Monday, April 8, 2013

Songs from the Wood* - Wood Fired Electricity in New Hampshire – Part 1

In my last post we took a look at energy sources and costs for home heating, and we saw how relatively inexpensive wood is compared to most other fuel sources in New Hampshire at the moment. Only natural gas is cheaper on a dollar per heat content basis. Wood as a fuel is particularly intriguing in the New Hampshire context. A great deal of NH is forested. In fact, 84% of the State is forested and, of the 5.7 million acres of land in New Hampshire, 81% is viable timberland. Therefore, tree harvesting for electricity or home heating applications is a way we can use a local in-state resource to reduce imports of fossil fuels like coal, oil or natural gas. Utilization of NH forests also provides a lot of local jobs and, over the 20 to 30 year life of a tree, wood combustion does have a lower carbon footprint when compared to fossil fuels. 

Many folks believe that, over the long term, the use of wood for energy purposes can be viewed as being carbon neutral. In other words, the carbon dioxide that is released in the combustion of wood is offset by the carbon that was absorbed by the tree during the process of photosynthesis which converted the carbon dioxide into the organic matrix of the tree. This is not entirely correct as there is not a one-to-one correspondence of carbon dioxide in = carbon dioxide out. We have to take into account that a great deal of fossil fuel energy is used to manage the forest, harvest the wood, transport it and convert it into a form, such as firewood, woodchips or wood pellets, which can then be combusted. As a result, the total amount of carbon emitted in utilizing wood as a fuel source is greater than that absorbed during photosynthesis.
We generate a good amount of electricity in New Hampshire from the combustion of wood. In 2011 we produced 1.03 million megawatt hours (MWh) of electricity from wood-fired operations, which is equivalent to 5.1% of the total electricity produced in the State. There are presently seven wood-burning plants that generate electricity from wood chips in New Hampshire and this year we should see the commissioning of the State's largest wood-fired power plant, the Burgess Biopower plant, which is located in Berlin on the site of the old Frasier Paper Mill. This new power plant will increase the overall percentage of electricity generated from wood from 5.1 to 7.5%.
The map below shows the location of the various wood-fired electricity plants in NH and the table that follows provides the key for the locations shown on the map as well as information about the various operations.

The data in the table show that most of the wood-fired power plants are smaller operations with capacities of the order of 15 to 20 MW. At this time, the largest operating plant in the state is the 50 MW Northern Wood Power plant. This facility is owned by our largest utility company, Public Service of New Hampshire, PSNH, and is located in Portsmouth on the Schiller power plant site. This plant was started up in 2006, when PSNH converted one of the three large boilers at the Schiller station from coal to a wood chip fuel source. Most of the other wood-fired operations were built in the late 1980s when New Hampshire was encouraging home grown energy and PSNH was required to sign 20-year power purchase agreements with these new wood-fired operations. Over time, there has been a fair amount of change in the ownership of some of these plants as the attractions and economics of renewable energy have waxed and waned.
The generation of electricity from wood is an intriguing business and one that is full of opportunities and challenges – some of which I will discuss my next post. In the figure below I have sketched the value chain for the wood-fired electricity business, along with some of the important inputs and outputs.

In the first step, we have the lengthy process of tree growing and the incorporation of carbon into the organic matrix of the tree and sunlight into stored chemical energy – both of which will eventually be released during combustion. Active and scientific forest management is required to allow trees to grow to their full potential. This has to be done while countering insect infestations, drought, fire and a host of other problems. The average life of a tree is typically 20 years before it is harvested.
In step 2, we have the harvesting of trees from public and private lands. This process is now highly mechanized and involves large machines to take down the trees, remove the brush and load them for transportation. Because it is highly mechanized, a lot of fossil fuel, in the form of diesel and gasoline, is used, so there is a net energy input into this step.
In step 3, the logs are transported to the saw mills where they are converted into lumber, wood chips for paper pulp, wood chips for combustion and waste sawdust. A great deal of the waste sawdust is converted into wood pellets which many folks use as fuel in their wood-fired home furnaces. The sawmill conversion process is also highly mechanized and automated, and it too requires a large energy input. I will note that it is possible to combine the harvesting and chipping operations right on site where the trees are harvested. In these operations, the trees are taken down and are immediately run through a large wood chipper. The wood chips are then directly loaded onto trucks for transportation to the wood-fired power plants, thus bypassing the saw mills.
Finally in step 4, wood chips from the saw mills or chipping operations are then transported to the wood-burning plant, where they are stored in enormous storage piles. The wood chips then become the fuel for the power plant, and they are burned in large furnaces where the heat is used to boil water. The steam is then used to drive a turbine which drives an electrical generator that produces electricity. Other than wood chips, there are inputs of water and skilled labor as well as fossil fuels used to transport the wood chips into the feeders.
These wood-burning operations produce electricity that is fed into the electrical grid but an enormous amount of waste heat is produced as well. In a previous post, I provided data that showed that only a small fraction, 23%, of the chemical energy in wood is converted into electricity. In contrast, coal-fired power plants have conversion efficiencies of 31% and natural gas plants, on average, have efficiencies of 45%. The 77% of waste heat from these wood-fired operations is dissipated by the evaporation of water and by the hot gases exiting the tall emissions stacks from these operations.
A fact that is not often appreciated is that a lot of water is utilized in the production of electricity. The water is used to produce steam and to cool the off-gases and, as such, a lot of water is lost to the atmosphere via evaporation. It has been estimated that water consumption for electricity generation is of the order of 4000 to 8000 gallons per MWh of electricity produced. One megawatt hour represents the approximate monthly electricity consumption for a US home so every month your electricity usage results in the consumption of 4000 to 8000 gallons of water. However, it should be noted that the water consumption numbers for wood-fired power plants are substantially lower. Saving on water consumption is just another reason to turn off the lights and save electricity. A point I often make to students in the Franklin Pierce University MBA in Energy and Sustainability Studies program is that if you are in the energy business, you are in the water business as well.
A small amount of wood ash is produced from these operations and this is a valuable soil additive that is used by local farmers.
So if we step back and look at the energy and carbon dioxide flow aspects of the wood-fired electricity business, it is clear that it is a lengthy and involved pipeline from photosynthesis to electricity. It takes time, money, energy, labor, and fossil fuels to get forests to incorporate sunlight and carbon dioxide into the body of tree and for us to release that energy (and carbon dioxide). As a result of all the various fossil fuel-based energy inputs all along the way, it is clear that wood burning cannot be viewed as entirely carbon neutral. However for us, here in forested NH, wood does represent a better fuel source than imported fossil fuels which have no offsetting carbon absorption and the wood-fired electricity does provide a lot of jobs and livelihoods as we move along the chain from photosynthesis to electricity. Now if we could only do something about that 77% of wasted energy…..
In my next post I will take a brief look at the economics of producing electricity from wood, and I will wrestle with the issue of just how much wood could we burn in New Hampshire before we overtax our forests. In the meantime, remember that it takes a lot of fossil fuel, labor and water to convert the chemical energy in wood into electricity we use in our homes. Many times the songs we play in our homes on our CD players are powered by our forests. These, indeed, might be Songs from the Wood.
Until next time, remember to turn off the lights when you leave the room. You will be saving energy and water.

Mike Mooiman
Franklin Pierce University

(*Songs from the Wood was the title of and a song from Jethro Tull's tenth album. This album, with its distinct British folk rock sound, was quite a departure from Tull's earlier heavy rock and blues influenced recordings. This recording cemented Jethro Tull's reputation as a bunch of odd ducks making interesting and influential music. The cover art on the album also features the results of some wood harvesting. An old tune but worth a listen).

1 comment:

  1. Living in the Sunapee area, we do take a bit of pride in our wood fired plant, however you rarely consider the entire supply chain from seedling to MW. The real plus from the wood fired plant in my view is also the diversification of energy sources.



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