Co-Firing Sewage Sludge, Biomass and Municipal Waste in Coal-fired Boilers was the Subject of the Hot Topic Hour on Thursday
Twenty-nine states and the District of Columbia have Renewable Energy Standards (RES) ranging from 10-33 percent with various timelines between the years 2015-2025, and pending Federal legislation would establish a national renewable energy/efficiency standard of up to 24 percent in 2020 increasing to 84 percent in 2035. As a result, coal-fired power plant and industrial boiler operators are exploring biomass as an option for RES compliance. On Thursday four speakers discussed the issues and implications of converting coal-fired boilers to full or partial use of biomass, sewage sludge or municipal waste.
Arie Verloop, P.E., Vice President of Technology and Client Relations with Jansen Combustion and Boiler Technologies, Inc. (JANSEN), discussed their experience with upgrading over 300 boilers to burn a variety of biomass. Although they have primarily worked with industrial stoker-fired boilers, Arie noted that they have also worked on fluidized bed combustion systems and are working with a major industrial gas supplier on Oxy-fueled combustion systems for burning biomass. According to Arie, the term biomass basically includes anything that can be combusted that is not a fossil fuel including all types of wood and agricultural wastes, construction and demolition (C&D) material, refuse derived fuel (RDF), tire derived fuel (TDF) and waste sludge from water treatment plants. He stated that obtaining the typical combustion goals of uninterrupted stable operation and optimum fuel economy and steam conditions for power generation, while meeting emission limits and minimizing erosion and corrosion problems, is much more difficult with biomass than fossil fuels. However, with their design approach they are able to increase biomass capacity significantly (five percent to 40 percent) and reduce or eliminate fossil fuel usage while improving emissions (CO, NOx, PM) and reducing carryover, erosion and unburned carbon losses (LOI) while increasing thermal efficiency.
Brandon Bell, P.E., Principal Mechanical Engineer at KBR Power & Industrial, clarified the EPA’s definition of biomass and reviewed the regulations governing the combustion of biomass and other solid wastes or “renewable fuels” such as C&D waste for independent power producers, agricultural cooperatives, and utilities. He then discussed typical problems associated with the conversion of coal-fired units to co-fire biomass, citing the high cost of specialized material handling equipment required and large space required for the storage and preparation of biomass. Because of the high moisture content and lower heat value of most biomass, three to six times the volumetric throughput is required to replace an equivalent amount of coal. He also discussed the potential problems associated with higher chlorine and alkali content typical in biomass. If the boiler is not designed properly to burn biomass there will be problems with tube plugging and corrosion.
Thomas (Tom) J. Maestri, a Consultant for and Director of Renewable Energy Programs at Synagro Technologies, Inc., discussed the regulations governing and issues related to co-combustion of bio-solids (i.e. sewage sludge) with non-solid waste fuels. He said the primary issue is that sewage sludge is defined by EPA as a solid waste. Therefore, when sewage sludge is co-combusted with a non-solid waste fuel (i.e., coal, biomass, etc.), the emissions standards shift to Section 129 of the CAA rather than Section 112. Since 129 has far more stringent emissions standards covering more pollutants than 112, this puts a capital burden on any project to combust sludge. To address this, Tom described the EPA’s petitioning process by which Non Hazardous Secondary Materials (NHSM) can be declared as Non-solid Waste (NSW) and combusted under the less stringent 112 rules. He stated that two facilities in Michigan have been successful with their petitions for co-combustion of bio-solids. Therefore, the solid waste definition rules should not solely preclude a project from moving forward. Tom also noted that in many cases facilities already comply with 129 standards vs. 112 and as such can co-combust solid waste and non-solid waste with little financial impact.
Dr. Kevin Davis, Vice-President for Business Development at Reaction Engineering International (REI), started his presentation by showing a chart prepared by the U.S. Energy Information Association projecting that by 2035 combustion of biomass will eclipse wind and become the leader in renewable power generation. During further discussions however, he stated that this may be true for Europe but with the advent of fracking and low gas prices he does not believe that it will be true in the U.S. He told of a conversation with an associate in New York that said that four of the six biomass facilities in the state had shut down because of the lower gas prices. The utilities involved were willing to accept penalties for the shutdown because of cheap gas prices. He then went on to discuss the drivers for biomass use as a fuel and the utilization issues related to biomass combustion. Utilization issues include fuel collection, storage, processing and handling, combustion, emissions, operational impacts, economics and regulatory. He focused on the problems of deposition, slagging, sintering and fouling and the potential for corrosion from the high chlorine and alkali content in biomass. He then discussed the “GLACIER” CFD software REI has developed to evaluate the effect of various biomass materials in specific boilers for emissions and LOI as well as their potential to create ash and corrosion problems. He gave an example of an analysis conducted with three different pelletized wood products and a pelletized straw combusted in a 660 MW opposed-wall pulverized coal- fired boiler. Overall simulation results indicated:
§ Modest increase in FEGT for biomass firing
§ Some reduction in wall heat transfer
§ 35-40 percent decrease in NOx emissions
§ Similar CO emissions
§ Slight decrease in carbon-in-flyash
Graphics showing ash deposition and sintering after four hours were also presented. Kevin concludes that by using advanced fuel characterization (such as CCSEM and partial chemical fractionation) and the application of multi-phase reacting CFD tools, predictions of deposition, sintering and corrosion provide valuable reassurance that fuel-specific design objectives can be met.
Replacement of coal with biomass has long been recognized as a potential means of mitigating carbon dioxide and other emissions from solid-fuel-fired power generating stations. International, national, regional and local drivers have resulted in a number of past and current conversions of this type. These conversions range from biomass co-firing with coal to 100 percent biomass conversions of pulverized coal and grate boilers. The combustion of biomass presents unique design and operational challenges and can benefit greatly from the application of cutting edge modeling tools. Recently, Reaction Engineering International has tailored in-house modeling capabilities to address deposition and corrosion concerns related to these conversions. Using advanced fuel characterization (such as CCSEM and partial chemical fractionation) and the application of multi-phase reacting CFD tools, predictions of deposition, sintering and corrosion provide valuable reassurance that fuel-specific design objectives can be met.
BIOS, ABSTRACTS AND PHOTOS can be seen at: BIOS, ABSTRACTS, PHOTOS - December 13, 2012.htm
The individual presentations are as follows: