Services to Technology Providers

Transcription

1 Services to Technology Providers Training and Capacity building activities with HP boiler manufacturers Session 3: firing systems Trainer: Frans Baltussen Date: first half 2015

2 Overview combustion technology for biomass Mostly used for biomass

3 Overfeed stokers Reciprograting grate: Can burn everything Water cooled inclined vibrating grate: Straw which has a large bulk and it very light Travelling grate Can be used for rice husk, nut shells, however very rear

4 Spreader stokers In case of volatile and not to light biomass then spreading is preferred above grate In order to make the grate and boiler smaller Travelling grate In case of higher ash content Stationary grate (vibrating and pinhole) In case of low ash content

5 Spreader stokers High amount of overfiring to mix the flue gasses and particles Fuel uniform spread Air distribution uniform

6 Spreader stokers Fuels are uniform thrown / spread over then grate area Fines ignite and burn in suspension The coarser particles fall on the grate and burn on a thin fast burning bed. The fuel and air needs to be uniformly distributed over the bed A part of the air (overfiring air) is injected above the grate into the furnace to mix the unburned gasses and particle with air The furnace is made large enough to ensure complete combustion before entering the convection part.

7 Sizing of fuels for spreading Fuels to be spread needs to be not: to fine to large Very small Will be blown easy away Too large pieces falls on grate and block it Sizing for wood and bark shredded

8 16700 m Fuel feed Multi drum Feeders for bagasse feeding: Proper feeding is very important to guarantee uniformity Also parts should be not stick together and form large lumps. We need loose material Chutes/silo with 2,5 min storage time. Including sightglasses on 3 heights for viewing the flow of bagasse. Chute angle not more then 30 degree from vertical axes. Should be opening towards feeder Multiple drum feeders Capacity sufficient for running 100% load when 1 feeder is stopped

9 Multi drum Feeders for bagasse feeding: Multi drum feeders are used and it is experienced that the bagasse amounts can be controlled accurately with this type. Twin rollers for proper bagasse control. Variable speed However, important is to keep the centerline of the twin rollers such that the bagasse will not cause a high pressure on the rollers. The speed of the electromotor of the twin rollers needs to be high enough for proper cooling Drum running at high speed for splitting the lumps in fines, Lumps will fall on the grate and pile up which result in poor combustion

10 Large storage silo Drum Feeders plus screw conveyor for bagasse feeding: Used in case of requirement for high bagasse storage quantity i.e. 10 minutes. Silo needs to be polished inside to lower resistance to avoid roof forming. Also opening angle (2 degree) should be sufficient. The drum needs to be more to the outer side to reduce the pressure from the bagasse column The teeth on the drum needs to be stronger due to the high friction caused by the weight of the bagasse. Stones and metals needs to be avoided

11 Drum Feeders plus screw conveyor for bagasse feeding: Bridge forming Above feeder Drum feeder with variable speed to control the bagasse flow No knives installed No provision to split the lumps in small parts

12 Pneumatic bagasse spreader spreader bagasse Counter weight Protection against back fire only for dry materials 2 Spreader fans. One as standby for supply of constant flow and pressure to the spreaders in order to guarantee proper spreading of bagasse over the depth of grate in all load cases. To improve the spreading cold air is used instead of hot air. The pressure is 75 mbar at fan outlet. Need approx. 50 mbar at spreader Approx 2000 Nm3/hr per spreader for maximum 1500 mm grate width Maximum spreading depth approx. 7 m Spreader width 40% of grate section width air Rotating damper Continuous Rotating valve in each air line to spreader for equalizing the spreading of bagasse over the fuel grade Set the air opening Plate for setting the spreading angle E E To bagasse spreaders

13 Type of spreader stokers 1. Fuels with low ash content ( less then 5%),high melting points. Like: wood, bagasse, bark, corn cob, Can be fired on a stationary grate like : Dumping grate Water cooled pin hole grate Water or air cooled vibrating grate 2. Fuels with high ash content ( more then 5%),high melting points. Like: rice husk Can be fired on a travelling grate

14 Dumping grate Split in 2 sections Pneumatic or hand lever All horizontal bars connected with lever for dumping Cast iron grate bars with venture openings for air supply installed on a horizontal bar which is supported in bearing at both side in frame

15 multiple dumping grate sections More sections besides each other

16 Dumping grate example 1 The dumping grate has a lot of moving parts and requires a high maintenance activity Support structure can be damaged due to fire in hopper Air distribution is poor due to large opening and low pressure drop

17 Dumping grate example 2 The dumping grate has a lot of moving parts and requires a high maintenance activity Support structure can be damaged due to fire in hopper Air distribution is poor due to large opening and low pressure drop

18 Procedure dumping Important is to dump in a proper way: Divided in several sections in order to clean the grate during operation Air and fuel supply to one section will be stopped and ash of section will be dumped. The other sections will continue with firing. Very important that the other sections will be increased in firing rate in order to avoid any steam flow loss. After dumping all ash should be removed from hopper before re-opening the air supply in order to avoid any firing in hopper The cycle of dumping takes some minutes.

19 Water cooled vibrating grate The grate is water cooled so that the supported structure underneath will be protected against overheating. The ash removal is by discontinuous slightly vibrating the grate in the direction of hopper The grate needs separate water supply for cooling or can be connected with natural circulation system with flexible piping connections.

20 Water cooled pinhole grate This system is worldwide applied for bagasse firing and used up to 220 tph steam capacity The grate is water cooled so that the supported structure underneath will be protected against overheating. The grate tiles supported on water cooled tubes The ash removal is by discontinuous by blowing steam through nozzles installed in the grate as a pin.

21 Pinhole grate as per design of MBTE south Africa Larger cast steel plates Water cooled tubes

22 Example of cast iron grate section of pinhole grate as per Joh Thompson design The cast iron grate sections protected with stainless steel plates Cast iron parts

23 Example watercooled pinhole grate as per design of TES from USA installed in Jamaica for 113 tph steam capacity Cast iron grate sections

24 Steam cleaning system Pinhole grate

25 Advantage watercooled grate against dumping grate A solid and simple design no mechanically moving elements water cooled tubes as supporting structure connected with boiler natural circulation system No special hopper required under grate Due to absence of hopper less height required Even air distribution over grate system by increased pressure drop and proper designed air distribution box On line ash removal system. No interruption in steam supply Air temperature can be far above 200 C. No limitation.

26 Travelling grate Re-firing of unburned particles Cashed separated from flue gasses possible Continuous ash removal Rotation direction Very slow speed Air supply

27 Large travelling grate sections Section maximum5, m width and 6,5 m depth Larger ones split in 2 sections with at each side a drive. The drive is normally hydraulic

28 Hydraulic drive

29 Need of special parts for smooth operation and to resist the wear and tear Special shaped Grate castings in alloy High alloy steel shafts and sleeve bearings High strength steel hardened ductile iron sprockets Forced steel chain links, case steel hardened pins and rollers. Supports and rails also hardened

30 The grate and furnace heat release The loading of the grate and furnace are important factors for firing the biomass. When there is high moisture we will need more time for drying the fuel before it ignites. How more fine it is how easier to dry. So the grate loading should be lower When there is a high moisture content it is important to supply the primary under the grate as high possible. The air distribution over the grate should be as equal as possible in order to lower the excess air as far as possible The upwards flue gas velocity should not be too high to ensure that particles will not be blown out to fast from furnace. The slagging and clinkering should be avoided There are many figures mentioned as heat release for grates by supplier which are confusing

31 Recommended heat release for grates with spreader stokers based on GCV value of fuel

32 Excess air in relation to moisture content of fuel

33 Furnace heat release Total volume of furnace should be such that residence time is minimum 3 seconds Velocity is lower and need more time for complete combustion Velocity is approx. same as flue gas and need less time for complete combustion Experience shows good burn out of 1 to 3% with a furnace heat release of 186 to 230 KW /m3 on GCV value. However, the overfiring system should then also made proper.

34 Overfiring system The distribution of air and fuel including sizing is never equal. There will be CO strains and areas where to less air is available for further combustion. Further the biomass has lighter particles which will be blown out easy from furnace. Mixing of all flue gasses and particles with air Therefore biomass firing requires always high over firing air required up to 50%

35 Overfiring system in bagasse fired boilers Openings in furnace walls of approx. 13 * 50 mm Overfiring system in furnace of bagasse fired boiler

36 Overfiring system with small jets at multilevel Originally from stoker fired coal boilers Ports on front and rear wall The small jets have not much penetration by which the mixing is not optimum. Channeling occurs

37 Some flow modelling of small air jets No deep penetration

38 Studied Alternative type of air jets in a cyclonic effect Test done by creating a cyclone effect in furnace installed above fuel distributors Intended to create swirl Could be made of larger capacity The larger jets had higher momentum However. Poor coverage still Also channeling Poor mixing Higher excess air was required Imbalance in gasses leaving furnace

39 Results are: Studied effect with large jets from front and rear wall Installed above fuel distributors Large pipes of approx. 100 m in one layer Outlet velocity approx. 40 to 50 m/s Could be made of higher capacity and momentum Interlaced or opposed jet arrangements Good coverage is possible with interlaced However in arrangement interference with fuel distributors, ah re injection and furnace openings Expansion at 320 C Expansion at 200 C Need distance and space For installation overfiring piping

40 Large jets from front and rear wall

41 Studied effect with large jets from side walls The effect was also positive However when furnace is 10,5 m width and 6 m deep then it is not possible to get a deep injection of air

42 Mass flow fuel feed One of the first firing grates: No moving parts The fuel slides down No control over load. Runs at full load low grate heat release of 800 KW /m2 Excess air needs to be higher and zoned Watercooled grate used for bark firing

43 Example of boiler with straw firing Water cooled vibrating grate Special Inconel materials in superheaters

44 Example Travelling grate Not much volume in silo available for large volume biomass Needs zoning of air supply

45 Reciprograting grate Can fire all type of fuels. High wear and tear Higher excess air Low grate heat release Air zoning Special arch with overfiring air to mix all flue gasses Recirculation of flue gasses used to keep the excess air low and air velocity for each fuel approx. the same

46 Pictures of reciprograting grate Overheated parts Not moving

47 Reciprograting grate in case of garbage firing

48 Atmospheric bubbling fluidized bed system Free board Over firing system Air spreading 850 C Velocity approx. 2 m/s This system operates at lower temperature of 850 C More suitable for biomass with high potassium The temperature is controlled by recirculation of flue gasses or in bed tubing part of evaporator Unburned losses very low due to excellent mixing of fuel.

49 Atmospheric bubbling fluidized bed system 500 mm bed height Special high alloy nozzles with sufficient pressure drop for even air distribution Large spaces so that larger parts can be drained during operation

50 Thank You for Your Kind Attention