The Effect of Biomass Fuel on Boiler Refractories

By Justin Teiken | December 27, 2018

W ith increasing energy demands and heavy dependence on fossil fuels, renewable energy has been gaining attention, interest and viability. As a clean and environmentally friendly fuel, biomass currently accounts for 6 to 8 percent of the world’s energy consumption, and is becoming an increasingly popular option. Companies investing in biomass as an effective alternative to traditional coal, petcoke or oil and gas should be aware they must incorporate changes in their refractory linings due to differences in fuel chemistry.

Biomass, mainly wood, was the primary energy source until coal emerged in the late 1800s. Coal was king until huge reserves of petroleum from the Middle East flooded the world with a lower-cost energy source in the 1950s, and reliance on petroleum has continued since then.

Cost-effective energy is critical to industry. With low-cost petroleum, in many cases, biomass is not cost-effective, but if the source is a waste byproduct, it can be a very economical fuel source. There will always be a wide variety of biomass byproducts available, and many are underutilized. As our supply of nonrenewables decreases over the decades, biomass use will be vital to ensure we meet our ever-increasing energy demands.

Petroleum, coal, and natural gas sources are consistent fuels, and their known impurities have been studied and understood for decades. With biomass, there are a wide variety of options, and each fuel source’s impurities are unique in type and level. For boilers and heaters that use biomass as fuel, the impurities can have a negative effect on refractory lining.

Chemistry Behind Corrosive Compounds, Slag
During service, corrosive compounds can accumulate from biomass combustion, resulting in slag formation. Slag is created from ash residue of the combusted biomass. Biomass ash is composed of oxides such as silica (SiO2) and alumina (Al2O3) and fluxing elements such as alkalies (sodium, calcium, magnesium, etc.). The amount of these alkalies can vary widely with biomass type and source. At temperatures above 1,500 degrees Fahrenheit (815 degrees Celsius), these alkalies react with the silica and alumina to form a viscous slag.  As temperatures reach 1,800 to 2,000-plus degrees F, these slags become less viscous, and thus increasingly reactive. Porosity levels in the refractory lining play a major role, as capillary suction from the pores can draw the slag into the refractory. Biomass typically needs higher combustion temperatures than coal or petroleum, so the slag penetration into the refractory, due to capillary action, is enhanced with the decreased slag viscosity.

To help visualize this, picture a chemist mixing test tubes of a liquid acid and base, resulting in a foamy, oozing mass, or worse—an explosion. Refractories and slags have a less dramatic reaction, but in the end, it can be very destructive over time. Refractories, like all materials, have a chemistry that is acidic or basic in nature. Ideally, the refractory aggregate and slag pH should be similar to minimize corrosion. 

Nature does not favor acids or bases, but prefers that surfaces be neutral. An acid slag, such as a high-silica slag, will try to neutralize basic oxide refractory components as it penetrates into the refractory lining.

Many refractories in boilers use acidic/neutral aggregates (silica, alumino-silicate and alumina are typical aggregate components). These aggregates are most commonly bonded with calcium aluminate cements.

These cements are composed of either three primary oxides: Al2O3-SiO2-CaO, or two oxides: Al2O3-CaO. Calcia (CaO) is the active phase in these cements and is basic. A silica slag will react with CaO in a refractory and form a corrosion layer of glass. In a well-operated boiler, slag formation is minimal, but if there are large variations in biomass source and combustion temperatures, slag corrosion may accelerate.

The original refractory castables, now called conventional types, used large amounts of cement (20-plus percent), thus there were high amounts of basic CaO present. In the 1980s, low cement technology created castable materials with greatly reduced amounts of CaO. These materials also utilized water reducers, reactive fine aluminas and fume silicas to create much denser packing. This denser packing creates a lower porosity material that is less permeable with reduced amount of CaO, creating a more slag- and alkali-resistant structure. Further advancement in ultralow cement and no-cement castable options utilize even lower cement amounts or alternate, noncement binders.

Reactions with cement are not the only cause of failure when alkalis come into contact with refractory linings.  The standard phase in which alumina exists in refractory material, a-alumina, can convert into b-alumina in the presence of alkai liquids and gases. This transformation results in significant volume expansion, and is one of the common causes of alkali-related failure of refractory linings in biomass boilers. Replacement of pure alumina with other raw materials can extend the life of such linings.

Alkali Cup Testing
Refractory performance in alkali-rich slag applications can be predicted with laboratory alkali cup testing. This testing is performed with 2” by 2” by 2” refractory cube samples with a center core drilled out. The core is filled with an alkali salt or salt combination. In such a test shown in the top right image, a number of standard refractory options were compared to predict performance.

In this example, the alkali testing was performed on a series of castables made with a synthetic mullite aggregate. Binders varied in CaO content from high levels (conventional castable) to no CaO (no cement castable). As the CaO is reduced (from left to right), the alkali reaction depth, the grey area, is decreased, with the no cement castable experiencing very little reaction and no cracking (alkali bursting) on the exterior cast surface of the cube.

Minimize Changes in Your Refractory Life
Biomass boilers are gaining in popularity due to their economics and renewable energy status. Changing the fuel source in some cases can have a negative effect on refractory linings. Using the correct refractory material can help to preserve the integrity of the refractory lining.

Biomass boilers utilize solid waste for fuel, which can contain a wide and variable amount of impurities as combustion ash. The ash chemistry typically includes silica and high levels of alkali. Biomass combusts at higher temperatures than petroleum, which can lead to the formation of alkali-rich silica slags. These slags are corrosive to standard refractories. In addition to the corrosion issues, alkali-related failure can be caused by phase transformations resulting from contact with alkali liquids and gases. In some applications, alternate refractory materials are needed. An alkali cup test is a good gauge for material selection. In alkali testing of synthetic mullite aggregates with various binders, reducing cement content improved alkali reaction and corrosion.

Experience refractory experts can help determine the best refractory products to meet your specific biomass boiler requirements.

Author: Justin Teiken
Vice President of Sourcing and Product Development, Plibrico Co. LLC

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