Engineering Chat on Biomass Co-firing

This session's chat room members:

Stephen Segrest (Common Purpose Institute)
Dr. Evan Hughes (EPRI -- Electric Power Research Institute)
Kevin Craig (NETL -- DOE's National Energy Technolgy Lab)
Dr. Doug Boylan (The Southern Company)
Dave Tillman (Foster Wheeler)

Steve Segrest: I ran across the following on biomass co-firing from a Southern Company filing at NETL's website. Some pretty powerful statements here. Could each of you comment, especially on the sentence in red?

Biomass cofiring offers a significant improvement for coal-fired power generation: potentially 20% less fossil carbon and 20% more kWh of generation from a coal-fired power plant for a given amount of coal input. In addition, reductions in S02 and NOx emissions have been quantified: SO2 reductions are proportional to the percentage of coal displaced, and NOx emissions may be reduced by an equivalent amount depending on cofiring specifics.

As with any technology that reduces the production of these pollutants, biomass cofiring should also improve plant availability by reducing the demand on components of the plant normally used to control the emission of these pollutants.

Kevin Craig: Some of this is obvious, some is speculative. As you know, most biomass has very little sulfur (compared to most coal) so SOx reductions are proportional to the fraction of biomass (by heat) cofired. Sounds like they're speculating cofiring up to 20% by heat which is quite high based on most tests to date. NOx has been shown to be reduced in amounts greater than the cofiring percentage if you do it just right in certain types of boilers. Much of this has been documented by Dave Tillman in EPRI reports funded by DOE.

Carbon emissions depend on a lot of things including the feedstock and it's alternate fate (speaking on a life cycle basis). Our studies have shown reduced greenhouse gas emission as much as several percentage points higher than the cofiring rate. The increase in availability (or reduction in maintenance costs) is, as far as I know, pure speculation. It might make some sense depending on what pollutant removal technology is used. However, with a second feed-preparation system to maintain, it's probably equally as likely that maintenance cost could go up and availability down. I don't think anyone has cofired long enough to evaluate the impact on cleanup equipment availability. It's worth including in proposed work in a proposal of this type however. There are very real concerns about the impact of biomass cofiring on SCR catalyst as well.

Dave Tillman: Kevin, I agree with your comments to a very large extent. We always document a SO2 reduction, almost always (but not always) see a NOx reduction, and always see a good fossil CO2 reduction--greater than just the coal offset. I agree particularly the comments on position on the dispatch laddar. That is quite speculative. In some cases the biomass is cheaper than coal--both at the plant gate and the burner tip. In most cases it is more expensive to fire biomass than coal. Position on the dispatch laddar is economically driven. And there is a very slight efficiency penalty we pay. This factors into fuel costs. We have documented reduced load on pulverizers and FD fans as a consequence of cofiring with separate injection. However, from a house load perspective, the ID fan amps may increase slightly (if you blend the biomass with coal then the energy consumption by the pulverizers also increases). Further, there is the house load for the biomass system--receiving fuel, screening the fuel, grinding the fuel, and transporting the fuel. Our data and calculations show that this is at least equal to the reductions associated with the coal handling system. We have to grind the oversized material, and the motors we use are not trivial.

Regarding maintenance -- well we are in the middle of a long program at Willow Island. And this is material handling equipment. It may be robust but it is not "set it and forget it" time. But maintenance can be minimized. You indicated that this may cause plant availability to go down. As a practical matter, with separate injection that doesn't happen because the cofiring system can be isolated from the plant as a whole. So, what we have seen is that cofiring does not impact position on the dispatch laddar, and does not increase generation at a site. Its impacts are too small. I would like to hear Doug Boylan's views on this.

Steve Segrest: In my experience with biomass co-firing test burns at coal fired power plants, several engineers have said: "Because of the high moisture value of green wood, some level of overall de-rate must always occur on a coal unit with co-firing".

Could you comment on this statement? Was I not listening correctly?, is this issue more complex than I perceived?

Dave Tillman: Steve, Derate implies a loss of capacity. That does not happen, except when you try to put the biomass through the mills. In reality, with separate injection, fuel feeding is not capacity limiting. There is a minor loss of efficiency -- increasing the fuel required to generate electricity.

Evan Hughes: Steve, The moisture must always cause an efficiency loss, as does the moisture in the coal. Not a capacity loss. My rule on efficiency is that 50% moisture wood vs. say 5% moisture coal will lead to the biomass fraction being turned to electricity at 11,500 Btu/kWh when the coal converts at 10,000 Btu/kWh. .

Kevin Craig: Dave, Thanks for your more detailed and practical info. Sorry if I implied that availability would for sure go down; I was speaking only in terms of possibilities given another feed system to operate and maintain. It would certainly depend on the biomass being used, the consistency of its properties and form, system design, and a bunch of other things.

Doug Boylan: Our experience [Southern Company] has been similar. The SO2 reduction is expected and real. Our NOx reduction history has been mixed - we have had small reductions in NOx in a few cases, but at least I do not believe we have made NOx worse anywhere. We have sometimes taken credit for larger-than-coal-offset CO2 reductions if the material was landfill bound when we took it. With switchgrass we typically don't try to take any credit for carbon sequestration in the soil as once the soil is plowed, I am told that most comes out of the ground again.

In most of our applications it is more expensive to co-fire biomass. An exception to this might be very low cost sander dust or sawdust mixed at the coal pile. With direct injection of biomass, our system pays an efficiency penalty for introducing cold transport air into the furnace.

Co-milling material I think increases our maintenance costs as there is often the potential to plug something up, especially if the wood fibers have any size. For direct injection, as Dave notes, the material has to be processed pretty fine and that takes some power and some cost. Our switchgrass system is not so robust as the Willow Island system, and we have spent a lot on maintaining as well as operating the size processing equipment. As he says, the fact that the separate injection system is separate means that loss of that biomass system has minimal impact on unit operation. I think that 20% co-firing by energy input is probably doable, and at 20%, loss of the system would not be trivial. On the other hand, if the plant fuel system is limited by wet coal or lost mill, part of the lost generation can be made up with the biomass system. However, I don't think we would allow the system to generate more as a result of the renewable energy program there.

Regarding dispatch, if we are selling green power, then the cost of the renewable energy is already accounted for by the green power premiums. For co-milling, the material is generally cheap and of low percentage so dispatch is unaffected.

In regard to de-rating, a lot depends on site specific conditions. In co-milling, the mill amps tend to increase which limits the amount of material that can be processed. As the density and Btu content of the green wood are low compared with coal, then anywhere volume is an issue in the system, the wood-coal blend uses up more of that volume capacity than coal alone. If there is not enough capacity to handle that volume of material, then the load must be reduced.

The points regarding issues related to ash sales and SCR catalyst effects are very important to us, and we as well as others are working on how to resolve them.

Kevin Craig: Doug, A small point... generally, you're right regarding soil carbon, but it depends very much on the soil. If it's been intensively farmed with traditional till methods, it could be very depleted in carbon and could take up carbon for quite a few years. If it's been fallow or in no-till, your assumption is probably right.