STUDIES ON PIPELINE DISPOSAL OF FLY ASH AND FLY ASH-BOTTOM ASH MIXTURE SLURRY AT HIGH CONCENTRATIONS

Transcription

1 STUDIES ON PIPELINE DISPOSAL OF FLY ASH AND FLY ASH-BOTTOM ASH MIXTURE SLURRY AT HIGH CONCENTRATIONS Implemented by: Institute of Minerals and Materials Technology (Council of Scientific and Industrial Research) Bhubaneswar For Fly Ash Unit, Department of Science & Technology (DST), New Delhi 2011

2 C O N T E N T S CHAPTER PARTICULARS Page No. Executive Summary 1. Previous Work 1 2. Objective and Scope of the Present Investigation 7 3. Characterization Studies on the Power Plant Ash Samples 3.1 Particle size distribution of ash samples 3.2 Particle density of ash samples 3.3 Chemicals analysis of ash samples 3.4 Effect of slurry ph with mixing time 3.5 Maximum static settled concentration of fly ash and fly ash- bottom ash mixture slurry Effect of addition of bottom ash fractions on maximum settled concentration of fly ash slurry Rheological Studies on Fly Ash and Fly Ash- Bottom Ash Mixture Slurry Samples 4.1 Description of the equipment 4.2 Experimental procedure 4.3 Results and discussion Rheological behavior of fly ash slurry samples Rheological behavior of fly ash- bottom ash mixture slurry samples Effect of addition of bottom ash fractions on viscosity 5. Pilot Plant Studies In High Concentration Test Loop 5.1 Commissioning of the high concentration slurry test loop at IMMT Commissioning of the Plunger Pump Commissioning of the Progressive Cavity Pump Commissioning of Bucket Elevator, Slurry Mixer and Screw Feeder etc. 5.2 Description of the test loop set-up 5.3 Experimental procedure 5.4 Results and discussion Head loss of water Head loss of fly ash slurry Head loss of fly ash- bottom ash mixture slurry Effect of addition of bottom ash on slurry head loss 5.5 Conclusion 6 Development of Head Loss Model for Fly Ash and Fly Ash Bottom Ash Mixture Slurry 6.1 Prediction of head loss for fly ash slurry samples 6.2 Results and discussion

3 6.3 Prediction of head loss for fly ash ash-bottom ash mixture slurry samples Prediction of head loss by power law model 6.4 Conclusion 7 Optimization of HIgh Concentration ESP Ash and ESP- Bottom Ash Mixture Slurry for Pipeline Transportation 7.1 Head loss of fly ash slurry and fly ash-bottom ash mixture slurry 7.2 Solids Flow rate 7.3 Specific Power Consumption 7.4 Optimum transport concentration Strength, Settling Properties and Load Bearing Capacities of High Concentration Ash Mixture Slurry 8.1 Compressive strength 8.2 Load bearing capacity, settling and erosion of settled mass of ash mixture slurry Appendix-I Computer Programme to Evaluate Pipe Roughness Using Water Run Data in 50 MM Diameter Pipe Appendix-II Computer Programme to Predict Head Loss of Fly Ash and Fly Ash-Bottom Ash Mixture Slurry Using Power Law Head Loss Model References

4 EXECUTIVE SUMMARY Coal is the only natural resource and fossil fuel available in abundance in India. Consequently it is used widely as a thermal energy source and also as fuel for thermal power plants producing electricity. India presently has installed capacity of MW of thermal generation (as on March, 2007) constituting 65% of total installed capacity. With the boom of population and industrial growth, the need for power has increased manifold. Nearly 73% of India s total installed power generation capacity is thermal, of which 90% is coal-based generation, with diesel, wind, gas and steam making up the rest. To meet the projected demand in , additional capacity requirement of about 7,800 MW is required to be added in 11 th Plan ( ).Thermal power generation is expected to continue to dominate in the power generation scenario. The problems associated with the use of coal are low calorific value and very high ash content. The ash content is as 55-60%, with an average value of about 35-40%. Due to low calorific value and ash content (up to 40%) of India coals, these produce massive amounts of coal ash to the tune of 5-6 tonnes per MW per day. On the other hand, many power stations in developed countries produce far lower quantum of ash, about tonnes per MW per day due to high calorific value and lower ash content (around 10%) in their coals. Fly ash constitutes nearly 80% of the total ash generated in thermal power plants. The bottom ash amounting to around 20% is separated in the furnace and being granular in form does not pose much pollution or disposal problem. The thermal power stations in India produce more than 150 million tonnes of coal ash per year. The conventional lean phase ash slurry pipeline systems presently adopted by these thermal power plants require more then 25, 000 hectares of land including agricultural and forest lands for disposal of the slurry. In the process more then 630 million cubic meters of water per annum are consumed (Sen and Kumur, 1995; CPCB report, 2003; Mishra, 2004; Thermal Power India Conference 2, 2007). Due to settling nature of the coarse bottom ash particles the transportation are generally done under turbulent conditions incurring higher pressure drops. Also the anticipated bed load transport of such slurries may not be ignored. These cumulative effects of turbulent flow and bed load transport conditions result in erosion problems requiring frequent replacement of piping and valves thus affecting pipe economics. A number of studies on rheological and pipe flow behaviour of coal ash slurry at low to medium concentration(c w =10-50% by weight) have been reported in literature (Iwanami & Techibana (1970), Wright & Brown (1979), Verkerk (1982), IMMT s Internal report, 1987; Parida et al., 1988, Verkerk(1985) and Verkerk et al.(1993), Lazarus and Sive (1984), Vlasak et al.(1993). The hydraulic transportation of fly ash- bottom ash mixture slurry at high concentration is very scanty in literature. Varkerk (1982) has conducted some very useful investigations on the hydraulic transportation of fly ash of South African power stations. A100 m long pipeline- test loop with a pipe dia of 100 mm was used with a reciprocating piston pump for high concentration pastes of 65 to 70% by weight. The slurry head loss results obtained by Verkerk from the loop tests were consistent with the typical

5 homogeneous slurry characteristic curves. A kink in the curve was observed when the head loss data plotted in logarithmic scale at a flow rate of about m 3 /hr which corresponded to the visual observation of the unset of deposition on the bottom of the pipeline. Tests at high concentrations indicated presence of a yield stress which was due to the non- Newtonian behaviour of the slurry. The slurry changed from fluid like character to one that tended to form sliding planes at 68-69% weight concentration. The pumping of high concentrarion paste above 69% concentrarion incurred high pressure loss. However it was observed that a paste having a fraction of bottom ash incurred substantially less pressure drop. But Streat (1986) in his paper has raised the certain questions on the decrease in pressure drop in presence of coarse particles, terming the phenomenon as surprising and needing explanation. Bunn et al.(1990,91) have undertaken a number of studies on high concentration slurries of fly ash from some of the Australian power stations. The fly ash slurries were found to be time independent and exhibited non- Newtonian behaviour for solids concentration greater that 60%. For solids concentrations close to 60%, a Bingham model closely fitted the measured curve. At high concentration, the rheology curve deviated from Bingham plastic model. At high strain rates, the agreement between the data and Bingham model was good but below a critical shear rate, pseudo- plastic exponential model closely fitted the measured data. Studies on the dense phase hydraulic conveying of fly ash at Vales Point power station in Australia have been reported by some authors(bunn, 1989; Bunn et al., 1993).It was indicated that the optimum concentration of slurry in the pipeline would be around 60% by weight. Heywood et al.(1993) assessed the flow characteristics of pulverized fuel ash slurries at high concentration in the range of 68% to 70% weight concentration, the d 50 of the sample being 38 microns. The authors established pressure loss- flow rate relationship by conducting experiments in a 72 mm ID, 8.34m long polypropylene plastic pipe. A power law model was fitted to the lower shear rate range data appropriate for prediction of frictional losses in 150mm and 200 mm dia pipelines over a distance of 8 km. The power law exponent n was found to be largely constant at around 0.46 for two types of pulverized fuel ash investigated. Singh et at.(1998) investigated the rheological properties of fly ash-water slurry at obtained from three different sources. The difference in the behaviour of the fly ash water slurries from different power stations was attributed to the nature of mineral matter in the coal, size distribution of the pulverized fuel and the combustion conditions in the boiler. He concluded that the development of high concentration slurry disposal system is very much dependent on the source of fly ash. Parida et al.(1995,1996) investigated the rheological and pipe flow behaviour of ash samples from Talcher Thermal Power Station, Orissa. The viscosity of the fly ash slurry was found to be Newtonian in nature upto a solids concentration (C w ) of 50% and above this concentration the viscosity is non- Newtonian. The power low law pseudo- plastic model correctly characterizes the non- Newtonian viscosity of fly ash slurry. By using appropriate Newtonian and non- Newtonian models the head loss of the slurry were predicted. It was indicated that the transportation cost of fly ash slurry reduces

6 drastically if the same is transported at high concentrations instead of low concentrations. Ward et al.(1999) and Hiromoto et al. (2001) investigated the hydraulic transportation of dense fly ash slurry using a stabilizing additive to prevent sedimentation of fly ash particles. But the additions of stabilizing additive increased the slurry viscosity for which a dispersing additive was to be used to slove the problem. Biswas et al. (2000) investigated on various solid properties of fly ash and bed ash collected from Indian thermal power plants. It was indicated that the properties of different ash samples (both fly ash and bed ash) vary over a wide range. The rheological properties of the ash slurries at different concentrations exhibit a wide variation and thus the design of ash disposal pipe line is very much dependant on the rheological parameters from optimization of energy and water consumption point of view. Gandhi et al. (2001) evaluated the performance characteristics of centrifugal slurry pumps using fly ash and bed ash samples of an Indian thermal power plant. The rheological characteristics of fly ash and bed ash indicated Newtonian behaviour up to a solid concentration of 30% and 50% by weight respectively. Beyond a solid concentration of 30% by weight, the fly ash slurry exhibited Bingham fluid characteristics. Therefore the pump characteristics were very much affected due to change in flow behaviour of fly ash and slurries at higher solids concentration. Parida et al. (2003) using the flow and head lose characteristics of high concentration fly ash-bottom ash mixture slurry carried out the pipeline design for hydraulic back-filling of coal mines by considering the effects of solids concentration, bottom ash fraction, pipe diameter and flow velocity. A design chart was formulated to determine the pipe size and design transport velocity for a given backfilling rate and given H/L ratio i.e. length of vertical section of pipe (m) to length of horizontal section of the pipe (m). Senapati et al. (2004, 2005, and 2006) conducted the static settling studies, rheological behavior and design scale-up procedure for predicting head loss for fly ash samples collected from NALCO, Anugul, India, Studies indicated that the maximum static settled concentration (C w max ) of around 69% can be achieved for the slurry and hence a slurry in the range of 60-67% by weight can be prepared for transporting through pipelines. Verma et al. (2006) conducted pipe loop tests in 50 mm NB mild steel pipe of 30 m length to establish the pressure loss across 90 0 horizontal circular pipe bend using fly ash slurry at higher solids concentration in the range of 50-65% by weight. It was indicated that the relative pressure drop across the pipe bend increases with increase in velocity and approaches a constant value at high velocity in the studied range of concentration. The bend loss coefficient is very much dependant on slurry Reynolds number. Further the increase in bend permanent loss with concentration is more at lower velocity than at higher velocity. The additional pressure loss due to flow

7 disturbances in the downstream of the pipe bend is insignificant for highly concentrated ash slurry. Vlasak et al. (2007) investigated the pipeline transportation of fly ash-bottom ash slurry at solid concentrations in the range of 23.4%31.2% and indicated that by adding bottom ash fractions the ash mixture slurries reached slightly higher maximum concentration. The addition of coarse bottom ash particles reduces the hydraulic gradient in the laminar region. This effect was explained due to destruction of the aggregates of fine ash particles present in the slurry and liberation of water originally present in the aggregates thereby the slurry becomes peptized and liquefied. This helps in reduction of yield stress of higher concentration fly ash-bottom ash mixture slurry. This effect depends on the total slurry concentration, velocity, the proportion of fine and coarse particles and on the colloidal particle content. The flow behavior of the slurry was approximated by Yield-power law model. Since the addition of bottom ash cause a decrease of head loss predominantly in the laminar region, therefore it is possible to use a lower operational velocity for the stabilized slurry, which brings a significant reduction in head loss. The effect of addition of Sodium hexametaphosphate at 0.1% concentration (by weight) as an additive on rheological flow behavior of ash slurry was studies by Seshadri et al. (2008). By using the rheological data the head loss of fly ash slurry at high concentrations (Cw=60%,65% and 68%) were predicted using a Bingham plastic model. Since the additive modified the rheological behaviour of the ash slurry a substantial reduction in head loss and energy consumption could be achieved. Chandel et al. (2010) investigated on rheological and pipe flow characteristics of mixture of fly ash and bottom ash at high solids concentration (>50% by weight).the pipe loop tests conducted in a 42 mm diameter pipeline of 50 m length indicated that the pressure drop for any given solid concentration increases with increase in velocity and at any given flow velocity, pressure drop increase with increase in solid concentration. The pressure drop predicted for a 42 mm diameter pipeline using rheological parameters was in good agreement with the experimental data. The pipeline transportation of ash slurry with wide size distribution material at high concentration is the way of the future and requires careful investigation in pilot scale. The increase in concentration of fine ash particles in the carrier fluid usually changes the flow behaviour of the slurry. Again the transport of coarse fractions of bottom ash particles in this slurry makes the flow behaviour more complex. For design of such slurry pipelines the head loss/pressure drop can be predicted from rheological data. But the presence of these coarse particles makes the rheological data alone insufficient to adequately predict the slurry behaviour for many important operational conditions. From the previous studies reported in this section the following points are highlighted. (i) Fly ash can be transported at much higher concentrations than being practiced for disposal purposes in the power plants in this country.

8 (ii) (iii) (iv) (v) (vi) (vii) The rheological behaviour and pipe flow characteristics of fly ash slurries are very much dependent on the particle size range of fly ash as well as on the type of coal generating the fly ash. Depending on the particle size and solid concentration the fly ash slurries exhibit Newtonian or non-newtonian behaviour. The transition between Newtonian and the non-newtonian flow varies depending on the properties of specific fly ash samples. Prediction of pipeline head loss of fly ash slurry at higher concentrations has not been found satisfactory. A detailed investigation on addition of bottom ash to fly ash affecting rheological and pipe flow characteristics is requires to be carried out. The optimization of high concentration ESP ash (fly ash) and ESP-bottom ash mixture slurries are to be carried out for designing of commercial ash slurry pipelines. The strength, settling properties and load bearing capacities of high concentration mixture slurry at the disposal site requires careful investigation. The present investigation aims at establishing the technical feasibility of fly ash and fly ash bottom ash slurry handling system with pipeline transportation of ash slurry at high concentration as a possible alternative to solving the pollution problems in the state of Orissa. Orissa is bestowed with good amount of coal reserves and majority of thermal power plants are located in Angul, Talcher, Sambalpur, OPGC,JSL to name a few are generating huge amount of ash which is to be either utilized or disposed off in an environment- friendly manner to prevent pollution. All these thermal power plants employ short distance ash slurry disposal pipelines which operate at low concentrations of 10-20% solids by weight. This has made the disposal practice highly uneconomical since a lot of electrical energy is consumed for pumping 80-90% of water in the ash slurry. Moreover, the excess water spilling over from the ash disposal ponds cause water pollution in the adjoining localities. On the other hand, transportation of fly ash slurry at high concentrations (above 50% by weight) will drastically reduce the consumption of water as well as power and will make the process highly cost effective. Apart from this, the ash slurry at high concentrations has a very good scope of application for backfilling the coal mine cavities and in this process, the practice of converting large areas of cultivable land into ash disposal sites can be avoided. In the above context, a detailed investigation was carried out to establish a viable technology for pipe line transportation of high concentration ash slurry with specific reference to there power plant ash: NALCO captive power plant, Angul, Ib Thermal Power Station (OPGC), Jhrasuguda and JSL Captive power plant, Jajpur, Accordingly, the broad objective and scope of the investigation are given as follows:

9 Objective: Studies on pipeline Disposal of fly ash and fly ash- Bottom ash mixture slurry at High Concentration Scope: a) To conduct studies on dense phase fly ash and fly ash-bottom ash slurry to determine its mining, pipeline transportation and filling characteristics and establish the viability of mine backfilling using such slurry. b) To achieved high concentration of 60-70% by weight for coal ash slurry by adopting appropriate mixing technique. c) To determine rheological characteristics of slurry by rotational viscometer with and without binding agents. d) To study the strength characteristics of the solidified coal as. e) To investigate the pipe line flow behaviour of the slurry in the pipe test loops of 50 mm/100 mm diameters and the relevant head loss model for scale up design. f) To study the settling characteristics of high concentration fly ash slurry in an open area (simulation ash pond) g) To study load bearing capacity and erosion of settled/dried high concentration fly ash slurry disposed. h) To study/generate data and optimize high concentration slurry disposal for ESP ash and ESP ash & bottom ash mixed together in the ratio of their generation at the thermal power stations. ***********************************

10