Optimisation of solid feeding systems performance

Pulverised fuels

Initially, to evaluate the suitability of existing/available feeding systems, three different solid feeding systems were used and coal/biomass flow rates measured: 1) Rospen feeding system [Rospen Industries (UK) Ltd., Stone-house, Gloucestershire]; 2) Accurate feeding system [Accurate Cutting Services (UK) Ltd., Redditich, Worcestershire]; 3) Hopper motorized feeding system [Energy Technology Centre Rig Lab Facilities, Cranfield University, Cranfield, UK].

Feeding systems 1 and 2 consisted of sealed feeders with a wider spiral and a narrow spiral horizontal auger screws respectively, which fed the fuel into a flow of pressurised air. The screws were an increasing pitch auger screws which were driven by a variable speed motor. Feeding system 3 was fitted with valve cylinder drive motor controlled by a speed controller. The hopper was purged with air intermittently which agitated the fuels to prevent slumping, bridging and agglomeration of the materials. Photographs of these feeding systems (includes feeder type) are shown in Figure 3.24. The solid fuels exited all feeding systems through an orifice and were pneumatically conveyed by a

controlled stream of air through a tube line leading to the combustor. The capacity of feeding systems 1, 2 and 3 provides fuel loading of 20, 10 and 10 kg of feedstock respectively. However feeding system 3 includes two hoppers attached to each other for switching and/or refilling for longer tests.

Figure 3.24 Solid feeding systems used in preliminary study

To evaluate the flow rate of pure or blend fuels performance on these feeding systems, the following work was undertaken;

 El-cerrejon coal (100 %, wt) and El-cerrejon coal: Cereal Co-product (80:20 %, wt) were sieved to 2 mm size (average size of 0.5 mm)

 The feeding systems were each filled with 2.0 kg of fuel (accurately weighed)  Each feeding system was operated at low, middle and full speed

 Feed rate was determined as a function of time  Repeat measurement were made

The average flow rate (n = 5) results for different feeding systems of both fuels used under full speed operation (as an example) are stated in Table 3.16.

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Table 3.16 Results of feeding rate of different fuels using solid feeding systems

a Not/Applicable

It can be seen that Hopper feeding system (3) gave average coal flow rate of 111 g/min (0.9 % RSD) and mixture coal: biomass flow rate of 104 g/min (1.2 % RSD). Feeding system 1 was not satisfactory for coal: biomass blend mixture fuel. The feeder could not deliver a consistent flow due to blockage or sticking in the feed pipe. Feeding system 2 gave poor precision and repeatability of fuel mixture flow rate with 21.3 % RSD.

However the hopper feeding system gave good precision but the amount of fuels delivered were too low (e.g. ~ 6.6 kg/hr Coal:CCP, 80:20 %,wt) and not suitable for co- firing tests in respect to the prediction of combustion products calculations (minimum feed ~ 8.1 kg/hr). Results from low and middle speed performance had difficulties in feeding blend fuels. These problems included: (1) feeding materials agglomerated and plugged the feed hopper and the valve cylinder feeder; (2) feed rates were not reproducible; and (3) feed rates were not consistent during a run.

In conclusion, these three feeding systems studied were not suitable for combustion/co- firing exposures. El-cerrejon Coal (100 %, wt) Coal :CCP (80:20 %, wt) Rospen Feeding 1 Accurate Feeding 2 Fuel Feed (kg) Hopper Feeding 3 Rospen Feeding 1 Accurate Feeding 2 Fuel Feed (kg) Hopper Feeding 3 2 (Flow Rate) 2 (Flow Rate) 2 (Flow Rate) 2 (Flow Rate) 2 (Flow Rate) 2 (Flow Rate) Mean g/min (n = 5) 166 224 111 N/Aa _{179 104} SD 10.05 2.75 0.96 N/A 38.31 1.29 % RSD 6.0 1.2 0.9 21.3 1.2

These problems have been solved by designing and constructing a new feeding system (described in Sec. 3.2.5). The feeding system was designed after a literature review to determine the conditions required to feed materials into a combustor at a constant mass flow rate (and with appropriate quality assurance and control). This is briefly explained as following;

 Literature review: There are only a few literature references for feeding systems. Most laboratory and/or pilot-scale feeders can be classified as either mechanical (e.g. screw feeder), gravity (depend on a smooth flow of material through a controlled opening) or aspirated (feeder suspends the material particles in a gas). The problems encountered in feeding solid materials are formidable. Generally, the irregular shape of the individual solid particles leads to variations in bulk density (due to differences in practical orientation and compaction). Also, the rough surfaces of particles in contact with each other produce varying inter-particle friction so that smooth flow is difficult to maintain. These factors combine to cause common feeder problems such as bridging and slagging. Wilen and Rautalin, 1993 stated the technical requirement of feeder design are: smooth and continuous feed, suitability for handling a variety of bulk material, insensitivity to variation in fuel quality, a sufficient pressure seal against back feeding, accurate feed control, a construction suited to measuring the feed rate, durability and availability. Green et al. 1991 constructed a biomass feeder for use with co-firing biomass and natural gas in a modular incinerator. The system had three hoppers with different size of augers and rotation speed. The injection chute system was connected to the primary combustion chamber in an almost vertical position. A wind box and external fan provided overpressure so the materials were forced into the chamber. From their own experiences, they recommended the following steps when designing biomass feeders for small modular combustor:

- Design hoppers with walls as close to vertical as possible (wherever possible have a negative slope)

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- Use vibrators on walls with sticking tendencies - Use drag chains when the above procedures fail - Use chutes with free fall for injection

 Quality assurance: the following studies/methods were used to ensure the quality of the designed pulverised solid feeding system performance:

- Feed materials: A wide variety of samples were used. Preliminary feeder tests were conducted with different percentage shares of biomass with coal to study the effect of the fuel properties changes on feeding system. The CCP and miscanthus were blended with the El-cerrejon coal at 0, 20, 40, 60 and 80 %, wt. The samples were screened and the sizes were fine particles in the range less than 2 mm (miscanthus, CCP average size of 0.8 and 0.5 mm, respectively). Fuel mixtures were prepared by mixing the appropriate amount of each in a bucket.

- Feeder calibration procedure/results: About 4 kg of sample was placed in the feed hopper (for each experiment) and the feed speed was controlled by adjusting the shaker setting (within a range 10-70 speed controller). The whole unit was placed on a weight balance to indicate the change in the weight of the solid fuel in the hopper with time in order to determine the actual feed flow rate of the solid fuel. The cumulative weight delivered (which was pneumatically conveyed by a constant flow of nitrogen, ~ 30 l/min at 2 bar) was measured at 10 minutes intervals during specific shaker setting speed controller (e.g. 10, 20, etc.) and then the procedure was repeated. The result of feeding pure El-cerrejon coal (100 %, wt) as an example is presented in Figure 3.25

Figure 3.25 Pulverised feeding system feed calibration

0 4 8 12 16 20 0 10 20 30 40 50 60 70 80 C o rr e s p o n d in g fe e d ra te (k g /h r)

It can be seen that increasing the speed of the feed (by the shaker setting controller) increased the feed rate linearly (however, no significant increase between speed of 10 to 20). The degree of reproducibility was very high (SD within a range of 0.15-0.30) as shown by scattering (error bar) data on the graph. The reproducibility decreased below speed control around 20 for such fuel (e.g. pure CCP biomass) indicated the feeding system set start speed at 20. Also, inspection revealed the type of fuel mixes influence the corresponding feeding rate signifying a necessary calibration procedure for each material samples. - Consistency of feed rate: The consistency of feed rates was studied over a period

time (30 minute). Figure 3.26 presents the constancy of feed rate at various sample tests under speed of feed shaking of 30. The results demonstrate that the pulverised feeding system was able to feed at a constant rate. As shown, feeding pure coal gave the highest feed rate (average of 8.3 kg/hr) while the lowest feed rate was from feeding higher shares (≥ 80 %) of biomass for El-cerrejon:CCP fuel mixes (average of 6.3 kg/hr) at the same speed of feed. It could be explained by the difference in density of materials, so the higher density of materials, the higher mass feed rate.

Figure 3.26 Consistency of feed rate of pulverised feeding system over period of time

Pelletised/lump fuels 4 8 12 0 5 10 15 20 25 30 F e e d ra te (k g /h r) Time (min) El-cerrejon coal (100 %, wt) El-cerrejon:CCP (80:20 %, wt) El-cerrejon:CCP (60:40 %, wt) El-cerrejon:CCP (40:60 %, wt) El-cerrejon:CCP (20:80 %,wt)

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feed rate (e.g. 40 %, 50 %, etc.) and the corresponding mass flow rate was measured at 10 minutes time period. Figure 3.27 shows the feed rate calibration of coppiced willow fuel (woodchips, up to 5.4 in size) as an example. The results gave a linear straight-line relationship (corresponding feed rate vs. adjusting speed) with an equation of y = 0.1321x – 1.65 and a correlation coefficient of (R2) 0.9549. Good precision and reproducibility were achieved (SD within a range of 0.7-0.1). Alternatively, the corresponding feed rate using pellets fuel (e.g. OSR straw) was higher (with % RSD significantly lower) than feeding woodchips at the same speed of feed. This can be explained by fuel in pellet form being homogenous than chopped willow, indicating that the larger solid particle size results in a lower feed rate. This proved that feeding miscanthus pellets (individual cylinders of about 7.2 mm long) gave higher feeding rate than OSR straw pellets (individual cylinders of ~ 16.0 mm long) at the same speed of feed.

Figure 3.27 Pelletised/lump feeding system feed calibration