Experimental Study to Reduce CO Emissions in Skellefteå Kraft's 16 MW Bubbling Fluidized Bed Boiler

Abstract: Skellefteå Kraft has had high CO emissions problems in a bubbling fluidized bed (BFB) boiler in Malå.Statistical emissions data from 2013 to 2019 shows plant average emissions of 1230 ppm. Thisexceeds the recommended 440 ppm (6 % O2) that Naturvårdsverket proposes (Naturvårdsverket,2005). The aim of this project was to identify and map the cause of the BFB boilers high CO yield withthe help of a literature study as well as practical experiments to reduce CO, and; furthermore,suggest possible changes that can be implemented to resolve this problem.A literature study was conducted to gain a better understanding of possible causes of high CO influidized bed combustion of biomass. The results from the literature study showed that bedagglomeration, airflow symmetry, biomass moisture content, biomass particle size and biomass beddistribution all are common factors that can cause high CO emissions.Based on the literature study, statistical data from 2013 – 2019 and employee operator experience aseries of tests, calibrations and experiments were conducted on the boiler. The following methodswere used; Boiler statistical data analysis; Primary, secondary and tertiary airflow mapping;Pneumatic boiler feed system test (biomass bed distribution) and a bed material change test.From the statistical data analysis, it was found that sudden load variations had a minor contributionto the CO emissions. However, load variations were not the biggest contributing factor to high COemissions in the boiler in Malå. The primary, secondary and tertiary airflow were measured andmapped, uneven airflows between left and right were found. By calibration of even air distribution inthe primary systems CO levels were slightly reduced by 14 %. The primary air calibration improvedleft to right boiler air distribution from a 5 % difference to only a 0.7 % difference.A series of 11 tests was conducted on the pneumatic boiler feed system to obtain optimal fueldistribution. The test series that consisted of maximum parameters for transport air, spreading airand casting air obtained the lowest CO emissions levels of 350 ppm. An increase in airflow oftransport air in the pneumatic boiler feed system resulted in more even biomass bed distribution andincreased airflow in the lower secondary zone. This resulted in a dramatic CO emission decrease by63 %. A 45 °C temperature increase was also noticed in the secondary zone, a decrease intemperature before super heater 2 and a decrease in temperature before super heater 1. The changein transport air caused combustion of flue gasses to occur a lot lower in the furnace.This was clearly also documented through visual images taken inside the boiler with a special camerawhile the boiler was in operation. The images taken inside the boiler before and after theadjustments clearly showed that an increase of lower secondary air moves combustion closer to theboiler bed and reduces the CO emissions. The images also showed that uneven combustion wasoccurring in the boiler, as more violent and turbulent combustion was occurring on the right side ofthe boiler. This was an interesting finding as the measured airflow distribution was even in the boiler.This suggests that there is a major leakage in the air distribution pipes on the left side of the boiler.By reducing the airflow to the right section and increasing airflow to maximum on the left, an evenairflow distribution was obtained resulting in a more even combustion throughout the entire boiler.An increase in bed material change frequency from 1 time (3 % regeneration of total bed mass) to 3times (9 % regeneration of total bed mass) resulted in a CO emission decrease by 20 %. During thebed material change test bed agglomerates were observed which may explain the possible emissionsimprovement as the bed material was changed more frequently.All different tests were conducted independently of each other. The change that resulted in thehighest CO reduction was the increase of transport air to the secondary zone. Emissions werereduced by a total of 68 % as the transport air was set to maximum flow, the sand was changed threetimes and the primary airflow from left to right was even. The proposed adjustments will likely alsoincrease the boiler efficiency and reduce the maintenance of super heater 1, super heater 2 as wellas the economizer. The boiler produces around 400 ppm at 6 % O2 after the changes have beenimplemented. This is significantly lower compared to the original operation settings that during 2013to February 2019 have produced a CO emission of 1230 ppm (on average). The significant COdecrease in Skellefteå Kraft’s boiler has opened opportunity to operate the boiler with lower excessair which ultimately decreases exhaust heat losses. If Skellefteå Kraft were to succeed in operatingthe boiler at 3 % O2 excess, a sum of 255 kkr could be saved annually.The following recommendations are given in order to achieve low CO emissions:- Increase the bed material change frequency.- Clean pneumatic airflow system during summer stop annually.- Ensure airflow symmetry within the boiler is achieved annually.- Increase airflow to lower secondary zone.- Use recommended airflows for pneumatic boiler feed system.- Invest in a buffer tank to reduce load variation by Setra Sawmill.- Reduce secondary air nozzle size to increase combustion air in the middle of the boiler.- Increase the feed port ramp angle to increase biomass casting length in the boiler.- Investigate left side secondary air flow (possible leakage suggested)