Last week I discussed a recent presentation on biofuels focusing on algal biodiesel. Algae is a big topic in the biomass industry at the moment, as it offers high growth rates as compared to other forms of biomass. The problem is that, as a new technology receiving a high level of interest only in the past 3-5 years, the optimum method for cultivating the biomass has yet to be determined. A recent article in Biomass Magazine looks at an alternative to biodiesel: catalyzed gasification.
What does it produce?
The technology, developed by the Pacific Northwest National Laboratory (PNNL), involves the catalytic conversion of aquatic biomass. The authors state that the process transforms the algae into methane, carbon dioxide, ammonia, and water. Methane, of course, is the chief component in natural gas, and the main product of the process. The authors suggest that the water and carbon dioxide can be pumped back to the growth ponds as feed. Ammonia and sulfur products come out in the water, and must be removed prior to reinjection. Using a train of an ammonia stripper and an ammonia scrubber, the ammonia can be converted into ammonium sulfate; possibly using the sulfur byproducts. So far, so good.
Is it efficient?
The real interesting part of the technology is its ability to create natural gas from the aquatic biomass without drying the biomass, which represents a huge drain on the thermal efficiency. The article suggests temperatures of 350C (662F), but under pressure, so the water remains a liquid. If true, rather than expending ~1115 BTU/lb to dry (evaporate) the water from the biomass, the process only uses ~670 BTU/lb to heat the water, a thermal savings of 40%.
Too good to be true?
I do have some reservations about the technology as presented. The biggest drain on biomass is the amount of water contained in the biomass. Water is an energy sink - it does not combust and reduces the combustion of heat of the biomass. Any savings offered by this process is dependent on the ability to generate a "dry" biomass. I typically see biomass sources containing 20% biomass and 80% water. An algae stream with 10% biomass would have twice as much water, and would lose the gain in thermal efficiency as it has to heat twice as much water. The math only works if the biomass is concentrated.
And that may be only half the problem. My experience with fluidized beds is that there is a limit to the concentration of solids in a fluidized bed. The simplest way around that is to recycle the water produced in the process (which presumably is still hot and pressurized). However, that is still a loss in the system, and the exact solids concentration limit will have an impact on the efficiency of the process.
Final thoughts
I believe, as stated in this article, that harvesting algal biomass is quite possibly the most critical step to its economic viability. Water is an energy drain on all of biomass. PNLL and Genifuel look to have found one way to possibly reduce the cost. Further, I like their approach at looking at waste streams first, as solving a wastewater discharge problem improves the economics of the process; it is one of the reasons that waste to energy projects have succeeded.
To read more about ethanol recovery, please download our presentation at the 2009 FEW conference.
