Wood chip biomass boiler emergency shutdown 1

[Schwatzernov]

I have some questions about the operation sequence when the wood chip biomass fired boiler trips and need to shutdown.

When the boiler is tripped as a emergency case and the normal AC power supply is unavailable as it is totally black out, since the fuel feeder is screw feeder, during shutdown process some unburned wood chips are still in the furnace while all motors are tripped e.g. boiler feed pump, circulating water pump, forced drafted fan, induced drafted fan,...etc.how to manage the latent heat in the boiler from the residue fuel in the furnace without causing damage to the boiler ?

During this period, there is the backup power from battery and UPS to supply power to DCS control system, DC for turning gear of steam turbine, DC emergency lube oil pump,...etc.

I checked the NFPA85 regrading to boiler safety code, there is no requirement to run boiler feed pump or induced drafted fan during this period. How to prevent the damage to the boiler tube if there is residue fuel and the drum water level is low ?

Thanks for any suggestions.

 

[davefitz]

you have not defined the type of combustion process- fluid bed or stoker fired.

tripping the fans would slump the bed in a bfb of cfb boiler and the heat transfer to the tubes would decrease quickly as an insulating layer of sand or ash or limestone formed and cooled near the membrane walls, so loss of the circ water pumps is not likely to fail the tubes.

Stoker firing- the fuel is gasified at the stoker grate and most of the heat is liberated above the stoker when secondary air is injected above the stoker and gases are combusted. Tripping the fans will stop the injection of air and the gasification would continue slightly so a buildup of CO is feasible.



"Whom the gods would destroy, they first make mad "

 

[MJCronin]

It sounds like the same problem as that encountered for a stoker fuel coal-fired boiler...... residual heat damage of the boiler with no water flow.

Have you considered a steam driven boiler feed pump ?

The plant will coast down with the BFP providing flow as long as heat is in the boiler

http://coffinpump.com/products/pumps/

http://www.energy-tech.com/article.cfm?id=18376

There are reliability improvements for the plant also.



MJCronin
Sr. Process Engineer

 

[chicopee]

or about an emergency generator dedicated to the boiler's pumps or about two independent power supply lines for the power plant ?

 

[racookpe1978]

The heat in the few wood chips in that no-longer-rotating conveyor is very, very small compared to the thermal mass of the boiler walls, supports, and boiler water mass.

It's a problem of course, but you need to supportr much more than the single wood feed screw.

As recommended above, get an outside diesel generator to provide lights, control, access, computers, and instrumentation, and - a single low flow emergency feed pump. Then you can replace the water as it leaves the boiler.

 

[Schwatzernov]

Thanks for all advices above.

The combustion process is the stoker fired. I still have some questions as below;

1. How about when the plant is subjected to emergency trip ? if the minimum plant safe shutdown requires to continue running BFP, how about other fans such as secondary air fan and induced draft fan, is it necessary to also run these fans together with BFP ?

2. If the minimum requirement for plant safe shutdown requires to also run these large AC motors, why didn't NFPA especially NFPA85 stipulate requirements to have emergency diesel generator to supply these large pumps/fans ? I have experienced many plant with same combustion process where they didn't have diesel generator except DC UPS power supply which the plant design doesn't require to run any these large motors but it is considered as safe shutdown. I agree on the point of view that when plant is emergency tripped, the fuel will be immediately tripped and as such the residual fuel shouldn't be much on the grate and the thermal capacity of the boiler tube should have enough capability to withstand the latent heat without damage even though the BFP is not running.

3. During emergency shutdown period, when the normal AC power supply is totally black out, all motors will certainly tripped by protective relays. The protection system will initiate plant safe shutdown process. However even though we have diesel generator to supply these loads, the generator needs at least 15 seconds to start. What will happen to the boiler if it needs to wait at least 15-20 seconds before the BfP, SAF, IDF get AC power ?

4.I wonder that since the boiler vendor know the type of fuel and they design and supply furnace together with boiler, they should already have consider the design to prepare for the case of residual fuel in the furnace during plant shutdown to avoid the condition that the water in the drum would decrease to the level that unsafe to the boiler by some safety margin.

 

[racookpe1978]

Lube Oil DC backup from the emergency oil pump battery should come on immediately!

15 seconds is not great, but not too long for a feed pump either. If it is the normal BFP and not a small emergency BFP just for cooling water to the boiler, you need the condensate pump to start first.

 

[Duwe6]

Steam-powered Feedwater pump. 'Old School', because it works well and is cost effective. Like MJC said.