by Nicholas Sikes
Not much research has been done on the cost of biomass until now. This article outlines the various hidden costs of biomass.
Biomass has become a popular topic of discussion among boiler owners and operators. With the recent federal incentives to convert existing infrastructure into a more “green” system, many companies have begun providing expertise to retrofit existing boilers to burn biomass.
Biomass was a significant energy source for the United States until the 1950s, before natural gas became the dominant heating source for residential homes. It regained prominence when companies started looking for alternatives to the high oil prices in the 1970s and 1980s, and now biomass is poised to play a key role in renewable portfolio standards (RPS) compliance for electric utilities. It is very likely that biomass will continue to grow in importance as climate change moves to the forefront of political agendas throughout the world.
Until recently, there has not been much discussion about the cost of biomass. Most of the biomass consumed for energy has been wood sourced as waste from other processors. Estimates from the Energy Information Administration place biomass energy growth at approximately 110 GW of new generating capacity within 15 years. If this happens, it is likely that there will no longer be much access to “waste” biomass sources. This increased demand will likely bring about significant changes in the pricing structure and availability of any type of biomass.
There are several “hidden” costs that come with utilizing biomass, and these costs are not normally presented to a customer or developer. Here are some costs that significantly impact the economics of utilizing biomass for energy.
Water and ash are not free
The dominant unit measurement for biomass has traditionally been tons. The difficulty with using tons is that the moisture and ash content of biomass varies greatly. Even within the same species of biomass, there can be widely varying ash contents. The Energy Center of the Netherlands (ECN) compiled a great database of more than 2,300 biomass samples, showing how much variation can be possible.
With typical ash content varying from 0.5-15 percent, knowing the content of the biomass feedstock you are purchasing is critical.
Whether for traditional combustion or more advanced thermochemical conversion technologies like gasification, moisture that enters the process with the biomass is vaporized. This vaporization results in an energy penalty of roughly 1,000 Btu/lb of moisture fed to the process. For a typical biomass feedstock, having a heating value of roughly 7,000-8,500 Btu/dry pound, this energy penalty is very significant, and must be made up by increased fuel consumption. In nearly dry feedstocks the penalty is reasonable, but as moisture increases the penalty also increases dramatically. If we account for this moisture energy penalty, an additional 10 percent of bone dry basis biomass is required to maintain the same energy input into a boiler when moisture goes from 20 percent to 50 percent. Not only is this an extra 10 percent fuel cost, but additional material handling, sorting and inventory is required. It is not enough to simply discount feedstock based on moisture content when additional energy is required to vaporize the additional water. The prevailing method of discounting feedstock based on the weight of the moisture does not account for the reduced net heating value of the feedstock.
The best unit measurement for biomass should be a moisture-corrected heating value. This would account for both the ash content of the biomass and the weight/energy penalty for moisture content. Purchasing feedstock on a moisture-corrected Btu basis places the burden on the feedstock supplier to provide a high quality, unadulterated product. The formula shown in Figure 1 can be used to perform this energy-based moisture correction, and Table 1 shows how this impacts the effective dry-basis heating value and true cost of the biomass fuel.
Biomass can cause significant increases in boiler maintenance
Biomass brings with it an inherent risk of corrosion and fouling due to the composition of its ash.
While boiler design can mitigate some of this risk, this also has a high price. The most important factors relating to feedstock composition when firing biomass are the levels of alkali metals and chlorine. Woody biomass is an attractive biomass fuel because its ash profile, characterized by reasonably low levels of chlorine and alkali, generally offers one of the lowest risks of boiler corrosion.
The danger comes when various grasses and straws are utilized in a boiler. Whereas wood might have 100 ppm of chlorine, herbaceous grasses and ag residues like straw, switchgrass, corn stover or corn cobs might have more than 2,000 ppm. The alkali chlorides are the most stable species of chlorine, and combustion results in the vaporization of alkali chlorides. These re-condense on the heat transfer surfaces of the boiler as highly aggressive, corrosive deposits. Studies by boiler manufacturers have shown these deposits to be among the most aggressive agents of corrosion on tube surfaces in the superheater and convection sections of a boiler. Utilizing bio-char or torrefied biomass in a boiler might also cause this deposition, since all of the ash components of the biomass used to create these char-based fuels are still in the feedstock.
Corrosion can be mitigated in two ways: 1. construction with exotic alloys impervious to corrosion, or 2. removing the ash constituents that cause corrosion prior to combustion in the boiler.
Retrofitting existing boilers with exotic alloys can be an expensive and time consuming process. If a gasification process is utilized on the front-end of a boiler to create a fuel gas from the biomass, there is opportunity to remove the ash from the fuel gas through a high temperature filtration system. This process allows the use of existing infrastructure without the danger of increased maintenance expenses caused by corrosion.
Biomass demand will cause significant increases in the costs of “easy” biomass
In 2010 there is still readily available access to “easy” biomass in most parts of the United States. This biomass is mostly woody biomass: slash, mill residue, bark and wood not suitable for pulping.
With the potential that 500 million dry tons/year of biomass will be needed to fulfill the electric utility renewable portfolio standards being put in place, this “easy” biomass will be locked into long term contracts very soon.
The USDA’s Billion Ton Study (2005, Perlack, R.D.) indicated a large amount of potential biomass that is currently not being utilized in any significant fashion. These feedstocks include ag residues like switchgrass, corn stover, corn cobs and other waste streams classified as biomass by some states, like refuse-derived fuels, waste paper/cardboard, etc. For a facility to capture this potential, though, they need to be feedstock flexible. The opportunity to utilize the most economic feedstock available requires a well thought out feed handling design during construction of a facility. Failure to accommodate a wide range of feedstocks is likely to result in a much lower ROI than anticipated in the long run.
Biomass represents a great opportunity to produce clean energy, both thermal and electrical. All of these “hidden” costs can be managed, though, if an experienced and capable technology partner is engaged in the project. There is no single solution to utilizing biomass, and every project needs a technology package that creates the most economic system for that location. A comprehensive preliminary engineering study needs to be completed prior to undertaking any biomass project. These three areas should be fully addressed by understanding the available feedstocks in the area surrounding the project and designing the system to handle the widest range of fuels possible.
There are great opportunities for bio-power worldwide, and several great solutions for many of the technical challenges facing most of the available feedstock. Utilization of the right technology, in the right application, will ensure the highest opportunity for a great return on investment.