The theory is that a pulverized coal (PC) power plant could be converted to use torreﬁed or steam-treated biomass instead of coal with very little CAPEX on modifying the existing plant infrastructure. The Canadian energy utility Ontario Power Generation (OPG) is the first to make a commercial conversion commitment and, in so doing, can soon verify if and to what extent this theory holds true.
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There have been numerous announcements over the last 18 months or so about biocoal, black pellets, brown pellets, torreﬁed, and steam-treated biomass projects. Leaving aside the technical diﬀerences between torreﬁed and steam-treated biomass and that suﬃcient commercial volumes have yet to appear, utilities it would seem have shown little interest apart from various tests and trial burns.
Undoubtedly this is about to change as the Canadian energy utility Ontario Power Generation (OPG) nears completion of modiﬁng its pulverized coal-ﬁred Thunder Bay Generating Station (TBGS) to use ”advanced wood pellet” fuel instead, more specifically steam treated. Once TBGS is commercially available early next year it will be the world’s ﬁrst and largest commercial plant to use such a fuel.
Repowering Thunder Bay
Located on the north western shore of Lake Superior in the City of Thunder Bay, Ontario TBGS is the oldest of OPG’s ﬁve thermal power stations. It was originally commissioned in 1963 as a single 100 MW bituminous coal PC unit.
In the early 1980s two 163 MW PC units were added (units 2 and 3) also using lignite coal and unit one was retired shortly thereafter. Since the economic downturn during the late 2000’s TBGS’s role has been to provide backup and peak demand power.
The goal of the TB unit three modiﬁcation project is to use the existing fuel handling and powerhouse equipment as required to produce dispatch-able energy with the same capabilities as TBGS on coal but with the beneﬁt of reduced emissions, said Faron Rollins, Project Director for OPG’s Northwest Operations, during a visit on a bitterly cold November day.
Northwest Operations is a newly formed internal merger of OPG’s Northwest hydro and thermal power generation operations under one regionalized management team.
In total 11 hydroelectric stations with a combined installed capacity of over 680 MW and two thermal generating stations that now use biomass; the 205 MW Atikokan GS, recently converted to use conventional “white” wood pellets, and Thunder Bay GS with one 163 MW boiler unit being modiﬁed to use steam exploded pellets.
However, using advanced biomass at TBGS wasn’t the original conversion case. Back in 2004, there were government plans to convert it to natural gas only to be shelved in 2006.
In November 2013, the Ontario government announced that the Province was to stop all coal-ﬁred power generation and that TBGS would be converted to use “advanced biomass” with coal-like properties as fuel. In April this year, Thunder Bay used its ﬁnal piece of coal.
As a result close to 100 percent of the energy OPG generates is virtually free of emissions that cause smog or contribute to climate to climate change and the Province no longer uses coal for energy.
Though the term conversion is used, modiﬁcation is perhaps a more accurate description since no new major pieces of fuel-speciﬁc handling, storage, or combustion equipment were needed.
The project can be categorized into three components; administrative, fuel supply, and technical modiﬁcations to the fuel handling and powerhouse equipment, explained Rollins adding that the focus through the entire project is on the safe handling of the advanced wood pellet fuel.
The administrative part included securing a 5-year power supply agreement with the OPA, an extension of the current testing Environmental Compliance Approval (ECA) until the end of 2015, and an exemption from a Renewable Energy Approval (REA) with the Ministry of Environment and Climate Change (MOECC).
On the fuel supply side OPG submitted earlier in 2014 a tender request for indicative prices for the supply of “advanced wood pellet” fuel, a tender subsequently awarded to the Norwegian advanced pellet producer Arbaﬂame A/S.
The fuel supply contract is for up to 7 500 tonnes and we have already begun to take deliveries from Arbaﬂame, said Rollins adding that a second request for proposal for future supply will be announced later.
As you know the test burn we carried out in September 2013 provided the proof of concept that showed we could manage the dust with some minor modiﬁcations to our fuel handling system. That together with the 5-year OPA energy supply deal were the two components we needed in place to be able to move ahead, commented Rollins.
The obvious yet deﬁnitive point of diﬀerence is the fuel. The coal previously used was high moisture (about 30 percent) bituminous coal whereas the advanced pellet fuel has a moisture content under 10 percent and a 30 percent greater energy content per unit of fuel.
The major technological challenges in the project have been the various modiﬁcations to material handling systems such as unloading, conveyance, and transfer points for dust and ﬁre control, and modiﬁcations to the fuel combustion systems such as pulverizers, controls, and burners to optimize the combustion of the fuel.
This is why our major focus has to be on dust and safety. Before, dust was not a major hazard as bituminous coal was a wet fuel. Now we start oﬀ with a dry fuel that has dust with a lower ignition point than coal dust. This means that along the entire delivery chain from the fuel yard to the furnace point of entry we need to think of soft handling to avoid dust generation and static build-up, dust suppression, ﬁre control systems, and electrical up-grades, explained Steve Carlson, Supervising Project Engineer with OPG.
The coal was supplied by train to Thunder Bay harbor and then conveyed by an almost 3 km long conveyor to the fuel yard.
Although we are right at the waterfront, we do not have any boat-receiving terminal at the station, said Carlson, as we braved the elements to tour the fuel yard.
The pellets are delivered by truck from the harbor and stockpiled out in the fuel yard. From here they are scooped or pushed by a front-loader into a ground-level receiving bin that gently and evenly feeds the fuel onto a conveyor belt that takes it into the boiler house.
Out in the fuel yard, we bypass some of the previous equipment such as the frozen coal crackers that have become redundant. The bituminous coal came in wet lumps that would freeze into huge aggregate chunks during the winter, remarked Steve.
The 8 mm advanced pellets are already conveniently sized, dry, and, as far as testing over the last three years has shown, remain hydrophobic and do not freeze into aggregates.
Boiler house modiﬁcations
Inside the boiler house, the fuel is distributed into one of ﬁve coal hoppers that are capable of holding up to 800 m3 each.
As the material is already dry and energy-rich we don’t need to have big volumes inside. Under normal operations we expect to be loading in 250 m3 batches, said Steve.
The chutes are lined to avoid static build-up and the hoppers have what was referred to as “steam inerting” installed.
We have developed a method to use moisture to act both as a dust suppressant and spark retardant in the hoppers, said Steve declining to reveal any speciﬁc details other than to conﬁrm it was a result of internal research.
From the hoppers the pellets are fed via rotary valves, to avoid back-ﬁring dust explosions, into the pulverizers that grind the pellets to a pre-determined particle size. Another important modiﬁcation was to cool the incoming air used to carry and blow the powder into the boiler. Previously this would have been heated to further dry the moist lignite coal particle.
The particle size, air volume, temperature, and velocity into the boiler will depend on the energy content of your fuel, the moisture content, and your boiler conﬁguration. The pellet needs to be durable enough to minimize dust and ﬁnes during handling and transportation yet not too hard that it takes a lot of energy to grind to the desired particle size, explained Carlson.
This would also seem to suggest that there may be some diﬀerences between the available types of steam-treated pellets.
Does the theory hold?
Looking ahead the fuel deliveries from Norway are underway, construction is expected to be complete by the end of January 2015 and commercial availability of TBGS is targeted for the end of March 2015.
Once completed it will mean that Thunder Bay is the world’s ﬁrst and largest advanced wood pellet power plant conversion. And it positions OPG at the forefront as a driver of ground-breaking low-carbon technology.
So does the theory hold true that the adjustments are indeed minor and thus CAPEX is signiﬁcantly lower? As the conversion is still a work in progress, neither Steve Carlson nor Faron Rollins would give any ﬁgures other than to say that the CAPEX is much less than a conventional pellet conversion is likely to have cost and a fraction of a new build which the gas route would have entailed. Especially bearing in mind the peak load and backup role of the plant.
It was though never an option for Atikokan as the fuel volumes weren’t available commercially, said Rollins, pre-empting a follow-up question.
It is not a stretch to suggest that as a result of the Thunder Bay project, volumes of Norwegian advanced wood pellet fuels may well become commercially available sooner rather than later, on both sides of the Atlantic.