Paper details

Title:

Study on Bed Agglomeration, Fouling and Slagging Remedies in Biomass Fired BFB Combustors Based on Laboratory Tests and Long Term Operational Experiences

Author(s):

Zuwała, J., Lasek, J., Głód, K.

Document(s):

Abstract:

Generation of near CO2 free energy (electricity and heat) in existing large scale co-generation units can be achieved by partial substitution of fossil fuels with biomass commonly regarded as CO2-neutral fuel. Fluidized bed boilers are often the technology of choice to combust biomass fuels while bubbling fluidized bed boilers (BFB) are most advantageous for large-scale applications, including utility boilers. During the recent years public subsidies supporting renewable fuels increased the opportunities and thus the demand for highly efficient boilers capable to co-fire woody biomass with large shares of agricultural residues (agro). The ash chemistry of such fuel mixtures is notoriously challenging for highly efficient boilers, where it can lead to intense bed agglomeration, fouling, and corrosion. The causes and nature of the bed agglomeration and the effect of the combustion temperature, the fuel chemical composition has been the subject of broad process research. The compositions of biomass among fuel types are variable, especially with respect to inorganic constituents important to the critical problems of fouling and slagging. Alkali and alkaline earth metals, in combination with other fuel elements such as silica and sulphur, and facilitated by the presence of chlorine, are responsible for many undesirable reactions during the combustion. The paper presents the study on the impact of the additives on the decrease of agglomeration tendencies as well as slagging and fouling risks. Research was carried out for different biomass blends (willow chips and corn straw chips– lab scale, willow chips and sunflower husk during industrial verification,). As potential additives, mineral, non – renewable compounds: kaolin and halloysite were used while as bed material regular sand, low-silica sand and bottom ash from coal-combustion BFB boiler were compared. Laboratory scale testing was carried out by use of BFB auto-thermal combustor consisting of a stainless steel tube with a reactor diameter of 105 mm & height of 1500 mm at fuel feeding of 0.5-3.0 kg/h. The tests were involving the preparation of fuels blends and additives, their combustion and evaluation of process parameters followed by investigation of solid combustion residuals (fly and bottom ash) after combustion processes. In-house made deposit probe, as well as gas and solid samplings and a Fourier transform infrared (FTIR) analyser were utilized to monitor the risk of slagging, and fouling. Fluidized bed behaviour was monitored by taking samples directly from the bed. Afterwards, an experimental campaign was carried out at two BFB power plants plant to specify the challenges related to fluidized bed combustion of agricultural fuels and demonstrate a technically viable concept for establishing a 20% mass share of agricultural fuel co-fired with woody biomass. The complex research results obtained enabled to evaluate the following conclusions: . utility-size BFB boilers can combust biomass with large shares of agricultural residues, . as a way of agglomeration risk elimination, the following process parameters change possibilities should be considered first however keeping within the technological limitations (boiler output, heating surfaces temperature distribution, boiler efficiency etc) like bed temperature decrease and fluidisation velocity increase, . bed agglomeration, fouling, and slagging can be controlled by the addition of natural additives, . bottom ash/bed material of coal fired BFB boiler can be a good alternative of silica sand however considering the legal aspects of renewable energy calculation procedures (addition of extra combustible material resulting from LOI contents), . kaolin and halloysite are suitable additives to limit bed agglomeration and boiler slagging and fouling, . methodology and tools proper for woody/agro biomass blends composition and additives shares can be developed using laboratory BFB combustors and verified by long term operational test runs.

Keywords:

agglomeration, biomass, co-combustion, fluidized bed

Topic:

Biomass Conversion Technologies for Heating, Cooling and Electricity

Subtopic:

Biomass combustion in large utilities

Event:

26th European Biomass Conference and Exhibition

Session:

2AO.8.4

Pages:

458 - 464

ISBN:

978-88-89407-18-9

Paper DOI:

10.5071/26thEUBCE2018-2AO.8.4

Price:

FREE