1 HOW LIME PRETREATMENT IMPROVES YOUR BIOGAS YIELD Heiszwolf, J.J. 1, Dobbe, R. 2 and Brognaux, N. 1 1 Lhoist Business Innovation Center, Rue de l Industrie 31, B-1400, Belgium, 2 Lhoist Western Europe, Galvanistraat 1, 6716 AE, Ede, the Netherlands Corresponding Author Tel , Abstract In this paper it is shown how pretreatment of biomass results in the significant improvement of the biogas yield in the anaerobic digestion (AD) process. The pretreatment is carried out with Lhoist Lime which is a biocompatible material manufactured from high purity calcium oxide originating from natural resources. Apart from AD process improvement, Lhoist Lime has a variety of applications in anaerobic digestion, agriculture and biomass upgrading. The paper addresses the scientific concepts and practical application of lime pretreatment of biomass. It is shown that during the pretreatment easy digestible carbohydrates are formed and the enzymatic accessibility of the cellulose is increased. With a series of test using grass biomass, it is be demonstrated that after Lhoist Lime pretreatment, the resulting biomass shows significant improvement of the enzymatic digestibility and methane yield. It was found that the methane production of the AD process increased with 25% which contributes directly to the bottom line. In addition the pretreatment improves the operation of the anaerobic digester because the viscosity of the biomass is lower, the grass material is less prone to agglomeration and the throughput can be increased. Keywords Lime, alkali, biomass, pretreatment, enzymatic, digestibility, accessibility, methane, biogas, anaerobic digestion, lignocellulosic, biomass, cellulose, hemicellulose Introduction Biogas is a renewable energy carrier generated via the anaerobic digestion (AD) of biomass. Biogas is rapidly growing in Europe and contributes to our energy independence, reduction of CO2 emissions, and sustainable energy generation. The feedstock for AD units ideally consists of waste streams but in practice animal feeds (e.g. corn) are commonly used. The use of agricultural waste streams such as straw, grass or manure is hampered by their low biogas yield because of the high lignocellulosic composition. In this paper we will show how the application of lime pretreatment using Lhoist Lime can unleash the potential of low cost waste streams and can improve the biogas yield of fibrous biomass. What is Lhoist Lime? Lhoist Lime is a biocompatible product consisting of high purity calcium oxide made from natural resources. Lhoist Lime has a variety of applications in anaerobic digestion, agriculture and biomass upgrading. Lhoist Lime can be used in the pretreatment of biomass, can correct the ph of acidic digester feeds, contributes to the digester buffer capacity, assist in phosphate and ammonia recovery from manure streams, and is used to add value to organic waste streams.
2 Lhoist Lime Pretreatment The Lhoist Lime pretreatment of biomass is a simple process which can easily be carried out with inexpensive equipment. The process is as follows: the biomass is mixed with Lhoist Lime, heated to about ( o C), and is allowed to react for a certain residence time. Because the temperature remains well below the boiling point of water, no expensive pressure equipment is required. Moreover, the heat for the pretreatment can be supplied by the waste heat streams from the AD unit, for example from the exhaust gas of the gas combustion engine. After the Lhoist Lime pretreatment, the biomass can be fed directly to the anaerobic digester due to the biocompatibility of Lhoist Lime. Any remaining alkalinity will be neutralized by the CO2 in the digester to form calcium carbonate which is an excellent buffer for the biological ph values (Park et al., 2010). In the Lhoist corporate R&D centre, the Lhoist Lime pretreatment has been carried out in 3 litre temperature controlled stirred vessels, see Figure 1. With this set-up the commercial pretreatment process can be accurately simulated to quantify the enhancement of biogas production for different biomass feeds. Figure 1: Laboratory equipment for lime pretreatment of biomass consisting of three temperature controlled stirred tank reactors with individual temperature and ph measurement. Enhancement Mechanism The improvement of biogas yield by Lhoist Lime pretreatment is based on increased fibre digestion facilitated by a partial decomposition of the hemicellulose. This process increases the accessibility of the enzymes to cellulose and leads to the release of easy digestible short chain carbohydrates. As a result of both mechanisms, the Lhoist Lime treatment increases the biogas yield. It is important to add that the improvements to the biomass structure by Lhoist Lime pretreatment is not only advantageous for anaerobic digestion but also for bioethanol production and animal feed applications. In addition to the above two mechanisms, the alkaline Lhoist Lime pretreatment also increases the cellulose crystallinity, improves hydrolysis and causes less sugar degradation than acid pretreatment (Kumar et al. 2009).
3 In general, a less valuable biomass is higher in fibre content and is thus more difficult to process. At the same time, high fibre feeds are ideally suited for Lhoist Lime pretreatment. With Lhoist Lime pretreatment the economic value of low cost feeds can be upgraded. Experimental Procedure To demonstrate Lhoist Lime pretreatment experiments with grass were carried out. Grass is normally not used in anaerobic digesters because it is difficult to digest. The composition of the grass material was determined by the Flemish Institute for Agriculture and Fisheries (ILVO, Merlebeke, Belgium) and consisted mainly of fibrous material, see Figure 2. The pretreatment was carried out in the Lhoist laboratory by adding Lhoist Lime at three levels (3, 5, and 10 wt% CaO / DM) to a mixture of grass and water. The mixture was subsequently heated and kept at the elevated temperature for certain reaction times. Note that the reference sample, without Lhoist Lime, was also mixed with the same amount of water and was also heated to the same temperature and maintained for the same time. This means that the reference sample has been subjected to thermal pretreatment. Figure 2: Composition of the dried grass sample showing the values of the different components (wt% on dry matter basis) using the van Soest (1991) method. Figure 3: Fibre components of the biomass (wt% on dry matter basis) after pretreatment with and without Lhoist Lime. The fibre components were measured according to van Soest (1991). After the pretreatment the samples were cooled down, the fibre content was analyzed (ILVO), the enzymatic digestibility was measured (ILVO) and the biogas potential was determined by a commercial laboratory (Innolab, Gent, Belgium). For the biogas test about 70 gram of pretreated biomass was added to 3 litres of inoculum obtained from a mix of different commercial digesters. The biogas production was measured volumetrically on a daily basis and the methane content was determined at the end of the test by gas chromatograph. The biogas measurement was carried out at 38 o C out in duplo and the gas measurement was corrected by subtracting the gas production of a blanco test (only inoculum).
4 Performance Improvement Table 1 and Figure 3 show the composition of the biomass after the pretreatment with and without Lhoist Lime. It is obvious that the hemicellulose content of the biomass decreases significantly with increasing Lhoist Lime loading. At the same time, the total fraction of non-fibre material has increased. These results lead to the conclusion that in the Lhoist Lime pretreatment hemicellulose is converted to non-fibre material. Although the cellulose should not be affected by alkaline Lhoist Lime pretreatment, we observe a small increase in cellulose content. Figure 4: Enzymatic digestibility (de Boever, 1986, 1988) of the biomass samples after pretreatment with and without lime. Figure 5: Methane yield (Nm 3 /tonos) after mesophilic anaerobic digestion of the biomass samples after pretreatment with and without lime. The accessibility of the cellulose was determined by measuring the enzymatic digestibility of the biomass by cellulase. This test was developed in the eighties as an alternative for in vivo and in vitro rumen digestibility (de Boever, 1986, 1988) and is used today to determine the nutritional value of grass. From Figure 4 it is clear that after pretreatment with Lhoist Lime the enzymatic digestibility increases by more than thirty percent. This clearly demonstrates that after Lhoist Lime pretreatment the accessibility of the biomass for cellulase enzymes has indeed increased. We conclude that Lhoist Lime pretreatment the accessible nutritional value of grass.
5 Table 1: Composition of the biomass (wt% on dry matter basis) after pretreatment with and without lime. The fibre components were measured according to van Soest (1991) and the digestibility was measured according to de Boever (1986, 1988) Figure 5 gives the measured methane potential for the different samples and shows a 25% improvement of methane yield. Apparently, the increase in methane production is in agreement with the observed increase of the cellulase digestibility and a consequence of the reduction of fibrous content. In the biogas tests it was further observed that both the biogas and methane yield increase, the H2S content was very low (7 21 ppm) and that although the initial ph value differed, the final ph of the digestate was similar for all tests. Process Implementation The industrial implementation of the pretreatment process requires a number of principal process steps: (a) blending biomass and lime, (b) heating the mixture, and (c) maintaining a certain reaction time. Compared to other pretreatment technologies our lime based process has the advantage that the temperature remains below the boiling point of water so that no pressure vessels are required and that any excess lime does not need to be neutralized prior to feeding it to the digester. Any remaining lime will react in-situ with CO2 to form calcium carbonate which acts as a buffer in the digester. For installations where the biogas is fed to a gas engine to produce electricity, the residual heat from the gas engine can be used to heat the lime-treated biomass. In separate process tests it has been found that the lime pretreatment has a pronounced positive effect on the operation of the anaerobic digester. The biomass viscosity is reduced to a value 30% lower compared to the non-pretreated biomass which results in better mixing requiring a lower energy input. In addition, and specifically for grass, the breakdown of the fibrous material prevents the formation of long strings of grass material that can get caught by the impeller and can cause problems. In addition, floating layers of grass on top of the liquid are prevented. Lime treatment improves the capability of digesters to process grass feedstock material. It has also been found that the speed of digestion increases which may lead to a higher throughput of an existing digester or may be used to reduce the volume for newly designed ones. The process equipment to carry out the above steps has been designed and built by the Dutch company Byosis for industrial units. Such equipment can be installed as turn-key installations in addition to an existing biogas unit or can be integrated as part of a new digester. Figure 6 shows the installation of such equipment.
6 A B Figure 6: Installation of pretreatment equipment on site of a customer (A) and detail of the pretreatment installation (B). Conclusions Lhoist Lime is a biocompatible material consisting of high purity calcium oxide made from natural resources. Lhoist Lime has a variety of applications in anaerobic digestion, agriculture and biomass upgrading. Lhoist Lime pretreatment of biomass results in generation of easy digestible carbohydrates and increase of the enzymatic accessibility of the cellulose. After Lhoist Lime pretreatment, the resulting biomass shows significantly improved enzymatic digestibility and methane yield. The increase in methane yield directly contributes to the bottom line. The Lhoist Lime pretreatment improves the operation of the anaerobic digester because the viscosity of the biomass is lower, the grass material is less prone to agglomeration and the throughput can be increased. References De Boever, J. L., Cottyn, B. G., Buysse, F. X., Wainman, F. W., & Vanacker, J. M. (1986). The use of an enzymatic technique to predict digestibility, metabolizable and net energy of compound feedstuffs for ruminants. Animal Feed Science and Technology, 14(595), De Boever, J. L., Cottyn, B. G., Andries, J. I., Buysse, F. X., & Vanacker, J. M. (1988). The use of an enzymatic technique to predict digestibility, metabolizable and net energy of Forages. Animal Feed Science and Technology, 19(649), Park, J., Shiroma, R., Al-Haq, M. I., Zhang, Y., Ike, M., Arai-Sanoh, Y., Tokuyasu, K. (2010). A novel lime pretreatment for subsequent bioethanol production from rice straw--calcium capturing by carbonation (CaCCO) process. Bioresource Technology, 101(17),
7 Kumar, P., Barrett, D. M., Delwiche, M. J., & Stroeve, P. (2009). Methods for Pretreatment of Lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production. Industrial & Engineering Chemistry Research, 48(8), Van Soest, P. J., Robertson, J. B., & Lewis, B. A. (1991). Methods for Dietary Fibre, Neutral Detergent Fibre and Nonstartch Polysaccharides in Relation to Animal Nutrition. J. Dairy Sci, 74,