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.
