Wood fired Side Walk Heating

[peakbagger]

This will be an interesting local project near me. https://www.conwaydailysun.com/berl...cle_b2f67a26-18bd-11ed-a98a-8be3b0dc0abb.html

I used to work at a large pulp mill at that site. The majority of the mill was demolished (about 130 acres) but the former chemical recovery boiler was converted to the largest wood chip fired power plant in New England. https://www.stantec.com/en/projects/united-states-projects/b/burgess-biopower. The city of Berlin was built around the large mill complex along the Androscoggin River at a series of waterfalls starting in the 1850s. For a time the mills in Berlin were the biggest pulp and paper complex in the world and the biggest employer in the state. When the pulp mill was closed in 2006 it was a major blow to the regional economy, the biomass power plant helped the local economy but it employs far less people and therefore the city is looking for ways to distinguish itself and heated sidewalks and the rebuild of lot of local infrastructure is another step. RIght now the turbine condenser water is just run through a big cooling tower. They have been trying for years to co-locate a greenhouse next to the boiler to use the waste heat but to date it has not happened. Berlin is now the southern entry point for the biggest ATV network in the Northeast. ATVs are allowed to drive almost everywhere in the city on public roads and people from all over New England drive up to access the network all summer and fall and some are buying home which are bargain compared to the rest of New England.

The boiler conversion is interesting, it is a bubbling fluidized bed boiler which is incredibly rare in the US for wood chips. The bottom of the boiler is a deep bed of sand with grid of nozzles at the base of the sand. Heated air is blown through the grid into the sand making it act like quicksand. Wood chips are distributed on top of the sand and drop into the hot sand where the hot bubbling sand and turbulence dries the chips and then breaks the chips into fine powder and then gases which burn above the sand bed and radiate heat down towards the bed to keep it hot. Any heavy particles that start to go up the boiler either drop back into the bed to break down more or finish breaking down in the fireball above the bed. The fuel is low grade complete with rocks that make it past the screening system. The rocks, metals and non burnables drop to the bottom of the bed and work their way to discharge ports. The same technology was used to burn tires at one point in California. The boiler is 11 stories tall with heated air ports along the way so the wood is fully combusted before hitting the superheaters and then the generating bank economizer and air preheater. There is bundle of emissions equipment including a wet electrostatic precipitator on the end of the boiler and it is probably the cleanest wood fired power plant in the country. I got to spend some time inside the boiler doing fuel distribution testing. It is pretty impressive looking up a 10 story shaft at the superheaters. The plant is literally right next to the downtown in the center of town so not that hard to pipe water over to the sidewalk system.

The sad thing is even a relatively state of the art wood boiler cannot compete with cheap natural gas generation. Given the run up in gas pricing currently and expected until the Europe situation settles out they will be making money but without renewable incentives and some value for carbon displacement, if gas prices go back to old levels, they lose money. Their fuel is a mix of waste from sawmills and chipped low grade wood from logging operations. Unlike other renewables, they can run 24/7 and can store well over 30 days of fuel on site. The ash from the operation is spread back in the woods and in demand by farmers.

 

[EbS-P]

Thanks for the post!

 

[Ashful]

Can't be less reliable than electric sidewalk heating. Not only are those systems so often found malfunctioning when needed, but winter storms often knock out electric service, right when you need that system to work.

 

[vbu]

That is fascinating to read how that boiler works. When the wood chips are vaporized, there's still ashes right? How does that get separated out of the sand bed?

 

[peakbagger]

In order to figure out the how the ash gets separated you need to visualize the boiler. The hot gases goes up to the top of the boiler fully burning as it goes until its fully burnt. Then through the superheater pendants (rows of tubes that superheat the steam) and exits out the side of the boiler. The hot gases then go down through a generating bank (where the water in the boiler turns to steam) and then takes a 180 and goes up through the economizer (where feedwater is preheated to near boiling). Heavy ash being carried along will collect in the bottom of the generating bank at that 180 bend and there are conveyors and rotary seals to discharge the hot ash (more on that later) out of that low spot. The downsteam wet electrostatic precipitator takes out any remaining dust down to micron level. Unlike a natural draft home boiler, there are various large fans moving and pulling the air and flue gas through the system. Every boiler configuration is different, and the Berlin unit is a retrofit of a black liquor recovery boiler in an existing building so they had to fit it in where it would fit.

The sand bed is 3 or 4 feet deep of sand when fluidized with a grid of air nozzles under the sand. Wood chips are distributed on top of the bed from side ports on the boiler. Its a balancing act, the air flow through the bed has to be enough to fluidize the sand but not too much to prevent chips from sinking in the bed. In theory with uniform chips, the chips sink down into the hot sand and turn to dust so there are no ash chunks, its just a very fine dusty powder. Everything needs to be balanced and adjusted correctly. The fuel also needs to be uniform in size and quality control on low grade fuel is not great. The combination of poor balancing and non-uniform fuel can lead to the fuel not dropping down into the sand bed to be broken up into powder, these non-uniform typically large surface area lightweight particles never sink into the bed and they can be carried up through the boiler and only partially burn, the volatiles on the surface are burned but the carbon core stays intact. This is called char and if things are out of control, this char clogs up the ash removal. There is always some mixed in with the ash and its separated and reinjected in the boiler to burn but too much and its a problem.

There are only a limited number of people in the country capable of tuning this equipment. Most are retired or near retirement and not a lot of opportunity or interest in training new folks. I used to tune simpler biomass boilers on occasion but do not represent I could do the initial tune up of bubbling bed B&W developed the technology 30 years ago and was capable of building the Berlin plant but when it came to balancing and tuning it, they had significant issues upon startup. Once they got through those startup issues, the plant has been good neighbor, its rarely in the local news which is surprising given its location adjacent to downtown and couple of nursing homes. Then again anyone who has lived in city in the long term remember the former pulp mill which made a far more significant impact as the entire valley had the smell of the kraft process which was a combination of skunk and rotten eggs. To locals it was the smell of money but to outsiders it stunk. There were ball fields next to the mill and Berlin had a well known home field advantage over visiting teams.

It has been close to 20 years since the pulp mill closed down but to many in New England, they still think its stinky pulp mill town which has kept housing prices low but the ATV folks coming to visit realize they could pick up homes in the city for dirt cheap and retire up here with what the difference in price they could sell their homes elsewhere. The prices of homes has doubled in town yet the median sales price is still quite low and not many places where someone can drive their ATV to work or the grocery store with a 1000 miles of trails starting in town all the way to the Canadian border.

 

[sloeffle]

I used to tune simpler biomass boilers on occasion but do not represent I could do the initial tune up of bubbling bed B&W developed the technology 30 years ago and was capable of building the Berlin plant but when it came to balancing and tuning it, they had significant issues upon startup.

B&W, ( Babcox and Wilcox ) that's an acronym that I haven't heard in awhile. A friend of mine from grade school, his dad worked for B&W at their headquarters in Akron. I believe they were / are big into nuclear boilers also.

How do these boilers deal with the dust from the chips ? When I think of dust and fire I think of explosions.

 

[peakbagger]

B&W is still around but definitely lower profile. They were sloppy with their trademarks over the years and there are several variations of the name floating around. They and their corporate parent got dragged into the asbestos litigation 30 years ago and that pulled a lot of money out of the company that could have been spent on development of new technologies. They still make nuclear reactors for the Navy and have line of smaller boilers. When they built Berlin their claim to fame was they had bought many of the suppliers of support equipment for power plant so they could supply a complete plant. They did the steam side but the turbine was from other firms.

The chips come in quite damp and they keep them turned over,. Berlin being in a valley along a river tends to be bit damper than elsewhere as dew tends to form at night so dust is not a major issue except in very dry spells. The have a bottom pile chip system, raw wet chips are dropped from a long overhead conveyor about 5 stories high to form a long pile of chips. There is long pair of underground tunnels under the pile and chips are extracted from the base of the pile along the length of the tunnel and then dumped on a long conveyor belt which is then routed up to the boiler building feed bins. Chips fires usually start at the bottom of wood piles that are stagnant so this keeps the chips under the pile moving.

 

[ABMax24]

We also have a biomass power plant in the middle of the city, albeit much smaller at 18MW. It uses waste wood from lumber production for fuel. Steam is also pulled off the backside of the turbine at 60lbs to heat the lumber kilns onsite. There has been some talk a couple times of using some waste heat to heat the local college a few blocks away. Although a couple main roads and businesses in the way seem to make it cost prohibitive.

This plant is the only one I'm aware of that uses a steam to air cooling tower to condense the steam. Which has posed immense challenges when the round tube filled with water and froze in -40. This has been rectified to some extent with oval tubes, allowing the tubes to freeze at least a few times before rounding out and then splitting.

I've always wondered if a snow melting pit could be created to utilize the waste heat, instead of hauling snow removed from roadways across town to the city "snow mountain". Cool to see it being done elsewhere.

 

[peakbagger]

We also have a biomass power plant in the middle of the city, albeit much smaller at 18MW. It uses waste wood from lumber production for fuel. Steam is also pulled off the backside of the turbine at 60lbs to heat the lumber kilns onsite. There has been some talk a couple times of using some waste heat to heat the local college a few blocks away. Although a couple main roads and businesses in the way seem to make it cost prohibitive.

This plant is the only one I'm aware of that uses a steam to air cooling tower to condense the steam. Which has posed immense challenges when the round tube filled with water and froze in -40. This has been rectified to some extent with oval tubes, allowing the tubes to freeze at least a few times before rounding out and then splitting.

I've always wondered if a snow melting pit could be created to utilize the waste heat, instead of hauling snow removed from roadways across town to the city "snow mountain". Cool to see it being done elsewhere.

There is an idled biomass plant of similar size in Sherman Mills Maine. I looked at moving it a few times. Its got a big air cooled condenser. Works great in winter with similar conditions but not so good in the summer as its working with dry bulb instead of wet bulb temps. I helped test a large combined cycle plant in Brampton Ontario once, it was 990 MWs with huge air cooled condenser. They are pricey and as mentioned not so good in summer.

These plants put out fairly cool water. Depends on the design but could be 50 degrees F. Maybe good for keeping a greenhouse warm but a lot of water needs to be moved to keep things from freezing.