to around 0.1kg of Na 2 O injection into the bed, which is about 3.5 times of the cofiring case. Therefore, relatively higher amount of sodium found in the bed in the case of cofiring may be due to most of the sodium being evaporated during coal firing. 12 Sodium (%) Sand Coal1 Coal2 Coal3 Coal+Vinasse Materials tested Potassium (%) Sand Coal1 Coal2 Coal3 Materials tested Coal+Vinasse The total amount of potassium (as K 2 O) added to the bed during the cofiring test was about kg. During three days of coal only testing potassium added into the bed was around 0.025kg, which is almost half of the cofiring case. Therefore, relatively higher amount of potassium found in the bed in the case of cofiring may be due to higher amount of potassium introduction into the bed. However, the amount of potassium found in the bed after cofiring test was around 35 times higher than the three days of coal testing. This suggests that during coal only firing most of the potassium may have been evaporated, while, during cofiring most of potassium stayed in the bed.

24 Gaseous emissions: 24 Nitrogen in biomass is mainly released as NH 3 while N in high rank coals is mainly released as HCN. Ammonia reacts with already formed NO to produce molecular nitrogen thus provide an in situ source of DeNO x Gas constituent Coal only Coal + Vinasse As Correcte As Correct measured dto6% measured ed to (12.7% O 2 ) O 2 (10.3% O 2 ) 6% O 2 CO 2 (%) CO (ppm) NO x (ppm) SO 2 (ppm) There is an increase in SO 2 emissions when vinasse is introduced. This is because vinasse has significant amounts of sulphur in it. However the increase in SO 2 does not reflect the high amounts of 2.1% (5% on dry basis) sulphur in vinasse. Thus it is thought that the sulphur behaved differently in the presence of water or is absorbed by water. Sulphur content in the bed material is considerably higher in co-firing case which indicates that a considerable proportion of the vinasse sulphur may have stayed in bed as CaSO 4.

25 Conclusions: 25 Agglomeration phenomenon is a function of many factors including operating temperature, properties of coal, mixing ratio of coal to biomass it is possible that the problem can be minimised at lower operating temperature or lower mixing ratio of biomass Agglomeration indices can be used as good indicators of agglomeration to pre-evaluate the behaviour of biomass fuels during combustion in a fluidised bed. Accumulative effect of vinasse top up and prolonged test time resulted in the formation of hard agglomerates. Agglomerates grow in size with time. Therefore, it can be concluded that alongside alkali concentration and bed temperature, the rate of alkali input into the bed is an important parameter to control agglomeration. Feeding high alkali material into the bed at a slower rate can help reducing agglomeration problem. Future studies should consider the use of alkali getters

26 26 Acknowledgement: The project was funded by British Coal Utilisation Research Alliance.

27 Vinasse a Potential Biomass Cofiring in a Fluidised Bed M. Akram 1, CK. Tan 2, SM. Thai 2, R. Garwood 2 1 University of Sheffield 2 University of South Wales 10 th ECCRIA conference September 2014 University of Hull