ARTICLE pubs.acs.org/EF
Combustion of Black Liquor Solid Biomass Mixtures in a Single Particle Reactor—Characteristics and Fate of Nitrogen Niklas V€ah€a-Savo,* Nikolai DeMartini, and Mikko Hupa Process Chemistry Centre, Åbo Akademi University Piispankatu 8, 20500 Turku, Finland ABSTRACT: Kraft black liquor is a by-product of the Kraft chemical pulping process. It is burned in a special boiler, called a Kraft recovery boiler, to recover energy and chemicals. One proposed concept is the mixing of biomass and black liquor for co-combustion in the recovery boiler. This laboratory scale study was done as a first step in evaluating this concept by comparing the combustion behavior of different levels of biomass addition and the fate of the fuel nitrogen. A unique feature of black liquor combustion is that up to 40% of the fuel nitrogen can be found in the molten salts after combustion, as sodium cyanate. This study found that part of the biomass nitrogen can also be found in the ash from the biomass black liquor mixtures. Combustion experiments were carried out with black liquor biomass mixtures made with wood or bark of up to 50% biomass. Single droplets were burned in a laboratory furnace at 900 °C and 10% O2 and at 1100 °C and 3.3% O2. Cyanate formation was studied by pyrolysis and gasification of individual droplets at 800 °C in 13% CO2/87% N2. The laboratory tests showed that there is an increase in combustion times, mostly the char burning time and an increase in NO formation with increased biomass addition. Cyanate formation tests indicated that black liquor promotes the conversion of biomass nitrogen to cyanate.
1. INTRODUCTION Black liquor is a by-product of the chemical pulping process, and it consists of the inorganic pulping chemicals and dissolved organics. World black liquor production is about 190 million tons dry solids per year.1 The spent cooking liquor is recovered by concentrating and then burning the black liquor in a Kraft recovery boiler. Energy is recovered from the combustion of the organics, and the inorganic salts are recovered as molten salt from the lower furnace. The pulp and paper industry is in a unique position, because the electricity/steam produced from black liquor, wood, or biomass combustion is considered CO2 neutral.2 The co-firing of black liquor biomass has been considered by companies with stand alone pulp mills with modern Kraft recovery boilers with excess recovery boiler capacity and condensing turbines. At such mills, the energy needs are fully met with the recovery boiler alone, and they do not have a biomass boiler. During periods when pulp production is low and the recovery boiler generating capacity is not fully used, biomass addition to the boiler would allow for additional electricity production and, therefore, additional revenue from existing facilities. During periods when pulp production is high, biomass addition could be reduced or eliminated entirely. Premixing of black liquor and biomass might allow a recovery boiler use existing black liquor firing systems for firing the fuel mixtures instead of installing separate burners for solid biomass firing. For mills with a biomass boiler, this co-firing concept would not be favorable, as the biomass could be more efficiently burned in the biomass boiler. The co-gasification of black liquor mixed with rice husk and straw cutter dust3 or sawdust4 has been considered earlier, and single droplet reactor combustion tests with black liquor mixed with bark, wood, peat, and biosludge have been reported earlier.5 Other waste streams like biosludge, methanol/turpentine, and non-condensable gases (NCG) have been successfully burned in r 2011 American Chemical Society
recovery boilers without any considerable problems in recovery boiler operation or increase in NO emissions.6,7 Several questions arose after the first study of biomass black liquor mixtures,5 and this work was conducted to clarify and answer these questions. The primary conclusion from the previous work was that while combustion times of black liquor biomass mixtures were longer, the combustion behavior for most mixtures up to 26% biomass addition was similar to the combustion behavior of black liquor alone. In the first study, only airdried biomass samples were used, and it was of interest to determine what kind of impact dry solids content of the mixed biomass has to the combustion characteristics the fuel mixtures. It was also of interest to study how much biomass can be added to black liquor before the combustion characteristics of the fuel mixture would more resemble the solid biomass than black liquor. Additionally, this study looks not only at NO formation but also cyanate (OCN ) formation with the addition of biomass. A unique feature in a recovery boiler compared to traditional solid biomass furnaces, is that during black liquor combustion part of the organic nitrogen is converted to form cyanate, and the formed cyanate exits the recovery boiler with smelt.8 The exact mechanism behind cyanate formation in black liquor combustion is still unknown,9 but the results of this work provide some possible insight. In the smelt dissolver process, water is mixed with smelt, and in the presence of water, cyanate decomposes to ammonia.10,11 The formed ammonia is commonly found in the smelt dissolver and causticizers in the Kraft pulp mill, and a part of it is removed as low concentration high volume gases. The rest of this ammonia is transferred to white liquor and is finally removed during cooking or in the early stages of weak black Received: July 4, 2011 Revised: October 13, 2011 Published: October 13, 2011 4944
dx.doi.org/10.1021/ef2009937 | Energy Fuels 2011, 25, 4944–4951
Energy & Fuels
ARTICLE
Table 1. Size, Dry Solids and Nitrogen Content dry solids sample
(wt %)
size
N (wt % d.s.)
black liquor
80.36
liquid
0.08
wood
92.23
length (up to 4 mm),
0.2
bark
92.28
